Advances in Agronomy, Volume 14

ADVANCES IN

AGRONOMY VOLUME 14

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ADVANCES IN

AGRONOMY Prepared under the Auspices of the AMERICANSocm-ry OF AGRONOMY

VOLUME 14 Edited by A.

G. NORMAN

The Uniuetsity of Michigan, Ann Arbor, Michigan

ADVISORY BOARD E. G. HEYNE F. L. PATTERSON R. W. PEARSON

W. H. h L A W A Y W. H. F ~ T E C. 0. G m m

1962

ACADEMIC PRESS

New York and London

COPYRIGHT @ 1962, BY ACADEMICPRESSINC. ALL RIGHTS RESERVED

NO PART O F THIS BOOK MAY BE REPRODUCED IN ANY FORM BY PHOTOSTAT, MICROFILM, OR ANY OTHER MEANS, WITHOUT WRITTEN PERMISSION FROM THE PUBLISHERS

ACADEMIC PRESS INC. 111 FIFTHAVENUE NEW YORK 3, N. Y.

United Kingdom Edition Published by ACADEMIC PRESS INC. (LONDON) LTD. BERKELEYSQUAREHOUSE,LONDON W. 1

Library of Congress Catalog Card Number 50-5598

PRINTED IN THE UNITED STATES O F AMERICA

CONTRIBUTORS TO VOLUME 14 C. ROY ADAIR,Research Agronomist, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland

L. T. ALEXANDER,Chief, Soil Survey Laboratoy, Soil Conservation Semice, United States Department of Agriculture, Plant Industy Station, Beltsville, M aylund H. M. BEACHELL, Research Agronomist, Crops Research Di&on, Agricultural Research Service, United States Department of Agriculture, Beaumont, Texas R. L. BERNARD, Research Geneticist, United States Regional Soybean Laboratory, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, Urbana, Illinois D. R. BOULDIN,Soil Chemist, Tennessee Valley Authority, Muscle Shoals, Alabama

J. G. CADY,Soil Scientist, Soil Survey Laboratory, Soil Conservation Service, United States Department of Agriculture, Plant Industry Station, Beltsville, Maryland J. L. CARTTER,Agronomist-in-charge, United States Regional Soybean Laboratory, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, Urbana, Illinois

MARLIN G. CLINE,Professor of Soil Science, Department of Agronomy, Cornell University, Ithaca, New York

E. E. HARTWIG, Research Agronomist, United States Regional Soybean Laboratoy, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, Stoneville, Mississippi HERBERT W. JOHNSON, Research Agronomist, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland DONKIRKHAM, Curtiss Distinguished Professor of Agriculture and Professor of Soils and Physics, Iowa State University, Ames, Iowa

RAYMONDJ. KUNZE, Assistant Professor of Soils, Department of Agronm y , Iowa State University, Ames, Iowa V

vi

CONTRIBmoRs

M . D. MILLER,Extension Agronomist, Agronomy Department, University of California, Davis, CaZifornia S. SNARAJASINGHAM, Assistant Chemist, Soil Surceys, Department of Agriculture, Peradeniya, Ceybn

DWIGHTD. SMJTH,Assistant Director for Water Management, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Beltsoilk, Maryland

GILBERT L. TERMAN, Agronomist, Tennessee Valley Authority, Muscle Shoals, Alabama

FRANKG. VETS, JR., Chief Soil Scientist, Northern Plains Branch, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Fort Collins, Colorado J. R. WEBB, Associate Professor of Soils, Department of Agronomy, Iowa State University, Ames, Iowa

WALTER H . WISCHMEIER, Research Investigations Leader for Water Erosion, Corn Belt Branch, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Purdue University, Lafayette, lndiana

PREFACE The eight chapters in this volume fall into the general pattern established for this series, which is to include reviews of research progress in soil and crop science and developments in agronomic practice. The central theme is the soil-plant relationship. Some European reviewers of this series have expressed the view that the range of subjects covered is far too wide to justify the implied suggestion that they are all branches of one science and that the literature reviewed is predominantly American. Essentially this criticism hinges on the definition of the word “agronomy” which in European usage and particularly British usage does not have the same connotation as in the U.S. Indeed one British reviewer states that “In England, little would be left for agronomy when the claims of chemistry, entomology, plant pathology and so on had been stated-perhaps the study of green manuring, seed rates and sowing dates.” As understood in the United States there is, however, a professional field of agronomy in which the above and many other disciplines have a part. There is a professional organization of agronomists with upwards of 4,000 members trained in a variety of disciplines which they bring to bear on a great diversity of problems relating to the soil, and its efficient use in the production of economic crops. Much of the science involved is international; but there are aspects that are regional and must be so. For example, in this issue there are two extensive reviews dealing respectively with the genetics of soybeans and the management of the soybean crop. Some sixty percent of the world soybean production is located in the United States. An even higher percentage of the total scientific work on this fascinating crop plant is carried on in the United States, and it is inevitable, therefore, that the literature should be predominantly American. Much the same applies to the article on rice production in the United States, where man hour per acre have been reduced to an astonishingly low figure. In contrast, attention should be drawn to the authoritative review on the subject of laterite by Sivarajasingham, Alexander, Cady and Cline, which reflects the world-wide distribution of the investigators of laterites rather than the distribution of lateritic soils. Greater fertilizer usage accounts in part for the steady yield increases recorded in most countries in recent years. In the development of fertilizers considerable attention is being directed towards new and unconventional materials, the evaluation of which presents challenging problems. Some of these are discussed by Terman, Bouldin and Webb. vii

viii

PmFACE

Viets, on the other hand, considers the involved relationships between fertilizer usage and the water requirement of crops, a very important issue in many areas of the world where rainfall is erratic and water reserves inadequate. The remaining articles deal directly with soil properties. Kirkham and Kunze discuss some of the applications of the use of isotopes and radiation to problems in soil physics, and Smith and Wischmeier the physical principles of soil erosion by rain. A. G . NORMAN Ann Arbor, Michigan July, 1962

CONTENTS

CONTRIBUTORS TO VOLUME14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Puge v

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii

LATERITE BY S. SIVARAJASJNGHAM. L. T. ALEXANDER.J . G . CADY.AND M. G. CLINE I. I1. I11. IV. V VI. VII .

.

The Term “Laterite” ........................................ The Nature of Laterite .................................... The Environment of Laterite ................................ Profiles Containing Laterite ................................. Formation of Laterite ...................................... Geomorphic Relationships .................................. Softening of Laterite ....................................... References ...............................................

1 5

14 20 26 53 55 56

RICE IMPROVEMENT AND CULTURE IN THE UNITED STATES

.

BY c. ROY ADAIR. M. D. MILLER.AND H . M BEACHELL

I. I1. I11. IV. V.

Introduction .............................................. Rice Culture in the United States ............................ Rice Field Pests ........................................... Origin, Botany. and Genetics of Rice ......................... Rice Breeding and Improvement in the United States . . . . . . . . . . . References ................................................

61 68 85

92 96 104

RAINFALL EROSION BY DWIGHTD . SMITHAND WALTERH . WBCHMEIER I. I1. I11 IV .

.

Introduction .............................................. Mechanics of Rainfall Erosion ............................... Basic Factors Affecting Field Soil Loss ........................ Soil Loss Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

109 113 123 137 144

CONTENTS

X

SOYBEAN GENETICS A N D BREEDING BY HERBERTW . I. I1 . 111. IV . V.

JOHNSON AND

RICHARDL . BERNARP

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reproduction ............................................. Genetics of Qualitative Characters ........................... Genetics of Quantitative Characters .......................... Breeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

149 152 157 172 199 218

FERTILIZERS AND THE EFFICIENT USE O F WATER

BY FRANK G . VIETS. JR. I. I1. I11. IV .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . ................. Definition of the Problem ............................. ........... Validity of Evapotranspiration Data . . . . . . . . . . . The Effects of Fertilizers on the Relationship of Evapotranspiration and Yield ......................... ..................... V . Fertilizers and Water-Use Efficiency in Terms of Applied Water . . VI . Fertilization and Water-Use Efficiency with Limited Moisture Supply ................................................. .... VII . Fertilization and Moisture Extraction by Roots . . . . . . . . VIII . Fertilizers and the Infiltration of Water ...................... IX . Fertilization. Crop Maturity. and Water Use . . . . . . . . . . . . . . . . . . x . Other Practices for Increasing Water-Use Efficiency . . . . . . . . . . . . XI . Is Maximum Water-Use Efficiency Desirable? . . . . . . . . . . . . . . . . . . ..... XI1. Conclusions .......................... References . . . . . . . . . . ........................

223 226 228 233 246 246 252 254 256 257 259 260 261

EVALUATION OF FERTILIZERS BY BIOLOGICAL METHODS

BY G. L TERMAN.D. R . BOULDIN.AND J . R . WEBB I . Introduction ......................... . . . . . . . . . . . . . . . . . . . . . 11. Chemical and Physical Characteristics of Fertilizers . . . . . . . . . . . . . I11. Concepts of Fertilizer Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Methods Used in Fertilizer Evaluation Tests . . . . . . . . . . . . . . . . . V . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References

...............................................

265 266 280 295 316 317

xi

CONTENTS

ISOTOPES METHODS A N D USES I N SOIL PHYSICS RESEARCH BY DONKIRKHAMAND RAYMONDJ . KUNZE

I. I1. I11. IV. V. VI . VII . VIII . IX . X.

Introduction .............................................. Soil Water ............................................... Soil Density and Soil Structure .............................. Soil Aeration ............................................. Soil Temperature ........................................... Soil Particle Movement .................................... Transformation of Soil Materials from One Form to Another ..... Soil Profile Formation and Dating ............................ Disposal of Radioactive Waste .............................. Proposed Future Work ..................................... References ................................................

321 322 342 347 348 348 350 352 353 354 355

THE MANAGEMENT OF SOYBEANS BY JACKSON L . CARTTER AND EDCARE . HARTWIG

.

1 Introduction .............................................. I1. Soil and Climatic Adaptation ............. I11. Time of Planting and Varietal Adaptation .................... IV Planting Methods and Equipment ............................ V Rotation Practices and Erosion Control ........................ ....... VI . Weed Control .................... ....... VII. Seed Quality and Seed Treatment ... VIII Nutrient Requirements ..................................... IX Water Requirements and Utilization .......................... X. Growth-Regulating Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI . Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI1 Seed Storage ............................................. XI11 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . .

. .

...................

360 365 372 378 383 386 389 390 401 403 404 406 407 . . . 408

AUTHOR INDEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

413

SUBJECT INDEX ......................................................

427

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LATERITE S. Sivarajasingham, L. T. Alexander, J. G. Cady, and M. G. Cline Department of Agriculture, Peradeniya, Ceylon, United States Department of Agriculture, Belhville, Maryland, and Carnell University, Ithaca, New York

Page I. The Term “Laterite” . . ...................... 11. The Nature of Laterite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Physical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Chemical Characteristics ................................ C . Mineralogical Characteristics . 111. The Environment of Laterite . . . . . A. Climate . . . . . . . . . . . . . . . . . . ......... ......... ......................................... C. Parent Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ IV. Profiles Containing Laterite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Soil Material Overlying Laterite .......................... B. Laterite within Soil Horizons .............................. C. Horizons or Layers Beneath Laterite ...................... V. Formation of Laterite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Weathering as a Preconditioner of Material of Laterite . . . . . . . . B. Development of Microstructures .......................... C. Hardening of Laterite .................................... D. Development of Laterite in Place without Enrichment from Outside Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Enrichment from Outside Sources ........................ F. Principal Processes Involved . .................. VI. Geomorphic Relationships . . . . . . . ...................... VII. Softening of Laterite . ............................ References . . . . . . . . . . ..................

1

5 5 7 14 15 16 20 20 21 22 24 26 28 38 39

45 45 53

55

1. The Term ”Laterite”

The term ‘laterite” was originally coined by Buchanan (1807) for a ferruginous, vesicular, and apparently unsbatified material occurring in immense masses over the country rock of Malabar in India. The freshly 1 This review constitutes Agronomy paper No. 542, Cornell University, Ithaca, New York.

1

2

S . SIVARAJASINGHAM ET AL.

dug material, as described by Buchanan, was soft enough to be readily cut into blocks by an iron instrument, but upon exposure to air it quickly became as hard as brick and remarkably resistant to the action of air and water. Since this material was used as building brick, and was called “brickstone” in several of the indigenous languages, Buchanan aptly called it ‘laterite” after later-the Latin for brick. Prescott (1954, pp. 1-2) has reported that Buchanan did not use the term “laterite” in his journals between 1807 and 1814 but used the term “brickstone” and that Babington (1821) was the first to use ‘laterite” in formal scientific literature. According to Prescott and Pendleton (1952, p. I ) , Buchanan used the word “brickstone” only in his later travels (1807-1813) through what is now Bihar to describe occurrences in the Rajmahal hills: “He noted the similarity of the Bihar occurrences to those of Malabar, but was puzzled by the fact that the former masses of material while still in the ground and excluded from the air retained their stony form.” At first in Malabar, he had used the terms ‘laterite” and “brickstone” interchangeably in his descriptions of the soft materials that harden; his later use of the term ‘8rickstone” in Malabar may have been out of a desire to reserve the term “laterite” for the soft ferruginous material that hardens. Though some of Buchanan’s immediate successors ( Voysey, 1833; Stirling, 1825) used “iron clay” as an alternative term, the word ‘laterite” gradually came into wide use in India. Detailed morphological descriptions of laterite, still considered to be among the most vivid, were given by Newbold (1844, 1846). Interest in laterite was stimulated in other parts of the world by the publication of a chapter by Blanford (1879) in the first Manual of the Geology of India, with which the name laterite became finally confirmed (Fox, 1936). Before the end of the nineteenth century, laterite as a surficial or shallow formation was identified on the basis of physical characteristics in many widely distributed areas of Australia, Africa, and South America. Fermor (1911) considered use of the term only for soft materials that could be cut into bricks, a severe restriction, though such use appears to conform with the intentions of Buchanan. Blanford (1859) had mentioned that in some cases the lithomarge underlying laterite becomes hard on exposure, and Harrison (1910) also recorded the occurrence of mottled, creamy white and dark red sesquioxide-poor deposits that harden on exposure. Thus, the property of hardening as a criterion of laterite became controversial. Later this aspect was further confused when Talbott in Australia (Prescott, 1931) extended the term

LATERITE

3

to include not only hard ferruginous surface formations, but also siliceous and travertine crusts, designating them ferruginous, siliceous, and calcareous laterite, respectively. Prescott, however, confined “laterite” to the ferruginous and aluminous forms and cited the earlier suggestion of Woolnough (1927), who introduced the term “duricrust” to cover the other kinds of crusts. Though many earlier observers had suggested the ferruginous and even the aluminous nature of laterite (Mallet, 1883), the fundamental chemical character of laterite was first established by Bauer (1898). His analyses revealed the low content of silica and the high contents of alumina and iron oxide in samples collected from the Seychelles. Subsequent investigations of samples from many different parts of the tropical region gave similar results (du Bois, 1903; Warth and Warth, 1903; Holland, 1903). Great interest developed because of the possible use of laterite as an ore for aluminum (Holland, 1905) and, in some cases, for manganese (Fermor, 1909). Consequently, much of the early work was confined to chemical analysis of bulk samples that were selected for high aluminum content. This prompted a reviewer (Bull. Imp. Inst. 1909, vii, p. 133) of Harrison’s work to suggest that the term laterite be restricted to products of weathering containing free alumina. As a result, controversy developed among geologists regarding the chemical properties of laterite. Fermor (1911) finally abandoned the physical property of hardness of a material in its natural state or on exposure as a criterion of laterite and developed a comprehensive system of nomenclature of lateritic materials on the basis of chemical composition, though he rejected the presence or absence of alumina in large quantity suggested by Crook (1909). He subscribed to the views of Evans (1910) that “though the chemical composition of laterite varies within wide limits, . . . one feature remains constant-the small amount of combined silica in proportion to the alumina present. . . . It is in this respect that laterites differ from clays, which also occur as tropical decomposition products.” Fermor, consequently, based his classification on arbitrary limits of the ‘lateritic” constituents, which he defined as the oxides of iron, aluminum, titanium, and manganese. Meanwhile Walther ( 1889,1915,1916) had erroneously assumed that the term “laterite” had been chosen to signify red color and “proposed that the word should be used for all red-colored alluvia” (Prescott and Pendleton, 1952, pp. 35-36). Ultimately, any tropical red earth came to be called ‘laterite” or “lateritic.” As studies of tropical soil progressed, attempts were made to standardize the use of these two terms on the

4

S. SIVhRAJASINGHAM ET AL.

basis of chemical composition. The silica-alumina ratio and, later, the silica-sesquioxide ratio were used to classify soils into “laterite,” “lateritic,” and “nonlateritic” (Martin and Doyne, 1921, 1930; Joachim and Kandiah, 1935). The terms received even wider connotation with the adoption of “laterite” and “lateritic” as the names of Great Soil Groups by the Lhited States Soil Survey Staff (Byers et al., 1938; Baldwin et al., 1938). Pendleton (1936) strongly urged that the term “laterite” be restricted to the original concepts of Buchanan, restated nearly 100 years later by Oldham ( 1893). As a consequence of this rigorous definition of “laterite,” Pendleton and Sharasuvana (1946, p. 434) defined a “laterite” soil as “one in which a laterite horizon is found in the profile.” They considered a “lateritic” soil to be “one in which there is an incipient or immaturely developed laterite horizon, and in which it is believed a true laterite horizon will develop if the prevailing conditions persist long enough.” The definitions put fonvad by du Preez (1949) for ‘laterite soil” and “lateritic soil” are essentially similar to those of Pendleton and Sharasuvana. His definition of laterite, like that of Pendleton and Sharasuvana, fails to recognize the importance of alumina, and while he covered some morphologicaI aspects of laterite comprehensively, he ignored the soft variety that hardens on exposure. Mohr and van Baren (1954) defended use of the terms ‘laterite” and “lateritic” for soil on grounds of similarity of weathering products that produce soil as well as material that hardens. Kellogg (1949, p. 79) confined the term ‘laterite” to four principal forms of sesquioxide-rich material that either are hard or that harden upon exposure: ( 1 ) soft mottled clays that change irreversibly to hardpans or crusts when exposed, ( 2 ) cellular and mottled hardpans and crusts, ( 3 ) concretions or nodules in a matrix of unconsolidated material, ( 4 ) consolidated masses of such concretions or nodules. The Soil Survey Staff of the United States Department of Agriculture (1960, p. 62) proposed a new term, plinthite (Gk. plinthos, brick), for essentially the same concept, defining it as “the sesquioxide rich, humus poor, highly weathered mixture of clay with quartz and other diluents, which commonly occurs as red mottles, usually in platy, polygonal, or reticulate patterns; plinthite changes irreversibly to hardpans or irregular ( hard) aggregates on repeated wetting and drying, or it is the hardened relicts of the soft red mottles.” The term “plinthite” was introduced to avoid the confusion arising from use of the word “laterite” without precise definition for many widely divergent materials. In this paper, the term laterite is retained as a term that would be recognized by most readers, though its use is restricted to material that conforms generally with the definitions of laterite by Kellogg (1919) and of plinthite by the Soil Survey Staff (1960).

LATERITE

5

II. The Nature of Laterite

The term laterite is restricted in the remainder of this paper to highly weathered material ( 1 ) rich in secondary forms of iron, aluminum, or both; ( 2 ) poor in humus; ( 3 ) depleted of bases and combined silica; ( 4 ) with or without nondiagnostic substances such as quartz, limited amounts of weatherable primary minerals, or silicate clays; and ( 5 ) either hard or subject to hardening upon exposure to alternate wetting and drying. The term as used implies no restrictions, other than those inherent in the properties defined, on size or shape of the masses, on their internal organization, on the processes by which diagnostic properties have developed, or specific conditions of place or time as factors essential to such development. In this sense it includes Buchanan’s laterite and hardened equivalents of it. In addition, it includes certain highly weathered material in sesquioxide-rich humus-poor nodules2 that are hard or that harden upon exposure, though they may be surrounded by earthy material that does not harden, as well as masses of such nodules cemented together by sesquioxide-rich material. It excludes ( a ) sesquioxide-rich earthy material, which has been called “laterite” or “lateritic soil,” that does not harden upon exposure; ( b ) ironrich masses or nodules with significant amounts of humus, which are characteristic of certain podzols; ( c ) hard masses cemented by silica, carbonates, or substances other than sesquioxides, though highly weathered sesquioxide-rich fragments or nodules within such masses might be included; and ( d ) certain hard pellets or “shot” found in slightly weathered material. A. PHYSICAL CHARACTERISTICS Laterite occurs in various morphological forms. Pendleton and Sharasuvana (1946, p. 438) recognized two distinct physical forms, vesicular and pisolitic, with many types intermediate between the two. Du Preez (1949, p. 57) has defined laterite as “a vesicular, concretionary, cellular, vermicular, slaglike, pisolitic or concrete-like mass.” The description of a vesicular laterite by Newbold (1844) is quoted in part here after Prescott and Pendleton (1952, p. 5). “The laterite . . . , generally speaking, is a purplish or brick red, porous rock, passing into liver brown perforated by numerous sinuous 2 The term “nodule” is used not only in the sense defined by Bryan (1952) to include “rounded lumps” of a variety of compositions whether formed by accretion or by centripetal enrichment, but also to include rounded fragments of laterite inherited from a laterite crust.

6

S. SIVARAJASINGHAM ET AL.

and tortuous tubular cavities either empty, filled, or partially filled with a greyish-white clay passing into an ochreous, reddish and yellowish brown dust; or with a lilac-tinted litheomargic earth. The sides of the cavities are usually ferruginous and often of a deep brown or chocolate color; though generally not more than a line or two in thickness, their laminar structure may frequently be distinguished by the naked eye. . . . The interior of the cavities has usually a smooth polished superficie, but sometimes mammillary, and stalactiform on a minute scale . . . . The surface masses of the softer kinds present a variegated appearance. The clay and lithomarge exhibit lively colored patches of yellow, lilac, and white, intersected by a network of red, purple, or brown. The softness of this rock is such that it may be cut with a spade; hardening by exposure to the sun and air, like the laterite of Malabar.” (Omissions are by the present authors.) Vesicular laterite may be soft or of varying hardness and commonly has earthy material in the cavities. It usually occurs near the surface. Cellular slag-like laterite is a scoriaceous mass. The many empty cavities are separated by ferruginow material similar in appearance to that which separates the earthy substance in vesicular laterite. Cellular laterite is usually dark colored and may have a dull or lustrous surface. It is of varying hardness and is brittle, being usually easily shattered when struck a sharp blow with a hammer. According to Fox (1936), cellular laterite is formed by removal of kaolin and other earthy material from the cavities in vesicular laterite when the latter is exposed to erosion and leaching at the surface. Falconer (1911), from his observations in nothern Nigeria gave a similar explanation, though he avoided using the term laterite for “surface ironstone.” Nodular laterite consists of individual concretions, pisolites or other crudely round masses, usually the size of a pea but commonly larger or smaller; it is generally ferruginous. The nodules may occur as a superficial covering or as a component in one or more horizons in the soil, varying in concentration from low or insignificant amounts to very high amounts. The nodules vary in hardness; some can be readily cut by a knife but most are hard and brittle. When the nodules of a layer are cemented together, hard “pisolitic” or “concrete-like” laterite is formed. It occurs mainly at or near the surface. The individual nodules may either be joined directly to one another or be discrete entities in a cementing matrix of similar, but usually less ferruginous, material. Recent studies by Alexander and Cady ( 1962) present enlightening detail on the physical arrangement of discrete components. Though various specimens exhibit a great variety of micromorphological features,

7

LATERITE

certain structures are common to many, but not necessarily all, varieties. Commonly under magnification in thin sections, tiny bodies ranging from perfect spheres to oblong rounded forms may be seen embedded in a matrix of fine particles; the matrix may be either very dense or spongelike. The rounded bodies may be individual units or, commonly, may be aggregates of smaller spherical units closely packed. Such rounded bodies may be widely spaced or closely packed in the matrix. Their boundaries may be smooth and definite or irregular and indefinite in various specimens. The matrix may be unorganized, may have a gridlike rectangular or reticulate network of oriented material, or may be largely oriented. Oriented material commonly lines pores and may appear as skins on the nodules. Crystalline oriented material is common as pseudomorphs after primary minerals, as porefillings, and as discrete bodies ranging from barely visible units to relatively large homogeneous masses. Rock structure may be preserved or may be entirely absent. Quartz particles may be included, and in some specimens weatherable minerals encased in a protective covering of weathered material have been observed.

B. CHENICAL CHARACTERISTICS Materials identified in the field as laterite have a wide range of chemical characteristics. A prominent feature common to all laterites, nevertheless, is a high content of either iron or aluminum or both relative to other constituents (Alexander et al., 1956). This is clearly illustrated by the following analyses, which are thought to be typical examples (Table I ) . Bases are almost completely absent. Combined silica is generally TABLE I Chemical Composition ( % ) of Selected Laterites Constituent Quartz Feldspar SiO, A1203 Fe7.03 TiO, CaO H,O (loss on ignition)

Site:a

1

2

3

4

5

0.76 NDb 1.77 4.32 80.02 6.06

ND ND 1.93 62.32 1.88 11.87

4.32 2.35 17.08 20.83 40.18 1.72

7.06

ND ND 0.37 43.83 26.61 4.45 0.86 23.88

21.54

11.05

ND ND 31.37 19.22 38.51 1.12 0.10 9.10

-

99.99

100.00

97.53

99.42

-

-

99.54

-

-

Site: 1, Coolgardie, Australia (Simpson, 1912). 2, Satara, Bombay, India (Warth and Warth, 1903). 3, Bagru Hill, Bihar, India (Fox, 1936). 4, Cheruvannur, India; Buchanan’s original site (Fox, 1936). 5, Djougou, Dahomey; laterite on granite (Alexander and Cady, 1962). 0 ND, not determined.

8

S. SIVAFUJASINGHAM ET AL.

low (sites 1, 2, and 3, Table I ) , but some varieties, such as the original laterite of Buchanan (site 4, Table I ) , may have significant amounts. This is probably largely in the form of kaolin, which has been found in recent work by Alexander and Cady (1!362) to be the principal or only identifiable silicate clay mineral in samples from Africa. Alumina may be the principal sesquioxide (site 3, Table I ) , but more commonly iron oxide (site 1, Table I ) or iron oxide and alumina together (sites 2, 4, and 5, Table I ) are the major constituents. Combined water, determined by loss on ignition, is appreciable but is generally higher in aluminous than in ferruginous varieties, as is shown in Table I. Titanium is also common in significant amounts in most varieties and may be a major constituent (site 3, Table I ) . Vanadium and chromium are found, but rarely in appreciable quantities. Quartz may be absent or present in only limited amounts, but on rocks high in quartz it is commonly a significant or major component, as on the granite of site 5 of Table I, for which petrographic studies showed that much of the total silica was contributed by quartz. Quartz is also common in laterite over nonquartzose rock, where it appears to be derived mainly from wind-blown or detrital material from outside sources. Ten samples of detrital laterite from various parts of India had an average of ,2074 quartz (Warth and Warth, 1903). Pendleton and Sharasuvana (1912, p. 10) have emphasized that differences in amount of quartz commonly contribute to major variation in SiO, among samples, even in the same profile. The impoverishment in combined silica and bases and concentration of sesquioxides during weathering and laterite formation on a dolerite is illustrated in Table 11. The “primary laterite” of Harrison is not to be confused with ‘laterite” as used in this review. It was a weathered earthy product that lay between the surficial hard laterite crust and the unweathered dolerite rock. Major differences in proportions of iron and aluminum and in amount of combined water between weathered material and laterite presumed to have formed in similar material are common, as in Table 11, but it is rarely possible to be certain that the laterite crust has indeed formed in material like that of the underlying weathered product. No consistent relationship seems to exist between the relative amounts of silica, iron, and alumina and the degree to which the physical properties of laterite are developed. The shortcoming of any chemical classification was shown by Fox (1936) from the analyses of laterite samples from Buchanan’s original sites (site 4, Table I ) . These would have been called ‘lateritic lithomarge” in Fermor’s ( 1911) classifkation because of the high content of combined silica, though the material was vermicular and was being quarried for building purposes.

9

LATERITE

The analyses considered so far refer to bulk samples of massive laterite without distinction between segregated nodular material and the matrix. The nodular material is, however, found either to be similar in composition to the matrix or to contain less combined silica and more TABLE I1 Chemical Composition ( ”/o ) of Dolerite, “Primary Laterite,” and Associated Laterite Ironstone at Eagle Mountain, British Guianaa Constituent

Dolerite rock

“Primary laterite”

Laterite ironstone

2.40 49.60 17.29 2.90 8.26 0.35 0.53 0.05 6.95 8.80 0.18 2.81

2.86 0.50 46.80 23.64 2.50 22.96 0.69 Nil Nil Nil Nil Nil

0.14 0.62 10.54 74.43 0.65 9.60 3.91 Nil Trace 0.02 Nil Nil

99.95

99.91

H,O (loss on ignition) TiO, MnO MgO CaO K,O Na,O a

-

-

100.12 From Harrison (1933).

TABLE I11 Selected Chemical Constituents (%) of Nodules of Laterite at 5 Sites and of the Matrix in Which the Nodules Were Embedded at One of Them 5 Constituent Site:a SiO, A1203 Fe,% TiO, MnO,

1

2

3

4

Soft nodules

Matrix

8.0 4.7 67.9

29.7 29.1 21.7 2.0 2.3

26.1 14.2 20.7 6.3 13.1

49.8 2.7 28.9 9.8 1.1

39.3 19.3 30.1 0.9 0.1

54.8 20.6 13.5 1.0 0.1

0.5 Nil

a Site: 1, Natal; ferruginous nodules (Beater, 1940). 2, Welimada, Ceylon; aluminous and silicious nodules (Joachim and Kandiah, 1941). 3, Peradeniya, Ceylon; manganiferous nodules (Joachim and Kandiah, 1941). 4, Hambantota, Ceylon; titaniferous nodules (Joachim and Kandiah, 1941), 5, Congo; soft nodules and matrix (Alexander and Cady, 1962).

ferric oxide. Site 5 of Table 111 illustrates the latter in soft nodules of a ground-water laterite of the Congo. Prescott and Pendleton (1952, p. 21) believed that nodules usually contain less free alumina than the more massive forms and that they are low in manganese.

10

S. SIVARAJASINGHAM ET

AL.

Nodules studied by Alexander et al. (1956) were high in sesquioxides and low in silica. Other workers have, however, reported appreciable contents of both quartz and combined silica (Joachim and Kandiah, 1941; Waegemans, 1954). The data in Table 111 illustrate the wide range of silica, alumina, iron, titanium, and manganese in nodules from different places. The work of Bennett and Allison (1928) also reveals variations in the composition of nodules in different soils.

L C. M W ~ L O G I C ACHARAC~ERISTLCS Chemical analysis alone is not sufficient to reveal the nature and origin of laterite ( Harrison, 1910; Campbell, 1917). Laterites having similar physical properties, such as hardness or morphology, may differ greatly in chemical composition, and, conversely, laterites having similar chemical compositions may have greatly different physical properties. Petrographic studies of thin sections ( Harrison, 1910, 1933), adsorption of dyes (Hardy and Rodrigues, 1939), differential thermal analysis (Humbert, 1948; Bonifas, 1959), and X-ray analysis (Alexander et al., 1956; Bonifas, 1959) have been used to supplement chemical determinations. Free alumina is mostly in the form of gibbsite (A1203.3H20),as boehmite ( A1203.H20), or as an amorphous hydrated form which has been called cliachite and a variety of other names (Hanlon, 1944; Palache d aE., 1944). Iron is found in the form of goethite (FeO-OH), hematite ( FeeOa),and as amorphous oxides or unidentifiable coatings on other minerals (Alexander et al., 1956). Free silica is mostly inherited quartz (Alexander et al., 1956),though Harrison ( 1933, p. 40) reported PLATE 1 Photomicrographs illustrating features of weathering and laterite formation A. Weathering diorite, North Carolina. Crossed Nicols. The lath-like forms are gibbsite pseudomorphs after feldspar. Some dark areas are allophane and some are iron oxide. €3. Soft laterite from granite, Nigeria. Crossed Nicols. The light areas of the crystal aggregate (upper left) and of the filled channel (lower right) are gibbsite formed upon weathering of kaolinite. Dark areas are iron-impregnated clays and iron oxides, which are isotropic or have a very low birefringence. C. Hard laterite from granite, Nigeria. Plain light. The dark areas are impregnated with iron by local redistribution from the light areas. The higher population of quartz grains (white areas) in the part that has lost iron indicates that these parts have collapsed. D. Hard laterite from granite, Kigeria. Crossed Nicols. The yellow parts are crystalline goethite which forms a continuous network, especially on the walls of the small channel at the left. The dark streak through the center is a former channel filled with fine-grained hematite. White spots are quartz grains.

A

C

B

D

PLATEI

This Page Intentionally Left Blank

LATERITE

13

secondary quartz in laterites presumed to have been derived from basic igneous rocks containing only small amounts of quartz. Chalcedony and opaline forms have been observed also. Presence of colloidal silica has been suspected. Besides quartz, other resistant minerals like magnetite, zircon, sphene, anatase, and ilmenite have been found. Combined silica is known to occur in kaolin and comparable silicate clay minerals that can be identified but is present in amorphous or subcrystalline forms as well. Montmorillonite and illite types of clays have not been identified in significant amounts. The distribution and form of constituents within the laterite has special significance. Recent studies by the authors have shown that the finely divided matrix is commonly largely unoriented and unidentifiable by petrographic techniques, though identsable portions range from none to most of the material. Grains and large patches of oriented aggregates ranging from kaolin slightly stained with iron oxide through kaolin highly impregnated with iron oxide to almost pure goethite or hematite are among the more prominent identifiable constituents. Variable optical density within the matrix is related to the degree of iron impregnation (Plate 1, C and D). Hardness of the mass appears to be related to the degree of crystalinity and continuity of the crystalline phase of the impregnating iron, which is largeIy in the form of goethite. Tiny spherical bodies comparable to incipient nodules of centripetal enrichment described by Bryan (1952) are largely unoriented earthy material having a higher degree of iron impregnation than the surrounding matrix. Some have films of crystalline goethite on the surface or as concentric shells within the body. Other spherical bodies embedded in the matrix may be concretionary or pisolitic forms of gibbsite, boehmite, goethite, or hematite. Gridlike networks of oriented materials are found in the matrix of many specimens and are composed principally of goethite (Alexander et al., 1956) or hematite ( Sivarajasingham, 1961). Gibbsite pseudomorphs after feldspar (Plate 1A) and goethite pseudomorphs after ferromagnesian minerals are common constituents. These appear to be most abundant in young laterites whereas concretionary or pisolitic nodular forms of the same minerals are more conspicuous in older varieties (Alexander et al., 1956). Gibbstite and boehmite are found commonly as fillings in cracks and pores, both in the matrix and in nodules. Pores and cracks may also be lined with oriented kaolin having varying degrees of iron impregnation, or even with oriented goethite (Plate 1D). In hard laterite crusts, hematite may be found as pore linings, as bands, or as discrete masses (Plate 1D). In varieties derived from quartzose material, quartz grains are generally distributed randomly through the matrix (Plate 1C) and earthy nodular bodies.

14

S. SIVARAJASINGHAM ET AL.

Quartz, apparently derived from outside sources, may also be present in laterite over nonquartzose rock (Alexander et aZ., 1956). 111. The Environment of Laterite

Laterite is widely distributed in Africa, Australia, Southeast Asia, and South America; Prescott and Pendleton (1952) have given a comprehensive review of its geographical distribution. The observed distribution of laterite relative to present environment, however, is not necessarily a criterion of conditions under which laterite forms (Hallsworth and Costin, 1953, p. 25; Pendleton, 1936, p. 107). A. CLIMATE Pendleton (1941) considered that an effective rainfall capable of supporting a forest is necessary and that laterite probably never develops in a climate that would support a savannah-type climax vegetation. Maignien (1958, p. 15) concluded that the Sudano-Guinea climate of 80 or more inches of rain per year with 2 to 4 months relatively dry is optimum for mobilization, accumulation, and induration of iron in laterite. Humbert ( 1948, pp. 281-282) concluded that continuously wet conditions do not favor laterite formation. The occurrence of a period of drought, which is observed in many areas where laterite occurs, was though to be a requisite by Maclaren (1906). This was disputed by Scrivenor ( 1909), however, who declared that laterite occurs in Malacca, where there is no such alternation of seasons. Campbell (1917) and Humhert (1948, p. 282) considered that a regular alternation of wet and dry seasons is not necessary if there are periods of wetness and dryness, even though irregularly distributed. Mohr and van Baren (1954, pp. 69-71) have suggested, on the basis of effective rainfall and evapotranspiration, that a monthly rainfall not exceeding 60 mm. would constitute an adequately “dry” season to provide no excess of rain over evapotranspiration in tropical regions. Simpson ( 1912) thought that laterite was forming under the semiarid conditions of western Australia. It appears clear that some minimum amount of water is necessary for weathering, the removal of bases and combined silica, and segregation of iron so evident in the resultant material. It also appears reasonable that periods of drying should favor the crystallization of goethite or similar minerals, which appear to be associated with hardening. Observations by the authors suggest that approximately equal wet and dry seasons favor crust formation and that some degree of alternating wet and dry season is probably essential for this process. It is not clear whether the material may or may not be conditioned for this final segregation and crystallization in permanently wet conditions. To some degree,

LATERITE

15

conflicting views may be the result of two confounding factors: (1) the occurrence of laterite under climates that are unlike those under which the laterite formed, and ( 2 ) differential effectiveness of climatic wetness or dryness in combination with differences of topography, parent material, and time. Laterite is found where temperatures are warm or are believed to have been warm at the time of formation. This does not preclude the possibility, however, that time, not temperature, is the controlling factor. If the principal effect of high temperature is to accelerate rates of reactions, the time interval necessary for laterite formation in temperate regions may exceed the period of time that appropriate landscapes have been stable. Similarly, observations of Oldham ( 1893), Joachim and Kandiah (1935), and Prescott and Pendleton (1952) that laterite has not formed under the cool temperatures of highlands in the tropics may be due less to cool temperature than to landscape instability. Nevertheless, field observation generally agree with the hypothesis that warm temperatures favor the formation of laterite.

B . VEGETATION A forest vegetation was considered necessary for laterite formation by Glinka (1927, pp. 33-34). More detailed observations have shown that, while laterite occurs in regions with a rain-forest vegetation, welldeveloped laterite is most commonly found under a low forest and that hard surficial laterite is a very common feature of the open savannah adjacent to the forest. Maignien (1958, p. 16) concluded that laterite is most extensive and strongly expressed at the boundary of forest and savannah. DHoore (1954,1957, p. 55) found iron mobilized to a greater degree and to greater depth under tropical grasses than under forest. Buchanan’s type of soft laterite is found in Ceylon in the forested areas of the southwestern lowland. Humbert (1948) suggested that laterite forms in a climate that has a wet and a dry season, and his descriptions and illustrations indicate that the laterite he observed was in an open savannah that was gradually replacing forest, a common condition where a dry season is prominent. The change from soft laterite to the hard form within a few years after man has cleared the forests has been reported by Alexander and Cady ( 1962) in French Guinea and other parts of West Africa. Blackie’s (1949) description of soils of Fiji and Aubert’s (1950) description of Dahomey also confirm the hardening of laterite following a change from forest to savannah caused by man’s activities. It would appear from the literature that laterite is most extensive in areas of savannah but that laterite forms under forest though its hardening is favored by lack of forest cover.

16

S. SIVARAJASLNGHAM ET AL.

Nodular forms are very common in forested areas, though they also appear to be most widespread in the savannah. Rosevear ( 1942) has reported a case where reforestation softened hard laterite significantly in as little as 16 years. The possible explanations of the effect of vegetation on hardening and softening will be considered later. C. PARENT MATERIAL Laterite is found overlying a variety of kinds of rock. It is common over basic igneous rocks such as basalt, norite, and diabase as well as over acid rocks such as granite, granulites, gneisses, and sericitic schists. Laterite has also been observed on shale, sandstone, and other sedimentary rocks including limestone (Jones, 1943; Stephens, 1946, p. 11). Marbut (1932, p. 76) has described laterite outcropping on the banks of the Amazon, presumably in alluvium, in association with a ground water table. Maignien (1958) has shown that fermginous laterites may develop on a variety of materials providing there i s a source of iron either in the parent rock or in adjacent higher-lying areas from which water may introduce ferruginous material. The laterite layers may be in material unrelated to the underlying bedrock. In some places, laterite clearly has formed in residuum of rocks that have weathered in place, as evidenced by features such as a constant proportion of iron oxides to alumina with depth or by a continuation of quartz veins from the rock into the laterite. It is also found in colluvial deposits; examples are described by Maignien ( 1958). Often laterite is associated with material that has been transported locally; recently recognition of stone lines in surhial mantles of this character has directed attention to the frequency of such conditions (Ruhe, 1959). Though such surficial deposits may not be directly related to the underlying bedrock, they may not be very different from residuum of the major rocks in the locality, since transport is often of a local nature as in cases cited by Nye (1955a) and by Ollier ( 1959). The parent material in which laterite forms may also be derived from material of different and highly contrasting strata than the underlying rock currently present. Whatever the source of material in which laterite forms, an adequate supply of iron appears to be essential. Alexander and Cady (1962) have noted that the thickness of laterite crusts is sometimes related to the iron-richness of associated rocks.

D. TOPOGRAPHY Laterite has been generally associated with a level or gently sloping surface. This characteristic was emphasized by Oldham (1893) in his review of numerous laterite locations in India and has been subsequently

LATERITE

17

confirmed by many workers from other parts of the tropics. Campbell ( 1917), from his many observations, reported that laterite currently being formed covers the flatter ground and stops where the slopes are steep. Holmes (1914) observed that in Mozambique laterite occurs only on gently undulating plateaus and never on steep slopes. Humbert (1948), from his study of laterite in Australian New Guinea, concluded that the best examples of laterite occur almost exclusively on areas of low relief and gentle slopes. Prescott and Pendleton (1952, pp. 25-26) also emphasized the nearly level nature of the terrain where laterite occurs. The horizontal disposition of laterite deposits is very striking in many relatively arid regions. Newbold (1846) observed that in the interior of South India laterite occurs in almost horizontal beds as cappings on the tops of mountains. Woolnough (1918) emphasized the significance of relic laterite deposits in western Australia on slightly elevated plateaus. Similarly in southwestern Australia, laterite is found in many localities as massive or concretionary deposits forming protective cappings on flattopped residuals (Mulcahy, 1960, 1962). Laterite is also present in extensive bodies on dissected tablelands (Stephens, 1946). Laterite is found along river banks and on terraces adjacent to higher ground. These masses of “gallery laterite” are believed by DHoore (1957, p. 97) to be formed by enrichment with iron during flood stage followed by its immobilization when the flood subsides. In the flat humid areas, where it is believed that laterite is forming currently, a high but fluctuating water table is common. The drainage is more or less imperfect, and the ground water at and below the water table is thought to be somewhat sluggish. This has led to a common opinion that topography conducive to a high water table may be essential. Soft laterite is found at a shallow depth on low hillocks in southwestern Ceylon, generally not much more than 100 to 200 feet above sea level (Prescott and Pendleton, 1952, p. 9 ) . The slopes are undulating and are certainly not level, Though Oldham (1893) dismissed these occurrences as “a more or less ferruginous subsoil which never passes into laterite,” the descriptions of Joachin and Kandiah (1935) indicate that these materials, locally called “cabook,” conform to Buchanan’s concept in every way. It should be noted, however, that the underlying rock in this region is charnockite, a quartzo-feldspathic gneiss or granulite with hypersthene, and that the rainfall is over 100 inches per annum but with dry intervals. High rainfall and high iron content of the parent rock may favor the formation of laterite even where the surface is not level but is stable owing to the nature of the weathered material and forest cover. Mulcahy (1960, p. 222, 1962) emphasized that peneplain condi-

18

S . SIVARAJASINCHAM ET AL.

tions of low relief are not essential, though low available relief conducive to stable land surfaces for long periods is most favorable. Laterite on areas of low relief, nevertheless, is the most common. Blanford (1879) classified the Indian laterites into high-level and lowlevel varieties, depending on the mode of occurrence. This distinction was at first intended only to signify two contrasting positions, high-level laterite capping the summits of hills and plateaus on the highlands of central and western India and low-level laterite covering large tracts in the coastal regions (Holland, 1903). Subsequent workers observed that low-level laterite is generally the more fenvginous and commonly does not exceed 30 feet in thickness. It also commonly contains inclusions of sand and pebbles, which indicate a multicycle or detrital origin (Oldham, 1893). Low-level laterite, however, does not everywhere contain such foreign inclusions. Since it was once thought that laterite can form only at or near the water table, the low-level laterite on areas with a high water table was called “live” laterite. The laterite on ground above the reach of the water table was thought to have been a product of an earlier period when the groundwater fluctuation reached the laterite zone. Hence, this was called “dead laterite ( Campbell, 1917). The high-level laterite is generally more homogeneous and may be relatively thick, as much as 100 to 200 feet according to Oldham (1893), though it is probable that such thickness is a feature of plateau margins. Though it was considered “dead” by Campbell ( 1917), Harrison (1933, p. 16) believed that laterite can form on high-level positions, as on the plateau of the Eagle Mountain range in British Guiana, under conditions of heavy rainfall though the water table is low. As it is presumably being formed currently, this would be called “live” laterite. Laterite in highlevel position may include pebbles from even higher surfaces, indicating detrital origin (Ruhe, 1954, p. 18). The laterite found on distinct slopes adjacent to higher ground may be called foot-slope laterite to indicate its topography. It is often detrital, being formed by the consolidation of fragments of the laterite of the higher level that have moved down the slope as erosion has advanced (Oldham, 1893). The second member of Greene’s ( 1945) “ironstone catena” would be this class. “Terrace” laterite occurs on terraces adjacent to high ground or along the banks of streams. It is thought to be formed by the deposition of dissolved material where ground water moving laterally and down the adjacent slope encounters the oxidizing conditions of the surface horizons (Campbell, 1917). Maignien (1958, 1959) emphasized the occurrence of laterite crusts at the borders of natural drainage areas, such as piedmonts, river banks, dissection forms, and

19

LATERITE

other abrupt breaks in slopes where the profile of a saturated zone approaches the surface. Another type recognized by Fermor (1911) is “lake” laterite formed in marshy areas by water flowing from the surrounding higher land, either along the surface or by seepage. Lake (1890) used the accompanying tabulation to classify the laterites in Malabar, India, according to topographic position, character, and origin: Group

Nature of the laterite

Origin

Plateau laterite Terrace laterite Valley laterite

Vesicular Pellety Partly vesicular Partly pellety

Nondetrital Detrital Partly nondetrital Partly detrital

More elaborate schemes of classification, based on other factors, have been published by DHoore (1955),Maignien (1958), and du Preez (1949). Four physiographically distinct landscapes with which laterite is commonly identified in the literature are: (1) high-level peneplain remnants, (2) colluvial footslopes subject to water seepage, ( 3 ) lowlevel plains having high water tables or receiving water from higher land, (4) residual uplands other than peneplain remnants. The first three are illustrated diagramatically in Fig. 1. Hlgh-Level Plateou wlth Loterite Cap

Laterits Detritus Laterite Enriched

FIG. 1. Relationships among physiographically distinct landscapes with which laterite is commonly associated.

Laterite on high-level peneplain remnants may be in residuum of rocks weathered in place or may be in transported material deposited prior to peneplain dissection. Its present position is a consequence of landscape inversion such as that described by Bonnault (1938); it once occupied a low-level position comparable to that illustrated in Fig. 1. With uplift or with lowering of base level, areas protected by laterite have remained as erosion lowered the surrounding areas. Such areas now occupy the highest positions and are being reduced slowly by

20

S. SIVARAJASINGHAM

ET AL.

lateral retreat of slopes, as on the two Tertiary surfaces described by Ruhe (1954, p. 18-25). At the base of peneplain remnants, detritus from above, including fragments of the crust, accumulates and is recemented to form younger laterite on the colluvial footslope (Maignien, 1958, 1959; D’Hoore, 1954, 1957). On low-level plains soft laterite is commonly currently forming above a water table; and on undulating upland surfaces, laterite may be forming locally in clayey iron-rich residuum whose impervious nature periodically causes saturation with water.

E. AGE Many existing laterites are clearly relics of geologic antiquity. Those of Queensland, Australia, are reported to be products of two humid periods of the Pliocene (Whitehouse, 1940). In Ceylon, of the crusts described most are thought to be products of the Pleistocene and some, of Pliocene or earlier periods (Fernando, 1948). Laterites of Nioka (Ituri), Congo have been related to mid- and late-tertiary surfaces by Ruhe (1954). Though laterite may be forming currently on some ancient peneplain remnants, many high-level crusts are considered to be “dead products of the age of peneplain development. Their existence may commonly be considered a factor in the preservation of the land forms on which they occur, and the period since the lowering of base level is a measure of the time required for landscape inversion under given conditions. Nevertheless, many examples of laterite believed to be forming currently have been reported ( Fermor, 1911; Simpson, 1912; Campbell, 1917; Marbut, 1932; Harrison, 1933; Joachim and Kandiah, 1941; Pendleton and Sharasuvana, 1942; Kellogg and Davol, 1949). Though it is commonly assumed that formation of laterite requires a very long time, some laterites of “absolute accumulation” ( D’Hoore, 1957, pp. 94-98) apparently may form rapidly. Obviously, formation of laterite from solid unweathered rock can be no more rapid than the time required to attain a high degree of weathering. In unconsolidated weathered material subject to enrichment in iron from outside sources, however, rate of development may be relatively rapid. Hardening of the soft preconditioned material may take place in a few years upon exposure (Alexander and Cady, 1962). IV. Profiles Containing Laterite

Though either nodular or vesicular laterite may lie within, and may be genetically related to, the solum of the modem soil, as defined by the Soil Survey Staff of U.S.D.A. (1951), many laterites appear to be

LATERITE

21

unrelated genetically to present soil horizons. Consequently, the Soil Survey Staff (1960) has defined “plinthite” independently of soil horizon definitions, though they use the presence of soft “plinthite” within the solum as a criterion of soil classification. This discussion is concerned with horizons or layers in the entire weathered section, which commonly is much thicker than the part that would be considered “solum.” A. SOILMATERIALOVERLYING LATERITE At sites where laterite is thought to be forming, it is generally found as a shallow but not surficial layer. Prescott and Pendleton (1952) reported laterite in Ceylon at various depths, averaging about 2 feet, and in Thailand from a fraction of a foot to 6 feet. Humbert (1948) described “red to yellow loam” 2 to 6 feet thick over laterite in Queensland. Some Iaterite layers have been described by Alexander and Cady (1!362) at a depth as great as 13 feet in Sierra Leone but others were found near or at the surface. Similar observations are very numerous in the literature and indicate that soil material over laterite is mainly less than 10 feet thick. Laterite crusts at the surface are very widespread ( Oldham, 1893; Maclaren, 1906; Simpson, 1912; Walther, 1915; Prescott and Pendleton, 1952, among many authors), but such exposure is generally attributed to erosion. Though hardening of laterite is thought to be a phenomenon favored by surface position, the literature implies that the initial development of material that will harden most commonly occurs at some depth below the surface. Hardened laterite crusts as thick as 200 feet (Oldham, 1893) suggest that development can proceed to this depth, but such an extreme may be only at the edge of peneplain remnants where the material is affected by vertical exposure. A crust 30 feet thick has been reported by Alexander and Cady (1962); Campbell (1917) observed that laterite seldom exceeds 30 feet in thickness. Mohr (1944) considered laterite to be essentially a soil horizon of sesquioxide accumulation to which the overlying soil material is related genetically. This concept has been elaborated by Mohr and van Baren (1954, pp. 300304) and was accepted by Pendleton (1936, p. 106). Marbut ( 1932) postulated a comparable genetic relationship between laterite and soil material above it. In all these cases, the authors have dealt with restricted conditions, comparable in many respects to the original laterite of Buchanan. The work of Maignien (1958, 1959), Mulcahy (1960), and D’Hoore (1954, 1957) and observations by the authors (Alexander and Cady, 1962) show clearly that the presence of a suficial mantle of soil over laterite is no assurance that the soil is related to the underlying laterite genetically, or that if genetic relationships are involved, they may be quite unlike those postulated.

22

S . SIVAEWJASINGHAM

ET AL.

The overlying soil material may be from sources different than the material in which the laterite has formed. Stone lines marking erosion surfaces (Ruhe, 19.59) are very common in the tropics and may mark major discontinuities of material vertically. Ollier ( 1959), however, has reported extensive areas in Uganda where the stone lines appear to be the result of sorting by termites, leaving coarse fragments below and moving fine earth to the surface. Nye (1954) also emphasized the activities of termites but reported substantial sorting and downslope creep of the sorted material. Berry and Ruxton (1959) also have emphasized an upper zone of migration. Resistant minerals, such as quartz, in soil over laterite from quartz-free rock are common and indicate at least contamination of the upper layers of material from outside sources. In some cases surface material can be demonstrated to be residual from the same rock as that from which underlying laterite has formed and either predates or is contemporaneous with the laterite. There are also cases known to the authors in which a surficial soil mantle has developed by disintegration of the upper part of a laterite crust. Such soils are commonly thin over the laterite and contain pieces of the disintegrating laterite. Thus, a great variety of soils may overlie laterite. Where such soil horizons are residual, they are composed of highly weathered material high in sesquioxides with or without kaolin and with some component of whatever highly resistant minerals may have been present in the parent rock. They may be uniform in character with depth, like Latosols, or may have genetic A-B horizon sequences not unlike Red-Yellow Podzolic Soils. Commonly, the first laterite encountered with depth is in the form of individual nodules within the soil horizons. These may reach a maximum with depth below which they decrease without being joined into masses as in examples given by Nye (1954, 1955a) and Radwanski and Ollier (1959), or they may pass into masses of nodular or vesicular laterite, as in profiles described by Joachim and Kandiah (1941) and Ollier (1959).

B. LA-

SOILHORIZONS Many soils of the tropics have laterite within genetic horizons. These may be detrital nodules or fragments from adjacent higher-lying landscapes containing laterite ( Greene, 1945; Ruhe, 1954), relics of disintegrating laterite crusts in which soils are forming (Mulcahy, 1960, 1962), or units developing concurrently with the modem soil (Ollier, 1959). The “murram” used for surfacing roads in India (Prescott and Pendleton, 1952) is mainly nodular laterite and may be in horizons of the modern WITHIN

23

LATERITE

soil. The authors have observed profiles comparable to both Red-Yellow Podzolic Soils and Latosols, both containing nodular laterite in various horizons, in widely separated areas of East and West Africa and in the Philippines. The Tifton series, a Red-Yellow Podzolic Soil of southeastern United States, contains laterite nodules in both Az and B horizons. The nodules in the B horizon of Tifton are intact and either forming or stable, whereas those in the A2 are apparently dissolving, leaving quartz grains protruding from the iron-rich matrix. Similar conditions in soils

0’

600

300

1200 1

900

Horizontal Scole, Feet

0-

12-

24

-

:

.z 3 6 -

e

z

48-

60

I. Dork. Sendy Loom

h r k . Sandy Loom

Red Sandy

Red So*

Lmm

with M Nodule.

Sandy

84 -

96

-

108

-

Loam

Nodules

Red Sandy Cloy with Few Fins

Nodular

-

Dork.Sandy Loom

Groyirh Brown

Cloy Loom rnth Commn

..

:a\/Cortkm

1.r

Mottled Sandy cloy Loam

r

Nodular

-

Mottled Red I and Brownish Yellow Sandy Clov Loom

,.,,’I

72

4

3. 0ork.Sondy Loom

GIOY Sandy Clay with Few Matller

FIG.2. Laterite nodules in soils of a catenary association of Southern Nigeria.

containing iron nodules have been observed in Southern Rhodesia and parts of Australia by the authors. Though laterite nodules are common in soils that have no obvious high water table ( Raychaudhuri, 1941) , such nodules commonly increase progressively downslope on a given land form. Figure 2 shows this relationship at a site near Ibadan, Xigeria where proximity of a zone of saturation to the surface appears to be a controlling factor. It is believed that these are developing concurrently with the modem soil and are analogous to the forms described by Nye (1954, 1955a) in Ghana. In the “Ground-Water Laterite” soil described by Kellogg and Davol

24

S. SWARAJASISGHASL ET AL.

(1949), the laterite is considered to be a genetic horizon of the modern soil. In this profile, laterite nodules are present at a depth of 8 inches, increase in numbers with depth, and occupy a major part of a weakly cemented horizon from 23 to 45 inches, which rests on soft massive laterite that hardens upon exposure to air. Mohr (1944) and Mohr and van Baren ( 1954) have postulated progressive soil development involving ( I ) laterite-free profiles in which an impervious substratum forms, ( 2 ) stages having horizons containing nodular laterite, and ( 3 ) a final stage involving massive laterite.

C. Homoss

OR

LAYERS BENEATH LATERITE

Obviously, detrital laterite fragments and nodules come to rest capriciously on whatever material may be present on footslopes of dissection forms, and in such areas of landscape inversion (Bonnault, 1938) the detrital laterite and the underlying material may be unrelated. The laterite detritus, however, commonly contributes iron to enrichment of adjacent layers (Maignien, 1958, 1959). A great variety of unrelated layers may be found under laterite zones in such positions. Where the laterite zone is apparently residual, however, the literature reveals some measure of consistency of kinds of underlying layers. Though Holland (1903) reported that laterite may rest on unaltered bedrock, most descriptions show highly weathered, commonly thick, earthy layers between the laterite zone and bedrock. In dry areas or where conditions contribute to good aeration, as on some high-level positions, the underlying material may be high in chroma (bright colored) though commonly variegated in color ( Kellogg and Davol, 1949, p. 52). More commonly, especially on low-level positions, the laterite zone is underlain by either a mottled zone, a light-colored layer, or both, suggestive of poor aeration, reduction of iron, and possible lateral leaching of sesquioxides. U‘alther ( 1915) introduced the terms “mottled zone” and “pallid zone” for comparable parts of profiles of western Australia, which contained the following layers: (1) ironstone crust, ( 2 ) mottled zone, ( 3 ) pallid zone, and (4)parent rock. Simpson (1912) described similar sequences on both granite and greenstone schist in the same region. Though Maclaren (1906) used different terms in describing a lSfoot section in India, his sequence, hard laterite-soft laterite-reddish buff sandy clay-white grit-decomposed biotite and quartz schist, is very similar. Marbut’s (1932, p. 74) idealized description of typical “Ground-Water Laterite” of the Amazon Valley and Cuba reveals the same layers: (1) soil, ( 2 ) iron-oxide layer,

LATERITE

25

porous, slaglike, (3) mottled layer, ( 4 ) gray layer, and (5) unconsolidated clay and sand. He reported that the gray layer (pallid zone) may be absent. Kellogg and Davol (1949) did not specify distinct mottled and pallid zones in a “Ground-Water Laterite” of the Congo, but their description shows mottling intermingled with high color values ( light colors) in the lower part of the profile. Mohr and van Baren ( 1954, p. 302) described similar layers in idealized profiles on volcanic ash in which laterite is formidg in Indonesia:

- Redearth B3- Layer of mottled A

B2B1C

-

clay differentiable into an upper layer of Fe203(incipient laterite) and a lower gibbsitic layer (mottled zone?) Spotted white clay (pallid zone) Layer with siliceous cement Unaltered ash of the basic suite

The numbering of “ B horizons by superscripts upward indicates the hypothesis of development upward above the slowly permeable silicacemented B1 layer, which supports a perched water table. Mulcahy (1960, 1962) described thick pallid zones in Australia under ancient laterite in high-level positions where mean annual rainfall is now less than 20 inches per year, their thickness decreasing from 60 or 80 feet under 20 inches of rain to about 10 feet under 13 inches. Jessop (1960) described pallid zones from 60 to 200 feet thick in the southeastern part of the Australian arid zone under a silicified cap on plateau remnants and concluded that the pallid zone is a relic of an ancient profile from which the ferruginous material has been stripped. From observations in Africa and Australia, Alexander has concluded that some pallid zones are consequences of exclusion of air by overlying laterite; they appear to be actively forming even on high-level positions in relatively dry climates when they lie beneath a crust dense enough to permit little access of air and where the zone is saturated for some period during a rainy season. A profile by Kellogg and Davol (1949, p. 52) in the Congo suggests that on relatively dry sites a thick (4-foot) layer of unmottled soil, comparable to laterite-free soils of the locality, may lie between a relic laterite crust and a mottled zone. Commonly immediately above unweathered rock is a soft layer that has undergone major chemical change while retaining the structural character of the rock from which it was formed. This was called “zersatz” by Harrassowitz (1926, 1930). This may lie beneath a mottled or pallid zone, as in the “Ground-Water Laterite” of Kellogg and Davol (1949).

26

S . Sn‘AEMJASINGHAhl ET AL.

may be separated from massive laterite by only a zone of concretionary laterite, as in a profile described by Humbert (1948), or may occur in profiles without laterite. In the more arid regions of western Australia, an additional horizon of siliceous nature has been recognized by Whitehouse (1940) in profiles containing laterite. Layers of silica-cemented sandy material, known as ‘billy,” or clayey material, known as “porcellanite,” were described beneath laterite and over sedimentary rocks. Jessop ( 1960) has described silicified layers within the pallid zones of profiles presumed to have once contained laterite and has concluded that they were formed in the manner postulated by Mohr and van Baren for silica-cemented layers under laterite in volcanic ash. Though such materials have been recognized in both western and southern Australia, their relationship to laterite, if any, has not been clearly demonstrated. They may be the result of accumulation of silica removed from iron-rich layers above in at least some cases. In summary, profiles containing laterite may have, from the surface downward, some combination of layers of the following sequence: ( 1 ) soil with or without nodular laterite in some horizon, ( 2 ) laterite, ( 3 ) “mottled zone,” ( 4 ) “pallid zone,” ( 5 ) silica-cemented layer, ( 6 ) “zersatz.” Descriptions of profiles with these layers in India have been presented by Satyanarayana and Thomas ( 1961 ) . All may be present, and one or all apparently may be related genetically to the laterite. None except the laterite itself appears to be essential to development of laterite in all environments, however. Accurate appraisal of relationships among such layers is enormously complicated by the common occurrence of laterite as relics of a former contrasting environment, the variety of geomorphic relationships involved in landscapes on which laterite is currently found, and the fact that the enrichment necessary for laterite formation can occur in a number of different ways. V. Formation of Laterite

The early theories of laterite formation were as varied as the nature of the material and the conditions under which it is found. Lake (1890) has given an excellent summary of these to 1890 in the appendix to his report of the geology of South Malabar. Briefly, one group including Babington (1821), Benza ( 1836), Blanford ( 1859), Buist ( 1860),Clark (1838), Kelaart (1853), McGee ( 1880), Wingate ( 1852), and others considered it to be a residual product of weathering in place. Blanford (1859), Cole (1836), King and Foote (18641, Newbold (1844, 1846), Theobald ( 1873), and Wynne ( 1872) recognized detrital, sedimentary,

LATERITE

27

or other origins not dependent upon residual weathering, though several of these authors recognized residual forms as well. Voysey (1833) and Carter ( 1852), among several, proposed volcanic origins with subsequent weathering. Issues of the Memoirs of the Geological Survey of India, starting with Volume 1 in 1859, the Madras Journal of Literature and Science during the first half of the nineteenth century, and the Record of the Geological Survey of India, starting with Volume 1 in 1868 are rich in articles on laterite. The various theories proposed were not without reasons. The slaglike character of laterite and its occurrence as horizontal masses over basalt flows of the Deccan plateau led Voysey (1833) to suggest a volcanic origin. Oldham ( 1893), possibly influenced by Mallet’s ( 1881) comparison of Indian laterite to certain ferruginous sedimentary beds of Ulster, postulated a lacustrine origin. Thickness as great as 200 feet locally, occurrence as extensive sheets overlying rocks as unlike as basalt and gneiss, high concentration of iron in laterite over rocks low in iron, and similar apparent anomalies led him to reject hypotheses of weathering in place advanced earlier by Newbold (1844, 1846), Foote (1876), and Lake (1890). Holland (1903) conceded that weathering of sorts is involved but concluded that a simple hypothesis of chemical weathering could not explain the abrupt transition from laterite to weathered rock nor the absence of laterite in regions having warm summers but cold winters. He postulated action by an organism capable of separating alumina from silica in silicates but destroyed by low temperatures-a process to be added to “the long list of tropical diseases against which the very rocks are not safe.” As DHoore (1955) has indicated, it is evident that the chemical composition of laterite can be a consequence of either or both of two processes: ( 1 ) concentration of sesquioxides by removal of silica and bases; or ( 2 ) concentration of sesquioxides by accumulation from outside sources. It is not often possible to decide on the basis of chemical analyses alone whether one or the other or both are involved. Commonly the nature of the parent material is not known with certainty, as the laterite may be in unidentifiable transported surficial deposits or may occupy positions that could receive iron-bearing water from adjacent areas. Even where the laterite is clearly in residuum of rock weathered in place, resistant index minerals or chemical constituents that can be used to reconstruct absolute changes in composition from rock to such highly weathered material are commonly not present in amounts or forms that would make such calculations unquestionably reliable. It is not surprising, therefore, that many conflicting theories have been propounded, even in recent years.

23

S. SIVARAJASIXGHAM ET AL.

Among the earlier workers, Glinka (1899), Holland (1903) and others suggested that the concentration of sesquioxides is primarily a consequence of removal of silica and bases. Maclaren (1906), Simpson (1912), Campbell (1917), Mohr ( 1944), Vine ( 1949), and others, however, postulated the probability, if not the necessity, of accumulation from outside sources. Vine postulated an aeolian mechanism; the others proposed accumulation from ground water, either from a high water table, from laterally moving water, or from both. There is, however, no specific basis for assuming that laterite can form in only one way or that all existing laterites have formed by the same combination of gross processes. Indeed, there is increasing evidence that although certain fundamental conditions are prerequisite to the chemical and mineralogical alterations that occur, these may be satisfied by a variety of local combinations of weathering reactions, water relationships, and other factors. These will be considered in more detail in the following pages. A. WEATHERING AS

A

PRECO~~ITIONER OF MATERIAL OF LATERITE

Whether or not a given laterite has been enriched in sesquioxides from outside sources, weathering is a major determinant of the ultimate composition and is considered here not only in relation to formation of laterite, but also as a preconditioner of material in which laterite may form. If the literature on chemical composition of laterite that is hard or that hardens upon exposure is compared with the literature on composition of soil material and weathering products that do not harden, it is apparent that the range of composition of laterite is included in the range of composition of what has been called ‘lateritic soils.” Thus, hardening is apparently not determined by gross chemical composition per se. Futhermore, the range of mineral species in materials that do not harden includes the mineral species in laterite, though differences may exist in proportions and continuity of crystallinity. For this reason, weathering and mineral transformations are treated in the pages that follow as processes and reactions that are not unique to laterite but that are essential to its development. Glinka emphasized the effects of rapid decomposition of organic matter in the tropics, with consequent release of energy and carbon dioxide, on weathering “so strong that laterite, essentially uniform in character, has been developed from rocks that vary widely in character” (Glinka, 1927, p. 35). He recognized intermediate stages in which “argillaceous aluminum silicates” accumulate with hydrated sesquioxides, and he inferred that secondary silicates are potential precursors of at least some of the sesquioxides that characterize the ultimate weathering product ( p. 42).

LATERITE

29

Harrassowitz ( 1926) applied the term “allophane” to weathering products consisting of various hydrous aluminum silicates or mixtures of hydroxides of aluminum and silicon whose crystallinity had not been demonstrated. The mass of clay in which “allophane” is a principal constituent but which contains the mineral kaolin was called siallite, and the process by which it had formed was called “siallitization.” Marbut ( 1928, p. 106) emphasized that “lateritization and siallitization are identical,” both being primarily processes of gradual “desilicification” leaving behind aluminum and iron after a very long period and going to completion only in hot humid climates. Marbut, like Glinka, believed firmly that “in cool climates it is apparent that siallite represents a product that is final so long as the environment remains constant” (p. 106). This distinction between weathering in temperate and in tropical climates and the concept of secondary silicates as intermediate products in the tropics appear repeatedly in the literature prior to 1933. Both have been questioned as consistent relationships subsequently. Harrison (1933), Alexander and co-workers (1941, 1956), D’Hoore (1954,1957), Leneuf ( 1959), Maignien ( 1958,1959), Briggs (1959), and others have clarified the mechanisms of weathering greatly since 1933 by detailed chemical and mineralogical studies; Harrison’s ( 1910) earlier work along similar lines had attracted little attention. As Mohr and van Baren (1954) have reviewed the literature prior to 1954 in more detail than is possible here, principally the work of Harrison and of selected individuals who worked after Mohr and van Baren’s book was published are considered in detail. Harrison (1933) found that weathering of basic igneous rocks in the tropics under more or less free drainage is accompanied by almost complete removal of silicon, calcium, magnesium, potassium, and sodium, leaving an earthy residue of gibbsite, ‘‘limonite,’’ a few unaltered fragments of feldspars, secondary quartz locally, and the various resistant minerals originally present in the rock. He called this earthy residuum “primary laterite,” a term which should not be confused with laterite as used in this paper. The loss of silicon relative to that of iron and aluminum is illustrated in Table IV by values based on Harrison’s data on composition of drainage water relative to the rock over which it had passed. Harrison concluded that the change from basic rock to primary weathering product is not gradual, as suggested by Glinka, Marbut, and others, but that sesquioxides are formed first, directly from their parent minerals, within a zone not greater than 1/6 inch from the hard rock. Farther from the rock, he found kaolin in significant quantities, which he attributed to resilication by small quantities of silica in percolating water. Mohr and van Baren (1954) have calculated the theoretical mineralogi-

30

S . SIVARAJASINCHAM ET AL.

cal composition of Harrison’s samples of dolerite and its weathered zones by allocating chemical constituents to specific mineral species ( Table V ) . Although the mineral species indicated are not necessarily identifiable in the amounts given, the theoretical mineralogical compositions of the dolerite rock and of the “latentic e a r t h more than 73 mm from the rock TABLE IV Silica, Iron Oxide, and Alumina in Water and in Weathering Rock Like That from Which It Had Drained5 Hornblende schist

Constituent SiO, Fez03 ~ 2 0 3

a b

Seepage water (mg/l)b 32.89 3.30 0.38

Epidiorite

spring Rock

water Rock (mg/l)b % 12.35 49.06 6.90 0.00 18.87 1.81

”/o 45.45 11.01 12.18

Basic rocks River Rock water (average) (mg/l)b % 24.69 48.95 3.99 11.34 - 15.97

Calculated from data of Harrison (1933, pp. 13, 15, 31, 35). Increase relative to rain water. TABLE V Theoretical Mineralogical Composition of Dolerite Rock and Successive Weathered Zonesa

Mineral Species Quartz Plagioclase Augite Magnetite Ilmenite Gibbsite Diaspore Goethite Hematite Kaolin Water 0

b c

Dolerite

Weathered material at indicated distance from the rock ( % )

%

Wmm

3-21mm

2 51 42 4 1

13 10 3 4 3 38 28

-

21-71mm

71-73mmb

73mm5mc

13

12

12

-

-

-

5 4 40

5 2 35

4

1 10

-

-

-

28

34

8 13 52

-

-

7 6

10 2

-

Mohr and van Baren ( 1954, p. 140). Hard crust. “Lateritic earth.”

are consistent with Harrison’s qualitative observations based on microscopic examination. The microscopic studies also showed segregation of alumina in nodules of gibbsite in a matrix of argillaceous material within the “zone of resilication.” Data such as these have been the principal evidence on which hypotheses of resilication of gibbsite have been based. In contrast to Harrison (1933) and Hardy and Rodrigues (1939),

LATERITE

31

who postulated, respectively, that silica for resilication comes from ascending and descending ground-water solutions, Alexander et al. ( 1941) concluded that the uniform thickness of the zone of resilication could be explained only on the basis that the source of silica was the core of weathering rock. The presence of a very thin zone relatively free of resilicated material was never satisfactorily explained on either basis, since the proposed roles of acidic and basic environments and of bases on mobilization and immobilization of silica under conditions of weathering with intense leaching are questionable. The very thin zone relatively free of resilicated material and the uniform thickness of the resilicated zone may be explained on the basis of relatively rapid rate of formation of gibbsite from the liberated alumina in relation to a slow rate of synthesis of kaolin from mobilized alumina under small but constant concentration of silica in soil water; the age of the zones of material are presumed to increase with increasing distance from the core of unaltered rock. Other studies of rock weathering where gibbsite is an early product have shown that allophane may also be present (Cady, 1951); this is illustrated in Plate 1A. The possibility exists that kaolin may in time form from allophane and that part or all of the gibbsite may be lost concurrently. Later studies by Alexander and Cady (1962) of weathering of dolerite at Mamou, Guinea, a site also studied by DHoore (1954, p. 52), allow more specific elaboration of the successive changes from rock to primary weathering product to laterite. The rock, on a ridge top, had weathered in tetrahedral form. Surrounding each tetrahedral core were successive shells of weathered material of about equal thickness, and the whole was surrounded by a shell of hard laterite. The rock, the altering margin of the rock, the weathered shells, and the hard laterite were sampled. Detailed chemical and mineralogical studies of these samples reveal certain significant relationships. Total chemical analyses of selected samples are reported in Table VI. Composition of scrapings from the hard rock may have been biased by differential removal of softer more weathered ferromagnesian minerals in preference to harder less-weathered feldspars, causing an apparent loss of aluminum and gain of iron and magnesium. The data do indicate, however, that conversion of ferrous to ferric iron is one of the first reactions. This was seen in thin sections of the material as a change in color associated with staining of the rock with ferric iron. Primary minerals in the shell immediately adjacent to the rock were completely weathered; successive shells outward had essentially the same chemical composition. Bases had been almost completely removed, and if either iron or aluminum is assumed to have been retained without loss, more than 90 per cent of the silica had also been removed. The ratio of aluminum to iron

32

S . SIVARAJASINGHAM ET AL.

remains essentially constant ( 1.23 to 1.29 as sesquioxides) throughout the rock and weathered shells. This indicates a high probability of little or no loss of either, as losses so consistently in equal proportions are most unlikely considering differential solubilities and chemical behavior of the compounds involved. The abrupt increase of combined water and its constancy through the weathering shells are signi6cant. TABLE VI Chemical Analyses of Selected Weathered Shells, of Hard Laterite, and of Parent Dolerite at Mamou, Guinea

Rock

Scrapings of rock margin

Shell adjacent to rock

Outer weathered shell

Hard laterite knobs

Constituent

(%I

(%I

(%I

(%I

(%I

SiO, TiO, AW, Fe,O:, FeO MnO CaO h4gO KzO Na,O

54.0 1.2 15.0 1.8 8.9 0.2 9.3 5.7

45.3 2.1 9.4 23.3 0.5 0.3 5.6 7.7 1.o 0.5 0.1 5.1

10.8 2.0 37.4 29.4 0.5 0.1

16.4 1.8 23.5 45.0 0.5 0.1

0.1 0.1

9.5 2.0 37.5 30.3 0.5 0.1 0.1 0.1

0.2 0.1 0.3 20.4

0.2 20.8 100.3

P,O, H,O (ignition loss)

1.0 2.4 0.1 0.2

-

-

-

99.8

100.4

101.4

0.1

0.1 0.1 0.1 0.2 13.3

100.2

The hard outer laterite knobs, however, show apparent gains of silica and iron and losses of alumina and combined water. If these are recalculated on the assumption of constant iron, however, the differences would appear to be almost entirely the result of loss of aluminum and combined water. Microscopic examination and bulk density measurements in relation to chemical composition indicated little change in volume from the rock to the weathered shells. Under the microscope it could be seen that feldspar laths of the original rock had been replaced by gibbsite. These were embedded in a reddish brown mass having the appearance of fine-grained iron-stained clay, unidentifiable as to mineral composition. The laths appeared spongy and were slightly farther apart than in the original rock; opaque minerals also appeared to be slightly more widely spaced than in the rock, suggesting possible volume increase. Bulk densities measured in the weathered shells ranged from 1.2 to 1.5 compared to 2.95 in the fresh rock. Assuming no volume change and either aluminum or iron constant

33

LATERITE

from rock to weathered shells, calculated bulk densities would be approximately 1.2, a figure which compares very favorably with the measured values on samples, particularly if the shrinkage due to airdrying is taken into account. Differential thermal and X-ray analyses of the five innermost weathered shells indicated gibbsite to be the principal crystalline mineral present. A low temperature endotherm indicated the presence of some amorphous material. In the outermost weathered shell, however, crystalline kaolin was present in significant amounts. In the hard laterite knobs, gibbsite was significantly less than in the weathered shells and crystalline kaolin and goethite or limonite were major components. Study of thin sections showed collapse and loss of rock-replica structure as well as loss of gibbsite in the outer weathered shell. Casual examination of these results suggest solution of gibbsite and its combination with silica in solution to form kaolin in the laterite shell. Allocation of silicon, iron, and aluminum to expected mineral species would give the distribution shown in the accompanying tabulation. Mineral species Kaolin Gibbsite Goethite

Weathered shells

Laterite knobs

%

%

22 45 32

35 15 50

It will be noted, however, that the ratios of theoretical kaolin to theoretical goethite are remarkably similar in the two materials. Although neither kaolin nor goethite was detected in appreciable amounts by either X-ray or differential thermal methods in the weathered shells, the constancy of the ratio of their theoretical values and the absence of quartz in amounts that would account for even a small fraction of the silica suggest that materials of their approximate composition were present but not crystallized to an extent that could be detected by the methods available. If this were the case, a simple solution Ioss of 79 per cent of the gibbsite as calculated would give almost the exact composition found in the laterite shell. This study has been reported in some detail, because it illustrates several significant aspects of the problem. First, it illustrates the very large loss of material during weathering, about two-thirds of the original rock. Second, it demonstrates that these changes may occur with little change in volume. Third, it shows dramatically the abrupt change from unweathered rock to a weathered material that has lost almost all its bases and more than 90 per cent of its silica within 1 mm from the rock.

34

S. SIVARAJASINGHAM ET AL.

These are common observations. In addition, however, this study emphasizes the possible role of poorly crystalline material, which is too commonly ignored because of lack of methods for identification, and the possibility that such material even with low amounts of silica may in time acquire continuity of crystal structure, which with loss of aluminum could account for apparent resilication of gibbsite in at least some cases. Further: it introduces the development of the crystalline state and its degree of continuity as a factor in hardening of laterite from weathered material, which will be considered in detail later. In contrast to his observations on basic rocks, Harrison (1933, pp. 60-70) found thick zones of transition from rock to weathered product over granite. Though general trends of changes in chemical composition were similar to those on dolerite, i.e., loss of silica and bases accompanied by concentration of sesquioxides, losses of iron were substantially greater and none of the sections studied had a discrete zone lacking in kaolin. In Table VII losses of major constihients in zones of alteration of a granite TABLE VII Cornpoution of a Granite and Losses of Chemical Constituents in Overlying Zone? of Alteration, Expressed as Per Cent of the Original Constituenta

Constituent

S102 A’20.3

Fe203 and FeO TiO,

MnO h1go CaO KZO Na,O

Per cent in underlying granite 72.35 14.32 1.93 0.70 0.31 1.02 1.24 4.86 2.99

Distance above the rock (feet)

2

2 112

12

% lost

70 lost

70 lost

9

16

41

14 lost 52

-

-

-

-

30b

20 26 100

30 21 100 87 98 74 95

63 11 100 94 99 94 97

0 100 57 73 27 68

87 98 58 95

a Recalculated from original data of Harrison (1933, pp. 71-72, Table 51, Layers 11, VIlI, IX, XVIII, and XXI), assuming aluminum constant. Apparent gain.

have been calculated from Harrison’s data, assuming aluminum constant. Aluminum was present in drainage waters only as traces. From the chemical data and field observations, Harrison concluded that a zone 2 feet thick above the rock was primarily a region of partial weathering of biotite and feldspars resulting in physical disintegration of the rock, small losses of silica, and substantial losses of bases. From 2 to 2% feet above the rock, however, was considered a zone of active ‘‘kaolinization’’ coincident with nearly, but not absolutely complete,

35

LATERITE

weathering of biotite and soda-lime feldspars, processes that were seemingly operative, though slower, throughout the next 10 feet of overlying material. Thus, ‘Icaolinization” and coincident weathering of primary minerals were apparently not complete in a zone more than 12 feet thick above the rock. No gibbsite was observed. It may be inferred that “kaolinization” proceeded directly from the primary minerals or that “resilication,” if it occurred, proceeded concurrently with disruption of the crystal lattice of the primary minerals. Harrison’s (1933, p. 46) studies of weathering of an intermediate igneous rock, quartz diorite, at Blue Mountain indicated gradual transition from rock to weathered product, as in granite, as well as the presence of minute crystalline flakes of gibbsite in the weathering material 2 feet above the rock and possible resilication in a zone immediately above that. The hypothesis for resilication was based on an apparent increase in SiOz calculated on the basis of constant aluminum, a dangerous assumption considering the evidence for potential loss of aluminum described above. Data of Alexander and Cady (1962) on samples collected at the Experiment Station at Samaru, Nigeria, a site also described and illustrated by D’Hoore (1954), indicate formation of kaolin in the zone nearest the granite rock and its dedication in overlying zones. Chemical and mineralogical data are presented in Table VIII. Plate 1B is a thin section from the soft laterite. The character and distribution of quartz revealed by studies of thin sections of the three layers indicated residual origin of the entire profile. TABLE VIII Chemical and Mineralogical Data of Laterite and Underlying Weathered Granite at Samaru, Nigeria Constituent or species SiO, TiO,

H,O (loss on ignition) Kaolinb Gibbsitec Boehmitec Goethiteo 0

b

c

Granite saprolite, 64-120 in.a

Soft laterite,

(%)

(%) 55.2 1.1

32-64 in.a

67.3 0.9 18.4 6.3 7.1 40 Detected

21.4 13.7 8.9 30 Detected

Detected

Detected

-

Depth below surface. Differential thermal analyses. X-Ray diffraction.

-

Hard laterite, 0-32 in.a

(%) 35.6 1.4 18.8 33.9 10.1 15 Detected Detected Moderate

36

S . SNARAJASINGHAM ET AL.

Petrographic studies of thin sections were consistent with the marked decrease of kaolin upward, as indicated by differential thermal analysis, and suggested increases in the proportions of boehmite and gibbsite, as well as goethite, in the surface crust, which were not demonstrated by X-ray diffraction. Quartz in the upper layer was spaced more closely than in the saprolite, suggesting loss of volume upward. If iron is assumed to have remained constant, the loss of S O z from saprolite to hard laterite was 90 per cent of that in the saprolite; the loss of A1203,81 per cent of that in the saprolite. Calculations of this kind, whatever assumptions may be made, indicate losses of S O 2 far in excess of that which can be attributed to weathering of kaolin, but decreasing kaolin upward is clearly evident. Considering that a large part of the Si02 is quartz, some mechanism of external enrichment in iron or a discontinuity in the profile is probable. Humbert (1948) studied weathering of a conglomerate of diabase and granite and found by differential thermal methods 75 per cent kaolin and no gibbsite in residuum of granite fragments and 50 per cent kaolin and a trace of gibbsite in residuum of fragments of diabase. The remainder was not identified, but a slight endothermic maximum between 100” and 200°C. for the diabase suggested some hydrous type of amorphous material. Kaolin pseudomorphs after feldspar without distinguishable gibbsite were observed in the residuum of diabase fragments. Humbert questioned the possibility that pseudomorphic form could be retained if kaolin were formed after crystallization of gibbsite and its subsequent resilication, since there would be associated changes in volume. Consequently, in contrast to views based on the more extensive work of Harrison (1933) and Alexander et d.(1941), he concluded: “It is apparent that primary laterite, as defined by Harrison, will not occur until the clay minerals have been broken by the agents of weathering with release and removal of silica and with accumulation of oxides of aluminum and iron in their hydrated states” (p. 286). This position is not tenable in the light of currently available information in the literature (Gordon and Tracy, 1952; Abbott, 1958; Cady, 1951). Humbert’s work is signscant to this discussion mainly as a case in which rock of highly contrasting composition in the same gross environment weathered to comparable products, suggesting that the presence of kaolin or gibbsite may be governed by the environment in which weathering proceeds, not solely by the character of the parent rock. Mohr and van Baren (1954), in their review of rock weathering, have shown that although many studies have shown gibbsite to be a product of weathering of basic rocks, and kaolin of acid rocks, this is by no means the universal pattern. Lacroix (1923), for example, found gibbsite

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37

derived from all feldspar-containing rocks including granite in Madagascar, but in Guinea he found gibbsite from syenites and gabbros and kaolin from granites, gneisses, and mica-schists. Mohr (1944) found both kaolin and gibbsite among the weathering products of andesite in Java whereas Idenberg (Mohr and van Baren, 1954) reported only kaolin on basalt in Sumatra. An amphibolite on Madagascar gave a product rich in gibbsite, whereas a similar rock on the island of Yap weathered to kaolinitic material. Mohr and van Baren concluded that these differences are probably related to conditions governing the composition, movement, and amount of soil water. The authors have observed in Hawaii that in some cases gibbsite appears to form directly as basalt weathers; in others halloysite forms. Both are found in regions of high rainfall, and reasons for the difference are unknown. The foregoing discussion suggests that material within the observed range of composition of laterite may be developed from rock in place by several possible courses of weathering and mineral transformations, all of which involve almost complete removal of bases and at least substantial losses of the combined silica of primary minerals. In the judgment of these authors, hypotheses of the probable course of weathering have been unduly influenced by the techniques available for mineral identification, which leave a major part of initial weathering products unidentified. The evidence appears clear that under some circumstances gibbsite is the first identifiable crystalline material to appear. It is equally clear that kaolin may be identified as a primary decomposition product without evidence of gibbsite as a precursor, and that gibbsite may form by desilication of kaolin. It is also evident that resilication of gibbsite to kaolin is a possible reaction. It is accepted as a well-established mechanism by many workers. It is likely, however, that it may not be as widely prevalent as has been postulated by earlier workers, who have tended to ignore the unidentifiable fraction. Resilication probably involves an intermediate mobilized form of alumina derived from gibbsite, if gibbsite is the precursor. It may also involve unorganized material in which alumina is a major component. Certainly, a consistent stepwise weathering sequence such as that from primary mineral to micalike clay to kaolin to gibbsite, has no basis in fact as the sole source of the material of laterite, though such a sequence may, indeed, have been the course through which some laterite materials have evolved. Jackson et al. (1948) imply such a sequence as a general trend but specifically suggest omission of steps, as in direct formation of gibbsite, and reversals, as resilication of gibbsite to kaolin, under appropriate conditions. The most significant feature, however, is that the mass of unidentified subcrystalline or amorphous material commonly present ( and commonly

38

S. SIVAR4JASINGHAM ET AL.

ignored) in the initial weathering product remains a major potential parent material of identifiable crystalline material upon aging. This with the potential for differential loss, local movement or enrichment of alumina or iron, potential resilication, and potential desilication make most attempts to reconstruct precisely the course of events extremely hazardous. It would appear that any one or all may be major determinants in certain environments.

B. DEVELOPMENT OF MICROSTRUCTURES Alexander et aZ. (1956) have described some of the variety of internal structures revealed by study of thin sections, but the most striking feature of their report is the evidence for movement, segregation, and development of organization of material in a variety of patterns. Some laterites retain some elements of rock structure, including “rough outlines of grain boundaries, organization of minerals following cleavage or twinning of feldspars and pyroxenes or amphiboles, or actual goethite, gibbsite or kaolin pseudomorphs after such rock minerals” ( p. 68). In addition, however, evident reorganization and segregation of constituents suggests local movement of material or, in some cases, enrichment from sources outside the sample sectioned. Their descriptions of these features and additional studies by the authors are reviewed here in some detail. Examples of some of the most common features are illustrated in Plate 1. A latticelike network of oriented Elms of kaolin stained or impregnated with iron oxide is very common on a microscale, and the network does not conform with expected structural elements of possible parent minerals of the original rock. It appears more like the arrangement of the oriented clay in a Latosol. This indicates either movement of part of the kaolin into such a regular pattern of films, which would appear unlikely, orientation by pressure, or crystallization in a regular pattern for some unknown reason. Commonly, some impregnating iron-bearing material, usually goethite if it is crystalline, is superimposed on such films, as in Plate 1, C and D. In many specimens goethite-rich tiny spherical aggregates of clayey material, a few microns in diameter, are assembled in popcorn ball-like clusters or surround pisolites, with which they have optical continuity. The density of packing of these tiny aggregates is associated with variations of apparent degree of impregnation of various parts of the specimen with iron compounds. Oriented films of iron-stained kaolin or pure goethite commonly line pores and cavities. These features suggest either movement of kaolin and iron oxide or crystallization of kaolin and iron oxide in an organized pattern, or both. The iron impregnation so commonly observed is consistent with expected mobility of iron and with the findings of Fripiat and Gastuche (1952) that iron is

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adsorbed and immobilized by kaolin. Thin sections commonly reveal evidence that such immobilized iron may be set free to move and recrystallize in various forms by destruction of kaolin. Alexander et al. (1956) and Alexander and Cady ( 1962) have found definitely defined separate bodies to be conspicuous in most laterites. Some are true pisolites with concentric structure, commonly with radial cracks indicative of shrinkage upon crystallization from a gel-like state. Pisolites having rhythmic bands of precipitates, such as those illustrated by Humbert (1948) are common. In addition to such structures and pseudomorphs after primary minerals, bodies of various shapes composed of a variety of segregated minerals are commonly present. Some appear to be fillings of former pores or cracks; others cannot be related to physical cavities of any kind. Some are pure goethite or hematite or pure gibbsite; others are amorphous or are unidentifiable mixtures of clay-size material stained with iron oxide ( see Plate 1 ) . The amount of material that has obviously moved, though perhaps only locally, to segregate either as definite crystalline material or as amorphous substances, or selectively to impregnate discrete parts of the matrix, is most impressive in thin sections under magnification. Although Hanlon (1944) suggested that pisolites form at or above a water table, many of the sites from which the observations above were derived could not have had a water table within many feet, and normal movement of soil water must have been the medium of transfer. Some pisolitic structures appear to have formed by crystallization from a gel state, as demonstrated by Bucher ( 1918) with ferric chloride. Schade (1910) demonstrated experimentally that “concretionary” bodies form when a substance passes from the “emulsoid to the solid state. He concluded that the resulting structure is radial if the substance is pure but is concentric if other material, either colloid or crystalloid, is present. Many of the forms observed can be attributed to such transformations, but others cannot. The processes involved are not evident, but the fact of at least local movement and segregation remains and is believed to be an important factor in differentiation of material that hardens from that which will not.

C. HARDENING OF LATERITE Mere concentration of sesquioxides does not insure the capacity to harden. The phenomenon of hardening has been discussed by Alexander et al. (1956), D’Hoore (1954), and Maignien ( 1958). Their data and unpublished work of authors of this review appear to confirm earlier opinions that iron plays a key role in the hardening process. Aluminous laterites do exist, as at site 3 of Table I, and it is known that alumina does

40

S . SIVARAJASINGHAM ET AL.

crystallize and cause hardness. The evidence indicates, however, that this probably involves crystallization of greater quantities of material and longer time intervals than are necessary if iron is involved. Most hardened laterites contain relatively high concentrations of iron, as in the examples of Table IX, Many contain almost no free aluminum, but TABLE IX Silicon, Aluminum, and Iron Oxides and Their Identifiable Mineral Species in Hardened Laterite and Underlying Soft Materiala, b SiO, A,O,

Location Sierra Leone ~

Sierra Leone Guinea

Ivory Coast Dahomey

h igeria Nigeria

Condition Hard Soft Hard Soft Hard Soft Hard Soft Hard Soft Hard Soft Hard Soft

Fe,O,

Qt

( 7 0 ) (70’0) ( % I 7.6 7.8 -

16.4 9.5 21.5 25.1 31.4 39.5 35.6 67.3

-

-

23.6 50.0

51.3 21.4

-

-

-

-

23.5 37.5 20.0 19.3 19.2 25.3 18.8 18.4

45.0 30.3 45.9 43.6 38.5 23.7 33.9 6.3

-

-

-

K1

Gb

Bh

Gt Hm

(70) (70) x

-

x

x

KK

20 65

-

xx

x

-

-

X

xx

X

X

-

xXx10

5

-

-

15 40 20 85

x x

-

-

-

xx x xx

-

x

x

x

- - - - - - - - - - x 20 - - xx x x 40 - x x xx 10 x x xx xx 40 - x x

XYX

XYX

-

-

x

-

-

Alexander and Cady (1962). Abbreviations and symbols: Qt, quartz; Kl, kaolin; Gb, gibbsite; Bh, Boehmite; Gt, goethite; Hm, hematite; x, detected; xx, moderate; xxx, abundant. a

b

few if any contain no free iron oxide. It appears, therefore, that while aluminum may be the major element involved in hardening of some laterites and may contribute significantly to the hardness of many, iron is by far the more common and more important agent, and probably the sole contributor to the rapid hardening, observed upon exposure, of many laterites. In most cases, some minimum amount of iron appears to be a primary requirement for hardening, that amount ranging widely among different materials and under different environments. Mere high concentrations of iron do not, however, ensure hardening. Many examples of earthy iron-rich material that is not hard and does not harden upon exposure are cited in the literature (Sivarajasingham, 1961; Marbut, 1928; Hough and Byers, 1937; Cline, 1955). Soft and hard material of similar iron content may occur in the same profile, as in material of the Ivory Coast (Table IX) .

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In every case known to the authors, the iron-bearing parts of a hardened laterite have a greater degree of crystallinity or greater continuity of the cystalline phase than the soft materials with which they are associated. The data on goethite and hematite in the six comparisons of TabIe IX illustrate the increase of amount of crystalline material from soft to hard material. Thin-section studies of these materials showed also that the crystalline phase of iron-oxide minerals in the hard material occurs in a framework having a high degree of continuity within the mass, though the exact form may vary widely from specimen to specimen. Such continuity is illustrated in Plate 1D. In most cases, the mineral involved is goethite, though hematite is prominent in some specimens. It would appear, therefore, that conditions that permit segregation and crystallization of iron in a pattern that will provide a rigid framework are essential to the hardening process of most laterites. This implies conditions favorable to the movement of iron, at least locally, and favorable not only for development of crystallinity, but also for development of a significant degree of continuity of the crystalline phase, The change from soft to hard laterite may involve only a small fraction of the mass; indeed, it may involve very little of the iron present. Buchanan’s original laterite, for example, appears from Fox’s (1936) data cited in Table I to be an iron-rich kaolinitic material having few unusual characteristics; yet it hardened quickly upon exposure. Its chemical composition is very similar to that of the laterite from the Ivory Coast reported in Table IX, thin-section studies of which showed the principal change upon hardening to be apparently minor reorganization, as indicated by the following descriptions of thin sections: Soft material: Large parts of the sections are composed of nets of small flakes of oriented iron-stained clay. Much of this oriented clay surrounds balls of more densely impregnated material. The sections contain more heavily impregnated areas than sections of material at greater depth, which does not harden, and the impregnated areas are more dense and have more definite boundaries, suggesting greater organization yet without the continuity of crystallinity that is associated with hardness. The sections contain more quartz per unit area than the deeper material, and this, with the discontinuity of clay nets and clay skins, suggests some breakdown of kaolin accompanied by loss of volume and release of iron that had been adsorbed on the clays. Some red highly birefringent pore linings, crack fillings, and other spots are goethite. Hard material: The main matrix of the thin sections appears quite clayey, and it is difficult to understand why the material is as hard as it is. It does not appear to be much more densely impregnated than the underlying soft material. The red areas are more extensive and more

42

S . SlVARAJAS1A'GIuh.I ET AL.

continuous, however, and show distinct oriented clay nets and some incipient ball-like aggregates. Some almost opaque heavily impregnated areas and definite concretions are present but are not abundant. Oriented clay coats in pores and cracks are less abundant than in the soft material; most of the crack and pore linings are goethite. The refractive index of even the pale parts of the matrix is high, indicating either a high iron content or loss of kaolin. It will be noted that chemical, mineralogical, and micromorphological differences between the soft and the hard material are not great. A decrease in kaolin is associated with an increase in crystallinity and continuity of goethite, which is believed to be the primary cause of the change from soft to hard material. Some materials, outwardly similar to Buchanan's laterite and to the soft specimens reported in Table IX, do not harden on exposure, however. At least some of such seemingly anomalous behavior can be explained by the work of Fripiat and Gastuche (1952), who demonstrated that kaolin has a substantial capacity to absorb and immobilize iron. Kaolin is a very common constituent of the soft material from which hard crusts form, and it has been noted that upon hardening the kaolin present is commonly isolated in pockets, encapsulated in a shell of crystalline goethite, or stained by iron materials. From the work of Fripiat and Gastuche, one would expect that kaolin to which iron in solution has access would have to be saturated with iron before goethite could crystallize. It will be noted in Table IX that the percentage of kaolin in the hardened product is lower in all cases than that in the underlying soft material. This is a common, but not a universal, relationship. Moreover it is impossible to conclude that the observed lower content of kaolin is due to an actual loss, since enrichment in iron from outside sources is obvious in some cases and possible in most. Yet the magnitude of decrease in many cases strongly suggests that kaolin has weathered, as suggested by Jackson et al. (1948). Such weathering of kaolin accompanied by release of iron and crystallization of gibbsite has been found in many thin sections by the authors and is illustrated in Plate IB. Such a loss of kaolin would decrease the capacity of the mass to immobilize iron and would increase the potential for development of the crystalline continuity of goethite so commonly observed. The potential for development of crystalline continuity of goethite could be due to either an absolute loss of kaolin or to absolute accumulation of iron from outside. The magnitude of decrease in kaolin from soft to hard material by itself cannot be used as good evidence to suggest either loss of kaolin or accumulation of iron. D'Hoore (1954) and

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43

Frankart et al. (1960) have used electron microscope photographs to decide between the two. Kaolin platelets with frayed edges indicate loss of kaolin as at least one possible mechanism for observed concentration of iron. Kaolin platelets with well-developed hexagonal outlines indicate that the concentration of iron is due to absolute accumulation from outside. Many factors may contribute to mobilization and movement of iron to sites where development of crystallinity results in hardening. Some iron, of course, is in soil solutions at all times, but conditions of high acidity, presence of reducing agents, and presence of complexing or chelating agents would increase the capacity for mobilization. The materials are commonly acid. In some cases iron may move as complexes with organic decomposition products comparable to those demonstrated by Bloomfield ( 1951) . This would be most likely in cases of iron enrichment by seep but is a potential mechanism in many cases. Some amounts of reducing agents from organic or other sources are potential components of most systems. The evidence at hand indicates that although reduction and complexing are among potential mechanisms, other processes also must contribute to mobilization and movement of iron in some laterites. A site in Guinea illustrates the influence of alternate wetting and drying on local rearrangement resulting in hardening. An ornamental ball and bricks for a house had been cut from a nearby laterite quarry 15 years before they were sampled by Alexander in 1951. The ball was exposed to the rain, but the walls of the house were protected by wide overhanging eaves. The walls of the house were soft enough to be cut by a thumbnail 15 years after exposure, but the ball was hard enough to require a moderate blow of a hammer to break it. The apparent difference in conditions was the alternate wetting and drying of the ball under the 70-inch annual rainfall of the area. X-ray and differential thermal analyses showed a decrease in kaolin and an increase in gibbsite, goethite, and hematite from the quarry to the ball. Chemical analyses and petrographic studies showed migration of iron within the ball from the matrix to channel linings and to an exterior shell, which were the hard portions. Comparable local rearrangement is illustrated in Plate 1C. Organic matter as a reducing or complexing agent could not have been a factor in the movement and segregation of iron in this case; only solution and precipitation as the ball was wet by the rains and dried between the rains could have been involved, yet the changes on a microscale had major consequences. This and similar observations, such as hardening on one side of an excavation exposed to the sun while the shady moist side remains soft, leads us to the conclusion that alternate wetting and drying, not drying alone,

44

S. SIVARAJASINGHAM ET AL.

is essential for hardening. This is consistent with the observed hardening after removal of vegetation and exposure of the surface to extreme drying-wetting cycles under the extremes of temperature induced by removal of vegetation. D'Hoore ( 1954, pp. 70-71) has measured temperatures of 50°C. in exposed surface material during the heat of midday while in an adjacent forest the temperature was only 27°C. It is also consistent with the observed tendency for little hardening where rainfall is very low or where rainfall is almost continuous. Not only wetting and drying related to pronounced seasonal distribution of rainfall but also the wetting and drying from rain to rain in cycles of several days or a few weeks should be important. It is also our conclusion that the zone of segregation must be maintained in a condition at or above field capacity for appreciable periods of time. This does not imply the necessity of a water table as such, though a fluctuating water table should contribute to the necessary wetting and drying cycles. The pallid zone so commonly found under laterite is a kaolin-rich, iron-poor layer that appears to be in an environment conducive to relative stability of kaolin and mobility of iron. This appears to be inconsistent with the discussion of immobilization of iron by kaolin in the preceding section, but it is possible that maintenance of the iron in immobilized form is related to wetting and drying cycles. The pallid zone is saturated for varying periods during the rainy season, with conditions favorable to mobilization of iron. As it occurs at a depth where the drying portion of the cycle is absent, or at least not intense, however, conditions for development of some degree of crystallinity may be lacking. The iron is clearly removed to other zones, probably over substantial periods of time. Although the pallid zone may be a source of iron for the laterite above it under some circumstances, its iron is probably lost mainly to water that moves downward or laterally to other zones or areas. The very thick cnists commonly found near the edges of plateau remnants probably derive their iron from such sources. If the hypothesis for a requirement of periodic near-saturation is correct, relatively flat surfaces that would favor accumulation of water during rainy periods would favor laterite development. This is consistent with the observed occurrence of laterite predominantly on flat surfaces. With appropriate rainfall pattern and consequent wetting and drying cycles, however, a flat surface should not be prerequisite; the cases of laterite on low hillocks in Ceylon reported by Prescott and Pendleton (1952) and the instances reported by Mulcahy (1960) substantiate this conclusion.

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45

D. DEVELOPMENT OF LATERITE IX PLACEWITHOUT ENRICHMENT FROM OUTSIDE SOURCES There is no doubt that some laterites have formed from residuum of rock weathered in place without enrichment from outside sources, as proposed by Newbold (1844), Glinka ( 189!3), and others. Hanlon ( 1944) observed that some laterites have the same proportions of iron and aluminum oxides as the parent rock and emphasized how unlikely it would be that two constituents of such divergent character could be introduced from outside sources or could be removed in so nearly the same proportions. A similar case has been presented in Table VI from a site in Guinea, and evidence has been given that the final hard product resulted primarily by “relative enrichment” as defined by D’Hoore (1955) through loss of bases and silica, followed by development of the firm skeleton of crystalline goethite from iron inherited from the parent rock. Such local reorganization is consistent with the hypotheses presented in preceding sections, assuming that the original rock contained iron in adequate amounts. Cases in which the property of hardening is weakly expressed in weathered material of rock low in iron are consistent with this view. The processes involved are discussed in detail in preceding sections. It is probable that a high proportion of the laterites of the world owe their character in whole or in part to these processes, but many are clearly affected to a major degree by enrichment from outside sources.

E. ENRICHMENTFROM OUTSIDE SOL~XS Much of the literature postulates that sesquioxides have accumulated from sources outside the immediate laterite zone. Though such enrichment appears not to be essential to the formation of laterite, the evidence clearly indicates that many laterites have been enriched, especially in iron, and such enrichment would be expected to favor development of a material that will harden. There are, in addition, some cases in which low iron content of the original rock would be expected to make enrichment essential. The authors find no basis in fact for postulating that laterite can form only by reorganization in place or only by enrichment.

1. Enrichment by Movement Downward in Solution Though combined silica and bases have been shown to be the dominant constituents that are mobilized from primary minerals and that are subject to loss, percolating waters contain small but significant amounts of aluminum and commonly greater amounts of iron (Table IV) . Profiles in which earthy material of variable thickness overlies the laterite

46

S . SIVAJ3AJhSIXCHAM ET AL.

are very common (Campbell, 1917; Marbut, 1928); indeed various authors, including Blanford as early as 1879, have suggested that laterite at the surface is the result of erosion of overlying earthy material. Such overlying material is commonly lower in iron than the laterite with which it is associated, and analyses of soils associated with laterite commonly show increasing iron with depth ( Marbut, 1928). It would be expected from the work of Bloomfield (1951) that iron in the upper layers would be more mobile than that deeper in the profile as a result of complexing with organic decomposition products. Bloomfield ( 1955) has also suggested immobilization of downward-moving ferrous iron by sorption on ferric iron particles or clays of deeper layers. Enrichment from above by such processes may be a contributing factor in many cases. That it could be the major source of enrichment in many laterites appears most unlikely, however, considering the relative amounts of iron involved in the laterite zone and in the overlying earthy material and the small amounts of organic matter present in these materials.

2. Enrichment by Capillury Rise Maclaren (1906) concluded that the pallid zone is a zone of reduction and solution of iron that moves upward by capillarity to enrich the overlying laterite. Data of Harrassowitz (1926) and others show that iron commonly increases upward from the pallid zone to the laterite, consistent with that hypothesis. Simpson ( 1912) suggested that laterite crusts are essentially efflorescences at the surface due to evaporation of water carrying iron upward by capillary rise. Though it is now recognized that upward capillary movement of water is less than was formerly thought, requiring at least a month to rise the maximum of 2 to 2.5 meters in the most favorable material (Baver, 1956; hlohr and van Baren, 1954), it could be significant immediately adjacent to a zone of saturation, either temporary or permanent, where extended periods without rain are common. Water from such a zone would be expected to carry significant amounts of reduced iron subject to oxidation upon contact with air. To what extent downward-moving water carrying reducing agents during rainy periods would reverse the process is unknown, but unless the forms of iron deposited were highly resistant, the reverse process should be appreciable in humid regions, as suggested by Nye ( 1955b). Enrichment by capiIlary rise would probably be most significant in regions having long dry seasons under special conditions that would maintain a zone of saturation well into the dry period. It would be expected to be effective only a short distance above the saturated zone (Sherman, 1950). Some of the Australian laterites underlain by a slowly permeable

LATERITE

47

siliceous layer may have developed in sediment-filled “bolsons” by capillary enrichment during a former arid regime. The unusual rodlike laterite described by du Preez (1949) in aeolian sands over a laterite sheet at Bornu, Nigeria, was probably enriched by such a process. The rodlike iron-rich forms were found in almost vertical orientation rising from a common base about 18 inches above a hard laterite sheet, suggesting deposition along capillary openings above a water table maintained by the laterite sheet. It is unlikely, however, that capillary rise alone could account for thick laterites or those far above a zone of saturation. Development of thick laterites by this process would require a slowly receding water table. Laterites far above a zone of saturation would be beyond the reach of capillary water. It is probable that capillary rise of iron-bearing water plays some role in enrichment of some part of many laterites but that it is not the major source of iron in most. Though the amounts of iron transported annually might be very small, the aggregate over the time scale visualized could be significant, but the effects should be seen only short distances above a zone of saturation. It is possible that the zone of laterite nodules found commonly in horizons of some soils having restricted internal drainage may originate in this manner (Fig. 2 ) . It is also possible that the zones of nodules in some soils of regions having very pronounced dry seasons, as in nothern Nigeria, may be in part a consequence of some upward movement of water after the rains without a distinct water table as such. Some upward movement does occur after a soil is brought to field capacity and evapotranspiration removes moisture near the surface. In the aggregate over very long periods of time, this could be a significant factor within the narrow zone in which water passes from the liquid to the vapor phase.

3. Enrichment by a Fluctuating Water Table The more widely accepted hypotheses of enrichment from outside have been based on assumption of a fluctuating water table at the upper boundary of which iron is precipitated as a result of oxidation (Campbell, 1917; Marbut, 1932; Mohr, 1944; Pendleton, 1941). Campbell ( 1917) postulated two-stage decomposition of rocks, each stage governed by access of oxygen and vadose water. To facilitate reference to these conditions, he defined three zones: ( 1 ) the zone of nonsaturation, including all the profile above the reach of vadose water; ( 2 ) the zone of intermittent saturation, including all the profile from the highest point to which capillarity raises vadose water to the lowest point at which atmospheric oxygen can penetrate; and ( 3 ) the zone of

46:

S. SIVARAJASINGHAM ET AL.

permanent saturation, including all the profile pervaded by vadose water and below the reach of atmospheric oxygen. One may disregard his hypotheses of weathering, which are not acceptable in the light of modern knowledge. He postulated enrichment of the zone of intermittent saturation, however, by iron carried downward by solutions rich in humic acids from the zone of nonsaturation and by ferrous compounds rising from the zone of permanent saturation. The latter could be precipitated by oxidation at the surface of the water table during the period of its recession or could be precipitated by oxidation from rising capillary solutions slightly above the water table. As the zone of intermittent saturation would be restricted to shallow depths by the requirement of access of oxygen, it was believed that this would account for the shallow occurrence and limited thickness of laterite observed. Development of laterite would not continue downward with recession of the water table because of lack of access of oxygen. If a down-cutting river established a new base level and developed a flood plain, laterite could form on the lower terraces but would not extend into the old land form because of lack of oxygen. Hence, different laterites would not form, one below the other. The hypothesis explained the occurrence of laterite overlying essentially all kinds of rocks on the basis of movement of lateritic constituents into the area in vadose water, whose variations in composition from area to area would also explain the variations of laterite composition observed. CampbelI’s hypotheses have been partial bases of many subsequent ideas on laterite formation, such as those of Marbut (1932, 1928) and Mohr (1944). Greene (1945) used the term “zone of alteration,” though he may have meant the “pallid zone” which implies no distinctive process such as that ascribed to this zone by Campbell. Mohr (1944) and later Mohr and van Baren (1954) used some of Campbell’s concepts, modified in the light of recent knowledge about weathering, to account for development of laterite in volcanic ash in Indonesia. It was proposed that during hydrolysis of volcanic glass and primaiy minerals in the ash, silica-bearing water would dissolve progressively more calcium and magnesium with depth until silica would precipitate, cementing the ash. Ultimately a siliceous hardpan would form, restricting permeability and causing a zone within which a water table would fluctuate. It was proposed that kaolin, gibbsite, and iron oxide would separate in this zone, the iron precipitating in the upper part where the water table fluctuates and the kaolin in the lower (pallid) part, further sealing the mass to movement of water. This was thought to result in a gradually rising water table with iron continuously subject to removal from sites of former precipitation to be redeposited in increasing

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concentration at the top of the fluctuating water table higher in the profile. Alumina was also thought to move gradually upward in the ascending water table, but at a slower rate than iron; the lower part of the laterite zone would be gibbsitic while the upper part would be characterized by iron and would harden upon exposure. This is in direct opposition to the hypothesis of Nye (1955b) that iron is dissolved from the top of the laterite and enriches underlying layers as normal erosion lowers the zone of saturation. While the explanation of precipitation of iron in the zone of fluctuation by conversion from ferrous to ferric state is consistent with principles of chemistry, the few explanations advanced for enrichment in aluminum from outside sources (Simpson, 1912; Campbell, 1917) are not tenable in the light of modem chemistry. Moreover, ground water proposed as a medium for enrichment in free alumina in the zone of fluctuation is also credited with resilication of gibbsite to kaolin in other zones, a seeming discrepancy that may be rationalized in terms of differing composition or environment but with little obvious basis in known chemical behavior. It would appear that enrichment directly by a fluctuating water table should apply principally to iron. It is not clear exactly how iron would be moved to the top of a fluctuating water table in an extensive plain such as those on which laterite is found. Movement of ground water is downward under the force of gravity, either downward vertically or laterally and downward. According to the Ghyben-Herzberg hypothesis ( Wentworth, 1955), the ground water at the end of a period of rains should have a lower layer consisting of the water present at the beginning of the rains on which should “float” an upper layer of water introduced by the rains and containing less dissolved material. This principle is employed on a practical basis in development of water supplies in Hawaii. Except as ground water moves laterally and downward to lower-lying landscapes and is mixed in the process, the upper layer of a rising ground water table should carry no more material for enrichment than is dissolved during its descent through the overlying soil and laterite zone. The authors can visualize no mechanism for the mechanical mixing implied by hypotheses of ground-water table fluctuation on land surfaces that do not lie adjacent to higher areas. Under these conditions, it is suggested that enrichment by a fluctuating water table would be confined to two principal effects: (1) temporary detention of material dissolved in overlying layers, with consequent increased opportunity for adsorption or precipitation in the zone of enrichment; ( 2 ) accelerated differential dissolution with associated differential losses, local translocation, and segregation of various constituents. Alternating oxidation and reduction,

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hydrolysis, and slow removal of the most soluble end products should be favored in the zone of fluctuation and should “enrich” the zone in lateritic constituents by removal of other components. Thus, it would appear that “enrichment” at sites where ground water is not introduced laterally from higher-lying land may be largely accomplished through transformations in place reinforced by introduction of solutes in water from overlying layers and intensified by the effects of ground-water fluctuation. Under these circumstances, a fluctuating ground-water table would be expected to intensify transformations by increasing the period of time during which conditions favorable for segregation of iron would prevail and by increasing the thickness of the zone in which such segregation would take place.

4. Enrichment b y Laterally Moving Watcr Under appropriate topographic conditions, iron especially, and to some degree aluminum, are clearly transported from one area to another, where they contribute in varying degrees to the sesquioxides of laterites. The increasing concentration of laterite nodules downslope in Fig. 2 is a case in which water moves laterally and downward in relation to topography, contributing to enrichment in the lower-lying positions. Greene (1945) described such a mechanism in relation to the catena concept. Gentle slopes at the base of higher-lying land, marshy areas or low-lying plains that receive water from other areas near or far, and similar topographic positions ( D’Hoore, 1957, pp. 48-51) are frequently sites of enrichment from outside sources. D’Hoore (1954, 1957, pp. 49) has described sites where iron has migrated from high-level laterites to successively younger laterites at lower levels down the slopes of dissection forms. Maignien (1958, pp. 185-187, 214-215) has emphasized the effects of a source of iron in old laterites at high levels on enrichment of new laterite on lower-lying areas. The enormous supply of sesquioxides at higher elevations permits extension of new laterite on nearby lower land much farther and much more rapidly than would be possible if the sole source of iron were that in primary minerals of the parent rock. The very thick laterite crusts found commonly near the edge of laterite-capped plateau remnants are apparently due to the issue of laterally moving iron-bearing water above a very slowly permeable zone at the face of the dissection form. In these cases, as in the Federal District of Brazil (Feuer, 1956, pp. 70-76), the laterite thins rapidly with distance from the face of the cliff and may be absent at some distance into the plateau. This is probably the situation of the very thick laterites

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reported by Oldham ( 1893 ) . Maignien ( 1958, pp. 184-185) has illustrated "corniche"laterite of this kind in Guinea. Cases of obvious enrichment from ground water are numerous, and the mechanisms of transport are easy to visualize where iron accumulates on a low-lying landscape from higher-lying land. This would include extensive areas of low-level laterite. Enrichment on one landscape, however, implies impoverishment on another higher one. It is probable that the pallid zone of high-level laterites is a major source in some areas, but the iron commonly present in water from weathering rock is probably adequate to account for slow enrichment on the lowlands generally without postulating any special zone of impoverishment. The process is conceived to be slow and wasteful, with small proportions of the reIatively small iron content of drainage waters contributing minute increments of enrichment over very long periods of time. At sites where a decomposing laterite crust or its detritus serves as an extraordinary source of iron, the enrichment is much more rapid.

F. PRINCIPAL PROCESSES INVOLVED The genesis of laterite requires enrichment of a material in iron, or sometimes aluminum. The enrichment required may be small if the material is coarse textured, i.e., if it has a small specific surface to take up the iron coatings; greater accumulations are required if texture is finer. The enrichment may come about by removal of other constituents. The potential for this type of accumulation would appear to be greatest in volcanic ash and basic rocks. The iron may come from outside the laterite layer itself; transport in seeping ground water from higher land is probably the most common external origin. Local enrichment sufficient to cause hardening or to develop a material that will harden on exposure also takes place as a result of local rearrangement of constituents within the layer itself, as illustrated in Plate 1C. Some parts of the mass lose iron; other parts gain. This process results in mottles which may develop into nodules or into a cellular framework arrangement. It can be accompanied, or caused by, weathering of kaolin to free the iron, but kaolin destruction is not required. The material may then have: ( a ) volumes high in iron because clay has been removed and iron has remained; ( b ) volumes high in iron because clay has been removed in adjacent volumes releasing the iron to move into the mottle or nodules; ( c ) volumes high in iron because of iron movement alone with no destruction or movement of clay. The iron may be mobilized by any of several mechanisms. The solubility of the hydrated ferric oxides is very low, probably of the same order

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of magnitude as that of Fe( OH)3 for which Latimer ( 1952) calculated a At pH 5.0 this implies Fe++ + theoretical solubility product of 6 x’ concentration of the order of lo-”) M. Such small amounts are essentially insignificant at any one time, though they could aggregate significant amounts over the time span involved in laterite development. Considering the relatively high solubility of ferrous forms, reduction is a much more potent mechanism in systems that contain the necessary reducing agents. Wetness alone is not enough; organic decomposition products are the most likely sources of reducing agents. Mobilization as complexes or chelates of organic substances may be more important than reduction. Some mechanism other than simple solution of ferric forms must be involved to account for iron of the order of a few milligrams per liter commonly found in drainage waters. Immobilization of iron at selected sites presents a more difficult problem, but on a macro scale it appears to be associated with sites within the vertical section or horizontally within landscapes where the rate of movement of solutions is slowed and loss of solvent occurs periodically or continuously. This may occur at sites within the landscape where laterally moving water approaches the surface and is reduced in volume by evaporation or transpiration, as where slope gradients change. Abrupt discontinuities in permeability, vertically or horizontally, can slow the rate of movement and enhance the chance of loss of water. Alternating wet and dry seasons, supplemented by transpiration, provide periodic excesses of losses over additions of water in various zones. Within this frame of reference, it would appear that oxidation of the ferrous to the ferric form upon introduction of oxygen, destruction of organic complexing or chelating agents, adsorption of ferrous forms either alone or complexed with organic compounds on ferric forms already present, adsorption on kaolin or similar compounds, and slow accretion on microcrystals with associated energy changes, are the most likely mechanisms of immobilization. There is no evidence that any single mechanism is responsible under all conditions. hlany of the characteristics of the resulting material are the same whether the enrichment was from outside or was a result of local rearrangement. It is difficult to prove the origin by examining a sample except in a case where the matrix material was originally devoid of iron or where it currently contains an abundance of easily weathered minerals that remain encapsulated within iron coatings. These would be obvious cases of external enrichment. Most laterites are developed by a combination of these processes of loss and gain and rearrangement of constituents. There are few clear examples of formation of laterites by a single mechanism.

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The hardening is also complex. Two processes appear to be involved in addition to gross or local enrichment: Crystallization of iron minerals into continuous aggregates or networks, as in Plate 1D. The iron may be combined with or mixed with clay but the hardest materials contain free crystalline goethite and hematite. Dehydrution is a factor in development of hard crusts. Hydrous, amorphous iron oxides lose water and crystallize as goethite. Old, very hard laterites are high in hematite. Goethite decreases and hematite increases with increasing age and increasing hardness in materials of similar origin. If gibbsite is present there is a correlative increase in boehmite. VI. Geomorphic Relationships

Any comprehensive concept of laterite as it exists must take into account not only the geomorphic relationships of the existing landscapes, but also the geomorphic history of the region. Ruhe (1954) has described in some detail the relationship in the Congo between remnants of a high-lying mid-Tertiary erosion surface capped by a laterite crust from which blocks and gravel are abundantly distributed on the slopes of the dissection forms and on an adjacent lower-lying end-Tertiary surface. Greene ( 1945), Andrew ( 1948), du Preez ( 1949), D’Hoore ( 1954) , Maignien ( 1958) , Mulcahy ( 1961), and others have described similar relationships between high-level laterite of ancient land forms and its detritus in soils and soil material of more recent erosion surfaces. The remnants of ancient erosion surfaces, uplifted and attacked in a new erosion cycle, are conspicuous features of many regions in which laterite occurs and stand as evidence of a more extensive plain cut to the control of a base level long since altered. The present multistepped land forms separated by steep slopes are consistent with the concept of back-wearing and parallel retreat of slopes propounded by Penck (1953) rather than the process involving reduction of slope gradients proposed by Davis ( 1902). As the ancient surface is worn away from the sides, fragments of its cap of laterite are let down upon the new surface, mantling the retreating slope with pieces ranging from huge blocks to fine detrital debris which gravitate to the pediments3 and the new plain in pieces of decreasing size and increasing roundness with distance from the source. An excellent discussion of these geomorphic relationships has been presented by Mulcahy (1962) with special reference to the land forms of western Australia. In this manner the new surface inherits the segregated material of 3 Pediment is used in the sense defined by Ruhe (1956, p. 442) and does not imply an arid region.

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the old, initially by physical transport under the force of gravity. Maignien (1958) and D’Hoore (1954) have shown that such detritus decomposes and serves as a rich source of iron in solution which moves downslope to enrich and cement new laterite at new sites on the lowerlying stepped landscapes, the “absolute accumulation” of DHoore ( 1955). The enrichment process appears to be much more rapid and more intense than that dependent wholly on sesquioxides from weathering primary minerals. Thus, the segregations of ages past tend to perpetuate themselves in part on the new erosion surface. On the footslopes the laterite matrix is an iron-cemented colluvial material frequently containing fragments of the older crust. In some places iron-bearing water moving over cemented material builds successive layers in planes parallel to the surface of the originaI hardened material, and the crust can be split along these planes. Transported material of various origins may be involved in the cemented matrix on different erosion surfaces in the same locality contributing to the wide variety of forms and compositions observed. On low-lying surfaces, detrital material may be found as nodular forms in soil profiles or as nodules cemented in a matrix, though the site may be far from present remnants of the higher and older erosion surface. On extensive low-lying plains, the surface is presumably in equilibrium with the drainage system controlled by the base level of the region. On such surfaces, the kinds of laterite described by Marbut (1932) would be expected to develop in unconsolidated sediments over the water table of the region. If one applies the geomorphic relationships of the present to the landscapes of the distant past, some of the variety found in high-level laterites may be explained. From the concept of succesive erosion surfaces presented by Ruhe (1954) and others and the concept of landscape inversion presented by Bonnault ( 1938), it is reasonable to believe that all the high-level landscape remnants of today were not always parts of featureless plains; at least some may have been low points in a plain or low-level surfaces among older high-level remnants. There is ample evidence of discontinuities vertically in the surficial mantles of such plains. Stone lines representing buried erosion surfaces are reported with increasing frequency ( Ruhe, 1959). Milne ( 1940), for example, found evidence of “a superposed dual morphology” when he visited some of Harrison’s sites, reporting not only truncation but also rejuvenation due to a change of ground-water relations. Indeed, mixing and translocation of surficial material appears to be the rule rather than the exception on old land surfaces. The fact that they are ancient erosion surfaces, in itself, implies a still higher surface, pediment development, and some inheritance of whatever material originally lay above them.

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One cannot regard them as the beginning, but as one stage in the endless transformations to which the earth‘s surface is subjected. If viewed in this perspective, it is reasonable to expect that many of the high-level forms were subjected to the same kinds of conditions of mass movement, lateral drainage, and enrichment observed on low-lying erosion surfaces today. VII. Softening of Laterite

Laterite does decompose; even it is not permanent in the landscape, though hard thick laterite crusts may persist for very long periods-even measured in geologic time. In some cases, decomposition occurs in a remarkably short period, as in the case described by Rosevear (1942) where a measurable reversal of hardening was accomplished under a teakwood plantation in 16 years. Maignien (1958, 1959) and DHoore (1954, 1957) have discussed decomposition of laterite as a normal process in the cycle of transfer of lateritic constituents from high- to lowlevel landscapes. Alexander et al. (1956) reported examples of disintegration or softening of laterite, including some of the same sites observed by Maignien and D’Hoore. Kellogg and Davol (1949, p. 8) considered that, although hard laterite is more resistant to mechanical and chemical breakdown than are rocks like granite, even it may weather to material of new soils or may contribute to colluvial or alluvial deposits in which new soils may form. Studies of soiI material over a disintegrating crust (Alexander et aE., 1956) showed that coarse fractions retained the composition of the underlying laterite but that fine fractions contained more silica and bases, related at least in part to introduction of fresh material from outside sources. Gottmann (1942) cited conclusions of Scaetta that introduction of weathering rock material would soften laterite, in direct contrast to Vine’s (1949) suggestion that introduction of aeolian material contributes to laterite development. Studies of a hard crust and a softened boulder from it (Alexander et al., 1956) showed increases of silica and iron and decreases of aluminum and combined water on a percentage basis. Thin-section studies and mineralogical analyses revealed an increase in porosity, removal of gibbsite near large pores and cavities, appearance of traces of kaolin, and conspicuous rearrangement of constituents in the softer specimens. Some parts were highly depleted of iron while others were very densely impregnated, suggesting that the softening was associated with a decrease in continuity of the impregnating material. The authors suggested that the impregnated parts would become residual nodules in a softened matrix that could be penetrated by water and roots. Studies by the same workers on detrital laterite

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fragments on low-lying areas in the Ivory Coast revealed loss of boehmite and gibbsite, with loss of iron from some fragments and apparent segregation of goethite in others within the same horizon. Kaolin was the principal clay mineral, and whether the aluminum of gibbsite and boehmite had been lost or had been silicated to kaolin could not be determined, but loss appears more likely. Iron was clearly mobile, but how much was lost and how much resegregated was unknown. These and similar observations indicate that laterites soften when processes comparable to reversal of the hardening mechanisms destroy the continuity of crystallinity. A decrease in amount, and especialIy in continuity, of crystalline material is most striking. Silication of aluminum compounds to kaolin may occur. Loss of aluminum is evident; loss or redistribution of iron and increase in hydration of iron compounds seem to be involved. Maintenance of a moist condition and actively growing vegetation appear to be associated with the process where it has been reported, and it is possible that the complexing of iron by organic compounds is involved. The time intervals involved in such softening are unknown, but from a very small and inadequate number of observations, it would appear that improvement of some areas that have had some degree of hardening may be possible in reasonable time providing there is material suitable for establishment and growth of trees. Improvement does not appear to be feasible on bare, hard laterite lacking soft parts with waterholding capacity great enough to support vegetation. Furthermore, the presence of encapsulated unweathered minerals from which plant nutrients may be available may be a critical factor in attempts to soften the material, and in its usefulness if it can be softened. The practicality of attempts to soften crusts remains very uncertain; that crusts do soften under natural processes over long time spans is a certainty. Prevention of hardening of laterite that is still soft is a more feasible enterprise. Use of the land in a manner that prevents erosion of the protecting unconsolidated soil cover, minimizes exposure to high temperatures and dehydration, and provides the mechanical disruption of extensive root systems is clearly helpful. Examples of hardening under opposite conditions are abundant. In some cases, maintenance of such areas in forest appears to be the only feasible use; in others, carefully managed agriculture is possible. Either is very difficult to accomplish in many areas in the face of mounting population pressure, ~FEFIENCES

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Ruhe, R. 17. 1954. Publs. inst. w t l . ktude agron. Congo Belge sSr. sci. 59. Ruhe, R. V. 1956. Soil Sci. 82, 441-455. Ruhe, R. V. 1959. Soil Sci. 87, 223-231. Satyanarayana, K. V. S., and Thomas, P. K. 1961. 1. Indian SOC. Soil Sci. 8, 110. Schade, H. 1910. Kolloidschem. Beih. 1, 375-390. Scrivenor, J. B. 1909. Geol. Mag. [V]6,574-575. Sherman, G. D. 1950. Pacific Sci. 4, 315-322. Simpson, E. S. 1912. Geol. Mag. [5]9, 399-406. Sivarajasingham, S. 1961. Ph.D. Thesis, Cornell University, Ithaca, New York. Soil Survey Staff. 1951. “Soil Survey Manual.” U. S. Dept. Agr. Handbook NO. 18. U. S. Govt. Printing Office, Washington, D. C. Soil Survey Staff. 1960. “Soil Classification, A Comprehensive System,” 7th Approximation. U. S. Govt. Printing Office, Washington, D. C. Stephens, C. G . 1946. C.S.I.R.O. (Australia) Bull. No. 206. Stirling, A. 1825. Asiatic Research 15, 163-338. Theobald, W. 1873. Mem. Geol. Survey India 10, 244. Vine, H. 1949. Commonwealth Bur. Soil Sci. (Gt. Brit.) Tech. Commun. NO. 46. Voysey, H. W. 1833. 1. Asiatic SOC. Bengal 2, 298-305. Waegemans, G . 1954. Publs. inst. natl. e‘tude agron. Congo Belge skr. sci. 60. Walther, J. 1889. Verhondl. Ges. Erdkunde 16, 318-328. Waither, J. 1915. Z. deut. geol. Ges. 67B, 113-140. Walther, J. 1916. Petennanns Geograph. Mitt. 62, 1-53. Warth, H.,and Warth, F. J. 1903. Geol. Mag. [IV] 10, 154-159. Wentworth, C. K. 1955. U . S . Dept. Agr. Soil Suruey Ser. I939 No. 25, 15-18. Whitehouse, F. W. 1940. Unb. Queensland Papers Dept. Geol. 2 (1). Wingate, G . 1852. Trans. Bombay Geograph. SOC. 10, 287. Woolnough, W. G. 1918. Geol. Mag. [6] 5, 385-393. Woolnough, W. G. 1927. Proc. Roy. SOC. New South Wales 61, 1-53. Wynne, A. B. 1872. Mem. Geol. Survey India 9, 68-70.

RICE IMPROVEMENT AND CULTURE IN THE UNITED STATES

.

.

. .

C Roy Adair. M D. Miller. and H M Beachell United States Department of Agriculture. Beltsville. Maryland. and the University of California. Davis. California

Page I . Introduction ................................................ 61 A . Importance as a Food Crop and Uses in the United States 61 and Elsewhere .......................................... B. History of Rice Growing in the United States ................ 62 I1. Rice Culture in the United States .............................. 68 A. Soil and Climatic Requirements .......................... 68 B. Cropping Systems ........................................ 69 C . Land Preparation ........................................ 71 D . Irrigation and Drainage .................................. 73 E . Seeding Rice ............................................ 77 F. Crop Fertilization ........................................ 79 G . Harvesting. Drying. Storing. and Milling .................... 83 I11. Rice Field Pests .............................................. 85 A . Weeds .................................................. 85 B. Diseases ................................................ 88 C . Insects and Other Pests .................................... 89 IV Origin. Botany, and Genetics of Rice ............................ 92 A . Origin and Species of Oym 92 B. Description of the Rice Plant .............................. 94 95 C . Genetics. Cytology. and Linkage Groups in Rice .............. V. Rice Breeding and Improvement in the United States .............. 96 A . History of Rice Breeding in the United States ................ 96 B. Current Objectives and Methods for the Rice-Breeding Program in 97 the United States ........................................ C . Results of the Rice-Breeding Program ...................... 99 References .................................................. 104

.

1

A . IMPORTANCE AS

.

introduction

FOODCROPAND USESIN THE UNITEDSTATES AND ELSEWHERE Rice is one of the most important food crops in the world. and it has been so for a long time. as agriculture and rice or food and rice are synonymous in the languages of China and India. its most probable A

61

62

C. ROY ADAIR,

M. D. MILLER, AND H. 1%. BEACHELL

area of origin. Rice is widely distributed throughout tropical, subtropical, and temperate zones of all continents. Asia produces over 90 per cent of the total world crop. Communist China, which produced about 160,OOO million pounds in 1960, leads in total production. North America is the fourth continent in production, and the United States is second to Brazil in the Western Hemisphere. Although the Agricultural Statistics, U.S. Department of Agriculture, 1960, showed that the average annual wheat production in the United States was about 13.5 times as much as rice during the 5-year period 1950-1954432,261,730 tons compared with 2,387,350 tons-the world production of these two important food crops was about equal. The average annual world production of rough rice was 197,962,000 tons compared with 209,400,000 tons of wheat. Since more wheat than rice is used for animal feed and seed, it is likely that as much rice as wheat is consumed by humans. Rice is the principal item in the diet of most of the people in southeast Asia and the adjacent islands and in some Latin American countries. Rice for food is used primarily as a whole-grain food product although the by-products from milling are used in other food products and beverages. During the milling process the hulls, bran, and embryos are removed. The bran and embryos are used primarily for animal feed. The whole grain (head rice) and larger pieces of broken grains (second heads) are used as table rice and processed cereals, the smaller pieces of broken kernels may be used for cereal products (grits) or ground into flour, and the smallest pieces of broken kernels (brewers' rice) are used in the brewing industry. Highly milled rice is low in protein and vitamins compared with some cereals. Milled rice does, however, contain 5 to 10 per cent protein with a good complement of the essential amino acids. Although highly milled rice is very low in vitamins, parboiled or slightly undermilled rice contains an appreciable amount of the B complex (Kik and Williams, 1945, p. 60).

B. HISTORY OF RICEGROWIK'C. IN THE UNITED STATES 1. Acreage and Prodtiction The history of rice growing in the United States and the method of culture used have been discused by many authors. Some of these are Copeland ( 1924), Gray and Thompson ( 1941 ) , Efferson ( 1952), Van Royen (1954), and Beachell (1959). Other authors have discussed phases or areas of rice culture in the United States. Several of these papers are referred to throughout this article. Rice has been grown as a commercial crop in the United States since

RICE IMPROVEMENT AND CULTURE I N UNITED STATES

63

the latter part of the seventeenth century. It was one of the crops the colonists tried to grow soon after settling at Jamestown, Virginia. Gray and Thompson (1941) reported that early records of the Virginia colony mention the intention to plant rice in 1609 and in 1622 a trial planting was made. Some upland rice for domestic use was grown in North Carolina and South Carolina before 1680. Successful rice culture was not established in the colonies until somewhat later than this. According to Salley (1936), Captain John Thurber, master of a New England brigantine, put in at Charles Town Harbor sometime prior to 1686. From him, Dr. Henry Woodward got about a peck of goldhulled seed rice obtained from Madagascar. Dr. Woodward planted the seed, had a very good yield, and distributed it to his friends for further planting. By 1690 the production of rice had so advanced that the planters asked that it be specified as one of the commodities with which they might pay their quitrents, and by 1700 its production was so great that there were not enough ships in Charles Town to export it all. Another version regarding the introduction of rice growing in the Colonies claims that Governor Langrave T. Smith obtained rice from a ship sailing from Madagascar that was forced to stop at Charles Town in 1694. This is probably an embellishment of the facts as it is known that rice had been in continuous production for several years before that date. Gray and Thompson (1941, p. 278) stated, “while rice was undoubtedly cultivated in South Carolina before 1694, the various accounts point to 1694 as a significant date, probably because varieties of superior quality were introduced which were better adapted to the physical condition of the Colony than were the varieties previously employed.” Thus it must be that 1685, a date frequently given, was the approximate date of the starting of continuous rice culture in the Colonies and that 1694, a date also frequently given, was about the time rice culture was firmly established. Rice production data by States are given for 1840, 1850, 1860, and 1870 and acreage and production for each tenth year from 1880 to 1960 in Table I. Holmes (1912) summarized the available rice production statistics for 1717 to 1911. He reported that the production in 1718 was over 17 million pounds of rough (paddy) rice, so there were about 12,OOO acres that year as 1350 pounds per acre was about the average yield. Rice production in the United States has fluctuated from year to year, and it has been influenced by economic conditions and production in ather countries and by wars, but data in Table I1 show that the trend in production has been upward recently. The production in 1960 was 53,363,000 bags of 100 pounds each, or about 300 times the production in 1718. The United States was a net exporter of rice until the Wax between

TABLE I Acreage and Production of Rice in the United States for Selected Years from 1840 to 1980k Year

Acreage ;tnd production

Total

Cal.

Miss.

N.C.

36.04 44.25 0.88 0.26 63.31 158.54 0.64 42.0 0.34 231.88 0.62 0.009 84.4 0.2 1.08 0.07 756.45 8.7 0.02 201.7 71.87 0.09 1,727.32 0.1 264.8 60.0 371.2 3,932.1 1.4 1,080.0 5,746.0 281.0 162.0 175.0 700.0 4,299.3 3,717.9 3,858.8 11,340.0 186.0 110.0 172.0 491.0 4,396.0 3,272.0 3,678.5 8,617.0 291.0 118.0 191.0 469.0 4,248.0 4,315.0 8,442.0 7,490.0 482.0 238.0 342.0 551.0 13,248.0 7,780.0 10,882.0 11,568.0 417.0 288.0 383.0 458.0 12.639.0 13,053.0 12.927.0 13.248.0

7.77 27.20 8.09 3.75 3.50 17.19 1.5 6.77 2.1 7.39 2.8 37.8 3.0 41.8

28.2 605.91 54.66 1,599.3 75.94 1,191.0 323.05 20.59 10.85 78.39 56.09 520.78 42.2 12.2 303.39 58.46 22.3 77.7 78.93 473.60 17.0 1.0 160.8 12.2 7.0 78.8

Ark.

La.

Texas

____I__

1840'A 1850" 1860" 187oC 1880 1890" 1900c

1910 1920 19301 1940f 19509 19SOn

l'rocluction'l Production Production Production Acrcagei Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreaee Productionj

808.41 2,153.13 1,871.67 736.35 174.17 1,101.31 161.3 1,285.9 351.3 2,837.23 722.8 11,029.5 1,336.0 23,429.8 959.0 19,934.5 1,069.O 24,495.0 1,620.0 38,889.0 1,595.3 531363.0

0.05 0.63 0.17 0.73

-

-

-

S.C.

7.0 189.0 45.0 1.350.0

_.

-

-

Ga. 123.85 389.50 525.08 222.77 34.97 253.70 18.1 145.58 22.0 111.75 4.0 39.6 4.0 46.8

-

Fla.

Ala.

4.81 1.49 10.75 23.12 2.24 4.93 2.23 4.02 1.58 2.55 12.95 8.11 0.8 1.8 10.12 3.99 23.0 5.4 9.27 22.54 0.9 1.o 8.8 11.2 1.0 3.0 32.4 14.0

-

-

-

-

-

-

-

-

-

-

-

-

-

P

E

-

H 3I

l?

Minor production reported in Ky., Tenn., Ill., Mo., \'a. b Minor Droduction renorted in Tenn., Mo., Va. c Minor production reported in Tenn. d Minor production reported in Va. e Minor iroduction reported in Hawaii and Va. f Minor production reported in Mo., Miss., S.C., Ga., Fla., Ala., N.C. a

Minor production reported in Mo., S.C., Ill., Tenn., Fla., Okla., Ariz., N.C. h .I000bags ( 100-pound bags). i 1000 acres, harvested. j Preliminary report. k Census Reports 1840-1900; U.S.D.A. Statistics 1910-1960. g

P

TABLE I1 Annual United States Rice Acreage and Production by States, 195619606 State Arkansas Louisiana Mississippi Texas California Other Statesa Total a b 0

Acreage and production Acreage Productionb Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production

1956 391,136 11,178.8 463,137 11,378.9 45,414 1,267.2 416,245 11,083.4 293,511 10,379.1 9,062 188.3 1,618,505 45,475.7

1957 337,246 10,355.4 427,990 10,660.7 33,073 999.8 346,434 10,666.5 231,807 9,602.9 6,678 152.8 1,383,228 42,438.2

1958 340,187 10,907.9 423,864 11,406.1 41,773 1,130.0 381,554 12,284.0 260,940 11,657.2 6,683 182.8 1,455,001 47,568.0

1959 391,901 12,642.9 463,437 12,915.7 45,242 1,355.5 416,054 13,388.0 291,511 11,944.2 8,897 267.5 1,617,052 52,513.8

Includes Florida, Illinois, Missouri, Oklahoma, North Carolina, South Carolina, and Tennessee. 1000 bags (100-pound bags). Statistics compiled by the Rice Millers Association, New Orleans, Louisiana.

1960

Average

385,982 12,583.4 462,573 13,161.0 45,139 1,297.7 417,039 13,484.6 288,458 12,921.8 9,226 290.6 1,608,417 53,739.2

369,291 11,533.7 448,200 11,904.5 42,130 1,210.0 395,465 12,181.3 273,246 11,301.0 8,109 216.4 1,536,441 48,347.0

E? 3

2

*

3 2

5 g 2!

9

8 ?

h

i

66

C.

ROY ADALR, M. D. MILLER, AND H. M. B E A W L L

the States (1861), but from that time until 1916 this country was a net importer. This position is now changed and in recent years the United States has been third or fourth in exports among rice-producing nations. The major rice-producing area of the United States was in South Carolina, North Carolina, and Georgia until about 1890. During 1884, 1885, and 1886, experimental plantings of rice were made in the prairie section of southwest Louisiana, using cultural methods and machinery employed in wheat production in the North Central States. After certain adjustments and modifications of these machines and methods had been made, and with irrigation water from streams, rice was grown successfully in this area (Knapp, 18%). Rice production expanded rapidly in the coastal prairies of southwest Louisiana and adjacent Texas. Production then declined rapidly in the southeastern States. Doar (1936), writing on rice production in South Carolina and the expansion of production in Louisiana, stated, “with these [rice producers in Louisiana], able to use machinery of all kinds, we could not compete with our system of hand labor, nor could we produce rice as cheaply as they.” A limited acreage of rice was grown along rivers in southeast Arkansas as early as 1840. The crop did not become of importance until rice growing was started on Grand Prairie in the early 1900’s. According to Vincenheller ( 1906), experimental plantings were made near Lonoke in 1902, and in 1904, 435 acres were sown and 5535 hundred-pound bags of rough rice were produced on the 260 acres harvested. The acreage expanded rapidly so that 60,000 acres were grown in Arkansas in 1910. Rice was not grown commercially in California until production was started in the Sacramento Valley about 1912. Chambliss (1912) reported that successful rice trials were conducted near Butte Creek in 1909. Starting in 1912, the acreage expanded so rapidly that rice was grown on 162,000 acres in California in 1920. Rice was grown along the rivers in Mississippi as early as 1840 (Table I ) , but it was not until 1948, when rice was produced by modern methods in the Greenville area, that rice became an important crop. The acreage expanded rapidly to about 79,000 acres in 1954. Because of the limited history of production, the acreage in Mississippi was reduced under the National Acreage Control Program to the present level of about 45,000 acres. Although rice has been grown in all southeastern States, as well as in Illinois, Missouri, Oklahoma, and Arizona, it now is an important commerical crop only in Arkansas, California, Louisiana, Mississippi, and Texas, with minor production in Missouri and the southeastern states. The annual acreage and production for 1956 to 1960 for each state are given in Table Ii.

RICE IMPROVEMENT AND CULTURE IN UNITED STATES

67

2. Cultural Methods Cultural methods used for rice in early times in the Carolinas were described by Gray and Thompson (1941), Doar (1936), and Allston (1854, 1855). The first rice was grown in the North American Colonies without irrigation but experience proved that irrigation throughout the growing season improved rice yields. A system was developed for irrigating rice along streams influenced by the tide. Hand labor was used to cbar timber from the land, to dig canals and ditches, and to plow or work the land with a hoe during the winter. Doar (1936) summarized a report made in 1850 of cultural methods then practiced by the best farmers. Rows were laid off in 13-inch centers with a 4-inch trenching hoe and seed was sown at the rate of 2% to 3 bushels (112 to 135 pounds) an acre. On “low gummy” soils the seed was not covered, but when soil was well prepared, it was covered. Water was then put on and held until the grain sprouted, 3 to 6 days after seeding. The water then was drained. The rice was irrigated and drained four times during the growing season. This made it possible to hoe and pull weeds and helped to control insects. The rice was cut by hand with a sickle, tied in sheaves, conveyed on “flats” through the canals, and stacked in the barnyard. Some planters used cradles to cut rice, but this was not a general practice. Some planters tried to use patent reapers, but this implement was not successful in the miry fields. When the grain was cured, it was threshed with a flail. The first mechanical thresher, which was operated by wind power, was tried in 1811 without success. A horse-powered thresher, introduced about 1829, was fairly successful and by 1851 steam-powered threshers that could thresh lo00 bushels a day were available. Mules and oxen replaced hand labor for seedbed preparation and hauling the sheaves to the flats, but much hand labor continued to be used in the “Carolina Low Country” until rice culture was discontinued. Production methods used in southwest Louisiana and southeast Texas in the late nineteenth and early twentieth centuries are described by Knapp (1899, 1900, 1910) and Bond and Keeney ( 1902). The best rice soil was considered to be a “medium loam” with about 50 per cent clay, underlain by an impervious subsoil. The fields were divided by small canals into subfields of “suitable” size, with a small levee on the border of each subfield. Each subfield was leveled so that the depth of irrigation water was uniform. The land was plowed, usually with a 2-gang, 10-inch moldboard plow. Sometimes plowing was done in the late fall or in the winter, but usually it was done in ‘the earIy spring. Plowing was followed in a short time by disking and then by harrowing. Seeding was

68

C. ROY ADAIR, M. D. MILLER, AND H. M. BEACHELL

done from mid-March to mid-May, but April seeding was considered the best. Seeding with a grain drill was recommended, but much rice was sown with a broadcast seeder and covered with a harrow. The seeding rate was 45 to 135 pounds per acre. When the soil was dry at seeding time, it was flooded and drained immediately so that the seed would sprout. The rice was imgated when the plants were 6 to 8 inches tall. The depth of water was 3 to 6 inches, and to avoid stagnation, water was renewed by continuous inflow and out0ow. The fields were drained about 10 days before the rice was ready to harvest. This allowed the soil to dry and become firm enough so the rice could be cut with a binder. Ten sheaves then were placed in a well-capped shock. As soon as the straw was cured and the grain was dry, it was threshed with a steampowered thresher. The grain was put in large burlap bags, usually about 185 pounds to a bag, and placed in a warehouse or hauled directly to the mill. Except for gradual improvements in the machinery and in irrigation systems, there apparently were no major changes in cultural methods from 1900 to about 1920. The use of tractors and gasoline-powered binders had become fairly common by about 1918. Soon after that, the power take-off-operated binder, which was a big improvement over the bullwheel and gasoline engine-powered binder, was perfected. As larger tractors and more efficient equipment were built, it was possible to time better and otherwise improve culture practices. The binderthresher method of harvesting continued for many years. The first change from this method was in about 1929 when “swathers” and pick-up combines were used to harvest rice in California. At about this time, studies were started by Smith et al. (1933) and Bainer ( 1932) to determine whether it would be possible to harvest rice with a combine. The extensive research that has been done in this field since about 1930 was reviewed and summarized by Dachtler (1959). The combine harvester is now universally used to harvest rice in the United States. To maintain the milling quality, rice must be harvested when it is too moist to be stored. With direct combine harvesting has come postharvest artificial drying of rice. Drying is done in farm or commercial drying plants. Bainer et al. (1955) reported that rice is produced in California with 7 9 man-hours per acre, as compared with 900 man-hours in Japan, where much hand labor is used. 11.

Rice Culture in the United States

A. SOIL~ ~ CLIMATIC i i REQUIREMENTS Rice generally is grown on silt loam, silty clay, and clay soils although it can be grown on all types of soils. It sometimes is grown on

RICE IMPROVEMENT AND CULTURE IN UNITED STATES

69

organic soils (Green, 1956), and sandy loams are often used. Satisfactory rice soils must have an impervious stratum within 2 to 5 feet of the surface or the texture must be fine enough to reduce water percolation so that seepage is reduced. Because of excessive percolation losses, rice grown on open permeable soil may require two to three times as much water as rice of comparable yield grown on "rice" soil. The surface of the soil must be level so that a uniform depth of about 6 inches of water can be maintained with a minimum number of levees, yet with enough slope so that surface water can be drained for seedbed preparation and harvest. Rice requires a relatively long, frost-free growing period. The optimum growing period for United States varieties is 105 to 170 days. The range of air temperature for rice production is about 70°F. minimum to about 95" maximum. Ormrod (1961) showed that CALORO rice seedlings maintain an appreciable net photosynthetic rate at low air temperature (40" or 60" ) and a wide range of light intensities. Low light intensities at a higher temperature (80' ) resulted in net losses of carbon dioxide from the plants. Rice often is considered to be a tropical crop, although it is grown throughout the tropical, subtropical, and temperate zones from the equator to nearly 50" latitude. The range in the United States is from the Gulf Coast (30"N) to about mid-United States (40"N). The yields generally are higher in the temperate than in the more tropical areas. This difference may be due partly to varieties grown, but climate and soil contribute to this difference.

B. CROPPING SYSTEMS The riceland cropping systems now used in the United States have evolved over the years. These systems are based on experience of growers and information gained from controlled experiments. The cropping system used depends on the soil type and climatic conditions. In most rice-producing areas of the United States, crop rotation is followed because, if cropped continuously, the soil usually becomes depleted in fertility, the organic content becomes so low that the deteriorating physical conditions make seedbed preparation difficult. The soil also becomes progressively infested with weeds that lower the yield and quality of the rice. In the early years in the Carolinas, rice was grown continuously in the same field, with occasional rest, according to Gray and Thompson ( 1941). Rice fields in that area were sometimes planted to oats in the fall, followed by potatoes the next year. Some farmers made a practice of growing rice and cotton on alternate years. This helped to control the weeds in both crops. In the South Central States, the fields were

70

C . ROY ADAIR, M. D. MILLER, AND H. M. BEACHELL

cropped to rice year after year until the yield became low and the quality was poor because of mixture of weed seeds and red rice in the threshed grain. Fields were then allowed to lay idle for a year or two and then put back into rice. This practice improved the yield but did not control the weeds satisfactorily, so the rice fields were grazed during years they were “laid-out.” This helped control the grass and weeds but did not control the red rice. Some farmers practiced summer fallowing for a year or two between rice crops and thus controlled the weeds and red rice more effectively than when the fields were not cultivated. In Arkansas, starting about 1930, rice was rotated with soybeans and lespedeza. Fall-sown oats had been rotated with rice to some extent before the use of the legume crops. A common rotation system for Arkansas was rice, soybeans, followed by fall-sown oats with lespedeza sown in the oats in the early spring. This is a difficult rotation to manage because of the likelihood of rain at a critical time which may prevent the seeding of the oats. However, when this rotation can be used, four crops are produced in three years, the soil is benefited, and weeds are controlled to some extent. Farming systems for rice farms in Arkansas are described and discussed by Mullins and Slusher ( 1950, 1951) . More recently, fish culture has been beneficially inserted into the rice rotation system in Arkansas and Mississippi (Green and Mullins, 1959). Because of weather conditions and diseases, oats and soybeans have not been grown in rotation with rice in the Gulf Coast area. The usual cropping system in this area is to rotate rice with improved pastures and to raise beef cattle as a supplemental enterprise. This system of rice farming and this method of establishing pastures are discussed by Moncrief and Weihing ( 1950). Walker and Sturgis (1946) conducted experiments on the establishment of pastures following rice on three soil types in Louisiana and described the results with respect to improving the pasture and the results of this treatment on the following rice crop. They found that fertilizer at the rate of 400 pounds of 3-12-6 and 1 ton of ground limestone an acre before seeding the pasture with white clover, Persian clover, common lespedeza, carpet grass, and bermudagrass, and 200 pounds an acre of the same fertilizer the next year almost doubled the production of the pasture compared with growth that occurred naturally following rice. Rice yields following the improved pasture were over 10oO pounds an acre higher than rice following unimproved pasture. This system or some modification of it is commonly used in Louisiana and Texas, according to Jones et al. (1952). In California, no definite system of crop rotation has yet emerged (Jones et al., 1950). To date no serious rice disease in that State has made crop rotation on riceland necessary. When rapidly advancing

RICE IMPROVEMENT AND CULTURE IN UNITED STATES

71

knowledge of weed control, fertilization, and other improved cultural practices are used, fields can be cropped continuously to rice with everincreasing per acre yields. Some California farmers follow rice with spring- or summer-plowed fallow, on which wheat or oats and vetch is fall-sown. After the ensuing grain crop, the field is returned to rice for an indefinite numbers of years. Currently, spring-sown grain sorghum, field beans, and saf3ower are being used on the medium-textured soil types for 1 or 2 years, the land then being returned to rice. To a limited extent, imgated ladino or strawberry clover-trefoil-grasspastures may be inserted in the rotation for a continuing period of 4 to 7 years. C. LANDPREPARATION The seedbed for rice is prepared in much the same manner as that for other small grains. The procedures generally followed are given by Davis (1950), Jones et al. (1950, 1952>,Reynolds (1954), and Finfrock and Miller (1958). The primary aim is to destroy weeds and to provide a suitable seedbed. Whether it should be rough or mellow on the surface will be governed by the seeding method used. If the rice is to be sown in the water, the soil surface should be rough; and if it is to be drilled, the soil surface should be mellow. Rice fields are usually plowed in the late summer, fall, or winter in Louisiana and Texas. Land that has been in pasture for several years, when plowed at that time, will be mellowed by the frost and rain during winter and early spring. A good seedbed can be prepared by disking, harrowing, and floating (dragging) in the spring. When the land is plowed in the spring, it is disked and harrowed immediately to prevent clod formation by baking. When cropped successively to rice, the land is disked as soon after harvest of the rice crop as soil condition permits. In Arkansas, rice fields usually are disked immediately after harvest. The tractors used to pull the grain carts while the combines are operating often are used early in the morning to disk the land harvested the previous day. Disking chops the stubble and straw and puts it in contact with the soil and thus hastens its decay. The fields may be disked another time or two during the winter as weather permits so that it can be seeded to lespedeza in early spring without further seedbed preparation. When rice follows soybeans that have been intertilled, a seedbed may be prepared by working the soil with a heavy disk, followed by a lighter disk and smoothing harrow. When rice follows lespedeza, the land is plowed, usually with a moldboard plow, disked, and harrowed. In California, the land is plowed in the spring to a depth of 4 to 6 inches, disked, and then harrowed or smoothed with a drag. Sometimes where rice follows beans, sorghum, or safflower, the seedbed may be

72

C . ROY AD=,

M. D. MILLER, A h 9 H. M.

BEACHELL

prepared by spring disking. When the rice is to be sown in the water, the soil should not be worked too soon after a rain and it should be left loose and rough on the surface. Since water seeding is the predominant method, a clod size ranging from 5 inch to 4 inches in diameter is the goal. As the clods slake down after flooding and seeding, the silt covers the seed. If the soil is compacted and puddled by working when it is too wet, algal growth is induced and oxygen essential for good root development is excluded from the soil. Usually, in Arkansas, the last field operation before flooding is to till the soil with a spring-tooth harrow or field cultivator. A heavy spike-tooth harrow is generally used for this last operation in California. In addition to the other advantage, a rough seedbed helps prevent drifting of the seed in the water. An essential part of rice culture is to have the soil surface as level as possible within each subfield. That is, knolls should be cut down and shallow sloughs and dead furrows filled so that the field is uniformly graded to the natural slope of the terrain. This will make it possible to keep the fields drained of winter water thus expediting seedbed preparation and speeding harvest by providing for rapid field drainage at maturity. The levees can be straightened when the fields are graded to a uniform slope from the upper to the lower side. This will reduce tillage and harvesting costs by cutting turning time and increase the rice yields by eliminating “drowned-out” and weedy spots. In most cases a rice field can be improved for water management by land planing before or during seedbed preparation. Regular earthmoving equipment is required to grade very rough fields. Grading rice fields while flooded has been successfully demonstrated in Louisiana (Faulkner and Miears, 1961). After a field has been adequately graded, it then can be maintained by regular land planing or floating prior to seeding the rice. Levee construction is an integral and important part of preparation for growing rice. Levees are the key device used to control the depth of application of water to the crops. The levees must be located accurately in order to maintain a uniform depth of water; they are constructed on the contour, that is, on lines of equal elevation. They should be located by an experienced surveyor or operator using an accurate instrument. The soil should be smooth, so the surveying can best be done immediately after the land has been floated. The contour interval between levees is usually 0.2 to 0.3 foot, depending upon the slope- of the land. On flat land they are built OR 0.1 to 0.2 foot contour, and on steep, sloping land on 0.3 foot. The levee should be as compact as possible and high enough to hold the water at the desired level. In the southern States, the levees

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have gently sloping sides so that the rice can be drilled and harvested over the levee. In this case the levees are complete after drilling the rice. In California, the levees are higher and have steep sides so that the area between pairs of levees is harvested as a field or unit. In the southern States, the base of the levee is made by plowing one round with a 3bottom plow and then finishing with a single-or a double-blade pusher on clayey soils or with a levee disk on sandy loam and loam. The levee base is built as early as possible so that it can settle. In California, a V-type diker or soil crowder that is 14 to 16 feet wide in front and 4 feet wide in the back is used. Two or more crawler-type tractors are used to pull the diker. The levees usually are 30 to 36 inches high when freshly made and settle to 16 to 20 inches. Levees usually are made in the fall and allowed to settle during the winter. A bulldozer or a tractor with front-end scoop is used to close the gap at the water-control boxes and at the end of each levee. Scott st al. (1961) reported on the possibilities of using plastic film rice levees in lieu of soil levees. Their studies show that plastic levees are physically feasible but of undetermined economic benefit. A machine to install plastic levees mechanically is required before this type of levee will be commercially feasible.

D. IRRIGATION AND DRAINAGE The water requirement of rice is high. Flood irrigation is used for all rice grown in the United States. That is, the soil is submerged in 4 to 8 inches of water most or all of the time from seeding until the grain is nearly ripe. Upland, or rain-fed, rice is grown in some areas of the world, but only where rain is almost a daily occurrence. Rice will not produce a profitable crop on stored soil moisture or infrequent rains as will some other cereals. Senewiratne and Mikkelsen (1Wl) suggest that differences in growth responses of flooded and unflooded rice may be due to differences in auxin metabolism. They found that plants grown under unfiooded conditions had a low catalase activity and a high peroxidase activity which favored accelerated auxin degradation. They suggested that high manganese levels in plants grown under unflooded conditions affect the indoleacetic oxidase mechanism resulting in retarded growth and depressed grain yields. Rice grown with ammonium nitrogen (flooded) collected small amounts of manganese, whereas plants grown with nitrate nitrogen (typical of upland rice) contained much more manganese. Clark et al. (1957) “concluded that the better growth of rice in submerged as compared to upland culture in at least some soils is due to greater Mn availability under submerged soil conditions.” A reliable source of comparatively salt-free irrigation water is required, The total amount required will depend upon seepage, average

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temperature, relative humidity of the air, and the amount and distribution of rainfall, as well as the length of growing season of the variety being produced. The amount varies from 1.5 to 3 acre-feet per acre in Arkansas and Louisiana to 3 to 8 acre-feet in California (Adair and Engler, 1955). A rate of %owequal to 1 cubic foot per second (450 gallons per minute) for each 50 acres being irrigated is usually required to maintain water levels in California rice fields ( Finfrock et aZ., 1960). In the United States, irrigation water is diverted from streams or is pumped from rivers, bayous, wells, lakes, or reservoirs. Water is conveyed to the fields in canals. If a large canal serves many farms, as in some areas in California, Louisiana, and Texas, the water is diverted to lateral canals for each farm or small group of farms. The water generally is turned into the top check of a field and flows through boxgates in the levee to reach each succeeding check. Some fields will have a lateral canal down the side so that each check can be watered separately. The water must be relatively free of salts toxic to rice. The characteristics of irrigation water which determine quality include: total concentration of soluble salts, relative ratio of sodium to other cations, concentration of boron or other toxic elements, and under some conditions, the bicarbonate concentration as related to the concentration of calcium plus magnesium. According to Finfrock et al. (1960), the qualities to look for in excellent to good water for rice irrigation are the following: specific electrical conductivity, ( K x lo6) less than 750; boron, parts per million (ppm) less than 1; S.A.R. Index (tendency to form alkali soil) less than 10. Other points to consider are the initial salinity of the soil, the effect of internal drainage while the soil is flooded, and the total salt content of the soil. The point of greatest importance is the nature of the soil solution or saturation extract found in the zone of rice roots. If soil saturation extracts have a conductivity index of 4 to 8 millimhos (mmhos) or more, the yield of CALORO rice may be reduced 50 per cent (Pearson, 1958). Rice seedlings are very sensitive to salinity during early development but are progressively less so at 3 and 6 weeks of age (Pearson, 1958). When soils are strongly saline, having an excessively high concentration of sodium, calcium, or potassium, the concentration of salts in the soil solution (including the standing water) may be so great that it will injure or kill the seedling rice (Pearson and Ayers, 1960). Excessive salt concentration results in restriction in downward percolation. Therefore the flood water is subject to a longer period of evaporation with an ensuing increase in salt concentration. Thus water having a higher salt

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concentration enters the soil, and the concentration of the soil solution is increased. Flooding, draining, and reflooding before seeding will help prevent such injury on fields known to give difficulty. Prompt draining and reflooding seedling stands in difficulty has usually corrected the condition. In Arkansas, the continued use of water from shallow wells, which may contain 75 ppm. of calcium and 22 ppm. of magnesium, for several years may change the soil reaction from acid (pH 5) to alkaline ( pH 8 ) . Hall (1959) reported that the condition can be corrected to some extent by alternate flooding and draining at about weekly intervals and by applying nitrogen fertilizer. When the rainfall is below normal, the amount of water pumped for rice from a bayou near the Gulf Coast may be more than the flow of the stream. In this case, brackish water may encroach from the Gulf. Quereau (1920) and Fraps (1909, 1927) reported the effect of brackish water on the growth of rice. The damage depends on the concentration of salts in the water, the time the salt water is applied, the length of time the injurious water is used, the amount of rainfall, and the variety of rice. In addition to causing a loss in yield of rice, the excessive use of salt water or the continued use of salt water for several years may deflocculate the soil so that stickiness, compactness, and impermeability increase and the soil is hard to cultivate. California experience has shown that rice culture is useful in the reclamation of saline soils, provided the fields to be reclaimed are first well drained. Mackie (1943) reported that one rice crop grown on Imperial clay near ImperiaI reduced the saline content 72 per cent to a depth of 6 feet. In his experiments he found the usual reduction in saline content from the first rice crop to be one-third to two-thirds. The temperature of the irrigation water should be not less than 70°F. or more than 85" for best results. Raney (1959) showed that the critical seasonal threshold of water temperature for normal growth of CALORO rice was near 69". If the mean temperature is 5" lower, maturity is delayed 30 days beyond the normal 160 days. Rice yield was highest when the mean water temperature was 80". At water temperature above 85", yield was reduced, and root development was poor, probably because of the low oxygen content of the water. Ehrler and Bernstein (1958) reported that at a constant root temperature of 64.4"F. CALORO shoot growth was twice that at 86" and root growth one and a half times as great; however, grain yield was only three-fourths as much at the lower root temperature. No significant interaction was found between root temperature and cationic concentration or cationic rates.

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Seeds germinate slowly when the temperature is less than 70°F. The temperature of water from rivers in northern California and shallow wells in some southern States may be 65" or lower. This temperature will retard and lower the germination of rice sown in the water and retard the development of plants so that the stand may be thin and late near the inlet to the field. According to Raney et al. (1957), field studies over a 3-year period showed that plant-free warming basins, 6 to 12 inches in depth, equal in size to 2 per cent of the area to be served, successfully raised the mean water temperature of 60" water to 70". Water management systems vary widely, depending upon method of seeding, soil type, climate, crop rotation, diseases, and insects. Methods of irrigating rice in Texas vary widely, according to Reynolds (1954). Where rice is drilled, the fields may be flushed for germination. Thereafter the drilled rice is irrigated for the first time when the plants reach a height of 4 to 6 inches. At this time water is applied to a depth of 1to 2 inches and is gradually increased to 4 to 6 inches as the plants increase in height. The water then is held at 5 inches until drained for harvest. Water may be drained once or twice during the growing season for fertilization and pest control. M'here rice is sown in water, some growers drain as soon as possible after seeding; others may delay draining as much as 36 hours. After the seedlings are well established, the practices are essentially as outlined for drilled rice. \Vater management is very similar in Louisiana and Arkansas. In Louisiana, water seeding is not common, but where it is practiced, the water is drained when the rice seedlings are 9 inch long ( Wasson and Walker, 1955). It then is allowed to grow until flooding is needed. Drilled rice may be flushed if necessary for uniform germination. Normally the rice is not flooded until it is 6 to 8 inches tall, and then only to a depth of 4% inches. Fields are drained as necessary for top-dressing with fertilizer and for pest control. In California, where japonica-derived varieties predominate, rice fields normally are flooded to a depth of 6 to 8 inches just before seeding and seeded by air with soaked seed. The water is maintained at that depth until drainage for harvest. Water may be lowered to about a 3-inch depth for weed control and at stand establishment time during cool weather. Good drainage, including winter rain water, of rice fields is necessary for several important reasons. Spring plowing and fall harvest are expedited by good drainage. Crops grown in rotation with rice may be damaged by impounded water from rains or irrigation if provision is not made for proper drainage. Drainways around and through the field that connect with main drainage ditches leading to natural drainage

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channels are essential. These ditches should be of sufficient size and depth to allow the removal of large quantities of water quickly. The time to drain the water before harvest varies with the soil type, rice variety, weather conditions, and drainage system. The water should be drained when the panicles are turned down and the grains at the tips of the panicles are ripe and those at the base of the panicle are in the hard-dough stage. This stage ordinarily is about 2 weeks before the crop is mature. When drained at this stage, there usually will be sufficient soil moisture so the rice matures normally, and within 2 weeks the soil will be firm enough to support the harvesting machinery.

E. SEEDINGRICE 1. Quality and Treatment of the Seed The choice of seed is an important part of rice culture. To achieve top yield, the variety to be grown must be adapted for the area. This is discussed in Section V, C. After deciding upon the variety, it is necessary to select a lot of rice that is free from varietal mixtures, does not contain red rice and weed seed, is high in percentage of viable seeds, and has high bushel weight. The seed requirements have been discussed by Jones et aE. (1950,1952) and Finfrock and Miller (1958).All rice-producing States have a rice seed certification program designed to provide high quality seed for the rice industry. The seed should be treated with a fungicide to control seedborne diseases, and with an insecticide in areas where there is likely to be a heavy infestation of rice water weevils. In California, soaking seed in a sodium hypochlorite solution (5.25 per cent by weight) at the rate of 1 gallon per 100 gallons of water is recommended as a seed protectant and to deactivate germination inhibitors located in rice hulls. Mikkelsen and Sinah (1961) reported the presence of six compounds in the hulls that inhibit germination of CALQRO rice. When present in large amounts, these compounds diffuse into the embryo and inhibit germination. Compounds identified include vanillic acid, ferulic acid, p-hydroxybenzoic acid, p-coumaric acid, p-hydroxybenzaldehyde, and possibly indoleacetic acid. In low concentrations, these chemical substances prove stimulatory to germination and ensuing growth. Leaching the seed by soaking with water reduces the concentration to stimulatory levels. The probable action of sodium hypochlorite solution is to modify these inhibitors, so that seedling growth is stimulated.

2. Methods of Seeding Several methods are used to seed rice in the United States. It may be sown with a grain drill much the same as other small grains, sown

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broadcast on dry soil and disked or harrowed to cover, or sown in water by airplane. On nonsubmerged soil, the rice is sown 1% to 2 inches deep. The field may be flushed and drained immediately to provide moisture for germination or there may be sufficient moisture in the soil or from rain for germination and seedling growth. When drilled rice is imgated before emergence, the water is drained immediately because the rice seedling cannot emerge through 1 to 3 inches of soil and thence through 4 to 8 inches of water. Several modifications of the water-seeding method are used. In California, the usual practice is to soak the seed for 18 to 24 hours, drain for 94 to 48 hours, and seed in the water. Seed rice will absorb the maximum amount of water in 18 to 24 hours. In Arkansas, rice sometimes is sown in water. When this is done, the fields are flooded to a depth of about 6 inches just before seeding. Dry seed is then sown by airplane, the water being maintained at a constant level for about 5 weeks; then it may be drained off to apply fertilizer or to control root maggots. In Texas (Reynolds, 1954) and sometimes in Louisiana, a water-seeding method that is different from the method used in California or Arkansas is used. In this method, which generally is used on clayey soils, a rough seedbed is prepared and levees are constructed. The field is flooded with enough water to cover the soil. The soil is then tilled with a spike-tooth or disk harrow, and sprouted seed is sown immediately. The soil suspended in the water lightly covers the seed when it settles out of the water. The water is drained from the field within 12 to 24 hours. In 7 to 10 days after draining, the field is reflooded to a shallow depth; and as the seedlings develop, the depth of water is increased gradually to 5 to 6 inches. When rice is sown with a drill in clayey soils in Texas, it is seldom sown over 1 inch deep and is flushed as soon after seeding as possible. In sandy loams, the rice is sown 3 or more inches deep and cultipacked immediately to conserve soil moisture. Frequently a harrow or a rotary hoe is used to break the crust to allow the rice to emerge. Water is not applied until the rice is 30 or more days old.

3. Time and Rate of Seeding Most rice in the southern States is sown from April 1 to May 30, but occasionally some fields are sown in late March and others as late as June 30. When rice is sown early while the soil or the water is relatively cool, it is subject to very slow germination and to rotting, so the seed should be treated with a fungicide. Seed germinates more quickly if sown later when the soil or water temperature is more nearly optimum. Date-of-seeding tests reported by Jenkins ( 1936), Adair ( 1940), Jodon

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(1953), and Reynolds (1954) show that there is a comparatively long seeding period for rice in the South. Most farmers take advantage of this fact to spread the work load both at seeding time and at harvest. By doing this, they can also make better use of the available irrigation water, especially if a well is the source of irrigation water. In California, the period for seeding rice is more restricted than in the South. Some rice is sown in California soon after April 1 and some as late as June 15, but most of it is sown from April 15 to May 15 (Finfrock and Miller, 1958). When rice is sown as late as May 30 in California, it should be fertilized at a moderate rate and a short-season variety such as COLUSA preferably should be grown. In the South, the seeding rate is about 90 to 100 pounds per acre when drilled and 110 to 140 pounds per acre when sown broadcast on dry soil or in the water. In California, seeding rates when sown in the water average about 150 pounds and range from 125 to 200 pounds (dry-weight basis) per acre. Rice seeds average 15,000 per pound; with a seeding rate of 150 pounds per acre, seeds are sown at the rate of 50 seeds per square foot. Excellent yields have been reported, from plant populations ranging from 8 to 30 per square foot. Seeding rates between these extremes apparently do not influence yields. Extremely dense stands lodge more readily than optimum stands. Rice in denser stands heads and matures more uniformly than thinner stands with abundant tillering. The higher seeding rates usually are used on old land.

F. CROPFERTILIZATION 1. Commercial Fertilizers Fertilizers were not commonly used for rice in the United States until comparatively recent years. Allston ( 1854) reviewed rice-fertilization practices up to about 1850. He reported that it was a fairly common practice to return the rice straw and chaff to the rice fields and to apply farmyard manure. At about that time some rice growers started to use lime ( 100 bushels an acre) and to apply rice-flour (bran) between the rows prior to the “long-flow.” Knapp (1899) reported that an application of 300 pounds of cottonseed meal, 150 pounds of acid phosphate, and 50 pounds of kainite plowed under before seeding increased the yield of grain and straw 25 per cent in Louisiana. Chambliss and Adams (1915) and Chambliss (1920b) do not mention the use of fertilizer for rice in California. Jones ( 1923), Dunshee (1928), and Davis and Jones (1940) report results of rice fertilizer experiments with good response to nitrogen. Jones et al. (1950) and Davis (1950) recommend ammonium sulfate at rates up to 250 pounds, 50 pounds of nitrogen an acre applied before flooding to 65 days after seeding.

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Mikkelsen and Finfrock (1957) subsequently reported that subsurface (drilled) placement of ammonium nitrogen produced better growth and yields of lowland rice than similar nitrogen applied on the soil surface (broadcast) before or after flooding. Mikkelsen et al. (1958) confirmed these findings and set forth conditions under which the use of phosphate fertilizers and top-dressing might be beneficial. Increases in yield of 3OOO pounds per acre as compared with the controls through use of improved fertilization practices were reported. Mikkelsen et al. (1961) reported that ammonium chloride as a source of nitrogen for rice was equal to other ammonium nitrogen sources. Ingebretsen et al. (1959) reported that the application of ferric sulfate to nonsaline-alkali (high-sodium) soils in California rice fields had greatly increased yields. They attribute the beneficial effects of Fe, to the correction of iron deficiency in such areas. ( Chambliss (1920a) reported that about 1920 the use of complete mineral fertilizer on rice was not a common practice in the South. Some phosphate and potash fertilizer was used and good results had been obtained with the use of ammonium sulfate. Experiments conducted in Louisiana during 1919 to 1923 (Chambliss and Jenkins, 1925) showed little or no benefit from various combinations of nitrogen, phosphate, potash, and lime; however, there was a substantial gain in rice yield when soybeans were plowed under before the rice crops. Jones et al. ( 1938) reported increased yields of rice from nitrogen and phosphate under most conditions. The best time of application of phosphate was at seeding. The best time to apply the nitrogen varied under different conditions from seeding time to 6 to 8 weeks after seeding. Jenkins and Jones (1944) reported that applications of an 8-10-6 fertilizer at rates of 100, 200,and 300 pounds per acre gave increases, respectively, of 4.5, 4.1, and 6 bushels per acre on alternately cropped land. Jones et al. (1952) reported that it was customary for farmers to use fertilizer in the South at that time. Good response was obtained from nitrogen on most soils although the time and rate of application vaned from one soil and type of management to another. They reported that phosphate sometimes was beneficial. The response to nitrogen appeared to be due in part to better cultural, irrigation, and weed control practice. Wasson and Walker (1955) recommended for Louisiana the use of 30 to 60 pounds of nitrogen, 20 to 40 pounds of phosphate, and 0 to 40 pounds per acre of potash, drilled with or below the seed. A top-dressing of 30 to 40 pounds per acre of nitrogen increases yields on the heavier soils. The extensive use of commercial fertilizer on rice in Arkansas has been a common practice for only a comparatively few years. Nelson

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(1908) conducted fertilizer trials in that State in 1907 to determine whether yields could be increased or maturity hastened. He stated that there was an apparent benefit from an application of a mixed (3-13-11) fertilizer at the rate of 200 pounds per acre and from cottonseed meaI at the same rate, but no yield data were reported. Nelson (1931) reported results of fertilizer trials conducted in Arkansas during 1927 to 1930 on weedy lands. He found that the fertilizer stimulated the grass and weeds more than it did the rice so that yields of rice were lower on fertilized than on unfertilized plots. Kapp ( 1933) conducted extensive experiments in the field and greenhouse on rice fertilization and concluded that “the lack of nitrogen is responsible for most of the abnormal growth and low yields of rice.” Nelson (1943) reported results of experiments on rice fertilizations in Arkansas during 1931 to 1940. Most of the experiments were on plots cropped continuously to rice. Under this type of culture, none of the treatments maintained yield at the level it was the first year, but the yield was higher with applications of 20 pounds per acre of nitrogen than when no fertilizer was applied. Beacher (1952) reported results of rice fertilization trials conducted over a wide range of conditions in Arkansas during 1946 to 1951. He found that when fertilizer was applied on dry soil 6 to 8 weeks after seeding, the average yield increase was 11 to 22 pounds of rice per pound of nitrogen applied at rates of 40 to 50 pounds per acre of nitrogen. The yield increase per pound of nitrogen was lower, but total yields and profits were high with rates of 80 to 100 pounds per acre of nitrogen. Responses from phosphate were not consistent. Beacher and Wells (1960) reported that in Arkansas ammonium sulfate, urea, anhydrous ammonia, liquid nonpressure mixture of urea and ammonium nitrate, and ammonium phosphate-sulfate and diammonium phosphate were equally effective when applied at equivalent nitrogen rates at any time up to 50 days after seeding. When applied 70 to 85 days after seeding, amonium nitrate and urea were equally effective. These workers also report that under some conditions nitrogen applications sometimes hastened maturity but that a single application of nitrogen at heavy rates of 135 to 180 pounds per acre before the second irrigation delayed maturity. They also reported that application of nitrogen increased the protein content of the grain. Rice fertilizer experiments were started in Texas in 1915. Results of experiments conducted from 1915 to 1940, inclusive, were sumarized by Wyche ( 1941). Experiments were conducted on Lake Charles clay, Beaumont clay, Lake Charles clay loam, and “light-colored soils, probably of the Katy series.” Rice on Beaumont clay responded to application of nitrogen alone with an additional increase when phosphate was used

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with nitrogen. Rice on the Lake Charles clay responded to phosphate

but not to nitrogen when these fertilizers were each used alone, the best yields being produced when the two were used together. On “lightcolored soil,” rice responded to phosphate alone but not to nitrogen either alone or in combination with phosphate. There was no response to application of the minor elements iron, manganese, copper, zinc, and boron. Best results were obtained when the nitrogen and phosphate were drilled with the seed. Reynolds (1954)reported that “applications of 80 pounds of nitrogen and 40 pounds of phosphoric acid per acre (8040-0)gave better results than other fertilizers on Beaumont clay, Lake Charles clay, and Lake Charles clay loam, and are recommended for these soils. The use of 40-40-20 is recommended for Katy fine sandy loam. Sulfate of ammonia, urea and cyanamid were better sources of nitrogen for rice than nitrate of soda.” He reported also that 80-40-0 also gave best results on Hockley fine sandy loam and Edna fine sandy loam. Evatt and Beachell (1958) reported a varietal difference in reaction to nitrogen fertilizer as their results indicated that short-season varieties and short-strawed varieties could utilize higher rates of nitrogen than long-season and tall varieties. 2. Green Manure and Crop-Residue Management Residue from preceding crops often are incorporated into the soil before seeding rice. Some of these practices are discussed in Section 11, B. Jones et al. (1950) recommended the use of burclover (Medicago hiqida Gaertn.). Recently, Williams et al. (1957) reported that the use of vetch green manure increased yields of rice about 1000 pounds an acre. Purple vetch (Vicia atropurpurea Desf.) is most commonly used. It can be sown in the rice after most of the water has been drained off but before the rice is cut. It also can be sown after other crops or after summer fallow. Other species of Vicia also are used, as well as field peas and burclover. Horsebeans (Vicia Juba L. ) are also used but are usually drilled into a prepared seedbed. Reed and Sturgis (1937) reviewed and reported that organic matter such as soybean hay increased rice yields and improved soil structure. However, they found that organic nitrogen and potash alone and in combination did not increase rice yields but were effective when applied with phosphate. Nelson (1944) reported increased rice yields following hairy vetch (Vicia o i l h a Roth.) and soybeans [Glycine max ( L.) Merr.]. Williams et al. (1957) reported that green manure crops added nitrogen to the highly carbonaceous rice crop residue. When this is done, decomposition of the subsequently incorporated material proceeds with-

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out tying up nitrogen needed by the ensuing rice crop and provides an alternative to the deleterious rice crop residue-burning practice. G. HARVESTING, DRYING,STORING, AND MILLING 1. Harvesting

Almost all rice grown in the United States is cut with a combine harvester and dried artificially before storing or milling. The only exception is a small percentage of seed rice which is cut and swathed and threshed when it has dried to 12 per cent moisture or less. Bainer (1932), Smith et al. (1933), Slusher and Mullins (1948), and McNeal (1950a) reported research which has led to greater efficiency in the mechanization of the rice harvest. Smith et al. (1938), McNeal (1950b), and Kester (1959) reported results of research related to time of rice harvest based upon maturity of the grain. It was clearly shown that maximum yield of head rice was obtained if harvested when the moisture content of the grain was 18 to 24 per cent and then artificially dried. Harvesting when the grain moisture content was higher caused a reduction of the yield and milling quality. If the moisture content is less than 18 per cent at harvest, the milling quality usually will be low because alternate wetting by dew and rain and drying of ripe rice cause a checking of the grain which can result in excessive kernel breakage when the rice is milled. In the United States rice is usually handled in bulk. The rice combine harvester is equipped with a tank for collecting the threshed grain. Rice is mechanically augered into a self-propelled “bank-out” or a tractor-drawn cart which takes the rice to a truck. After the truck is mechanically loaded from the “bank out” or cart, the rice is then taken to a drier or to a bin where it is aerated. 2. Drying and Storing

Rice in the United States is harvested or cut with too much moisture for safe storage; so it must be dried or aerated soon after threshing. If the grain is not dried or aerated, it will be damaged and the quality lowered. It is dried to about 13 per cent moisture with forced air, which may be heated or unheated. Dachtler (1959) reviewed United States research work on conditioning and storing rough and milled rice. For the proper drying of rice, moisture must be removed from inside the kernel. If rice is dried too fast or if the temperature of the drying air is too high, serious loss of quality will result. To prevent checking or shattering of the rice kernels from drying too rapidly, the drying usually is done in 3 to 5 stages. In each stage the rice passes through

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the drier and then is tempered in a bin so that the kernel moisture will equilibrate. Stored rice is subject to attack by a number of insects. The larvae of the lesser grain borer, the rice weevil, and the Angoumois grain moth attack sound rough rice kernels. The grain and flour beetles (cadelle, saw-toothed grain beetle, the flat grain beetle, red flour beetle, and the confused flour beetle) attack broken kernels or dehulled kernels. Moths that infest the surface of bulk or bagged rough rice and that spin webbing in profusion include the Indian-meal moth, the almond moth, and the rice moth, In bran and milled rice, bran bugs including the flour beetles are important (U.S. Dept. Agr., 1958; Rouse et al., 1958).

3. Milling The milling process and the machinery used were described by Kik and IVilliams (1945). Although improvements have been made in many of the machines used and the over-all organization of rice milling is more efficient, the present-day milling process is basically the same as it has been for many years. Modern rice mills perform the milling process mechanically, starting with cleaning the field-run grain and ending with milled rice which has been sorted into grades. The cleaning process involves (1) scalping to remove sticks, mud lumps, etc.; ( 2 ) bearding to remove the awns, culms, and leaves; and ( 3 ) monitoring or cleaning to remove sterile florets and any remaining foreign matter. In succession the grain passes to the hulling machine or “stones” to remove the hulls, leaving “brown rice”; to the ‘hullers” where the “bran” is removed; and to the brush which removes the “polish” that consists of the remainder of the innermost or aleurone layer and adhering floury particles. The “milled rice” is then graded into four size classes. The whole grains or “head rice” may not contain more than 4 per cent of large broken grains, large broken grains or “second heads,” smaller broken grains or “screenings,” and finely broken grains or “brewers’ rice.” Milled rice may also pass through an additional step whereby it is coated with glucose and talc powder and in some cases enriched with thiamine, riboflavin, niacin, and iron, and given a final polishing in a trumbal. The milling quality of United States varieties varies over a rather wide range. The conditions under which the rice is grown, dried, and stored also influence the milling quality. The milling results for trash-free rough rice usually are in the range of 17 to 20 per cent hulls, 48 to 65 per cent head rice, 4 to 8 per cent second heads, 5 to 10 per cent screenings, 1 to 3 per cent brewers’ rice, and 8 to 11 per cent bran and polish.

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111. Rice Field Pests

A. WEEDS

1. Important Weeds in Rice Fields More than 50 grasses, sedges, and broadleaf weeds are troublesome pests in United States rice fields. The more important weeds were listed by Beachell ( 1959).Certain of the more important weeds were discussed by Prince (1927), Smith ( 1940), Jones et al. ( 1950), Davis (1950), Williams (1955, 1956), and Finfrock et al. ( 1958). Smith et al. (1959) list the principal troublesome United States rice weeds as being barnyardgrasses ( Echinochloa spp. ), red rice ( Oryza sativa L.), coffeeweed (Sesbania exaltata (Raf.) Cory), curly indigo ( Aeschynomene virginica (L. ) B.S.P. ), Mexican weed (Caperonia castaneaefolia ( L. ) St. Hil. ), mudplantain ( Heteranthera sp. ), arrowheads ( Sagittaria spp. ), gooseweed ( Sphenoclea zeylanica Gaertn. ), redstem ( Ammunnia coccinea Rottb. ), bulrush ( Scirpus sp. ), umbrellasedges (Cyperus spp.), and spikerushes (Eleocharis spp.). These authors estimated that barnyardgrass may reduce rice yields by one-fourth or more in seriously infested fields and may be costing United States rice growers as much as twenty million dollars yearly in reduced yield. Among the many widely dispersed serious broadleaf weeds, mudplantain reduced potential rice yields by 18 to 48 per cent when allowed to compete with rice the first month after seeding. Weed control is an important problem wherever rice is grown (see Section I, B). Lack of effective weed control has frequently been the main cause of limited production. Rice usually is grown under a moisture regime which favors rapid growth and reproduction of aquatic and semiaquatic weeds. Most of the species involved are prolific seed producers and quickly infest land only a season or two after it is first seeded to rice, especially if a planned control program is not initiated early. Weeds reduce yields by competing with the developing rice plants for light, nutrients, water, and space. They increase harvesting costs by their presence and by promoting lodging. Weed seeds in the threshed grain cause lowered grain grade. Current United States standards for rough rice prescribe a maximum tolerance for No. 1 rough rice of two weed seeds per 500 grams or 0.5 per cent of red rice. 2. Control of K'eeds Research and field experience contributed much in the past decade to improved chemical methods of selective weed control in rice; however,

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cultural methods still remain the most economical and most widely employed procedures. In practice, a modem rice weed control program proceeds on three fronts: ( 1 ) Weed infestation prevention by seeding weed-free seed and by forestalling seed formation of weeds that volunteer. For example, in California some growers use sheep to keep weeds grazed from the levees. ( 2 ) Seedbed preparation done so as to destroy the maximum amount of weeds before seeding the rice crop. Proper time and method of crop fertilization play an important role in weed control. ( 3 ) Use of appropriate herbicides for grass and broadleaf weed control. Crop rotation such as rice-pasture in Louisiana and Texas, rice-fish or rice-soybeans-oats in Arkansas, and rice-field beans, grain-sorghum, or safflower in California can ameliorate the weed problem in rice fields. For long-range effectiveness, the entire rotation period must be weed free. The system that includes summer or fall fallow is especially effective. Well-leveled land, appropriately engineered levees, and a dependable water supply are important in weed control. These factors collectively make it possible to reduce the weed problem by controlling the depth of irrigation water. They also facilitate drainage, which helps to control algae and certain aquatic weeds. Seeding into the water as soon as possible ( 1 to 7 days after flooding) is generally practiced in California to control barnyardgrass, spikerush, and other weeds. Except possibly in cool years or in emergencies, the fields are continuously flooded to a depth of 4 to 8 inches until harvest. The germinating and seedling watergrass is inhibited or killed under flooded conditions. Such a procedure, however, may cause an increase of algae and other weeds. In the South, seedling weeds are partially controlled by judicious flooding of drill-planted seedling rice. For watergrass control, the first flood is usually for 2 to 3 weeks to a depth of 4 to 6 inches. Frequent cultivation at 1- to 3-week intervals in the spring before seeding is regularly followed to reduce weed infestations. The last operation usually should be shallow so that more viable weed seed is not brought to the surface. Roughly prepared seedbeds tend to discourage weed seed germination. Rice fertilization practices influence the rice field weed population. According to Smith et al. (1959), applying both phosphate and nitrogen fertilizer too soon before seeding the rice will greatly increase the competitive ability of the native weeds. Applying phosphate to the preceding rotational crop or delaying fertilizer application to just before the rice is first flooded is helpful in reducing weed activity. In some of the southern States when weed grass infestations are high, rice yields are almost doubled by delaying nitrogen application until the barnyardgrass has

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headed. When the fertilizer is applied earlier, the barnyardgrass utilizes the applied nutrients; its competitiveness with the rice is enhanced. In California, a correctly integrated fertilizer-weed control program calls for the drilling or disking of the fertilizer to a depth of 2 to 4 inches beneath the soil surface just ahead of flooding for seeding. Properly placed fertilizer usually enables the rice to emerge from the water and to compete successfully with weeds. Phosphate fertilizer applied to the dry surface without incorporation into the seedbed or if applied to standing water accelerates development of algae and watergrass. A seed certification program has effectively reduced red rice. Smith et al. (1959) estimated that 40 to 60 per cent of the United States rice crop now is treated annually with one of the phenoxyacetic acid herbicides for the selective removal of weeds from the growing crop. Kaufman and Crafts (1956) and Ryker and Brown (1947) described the effects of the several formulations, rates, dates, and methods of application. Although the compounds can be applied by groundrig, the greater part of the acreage is treated by airplane application. The recommended rates are 6 to 24 ounces of acid equivalent per acre in 5 to 10 gallons of aqueous solution. The most frequent time of application is 55 to 65 days after seeding the rice. In California, MCPA (2-methyl4-chlorophenoxyacetic acid) has proved less likely to cause injury to the rice than 2,4-D (2,4-dichlorophenoxyaceticacid) and is now the recommended herbicide for control of many monocotyledonous weeds as well as dicotyledons. Barnyardgrasses (Echinochloa spp. ), the most serious weedy grasses common to all rice areas in the United States, recently have yielded to chemical methods of control. Smith (1960a) found that pre-emergence application to drilled rice of isopropyl N-( 3-chloropheny1)carbamate (CIPC ) at 6 to 8 pounds per acre provided good control of barnyardgrasses and did not injure rice that had been planted at a depth of 1 to 2 inches. Barnyardgrass was not killed if sprayed at the 3- or 4-leaf stage. Selectivity depended upon the location of roots in the soil so that deeper planting protected rice from the chemical. Baker (1960) mentioned a morphological characteristic of barnyardgrass in which elongation of the first internode causes the roots to be formed at the soil surface regardless of the depth of the seed. The seminal rice roots arise very near where the seed is placed and, therefore, can be protected by deep sowing. While CIPC has been recommended for barnyardgrass control in Arkansas, the method and depth of seeding, time of application, and water management are critical factors in its effectiveness. This chemical

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is not widely used because poor control of the grass or rice injury often results from misuse. Smith (196Ob, 1961) reported on the evaluation of 3,4-dichloropropionanilide (DPA) as a selective postemergence herbicide for barnyardgrass control in rice. He found that in Arkansas, DPA was effective at 4 pounds per acre when the grass was in the 1- to 3-leaf stages. Rice in the 3- or 4-leaf stage was injured by application of 8 pounds per acre. Method of seeding, depth of seeding, water management, and volume of application did not affect results. In California, Viste (1961) reported that control was best when DPA was applied at 8 pounds an acre 4 weeks after seeding. Control was poor for earlier applications because grass seeds continued to germinate after treatment. Results of research and commercial-scale field trials in all rice-growing areas in 1961indicate that DPA at 3 to 4 pounds per acre will be a commercially acceptable herbicide for the selective control of barnyardgrass in rice. “Scum” or ‘moss” found in rice fields consists of algae. Scum may develop soon after flooding and interfere with the emergence of young seedlings from the water. In California, the scum problem is accentuated by incomplete incorporation of previous crop residues into the seedbed just before flooding. Chemical control has been erratically successful (Olsen, 1957). Chemicals that sometimes have been effective in Arkansas and California include copper sulfate, dichlone, /3-[2-( 3,5-dimethyl-2oxocyclohexyl ) -2-hydroxyethyl]glutarimide, and dehydroabietylamine acetate. Effective cultural control methods include deep plowing, early sowing, and judicious draining if scum appears.

B. DISEASES Most of the known important diseases of rice occur in the United States. Exceptions are rice dwarf virus, rice stripe virus, and some diseases caused by bacteria and physiological disturbances reported from Japan and other countries. Some minor fungal parasites of rice have not been reported in the United States. The rice diseases that occur in the United States were described by Atkins ( 1958). The principal ones are blast (Piriczrlaria ovyzae Cav. ), brown leaf spot (Helminthosporium oryzae de Haan), narrow brown leaf spot (Cercospora o y m e I. Miyake), root rot and seedling blight caused by various fungi, stem rot (Sclerotium o y m e Catt.), straighthead (nonparasitic), and white tip (Aphelenchoides oyzae Yokoo.). Kernel smut (Neouossia barclayanu Bref.) sometimes caused fairly heavy losses. Hoja blanca is a virus disease of rice that causes serious losses in some areas in Central and South America. It has been reported in the United States (Atkins et al., 1960) b u t has not caused important

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losses. Many of these diseases can be controlled by growing resistant varieties, following recommended cultural and water management practices, or treating the seed with a fungicide or with hot water. Leaf spots are very common in the South. Losses usually are not severe, but occasionally narrow brown leaf spot causes lodging. When very severe, brown leaf spot blights the panicle. Blast sometimes is serious when nitrogen is applied in excessive amounts and the weather is damp and nights are warm. Varieties now available are resistant to the races of the causal fungus commonly occuring in the United States. Straighthead is one of the most serious diseases of rice in the South. It is a physiological disorder that can be controlled by proper water management. Tisdale and Jenkins ( 1921) reported that straighthead could be controlled by draining and drying the soil. Todd and Beachell (1954) studied this problem further and found that the water should be drained off the field and the soil thoroughly dried just before floral initiation. No variety grown in the United States is highly resistant but some are fairly resistant. In California, germinating seeds and young rice seedlings are frequently killed by products from the anaerobic decomposition of highly carbonaceous crop residues. Early draining and aerating affected fields as soon as the condition is observed usualry will correct it. White tip is caused by a seedborne foliar nematode. This disease can be controlled by treating the seed with hot water (Cralley, 1952) or with certain chemicals (Todd and Atkins, 1959). Some varieties are resistant and nematode-free seed of susceptible varieties is now sown, so this disease is no longer a problem in the United States. Several species of parasitic root nematodes ( Atkins and Fielding, 1956) and a root knot nematode (Tullis, 1934) were reported on rice in the United States, but the loss they caused has not been determined. Stem rot sometimes causes severe losses in local areas in the South. Diseased plants lodge and the grain is chalky and light. None of the varieties grown in the United States is resistant, but some are less susceptible than others. Although the sclerotia of the causal fungus live for many years in the soil ( T u b and Cralley, 1941), crop rotation, use of potash fertilizer (Cralley, 1939), and early draining (Cralley and Adair, 1943) also aid in the control of stem rot. This disease was reported in California (Tullis et al., 1934), but it has not caused losses in-that state. C. INSECTS AND OTHERPESTS There are a number of insect pests of rice in the South. The most important insects are listed (Table 111). Douglas and Ingram (1942) and more recently Bowling (1960 and

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Rolston and Rouse (1960) reported studies on rice insects and their control in the South. TABLE I11 Insect Pests of Rice in the United States Common name Rice stinkbug Sugarcane borer Rice stalk borer Sugarcane beetle Rice water weevil Grape Colaspis

Fall armyworm Southern corn rootworm Chinch bug Flea beetle Grasshopper Rice leaf miner

Midge

Western yellow-striped armyworm Leafhopper

Onion thrips

Scientifk name Oebalus pugnax (Fabricius) Diutraea saccharalis (Fabricius) Chilo pkjahllus Zincken Euetheola rugiceps (Le Conte) Lissorhoptnrs oryzophilus ( Kuschel) Maecolaspis Maecoluspis jluvida (Say) Paromius longulus (Dallas) Laphygmu frugiperdu (J. E. Smith) Diabrotica undecimpunctata howardi Barber Blissus leucopterus (Say) Systena hudsonias (Forster) Conocephulus fmciatus (De Geer) Hydrellia griseolu (Fallen) Lerodeu eufnlo (Edwards) Cricotopus syEueStris ( Fabricius ) Tunytarsus sp. Paralauterborniellu subcincta ( Tomes) Prodenia praefiCu Grote Draeculacephala sp. Ezitianus aitiosus (Uhler) Nesosteles neglectus ( DeLong & Davidson) Macrosteles fmcifrom (Stal) Thrips tabaci Lindeman

The rice stinkbug feeds on the grain in the milk and soft-dough stages and reduces yield and quality (Douglas and Tullis, 1950).The “pecky rice” grains usually break when milled, and kernels that do not break are damaged. Only 0.5 per cent “peck‘‘-damaged grains are allowed in No. 1 milled rice. This insect sometimes can be controlled with aldrin, dieldrin, and toxaphene. However, in Texas malathion gives the best control. The insecticide should be applied when the rice starts to head. The eggs are parasitized by two species of wasplike insects that are an important means of control. Stalk borers cause some damage to rice, especially in the Gulf Coast area. Douglas and Ingram (1942) reported losses of 2.3 to 10.3 per cent in Louisiana. Damage was least in varieties with small culms and most in varieties with large culms. The mortality of borers due to cold in the

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winter is high. Fall plowing gives effective control, and heavy pasturing of stubble will reduce survival about 75 per cent. The sugarcane beetle damages rice in the seedling stage and at maturity after the fields are drained before harvest. Flooding the field will control this insect early in the season. Delayed drainage when the rice is ripe will reduce the time when damage might occur at that stage. The larvae of the rice water weevil damage rice by feeding on the roots. The damage seems to be greater in Arkansas than farther south, but this insect is sometimes a serious pest in Texas. Isely and Schwardt (1934) reported good control and increased yields by draining infested fields about 3 weeks after flooding and allowing the soil to dry before re3ooding. Seed treatment with aldrin at 1/4 pound per bushel was reported by Rolston and Rouse (1960) to give effective control. The overwintering larvae of the grape colaspis also reduce rice stands by feeding on germinating seeds and young seedlings. This insect can be controlled by treating the seed with aldrin at 1/4 pound per bushel (Rolston and Rouse, 1960). The other insects listed in Table I11 cause minor injury to rice. Usually they occur in localized areas and can be treated by hand. Grasshoppers sometimes become serious enough to warrant control by treating the field with an insecticide as recommended by Bowling ( 1960). The planthopper Sogato orixicola is the vector of the virus which causes hoja blanca. This insect has not become established in the United States, but it was found in Florida, Mississippi, and Louisiana in 1957 to 1959 (Atkins et al., 1960). Although California1 is free from most of the serious rice insect pests of the world, it does harbor many pests capable of “explosive” outbreaks (Table 111). This was demonstrated in the 1922 and 1953 outbreaks of the rice leaf miner (Lange et al., 1953). About 165,000 acres of rice were treated with dieldrin in 1953. Grigarick (1959) reported on the complicated ecological relationships of this species, existing between weather conditions, increase on native and introduced grasses, and migration of adults into rice fields. The leaf miner has been a rather minor factor in rice production since the 1953 attack, but 20,000 to 30,000 acres are treated in some years. The rice water weevil of the southern States was first found in California on June 1, 1959, near Biggs (Lange and Grigarick, 1959). This insect has not been serious on rice in California, probably because of the rather wide use of dieldrin for control of the rice leaf miner. The crustacean, commonly referred to as the tadpole shrimp, has 1 The review of the California insect pest situation was prepared in cooperation with Dr. W. H. Lange of the Entomology Department, University of California.

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been a pest of rice in California since 1947 (Rosenberg, 1947). The tadpole shrimp (Triops Eongicuudutus) feeds on the young seedlings and also muddies up the water, creating conditions considered adverse to rice production. Later, Portman and Williams (1952) reported the use of DDT for control. This still is a standard control measure for this pest, although Grigarick et uZ. (1961) reported that malathion, diazinon, and sevin all have possibilities for the control of the tadpole shrimp. The larvae of midges belonging to the family Chironomidae (Tendipedidue) occasionally cause damage to rice by feeding on germinating rice or eating holes in rice blades lying on the surface of the water. Dieldrin applied when attacks occur has given satisfactory control. Darby (19Sl) found 30 species of midges breeding in rice fields and pointed out the many ecological relationships involved between midges and rice fields as freshwater habitats. In California, many other insects occasionally attack rice in certain years and under particular conditions. A butterfly larva, the rice leaf folder, causes characteristic feeding and folding of rice blades. The omnivorous yellow-striped armyworm is sometimes a severe pest of rice in localized areas. Four species of leafhoppers are associated with rice and may cause stunting and yellowing of rice foliage. Several thrips cause damage to young rice in certain years. IV. Origin, Botany, and Genetics of Rice

A.

ORIGIN Ah?) SPECIES OF O T y Z U

The many species of O y u r (Table IV) are widely distributed throughout tropical and subtropical areas of the world. The generally accepted origin of the principal cultivated species, 0. sutivu L., is southeast Asia and the adjacent islands (Chattejee, 1951). Orym glaberrima Steud., a species cultivated in West Tropical Africa, had its origin in that area. Rice probably was first cultivated in India as early as 3000 to 2500 B.C. (Chatterjee, 1947). Rice cultivation spread from India to China and the southeastern countries of Asia at a very early time. Rice was grown in countries to the west of India as early as 400 to 300 B.C. It spread to Egypt early in the Christian era and later to Europe, Africa, and the Western Hemisphere. The progenitor of Orym sutiua has not been clearly defined. Sakai (1935) and Nandi (1936) concluded from cytological studies that 0. sutiva is a secondary balanced allotetraploid which originated through hybridization between two different five-chromosome species in which two chromosomes were duplicated, owing probably to meiotic irregu-

TABLE IV Chromosome Number and Distribution of O q z a Species Species 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.

sativa L. sativa L. forma spontanea Roschev. perennis Moench grandiglumis ( Doell. ) Prod. punctata Kotschy ex Steud. stapfii Roschev. breviligukta A. Cheval. & Roehr. australiensis Domin gkberrima Steud. alta Swallen latifolia Dew. oficinalis Wall. ex Watt eichingeri Peter minuta Presl granulata Nees & Am. ex Hook. f. meyerianu (Zoll. & Mor.) Baill. schlechteri Pilger ridleyi Hook. f. COaTCtata Roxb. brachyantha A. Cheval. & Roehr. tisseranti A. Cheval. perrieri A. Camus subulata Nees

Chromosome no. (2n) 24 and 48 24 24 24 and 48

-

24 24 24 24 24 and 48 48 24 48 48 24 and 48 24

48 48 24

-

24

Distribution India and Indochina Asia Tropical America, West Indies, Tropical Africa, Ceylon, and India South America North East Tropical Africa West Tropical Africa West Tropical Africa Australia West Tropical Africa South America and Central America Central America, South America, and West Indies India and Burma East Africa Malay Peninsula, Philippines, Sumatra, Java, and Bomeo India, Burma, Java, and Siam Java, Bomeo, Philippines, and Siam New Guinea Malay Peninsula, Siam, Borneo, and New Guinea India and Burma West Tropical and Central Africa Central Africa Madagascar South America

2

j 5

2

2 a 5E 2 d

i ?

i!

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C. ROY ADAJR, M. D.

MILLER, AND H. M. BEACHELL

larities in the hybrid. A subsequent doubling of the chromosomes attained the secondary balance of n = 12, the present existing number in 0. satiua. More recently, Shastry et al. (1960) reported that on the basis of their length and arm ratios, 12 pachytene bivalents in a s t r a i n of 0. satioa could be identified. This report cast some doubt on the validity that 0. sativa is a secondary balanced allotetraploid. It was suggested by G. Watt, according to Chattejee (1948), that OT~JZU safiuu might have been derived from wild rices in India. Ramiah sativa and Chose (1951) stated that “there is clear evidence that OTYZU var. futzta which occurs mainly in South and East Asia in a large number of forms and crossed freely with 0. sativa has contributed to the enormous varietal diversity existing in India.” Sampath and Govindaswami (1958) presented evidence in support of their theory that 0. satiua var. fatua Prain is a progeny of the cross 0. sativa x 0. perennis Moench., and that 0. perennis was the progenitor of 0. satiua. Results reported by Yeh and Henderson (1961) support this theory. However, 0. sutiua var. fatw occurs as a weed in rice fields in South and East Asia and crosses readiIy with cultivated varieties (0.sativu); so its does contribute to the “varietal diversity.” Much is known regarding interrelationships between species of 0y z a as a result of investigations presented by Roschevicz (1931), Kihara ( 1959), Chattejee (1951), Sampath and Rao (1951), Ramiah and Chose ( 1951), and Yeh and Henderson ( 19sl). However, as Kihara ( 1959) pointed out, much study is needed to elucidate fully the origin and evolution of the genus Oyza. The 23 species of Oyzn and the chromosome number and distribution as enumerated by Chatterjee (1948) and Kihara (1959) are shown in Table IV. Since the synonomy used by Chatterjee was followed, some of the species shown by Kihara ( 1959) and Roschevicz (1931) are not listed. “Oryzu perennis var. b a h g a ” is not listed although Yeh and Henderson (1961) presented evidence that this form should have specific status as 0. balunga.”

B. DESCRIPTION OF THE RICE PLANT Rice ( O y z a sutiua) in the Gramineae tribe Oryzeae usually is described as an annual although it is a perennial when soil moisture and temperature are optimum for continued growth. Rice plants have been maintained in the greenhouse for over twenty years. The roots which arise from the lower nodes spread laterally and do not usually penetrate more than 6 inches in depth. The primary culm has 12 to 18 nodes, depending upon the variety, but only 4 to 7 elongate. There is a bud or potential bud in the axil of the leaf at each node that can give

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rise to a tiller. When space and soil fertility are optimal, some varieties may produce as many as 100 tillers although when sown at the rate usually followed in the United States, the number of tillers is 2 or 3. The culms have hollow stems and solid nodes and vary in length from 2 to 6 or more feet, depending upon variety, soil fertility, and other factors. Most varieties in the United States have culms 3 to 5 feet long. The leaf consists of a sheath, auricle, ligule, junctura, and blade. There is a leaf at each node, but the number of functional leaves at any one time is usually 4 or 5 because the older, lower leaves die as new leaves are produced. The leaf blades vary in width from about a fourth to more than a half inch, and the color ranges from light yellowish green to dark green. The inflorescence is a panicle with the branches arranged singly or in pairs. It varies from compact and erect to open and drooping. The spikelet has one flower; it is strongly compressed laterally and articulates below the two small outer glumes. Two bract-like organs occur at the enlarged end of the pedicel. Weatherwax (1929) stated that these bractlets are sometimes sufficiently elongated to be mistaken for glumes. The true glumes are above the articulation. The “flowering glumes” are the lemma, which has five nerves and may terminate in an awn, and the palea, which has three nerves, two of which may terminate in short conical teeth. There are two lodicules at the base of the palea, which become turgid and open the flowering glumes at anthesis. The flower consists of six stamens and a two-branched plumose stigma. Anthesis usually occurs between 9:00 A.M. and 2:OO P.M. (Adair, 1934). The time the flowers open is influenced by temperature and sunlight. They open earlier on warm and sunny days than on cool or cloudy days. Rice is normally self-pollinated although as much as 0.5 per cent natural crossing may occur in the United States (Beachell et al., 1938). The amount of natural crossing is higher when the relative humidity of the air is high. C. GENETICS, CYTOLOGY, AND LINKAGE GROUPSIN RICE Most genetic studies on rice have been conducted in Japan, India, China, or the United States. Most cytological studies that have been reported were done in Japan, India, or the United States. Several papers summarized rice genetic studies and two or three attempts were made during the last twenty years to standardize the system of gene nomenclature in rice. Jones (1936) summarized information on the mode of inheritance of characters in rice. Kadam and Ramiah (1943) pointed out the need of a standard system of gene nomenclature and suggested a system that

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might be used. They also summarized results of all available genetic studies on rice. Nagao (1951) summarized the rice genetic data and proposed a modification of the system outlined by Kadam and Ramiah. Nagao also defined 4 linkage groups. Jodon (1955) summarized the information on rice genetics and defined 8 linkage groups, and Nagao and Takahashi (1960) gave a preliminary report on 12 linkage groups in rice. The International Rice Commission (FAO) Working Party on Rice Breeding in the 1955 meeting recognized the need for a standardized system of genic nomenclature and for a comprehensive survey of the information available on linkage relationships in rice. A committee then was appointed to undertake the work of proposing a system of nomenclature and summarizing the data on linkage groups. A preliminary summary of the committee report (Anonymous, 1959) outlines the system suggested for nomenclature of rice genes and gives the recommended symbols for each gene that has been studied. The system proposed by this committee follows the rules of the International Genetic Congress. The report of this committee also summarized the information on linkage groups in rice which followed closely the summary by Nagao and Takahashi (1960). A more extensive review of the subject has been prepared by this committee (Anonymous, 1962). V. Rice Breeding and Improvement in the United States

A. HISTORYOF RICEBREEDING IN THE UNITED STATES Although rice varieties were introduced subsequent to the establishment of rice culture in South Carolina, no effort was made by Federal or State agencies to improve rice varieties for the United States until rather recently. An abortive attempt was made in 1866, but the seed of the four varieties introduced failed to germinate. However, work was started by the U. S. Department of Agriculture in 1899 when Seaman A. Knapp, then an explorer in the Division of Botany, introduced from Japan 10 tons of Kyushu rice which was distributed in southwestern Louisiana and probably eastern Texas, where he arranged farm demonstrations of rice varieties and cultural methods. Many varieties were introduced and tested by Department workers on demonstration farms in Louisiana and Texas and later in Arkansas and California before the establishment of Rice Experiment Stations in these states. The Rice Experiment Station at Crowley, Louisiana, was established in 1909. The Biggs Rice Field Station, Biggs, California, and the Rice Experiment Station, Beaumont, Texas, were established in 1912. The Rice Branch Experiment Station, Stuttgart, Arkansas, was established

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in 1926. Rice experiments were conducted also at Elsbeny from 1928 to 1941 and later at Palmira in Missouri. Rice investigations were started at the Delta Branch Experiment Station, Stoneville, Mississippi, in a small way about 1950, and a fairly comprehensive breeding program now is being conducted. Breeding investigations are conducted cooperatively by the U.S. Department of Agriculture with State Agricultural Experiment Stations at each of these locations. Most of the earlier work consisted of testing selections from foreign introductions. Some of the varieties developed by selection from 1909 to about 1930 were CALORO, COLUSA, FORTUNA, NIRA, and REXORO. CALORO and COLUSA are the leading varieties at this time in California, and REXORO is grown on a significant acreage in Louisiana and Texas. Salmon L. Wright, a rice farmer in Louisiana, obtained material from varietal experiments on demonstration farms before the establishment of the Rice Experiment Station at Crowley. From this material he selected the medium-grain varieties BLUE ROSE and EARLY PROLIFIC and the long-grain varieties EDITH and LADY WRIGHT. None of these varieties is now in production, but for many years, about 1915 to 1944, they were the leading varieties in the South. The varieties developed during these earlier years were described by Chamliss and Jenkins (1923), Jones ( 1936), and Jones et al. ( 1941). Rice improvement investigations were summarized by Jones ( 1936). He stated that at that time the objectives were “to develop varieties that are resistant to diseases, that do not lodge or shatter, that mature at the desired time and that produce high field and mill yields of good table quality.’’ It is seen that the present-day objectives, as given in Section V, B, are simply a refinement and more detailed statement of the earlier objective.

B. CURRENT OBJECTIVES AND METHODS FOR

THE RICE-BREEDING PROGRAM IN THE UNITED STATES

Rice-breeding research at all experiment stations in the United States is closely coordinated. This coordination is achieved because most of this work is conducted cooperatively by the U. S . Department of Agriculture and the State Experiment Stations. The Rice Technical Working Group meets biennially, so all rice breeders and geneticists can discuss their common problems with other rice research workers. Uniform yield and disease experiments conducted in the South further unify the work. Objectives of the rice-breeding program are to develop short-, medium-, and long-grain varieties that germinate quickly and have seedling vigor; tolerate low temperatures in the germinating and seedling stages; are resistant to alkaline soils and salt in the irrigation water; are

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resistant to diseases and insects; respond to maximum rates of fertilizers; have short, stifE straw and resist lodging; mature uniformly; produce high field and mill yields over a wide range of environmental conditions; and have the desired cooking and processing characteristics. Breeding methods used for rice have followed the usual pattern for all small grains. Up until about 1920, rice varieties were introduced from most rice-producing countries and tested for adaptation in the United States. Selections were made from the better varieties and several fairly well-adapted varieties were obtained. These are mentioned in Section V, A. It was not possible to attain all breeding objectives by these methods; so, about 1920, some of the rice breeders started to make crosses in an attempt to combine desired characteristics. All varieties that have been released since 1942 are progeny of hybrids. The backcross or a modified backcross method now is commonly used. One example of a method now commonly used to achieve a breeding objective is described: In 1956, the potential threat of hoja blanca, a virus disease, to rice production in the United States was recognized. In 1957, experiments were started in Cuba and Venezuela to test all available material to determine the reaction to the hoja blanca disease. It was learned that many varieties and breeding lines were resistant. All the leading commercial varieties grown in the United States were susceptible to hoja blanca, but short- and medium-grain types were resistant and could be grown if the disease became established in this country. There were no hoja blanca-resistant long-grain varieties adapted to the environmental and cultural practices in the United States. Crosses between resistant types and leading United States long-grain varieties were made in the field in 1957. The F1 plants were grown in the greenhouse and some were backcrossed to the long-grain parent during the winter of 1957-1958. When this program started, the backcrossed plants were saved until their progeny had been tested to see which carried genes for resistance to hoja blanca. The plants carrying resistance were then backcrossed to the long-grain parent. It now i s possible to plant the backcross seeds; and as soon as the plants tiller, they can be divided and a part of each plant tested for reaction to hoja blanca in the greenhouse. Results of these tests are available by the time the plants flower so that resistant plants can be backcrossed to the recurring parent. Plants have been selected from each round of crossing and the progeny continued in the selecting and testing program. Concurrently with testing this material for reaction to hoja blanca, the lines were tested for cooking and processing characteristics as described in Section V, C.

RICE IMPROVEMENT AND CULTURE IN UNITED STATES

99

C. RESULTS OF THE RICE-BREEDING PROGRAM All the rice varieties now grown in the United States were developed by the U. S. Department of Agriculture in cooperation with the State Agricultural Experiment Stations and other agencies. All varieties being grown on farms and a few varieties that had been used as parents but were not in production were described by Johnston (1958). Of the 22 varieties described by this author, 16 were developed and released after about 1930, and 6 had been developed and released earlier. One of these older varieties is BLUE ROSE, one of the Salmon L. Wright varieties mentioned in Section V, A. Most of these varieties were described by Jones et aE. (1953). Two additional varieties, GULFROSE and BELLE PATNA, have been released since 1958. The annual acreage and production of rice varieties in the United States for 1956 to 1960, inclusive, are given (Table V ) . The total acreage was about the same in 1960 as in 1956 but it was lower in 1957 and 1958. The acreage of the R E x o R o - t y p e varieties declined sharply during this period, but there was an increase in acreage in 1960 as compared with that in 1959. The acreage of CENTURY PATNA 231 declined sharply. The acreage of BLUEBONNET 50 increased slightly as did that of ARKROSE. The CALROSE acreage increased in 1960 in California with a corresponding decrease in the acreage of the short-gain varieties CALORO and COLUSA. The acreage of TORO increased sharply in 1957 and 1958 and then declined in 1959 and leveled off in 1960. The greatest change in varieties was the rapid increase in the acreage of NATO and the corresponding decline in the acreage of ZENITH and MAGNOLIA. The days from seeding to maturity, plant height, and grain yields for 13 of the leading varieties are given (Table VI). The long-grain, late-maturing varieties TEXAS PATNA, TP 49, and REXORO are grown only in Louisiana and Texas. These varieties normally will not mature before killing frosts in the other areas. The midseason long-grain varieties BLUEBONNET 50, TORO, and SUNBONNET are grown throughout the Southern area as is the early long-grain variety CENTURY PATNA 231. The midseason medium-grain variety ARKROSE is grown only in Arkansas. The early-maturing medium-grain varieties NATO, ZENITH, and MAGNOLIA are grown in all the southern rice-producing states. The medium-grain variety CALROSE and the short-grain varieties CALORO and COLUSA are grown in California. These three varieties sometimes are fairly productive in the South but seldom are grown there because their yields are erratic in the area and because of limited market demands for the rices. The short-grain varieties sometimes are grown in Arkansas when seeding is delayed because of inclement weather.

" Annual United States Rice Acreage and Production by Varieties, 1956-19W II1ULIa.4

Acreage and production

Variety

Acreagc

REXORO"

CENTWHY PATNA BLUEHONNET

506

Tono MAGNOLIA AnmOSEC

ZENITH NATO CALROSE Short-grain Minor Varietiesd

Total

231

Production? Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production Acreage Production

1956

___

179,086 4,209.6 253,008 7,334.2 527,383 14,179.0 8,326 239.5 50,854 1,244.7 11,490 340.9 269,112 6,869.6 I

-

20,151 688.4 290,686 10,168.6 8,409 201.2 1,618,505 45,475.7

Includes TEXAS PATNA and TP 49. Includes BLUEBONNET and SUNBONNET. 0 Includes a small acreage of KAMROSE and BLUE ROSE. d Not the same each year, but includes REXARK, NIRA, R-N, R-D, LACROSSE, and others. a b

1957

- _._ _____ .

91,905 1,971.5 201,344 6,729.0 445,536 13,211.6 51,424 1,905.1 45,838 1,082.9 16,525 630.4 268.069 6,404.6 6,555 190.8 11,050 462.6 241,438 9,752.2 3,471 91.0 1,383,228 42,438.2

.._.

1958

___

73,644 1,655.0 196,029 7,341.6 518,553 15,644.3 61,942 1,802.2 53,873 1,365.1 20,454 597.7 184,632 4,577.2 72,811 2,599.7 23,082 977.0 249,505 10,997.5 476 10.7 1,455,001 47,568.0

w

8 1959

1960

7 1,004 2,226.6 200,350 5,550.2 540,498 17,535.7 42,506 1,163.7 18,885 505.7 18,640 514.7 113,916 2,773.9 312,021 10,173.5 34,401 1,340.5 264,276 10,729.3 555

89,363 2,743.1 111,696 3,571.0 596,120 18,357.7 44,593 1,261.3 11,884 346.0 26,761 850.0 37,121 1,131.8 401,184 12,516.0 90,438 4,010.9 199,007 8,943.5 250 8.0 1,608,417 53,739.2

-g

1,617,052 52,513.8

e 1000 bags of 100 pounds each. I Statistics compiled by the Rice Millers Association, New

Orleans, Louisiana. g Included with

BLUEBONNET

50 in report.

n

2 & "

p p

Y +

8 g

E2

RICE IMPROVEMENT AND CWLTVRE IN UNITED STATES

101

The cooking and processing characteristics of rice varieties and breeding lines have been tested in the cooperative quality laboratory at Beaumont, Texas. Typical United States long-grain varieties cook dry and flaky and are suitable for use in quick-cooking and canned soup products. Typical United States medium- and short-grain varieties cook moist and somewhat cohesive and are more suitable for making “dry” cereals and for use in baby foods and in brewing. Eating habits of different ethnic groups vary; so there is a demand for all types for use as TABLE VI Length of Growing Period, Plant Height, and Grain Yield of 13 Rice Varieties Yield per acre

Seeding to Variety

C.I. no.

TP 49 REXORO CENTURY PATNA 231 BLUEBONNET 50 TORO SUNBONNET MAGNOLIA ARICROSE

ZENITH NATO CALROSE CALORO COLUSA a

b c

d

Results Results Results Results

for for for for

8991 1779 8993 8990 9013 8989 8318 8310 7787 8998 8988 1561-1 1600

maturity (days)

1660 1730 1230 138-148b 137-148b 139-147b 126-132b 140C 126-132b 125-131b 1523 1526 137d

Plant Ark. La. Texas Cal. height 1953-58 1953-59 1955-58 1951-60 (in.)

53a 550 460 44-48” 44-47” 46-53b 48-52’’ 45C 46-52b 4348b 364 36d 35a

(lb.)

3969 3636 4100 3712 3681 4648 4036 4293

-

4500

-

(Ib.)

(lb.)

- 2786

-

(lb.) 1945

-

2754 3139 2516 2933

-

2923 3228

-

2538 3287 3817 3985

-

3902 3770 3832 3824

-

-

3112 3097 3280

’Texas. Arkansas, Louisiana, and Texas. Arkansas. California.

home-cooked table rice. The dry-flaky cooking characteristic is associated with a high percentage of amylose, a medium-high gelatinization temperature, and a maximum viscosity of the cooked paste when cooled to 55°C. The amylose content is estimated by the starch-iodine method ( Halick and Keneaster, 195s), which requires only a small sample, and quantitatively by the method described by Williams et al. (1958).The gelatinization temperature is estimated by the digestion in dilute alkali (Little et al., 1958) and determined fairly accurately by using the birefringence-end point-temperature or granule-swelling methods ( Halick et al., 1960).For final testing of a new variety, the gelatinization temperature and pasting characteristics are determined with an amylograph ( Halick and Kelly, 1959).

TABLE VII Dimmsions, Physical, and Chemical Characteristics of Milled Kernels of 14 Rice Varietie9 Starch content

Variety and grain type Long-grain CENTUIIYPATNA 231 BLUEBONNET 50 TP 49 REXOHO

TORO SUNBONNET Medium-grain NATO MAGNOLIA ZENITH ARKROSE CALROSE Short-grain COLUSA CALOHO MOCHIGOMI

Dimensions

GelatiWeight, nization 100 temperakernels ture (gram) (“C.)

Width (mm.)

Thickness (mm.)

W:L

Length (mm.)

6.60 7.08 6.93 6.69 6.50 6.76

1.92 2.08 2.00 1.94 1.98 2.10

1.55 1.64 1.64 1.56 1.52 1.69

3.44 3.41 3.47 3.44 3.28 3.27

1.53 1.88 1.73 1.61 1.68 1.93

5.53

2.51 2.51 2.49 2.66 2.57

1.65 1.77 1.68 1.83 1.85

2.21 2.46 2.25 2.41 2.09

1.76 2.08 1.77 2.33 2.00

6.06 5.62 5.80 5.37

ratio (1:)

Amylopectin: Cooking amylose quality ratio (cohesive(1:) ness No.)b

Total (%)

Amylose (%)

75.0 72.0 69.0 69.0 66.0 73.5

88.66 89.24 87.76 87.40 89.45 87.43

20.21 23.47 25.18 25.10 17.90 24.04

0.295 0.356 0.402 0.402 0.250 0.379

5.4 6.0 5.1 6.0 5.5 6.1

67.5 68.0 66.0 61.5 61.5

89.45 87.36 89.14 89.71 90.07

16.10 14.16 20.47 23.77 18.80

0.219 0.193 0.298 0.360 0.263

5.4 6.1 4.1

3.8 4.0

4.92 2.97 2.05 1.66 2.22 63.0 91.15 20.70 0.294 4.4 5.01 2.82 2.08 1.77 63.0 90.45 21.17 0.305 3.6 4.56 2.62 2.09 1.75 1.78 60.0 89.11 2.10 0.024 2.9 a Data from a study conducted by the Crops, Human Nutrition, Western Utilization, and Southern Utilization Research Divisions, Agricultural Research Service and Foreign Agricultural Service, U. S. Department of Agriculture. b Batcher et al. (1956).

0 m

2

RICE IMPROVEMENT AND CULTURE IN UNJTED STATES

103

Data for some of the physical and chemical characteristics of 14 rice varieties are given (Table VII) . MOCHIGOMI is a “glutinous” variety; that is, it has a very low percentage of amylose. The other 13 are “common” varieties and have a comparatively high amylose content although the range in the amylose:amylopectin ratio among the varieties in this group is fairly wide. The kernels of long-grain varieties (Table VII) are 6.5 mm. or more in length and the 1ength:width ratio is 3.27 to 3.47:l. The kernels of the medium-grain varieties range from 5.37 to 6.06mm. in length and have

FIG.1. Spikelets and milled kernels of: ( a ) long-grain variety BLUEBONNET 50; ( b ) medium-grain variety NATO;and ( c ) short-grain variety CALORO.

a 1ength:width ratio of from 2.09 to 2.46:l. KerneIs of the short-grain varieties range from 4.56 to 5.01mm. in length, and the 1ength:width ratio varies from 1.66 to 1.77:l. Kernels of all long-grain varieties were lighter in weight than kernels of most medium- and short-grain varieties. Spikelets and milled kernels of typical long-, medium-, and short-grain varieties are shown in Fig. 1. The gelatinization temperature of the starch was low for all shortand medium-grain varieties and intermediate for most long-grain varieties. The two exceptions were CENTURY PATNA 231, which had a high, and TORO, which had a low, gelatinization temperature. The long-grain varieties TP 49 and REXORO had the highest amylose content. BLUEBONNET 50 and SUNBONNET had somewhat lower amylose

104

c.

ROY

-rim,

M. D. MMILLER,AND H. M. BEACHELL

content. The amylose content of CENTURY PATNA and range as most medium- and short-grain varieties.

TORO

is in the same

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106

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M. BEACHELL

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Jones, J. W., Davis, L. L., and Williams, A. H. 1950. U . S. Dept. Agr. Farmers’ BuU. 2023. Jones, J. W., Dockins, J. O., Walker, R. K., and Davis, W. C. 1952. U . S. Dept. Agr. Fanners’ Bull. 2043. Jones, J. W., Adair, C. R., Beachell, H. M., Jodon, N. E., and Williams, A. H. 1953. U. S. Dept. Agr. Circ. 916. Kadam, B. S., and Ramiah, K. 1943. Indian J. Genet. and Pkant Breeding 3(1), 7-27. Kapp, L. C. 1933. Arkansas Uniu. (FayetteuiUe) Agr. Expt. Sta. Bull. 291. Kaufman, P, B., and Crafts, A. S. 1958. Hilgardia 24( 15), 411-453. Kester, E. B. 1959. Proc. Calif. Rice Research Symposium Albany, Calif. pp. 6-12. Kihara, H. 1959. Seiken Ziho 10, 68-83. Kik, M. C., and Williams, R. R. 1945. Natl. Acad. Sci. Natl. Research Council Bull. 112. Knapp, S. A. 1899. U. S. Dept. Agr. Diu. Botan. BuU. 22. Knapp, S. A. 1900. U. S. Dept. Agr. Farmers’ Bull. 110. Knapp, S. A. 1910. U. S. Dept. Agr. Farmers’ Bull. 417. Lange, W. H., and Grigarick, A. A. 1959. Calif. Agr. 19(8), 10-11. Lange, W. H., Ingebretsen, K. H., and Davis, L. L. 1953. Calif. Agr. 7 ( 8 ) , 8-9. Little, R. R., Hilder, C. B., and Dawson, E. H. 1958. Cereal Chem. 35(2), 111126. Mackie, W. W. 1943. Rice in the Imperial Valley (Calif.) 1943. Imperial Rice Growers’ Cooperative Association. Unnumbered Rept. McNeal, X. 1950a. Arkansas Uniu. (Fayetteuille) Agr. Expt. Sta. Bull. 600. McNeal, X. 1950b. Arkansas Uniu. (Fayetteuille) Agr. Expt. Sta. Bull. 604. Mikkelsen, D. S., and Finfrock, D. C. 1957. Agron. J. 49(6), 296-300. Mikkelsen, D. S., and Sinah, M. N. 1961. Crop Sci. 1(5), 332-335. Mikkelsen, D. S., Finfrock, D. C., and Miller, M. D. 1958. Calif. Uniu. Agr. Expt. Sta. Ext. Ser. Lcafiet 96. Mikkelsen, D. S., Miller, M. D., and Jordon, G. S. 1961. California Cooperative Rice Research Foundation, Inc. (mimeograph). Moncrief, J. B., and Weihing, R. M. 1950. Texas Agr. Expt. Sta. Bull. 729. Mullins, T., and Slusher, M. W. 1950. Arkansas Uniu. (Fayetteuille) Agr. Expt. Sta. Bull. 498. Mullins, T., and Slusher, M. W. 1951. Arkansas Unio. (Fayetteuille) Agr. Expt. Sta. BuU. 609. Nagao, S. 1951. Advances in Genet. 4, 181-212. Nagao, S., and Takahashi, M. E. 1960. 1. Fac. Agr. Hokkaido Uniu. 61(2), 289-298. Kandi, H. K. 1936. J. Genet. 33(2), 315-336. Nelson, M. 1931. Arkonsas Uniu. (Foyetteuille) Agr. Expt. Sta. BuU. 264. Nelson, M. 1943. Arkansas Uniu. (Fayetteuille) Agr. Expt. Sta. Bull. 450. Nelson, M. 1944. Arkansas Uniu. (Fuyetteuik) Agr. Expt. Sta. Bull. 446. Nelson, R. J. 1908. Arkansas Uniu. (Fayetteuilk) Agr. Erpt. Sta. Bull. 98. Olsen, K. L. 1957. Arkansas Uniu. (FayetteuiUe) Agr. Expt. Sta. Rept. Ser. Bull. 69.

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Ormrod, D. P. 1981. Agron. 1. 53, 93-95. Pearson, C. A. 1958. Soil Sci. 87(4), 198-206. Pearson, C. A., and Ayers, A. D. 1960. U. S. Dept. Agr. Production Research Rept. No. 43. Portman, R. F., and Williams, A. H. 1952. J. Econ. Entomol. 45(4), 712-716. Prince, A. H. 1927. Arkansas Uniu. Agr. Ext. Circ. 253. Quereau, F. C. 1920. Louisiana Agr. Expt. Sta. Bull. 171. Ramiah, K., and Chose, R. L. M. 1951. lndian 1. Genet. and Plant Breeding 11( l ) ,7-13. Raney, F. C. 1959. Proc. Calif. Rice Research Symposium Albany Calif. pp. 20-23. Raney, F. C., Hagan, R. M., and Finfrock, D. C. 1957. Calif. Agr. 11(4), 19-20, 37. Reed, J. F., and Sturgis, M. B. 1937. Louisiana Agr. Expt. Sta. Bull. 292. Reynolds, E. B. 1954. Texas Agr. Expt. Sta. BuU. 775. Rolston, L. H., and Rouse, P. 1960. Arkansas Uniu. (Fayetteuille) Agr. Expt. Sta. Bull. 624. Roschevicz, R. J. 1931. Bull. Appl. Botany Genet. Plunt Breeding (Leningrad) 27( 4 ) , 1-133. Rosenberg, L. E. 1947. Calif. Dept. Agr. Bull. 36(2), 42-48. Rouse, P., Rolston, L. H., and Lincoln, C. 1958. Arkansas Uniu. (Fayetteoille) Agr. Expt. Sta. Bull. 600. Ryker, T. C., and Brown, C. A. 1947. Louisiana Agr. Expt. Sta. Bull. 411. Sakai, K. I. 1935. Japan. J. Genet. 11(3), 145-156. Salley, A. S. 1936. Contdibs. Charleston Museum 8, 51-53. Sampath, S., and Govindaswami, S. 1958. Rice News Teller. Sampath, S., and Rao, M. B. V. N. 1951. lndian 1. Genet. and Plant Breeding 11, 14-17. Scott, V. H., Lewis, D. C., Fox, D. R., and Babb, A. F. 1961. Calif. Agr. 16(11). Senewiratne, S. T., and Mikkelsen, D. S. 1961. Plant and Soil 14(2), 127-146. Shastry, S. V. S., Rao, D. R. R., and Misra, R. N. 1980. lndian J. Genet. and Plant Breeding aO( 1 ), 15-21. Slusher, M. W., and Mullins, T. 1948. Arkamas Uniu. (Fayetteuille) Agr. Expt. Sta. Rept. Ser. 11. Smith, R. J., Jr. 1960a. Weeds 8, 256-267. Smith, R. J., Jr. 1960b. Proc. Southern Weed Control Conf. 13, 245-247. Smith, R. J., Jr. 1961. Weeds 9 ( 2 ) , 318-322. Smith, R. J., Jr., Viste, K. L., and Shaw, W. C. 1959. International Rice Commission News Letter 8(3 ) , 1-6. Smith, W. D. 1940. U . S. Dept. Agr. Farmers’ Bull. 1420. Smith, W. D., Deffes, J. J., Bennett, C. H., Hurst, W. M., and Redit, W. H. 1933. U. S. Dept. Agr. Circ. 292. Smith, W. D., Deffes, J. J., Bennett, C. H., Adair, C. R., and Beachell, H. M. 1938. U . S. Dept. Agr. Circ. 484. Tisdale, W. H., and Jenkins, J. M. 1921. U. S. Dept. Agr. Farmers’ Bull. l212. Todd, E. H., and Atkins, J. G . 1959. Phytopathology 49(4), 184-188. Todd, E. H., and Beachell, H. M. 1954. Texas Agr. Expt. Sta. Progr. Rept. 1650. Tullis, E. C. 1934. Phytopathology 24( 8 ) , 938-942. Tullis, E. C., and Cralley, E. M. 1941. Phytopathology 31(3), 279-281. T u b , E. C., Jones, J. W., and Davis, L. L. 1934. Phytopathology 24(9), 1047.

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U. S. Dept. Agr. 1958. Agricultural Marketing Service. Agriculture Handbook No.

m.

Van Royen, W. 1954. “The Agricultural Resources of the World,” pp. 83-92. Prentice-Hall, Englewood Cliffs, New Jersey, Vincenheller, W. G. 1906. Arkansas Unio. (Fayetteoille) Agr. Erpt. Sta. Bull. 89.

Viste, K. L. 1961. Research Progress Report Research Committee, Western Weed Control Conference, Salt Lake City, Utah (abstr.) p. 46. Walker, R. K., and Sturgis, hi. B. 1946. Louisiana Agr. Expt. Ste. Bull. 407. Wasson, R. A., and Walker, R. K. 1953. Louisiana Stute Uniu. Rice Expt. Sta. Agr. Ext. Ser. Publ. 1182. N‘eathenvas, P. 1929. Am. J. Botany 16(7 ) , 547-555. Williams, R. E. 1955. Rice 1. 68(13), 18-19. W’illiam, R. E. 1956. Rice J. 69(1), 8-9; ( 2 ) , 8; ( 3 ) , 8-9; ( 4 ) , 14; ( 5 ) , 24; (6),34; ( 8 ) , 22-23; ( 9 ) , 15; ( l o ) , 16-17. Williams, V. R., Wu, W. T., Tsai, H. Y., and Bates, H. C . 1958. J. Agr. Food C h e n ~6( . l ) , 47-48. Williams, W. A., Finfrock, D. C., and Miller, M. D. 1957. Culif. Uniu. Agr. Expt. Sta. Ext. Ser. Leaflet 90. Wyche, R. H. 1941. Texas Agr. Expt. Sta. Bull. 602. Yeh, B. and Henderson, M. T. 1961. Proc. Rice Tech. Working Group NIP-488.

RAINFALL EROSION Dwight D. Smith and Walter H. Wischmeier United States Department of Agriculture, Beltsville, Maryland, and Purdue Agricultural Experiment Station, Lafayette, Indiana

I. Introduction .................................................... A. Historical Sketch of Erosion Research .......................... B. Tools Used in Erosion Research .............................. 11. Mechanics of Rainfall Erosion .................................... A. Raindrop Characteristics .................................... B. Raindrop Impact and Splash .................................. C. Sheet and Microchannel Flow ................................ 111. Basic Factors Affecting Field Soil Loss ............................ A. Rainfall .................................................... B. Soil Erodibility .............................................. C. Topography ................................................ D. Cover and Management ...................................... E. Erosion-Control Practices .................................... IV. Soil Loss Prediction ............................................ A. Historical .................................................. B. The Universal Rainfall Erosion Equation ........................ References .....................................................

Page 109 110 111 113 114 119 122 123 123 125 126 129 134 137 137 138 144

1. Introduction

Rainfall erosion is a serious problem of farmland over a large part of the world. It is particularly acute on gently to steeply sloping land of both humid and semiarid areas. In the latter areas, seasonal or annual drought cycles which prevent the establishment and maintenance of plant cover create an erosion problem fully as severe as that in areas of heavier rainfall. Four factors and their interrelations have long been considered the basic determiners of the rate of rainfall erosion. They are: ( 1 ) climate, largely rainfall and temperature; ( 2 ) soil, its inherent resistance to dispersion and its water intake and transmission rates; ( 3 ) topography, particularly steepness and length of slope; and (4) plant couer, either living or the residues of dead vegetation. Any one of these factors can assume values which, alone, may create a rainfall erosion problem. A 109

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DWIGHT D. SMITH AND WALTER H. WISCHMEIER

soil with a very low intake rate or with extreme slope conditions will, when without plant cover, be subject to high erosion rates, even in areas where the erosive forces of the rainfall are relatively low. The Palouse hills of southeastern Washington with their steep, long slopes are an example. Lush vegetation or a combination of living and dead vegetation, as in a forest, eliminates practically all erosion in extreme conditions of both rainfall and topography. Sheet erosion, which has been defined (Soil Conservation Society of America, 1952) as “removal of a fairly uniform layer of soil or material from the land surface by the action of rainfall and runoff,” is the first stage of erosion. But the removal is uniform only from raindrop splash. After runoff starts, channelization soon begins and erosion is no longer uniform. This stage is referred to as rill or microchannel erosion. The channels are of a size that can be easily removed from sight by field tillage. But as repeated microchannels are created and then obliterated by cultivation, a topographic change takes place which in time results in increased concentration or channelization of runoff. Gullies soon appear in these locations. They are the mark of continual and advanced sheet and microchannel erosion on cultivated fields. These field gullies with their small drainage areas are not the same as main drainageway gullies which may develop in noncultivated areas.

A. HISTORICAL SKETCHOF EROSION RESEARCH Rainfall erosion research began with the work of Wollny in Germany in the latter half of the nineteenth century. His studies included the relation of erosion to steepness and orientation of slope, density of vegetative cover, and soil type. His work was discussed by Baver (1939) and Stallings (1957). Nelson { 1958) refers to Wollny as the father of soil conservation research. Erosion research in the United States began in 1917 with establishment of plots for study of the effect of soils, slope, and crops on runoff and erosion by Professor M. F. Miller of the University of Missouri. Others soon followed Professor Miller’s lead, using techniques developed by him and his early assistant, Dr. F. L. Duley. Similar work was started between 19-29 and 1933 by H. H. Bennett and L. A. Jones of the U .S Department of Agriculture. They established ten Federal-State experiment stations on the more critical erosion areas of the United States. Studies were started on eighteen additional soils during the next decade. In addition to soil, crop, and slope studies, several of the stations added terrace and strip crop design, gully control and small single practice watersheds. Most of these early studies were discontinued by 1943. In 1960 studies were underway on eighteen soils.

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Fundamental studies dealing with rainfall characteristics, drop velocity, drop size distribution, and splash were conducted by Laws and Parsons of the Soil Conservation Service, U. S. Department of Agriculture, during the period 1936 to 1940. Additional basic studies were started during and after World War I1 by Lutz and Hargrove (1944), Ellison (1944a), Ekern (1951), Mihara (1952), McIntyre (1958), Rose (1960), and Bisal (1950) and more recently by Hudson (1961%b) and the Soil and Water Conservation Research Division of the Agricultural Research Service, U. S . Department of Agriculture. Beginning in 1941, empirical equations were developed for estimating average annual field soil loss for different combinations of soil, slope, cropping, management, and conservation practices. Data for derivation and field use of the equations were those from the field plots established during the early 1930's. In 1953, a Runoff and Soil Loss Data Laboratory was established at Lafayette, Indiana, by the Soil and Water Conservation Research Division of the Agricultural Research Service, U. S. Department of Agriculture, in cooperation with Purdue University (Wischmeier, 1955). Practically all runoff and soil loss data from field plots in the United States have been assembled at this Laboratory, and research has been directed toward development of improved methods for the prediction of field soil loss and runoff.

B. TOOLS USEDrn EROSION RESEARCH Erosion research has been largely of an applied nature, conducted on field plots ranging in size from 0.005 acre to 2.0 acres. The early plots were generally 0.01 acre with a slope length of 72.6 feet. All runoff from the plots was caught in large tanks. To reduce the large volume of soil and water to be handled, and to allow the use of larger plots, several types of fractionating devices have been developed. The most accurate and reliable is the multislot divisor (Harrold and Krimgold, 1948). It is a plate containing a series of vertical slots mounted at the end of a rectangular box which is placed between two catchment tanks. A fraction of the runoff, either 1/5, 1/7, 1/9, or 1/11, is channeled into the second tank. Sometimes three tanks and two divisor units are used in series. A more recent development is the Coshocton-type rotating vane sampler (Fig. 1). It is a small rate-measuring flume that discharges onto a horizontal water wheel with a sampling slot. The rate-measuring ffume, when equipped with a water-stage recorder, provides rate of runoff as well as total amount of runoff and soil loss per storm (Parsons, 1954). The rotating wheel of this device secures an aliquot sample of the runoff. On areas producing heavy silt loads, a b d e arrangement (Par-

112

DWIGHT D. SMITH AND WALTER H. WISCHMEXER

sons, 1955) must be used in the flume approach section to prevent sediment deposits in the flume that cause inaccuracies in sampling. The use of devices to .apply water drops or simulated rain in the study of runoff and erosion began intensively during the 1930s. It included the work by Lowdermilk (1930), Nichols and Sexton (1932), Beutner et al. (1940), Borst and Woodburn (1940), Hendrickson (1934), Ellison and Pomerene (1944), Duley and Hays (1932), Neal (1938), Zingg (1940),

FIG. 1. Coshocton-type rotating vane sampler installation utilizing a large-plot concentrating trough designed to trap coarse sediment before it reaches the flume of the sampler.

Rowe (1940), Sharp and Holtan (1940), and Wilm (1941, 1943). In these investigations, water was applied by various means, ranging from hand-held sprinkling cans to well-developed automatic application devices. Development work was by both the Soil Conservation Service and the Forest Service of the U. S. Department of Agriculture. Unfortunately, little of the development work was recorded in published articles (Parsons, 1943). These devices generally had one common weakness. Either drop size or velocity, or both, was appreciably lower than in natural rainfall of

RAINFALL EROSION

113

medium to high intensity. Three of the smaller and more recent units have been described by Dortignac (1951), Packer (1957), and Bertrand and Parr (1961). A rainfall simulator that applied water drops uniformly on three or four field plots simultaneously was developed by Meyer and McCune (1958). The drop size distribution and energy of impact attained closely approximated that of high intensity natural rainfall (Meyer and McCune, 1958; Meyer, 1960). Other large-field plot simulators have been developed that use irrigation equipment for water application. One unit (Passerini, 1957) used a battery of six parallel oscillating pipes equipped with nozzles and located between pairs of plots. Swanson (1960) used an irrigational pipeline with closely spaced irrigation sprinklers. Smaller laboratory simulators and drop towers with related equipment are currently used in studies of mechanics of erosion by the Soil and Water Conservation Research Division of the Agricultural Research Service, U. S. Department of Agriculture and other groups. These include work by Rose (1960, 1961) and Bisal ( 1960). This review reports developments in mechanics of rainfall erosion, basic factor relationships, and soil loss prediction methods. Gully, irrigation, and wind erosion are not considered. II. Mechanics of Rainfall Erosion

Early research workers on rainfall erosion recognized the effect of plant cover in reducing runoff and erosion. The mechanics of how this occurred was, however, not fully understood for many years. Complete quantitative evaluation of each step in this process has not yet been made. The clogging of soil pores with the associated increase in runoff resulting from raindrop impact on bare soil and the prevention of these effects by plant cover was recognized by Wollny. Early experiments by Miller (1936), Lowdermilk (1930), Hendrickson (1934), and others in the United States established this concept. The general belief was that erosion resulted from the flow of water downslope, although the loosening of soil particles by raindrop impact prior to movement in the runoff was mentioned. Bennett (1939) described the erosion process as a disturbance, either compaction or a geyser-like loosening, of the immediate soil surface by impact of the raindrop. This was later called raindrop splash. A clear understanding of fundamental factors involved in the erosion processes and the development of practical methods for changing them (Cook, 1936) was recognized as the means for accomplishing the objectives of a soil conservation research program. It was under Cook's planning and guidance that D. A. Parsons and J. 0. Laws of the Soil Con-

114

DWIGHT D. SMITH AND WALTER H. WISCHMEIER

servation Service started their work on raindrop velocity, size distribution, and energy in relation to intensity. This marked a turning point in water erosion research. It was the beginning of the concept that erosion is a work process for which the energy is supplied by the falling raindrops and the slope of the land down which the runoff flows. A. R A ~ ~ R O CHARA~~ERISTICS P Study of rainfall momentum and energy in relation to erosion requires knowledge of the determining factors-raindrop mass, size, size distribution, shape, velocity, and direction. Direct measurement of momentum has been attempted by use of torsion balances (Neal and Baver, 1937) as discussed in an unpublished manuscript by N. W. Hudson,l but difficulties of wind shielding and adhering water drops have prevented successful development of the instrument. Hudson experimented with a kinetic energy recorder using a circular fan or paddle wheel. His most successful device utilizes a receiving diaphragm over a metal cylinder in which the noise of the impinging drop is picked up by a sensitive microphone. The output voltage from the microphone is passed through an amplifier, rectifier, and special recording milliammeter to give a trace with time that is proportional to the momentum of the drops. Drop size distribution in natural rain was first investigated with respect to erosion as a phase of rainfall simulator development (Laws and Parsons, 1943). Laws and Parsons used the flour method of drop size measurement (Bentley, 1904). Their data and those of others show an increase in median drop size with increases in rain intensity. Drop size distribution is described by a parameter, D50,commonly called the median drop size. Median, in this case, refers to the midpoint of the total volume. The combined volume of all drops smaller than D5,, equals the combined volume of all drops larger than D50. Drop size distributions measured by different investigators are frequently compared by the relationships of median drop size to rainfall intensity in the respective sets of data. Laws and Parsons combined data by Defant and Lenard with their own measurements secured at Washington, D. C. The relationship of median drop size to intensity was described by the equation: DjO = 2.23 ZOJS2 (1) in which Z is intensity in inches per hour. Another equation relating the two parameters was reported by Best (1950): DZo = 0.69l”’ APT (2) 1 Senior Research Engineer, Federal Department of Conservation and Extension, Henderson Research Station, P.B. 4, Mazoe, Southern Rhodesia.

RAINFALL EROSION

115

in which n, A, and p are empirically derived constants, and I is intensity. Spilhaus (194813) concluded that a rain can be completely described by the median drop diameter and rain intensity. The increased interest in measuring drop size distribution in the late 1940’s was due to measurement of rainfall by radar and the need for understanding the mechanics of precipitation (Atlas and Plank, 19%). These studies related drop size distribution and liquid water volumes per unit volume of the atmosphere (m?) to the radar reflectivity factor 2 by empirical equations of the exponential type. Drop size measurements of nonorographic rain have all shown fair to good agreement when compared on the basis of the relationship of median drop diameter to intensity. Variations within groups, however, has been high. These data were from widely scattered locations including, in addition to that of Laws and Parsons, Ottawa, Canada (Marshall and Palmer, 1948), Hawaii (Anderson, 1948; Blanchard, 1953; Blanchard and Spencer, 1957), Japan (Mihara, 1952), Illinois (Jones, 1956), England (Mason and Andrews, M O ) , and Southern Rhodesia (Hudson, 1961a). Several of the investigations recognized a different relationship of diameter to intensity for different types of rain. Orographic rain, in which drops are formed at low altitude and in warm cloud conditions, is an example. In these rains, drops seldom exceed 2.0 mm. in diameter and intensities generally do not exceed 1 inch per hour. Median drop diameters for this type rain are about half those in nonorographic rain of the same intensity (Blanchard, 1953). The number of drop size samples secured at intensities higher than 2 inches per hour has been limited. Maximum intensities sampled were approximately as follows: Laws and Parsons 4.5; Jones 4.0; Mason and Andrews 4.0; Mihara 5.0; and, Hudson 7.6 inches per hour. Hudson found a progressive trend toward larger drop sizes up to and including his class interval ending at 4.5 inches per hour. His median drop diameter peaked between 3.0 and 4.0 inches per hour and decreased thereafter. A similar trend had been noted by Mihara. The shape of raindrops as they strike the surface of the earth is not spherical owing to differential air pressure created by the falling drop. The resultant shape approximates an ellipsoid flattened on the bottom. This change in shape is of little consequence for drops less than 2.6 mm. diameter (Spilhaus, 1948a). The three principal factors that control the shape of larger water drops are surface tension, hydrostatic pressure, and external aerodynamic pressure ( McDonald, 1954). The change in shape of a raindrop has significance from an erosion standpoint in that it affects the velocity (Laws, 1941) and the impaot

116

DWIGHT D. SMITH AND WALTER H. WISMMEIER

force per unit area of soil (Ekern, 1951). Laws describes the action of the large drops as vibrating between vertical and horizontal oblateness with a frequency depending upon size. It is generally agreed, however, that drop shape is stable after terminal velocity has been attained. The phenomenon of drop vibration has been studied by other meteorologists (Blanchard, 1949, 1950; Magono, 1954; Gunn, 1949). This change in shape of larger drops is of high importance in rain simulator work where large drops at low fall height are used to secure high impact energy values. Study of the change in shape of water drops has been conducted largely in the laboratory. Natural raindrops were measured in Illinois by photographic methods (Jones, 1959). Of 1783 drops measured, 569 were spherical, 496 oblate, 331 prolate, and 387 unclassified. All drops of a given size did not have the same axis ratio, although definite trends were apparent with change in drop diameter. When all drops were included and averaged by class intervals, ratios were generally larger than when only oblate drops were considered. Values taken from Jones’ hand-fitted curves are shown in Table I. TABLE I Axial Ratios of Raindrops As Measured by Jonesa

D

All raindropsb

( mm. 1

( b’a) 0.99

0.92

0.85 0.78 0.71 a b

Oblate dropsb ( b’a) 0.93 0.87 0.81 0.74 0.68

From Table 2 of Jones ( 1959 ) . Ratio of the vertical to the horizontal drop axes = b/*.

The theoretical values of Spilhaus are in good agreement with the oblate values of Jones except for the maximum size drops. The fall velocity of raindrops was studied by Laws to assist in understanding the action of rain in the eroding of soil (Laws, 1941). He used photographic equipment to measure drop velocity. His studies covered a drop range from a little over 1 mm. to 6 mm. diameter. Laws’ values were significantly higher than earlier velocity measurements, probably because of less air turbulence and a more accurate method of measurement. They were essentially equal to the terminal velocities of water drops in stagnant air that Gunn and Kinzer (1949) measured by inducing an electric charge and producing pulses on an oscillograph record. Terminal velocities read from their data and that of Mihara (1952) are shown in Table 11.

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RAINFALL EROSION

In natural rain, air turbulence can act either to increase or to decrease drop velocity. The magnitude of air turbulence during rainfall and the effects of drop velocity have not been studied. Winds, however, have an appreciable effect on drop velocity. A horizontal wind increases terminal drop velocity by the reciprocal of the cosine of the angle of inclination of the rain with the vertical. In a heavy, driving rain with a 3-mm. median drop size and a 30-degree angle of inclination, the velocity would be increased 17 per cent and the kinetic energy would be increased 36 per cent. In detailed erosion studies where rain intensity, momentum or kinetic energy are related to soil movement, this factor cannot be neglected (Hudson, 1961a). It could not be considered in analyses of TABLE I1 Terminal Velocity of Waterdrops and Distance Necessary to Attain 95 Per Cent Terminal Velocity

Drop sue

(mm. 1 0.25 0.50 1.00 2.00 3.00 4.00 5.00 6.00 5

b c

Terminal velocitya (m./sec.) 1.0 2.0 4.0 6.5 8.1 8.8 9.1 9.3

Fall to reach 95 per cent terminal velocityb (m.)

2.2 5.0 7.2

Terminal velocityc (m./sec.)

2.0 4.1 6.3 7.6

7.8

8.5

7.6 7.2

8.8 9.0

Laws (1941) and GUM and Kinzer (1949). Laws (1941). Mihara ( 1952).

United States data because only recently were wind recording instruments installed at runoff plot sites. Kinetic energy of rainfall is important in erosion studies since erosion is a work process and much of the energy required to accomplish this work is derived from the falling raindrops. Kinetic energy of rainfall can be more easily computed than directly measured. The key to the computation is the intensity-drop size relationship since intensities may be secured from recording rain-gauge records. With terminal velocities and drop mass known, the calculation may be easily made (Wischmeier and Smith, 1958; Mihara, 1952; Hudson, 1961a). Drop size distribution as published by Laws and Parsons provided the desired intensity classes for this purpose. Energy values computed from median drop size data alone are from 10 to 15 per cent too high.

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DWIGHT D. SMITH A N D WALTER H. WISCHMEIER

The energy equation by Wischmeier and Smith is as follows:

El, = 916

+ 331 loglo Z

(3)

in which Ek is kinetic energy in foot-tons per acre-inch of rain, and Z is intensity in inches per hour. Hudson compared kinetic energies computed from rain-intensity relationships by several investigators. These are shown in Fig. 2. The difference in energy is probably accounted for by the drop size distribu-

W I

KW

B w 5 200-

0'

7.0 4.4

Hudron

* 300-

Miham

i

i

i

3.8

4

i

Rain intensity- Inches per Hour

-

i

-!

FIG.2. Kinetic energy of rain as determined by different investigators. (From Hudson, 1961a.)

tion-intensity relationship used in the computations since drop velocity measurements all agree rather closely. The increasing spread above 4 inches per hour between the curve of Hudson and that of Wischmeier and Smith may indicate an overestimation of kinetic energy by the Wischmeier and Smith equation for the higher intensities. Laws and Parsons' data used in the computations contained few samples above 2 inches per hour intensity. Both Hudson and Mihara observed a change in drop size distribution for the higher intensities with the median size drops tending to predominate. This could result in leveling of the energy curve for the higher intensities.

RAINFALL EROSION

119

B. RAINDROP IMPACT AND SPLASH The place of raindrop impact in water erosion began to emerge during the 1930's. The concentration of soil in runoff water was found to increase rapidly with raindrop energy (Laws, 1940). Intercepting raindrop impact with a straw mulch reduced erosion 95 per cent in rain simulator studies in Ohio (Borst and Woodburn, 1942). To isolate the mechanism of the reduction, a wire platform was used to support the straw mulch 1 inch off the bare soil so as to remove the energy of the drops but not to retard the flow of runoff. Raising the mulch off the ground did not decrease its effectiveness. This demonstrated that it was the impact of the drops on the bare soil and not the runoff velocity that detached the large quantities of soil washed from the unprotected plot. Splash erosion has been established as the initial phase of the water erosion process (Ellison, 1944a, b, 1947a,b, c). It is the true sheet erosion process. Ellison's studies demonstrated that erosion can proceed without runoff due to progressive movement of soil particles downslope by splash action. His studies based on direct measurement of splash showed that 75 per cent of soil splash on a 10 per cent slope moves downslope and only 25 per cent moves upslope. This compares with 60 per cent downslope and 40 per cent upslope movement of spashed sand on a slope of the same steepness (Ekern and Muckenhim, 1947). Ekern developed the equation that relative downslope movement by splash is approximately equal to 50 per cent plus the per cent slope; he substantiated the equation by laboratory measurements. This differential movement by vertical raindrops is explained by the fact that the downhill splash travels farther before recontacting the soil surface and that the angle of impact results in a greater downslope conponent. This is particularly important on extremely steep slopes (Mihara, 1952). Wind effects in the field, however, may upset this pattern. Losses from oriented soil pans exposed to natural rainfall in New York (Free, 1952) showed three times the losses from pans facing the direction of the storm as from pans facing the opposite direction. Sand particles apparently are more readily moved in splash than are finer soil particles (Woodburn, 1948; Woodburn and Kozachyn, 1956). Ellison reported that splash samples contained higher percentages of sand and gravel than did the original soil, but that the size of particles moved decreased with a decrease in drop size and velocity. The quantity splashed increased with drop size, drop velocity, and rain intensity. Sand particles 1 to 2 mm. in size were readily splashed by the larger drops, and there was an apparent downslope movement of particles as large as 8 mm. (Ellison, 1944a).

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DWIGHT D. ShlITH AND WALTER H. WISCHMEIER

Sand transport by splash from a 5.8-mm. drop showed a net increase with increased height of fall (Ekern, 1951). However, a series of rises and falls in amount of splash occurred with changes in height of fall that were less than the height required for terminal velocity. This resulted from drop oscillation between oblate to prolate shapes which affected the impact force per unit area of drop impact. The height and distance of splash depends upon the soil surface condition and the fall velocity of the drop (Mihara, 1952). The maximum splash distance reported by Mihara was 37 inches and the maximum height 12 inches when drops impacted on cultivated soil. On compacted soil, the splash from 6-mm. drops reached a distance of 59 inches. Studies in England showed that fragments of drops falling on wet paving stones rebounded to maximum heights of 12 to 24 inches (Mason and Andrews, 1960). Ellison reported that the maximum distance of splash resulting from a 5.9-mm. drop falling at 18 feet per second was 60 inches and the height about 15 inches. Some stone fragments of 4-mm. size were splashed 8 inches, and 2-mm. particles were splashed 16 inches. With drop size and velocity constant the important aspects affecting the magnitude of splash from a smooth soil surface without clods are the resistance of the soil to deformation by the drop and the depth of water film. The condition represented by the flour pan used in measuring drop size distribution illustrates the extreme ease of penetration and resulting absence of splash. The other extreme, which produces maximum splash or shattering of the drops, is illustrated by the paving stones with a thin film of water (Mason and Andrews, 1960). Ellison reported maximum splash shortly after the surface was wetted. Thereafter splash decreased with increased time of water application, possibly from development of a deeper water film or the removal of readily detached soil particles. Investigations in Germany (Kuron and Steinmetz, 1958) showed first an increase in splash with increasing depth of water film, followed by a decrease in splash with continued increase in depth. However, total soil transport was increased owing to increased turbulence of the runoff. Similar results were reported from Japan (Mihara, 1952). In this study the measured angle of splash was about 32 degrees when the water level in the sand was at the sand surface, but as the depth increased the angle increased rapidly until at about 2 mm. depth it was 75 degrees and at 10 mm. depth and above it was essentially 90 degrees. Mihara studied sand deformation under raindrop impact. He found that the diameter of the drop crater was always greater than the drop diameter and that it increased with increasing velocity. The depth of penetration varied with the compactness of the sand and the velocity of

121

RAINFALL EROSION

the drop. Figure 3 shows the steps in drop crater formation (Mihara, 1952). The different investigators are not agreed on the relation of splash to waterdrop parameters. Bisal ( 1960) shows detachment proportional to the 1.4 power of drop velocity. Ekern (1951) shows splash proportional to kinetic energy when amount of applied water is constant. Rose (1960) reports that soil detachment is more closely related to momentum per unit area and time of rain than to kinetic energy. Free (196Ob) found that splash losses from sand varied as the 0.9 power of drop energy, but from a silt loam soil as the 1.5 power of the energy. 0

sec

I I400 I 700

1 400

I I50 1 70

FIG.3. Steps in drop crater formation. (From Mihara, 1952.)

The dispersing action of raindrop impact on bare soil, which results in formation of a soil crust that reduces infiltration and increases surface runoff, has been shown by many investigators, including Lowdermilk (1930), Hendrickson (1934), Duley and Kelly (1939), Laws (1940), Borst and Woodburn (1942), Ellison (1947c), Levine (1952), and McIntyre (1958). It is most pronounced on fine-textured and weakly aggregated soils. Coarse-textured soils show the greater amount of splash, but with fine-textured soils the particles tend to compact and seal during the process. The relative importance of splash and runoff erosion have not been clearly established (Ellison, 1947d; Bennett et al., 1951). Splash erosion denudes the top of the slope owing to differential downslope splash movement; runoff erosion tends to increase as it creates rills in ever-

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DWIGHT D. SMITH AND WALTER H. WISCHMEIER

increasing size toward the bottom of the slope. For the Shelby and Marshall soils of the Midwest (Smith et nZ., 1945) depth of surface soil decreased down from the crest of the slope as steepness and length increased, but when the land slope flattened the depth increased.

C. SHEETAND MICROCHANNEL FLOW Runoff as sheet and microchannel flow is the second phase of the water erosion process. The raindrop impact-splash process has been shown to have high detachment but low transport capacity. Sheet and microchannel flow has low detachment capacity and high transport capacity. IYhen both act together on an unprotected soil, erosion soon reaches serious proportions. Little (1940) considered that the erosivity of flow was a function of turbulence and, therefore, a function of velocity squared per unit of flow depth. He developed equations for flow in terms of rainfall, hydraulic roughness, runoff coefficients, and ground slope profiles. He recognized the extreme complexity of the problem and suggested that compromise with theory, use of empirical constants, and judgment would enter into practical applications to field problems. In a study of runoff from a paved surface under simulated rainfall, Izzard and Augustine (1943) found that surface detention required to maintain a specific rate of flow under raindrop impact was appreciably greater than the detention required after rain ceased. The impact force of the raindrop nearly normal to the direction of flow acted to retard the downslope flow and created the type of turbulence that is very effective in detachment of soil particles on a bare field slope. In a study of the results of several investigations, Ekern (1954) concluded that erosivity was proportional to the additive kinetic energy of the raindrop at impact and of the shallow flow of water. The effect of shallow sheet flows without water drop impact on movement of noncohesive soil particles ranging in size from fine sand to fine gravel was studied by Lutz and Hargrove (1944) on a laboratory plot. They concluded that particle movement was expressed by the general relationship: (4) Ep = K S”Q”’ where Ep is particle movement, S is slope, and Q is flow discharge. K , n, and m were found to vary for the different-sized particles. Their Q values were much higher than those that would occur from rain falling on a short slope length equal to that of their plot but may be considered as representing quantities of flow (without silt) experienced with severe storms on segments of field slopes of around 100 to 200 feet in length.

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The importance of slope on particle movement decreased as particle size increased, but the importance of flow increased. The greatest loss occurred when the point of maximum velocity of flow was at a depth between the radius and diameter of the particle. Woodruff (1947) used the laboratory plot of Neal (1938) to compare soil loss when water was applied as sheet flow at the upper end of the plot and as simulated rain. His results showed that for sheet flow without drop impact the soil per cubic foot of runoff was only about 10 per cent of that with the water applied as simulated rain for slopes of 8 per cent and less. Doubling the slope to 16 per cent resulted in much greater detachment by the flowing water. The soil carried by the sheet flow in this latter case was nearly 60 per cent of that carried with the water applied as simulated rain. This indicates, for the condition of this study, that for the flatter slopes drop impact was responsible for the bulk of erosion but for the 16 per cent slope the runoff water with its higher energy contributed an important part of soil detachment and total loss from the plot. 111.

Basic Factors Affecting Field Soil Loss

Relationships of the basic factors to soil erosion under numerous field conditions have been empirically evaluated from plot studies throughout the United States and in several other countries. Recent assembly of most of these data in the Runoff and Soil Loss Data Laboratory ( Wischmeier, 1955) facilitated analyses to identify the factors and interaction effects responsible for the frequently wide differences in results reported from the localized studies. The basic factors affecting field soil loss are discussed under five major classifications.

A. RAINFALL Climatic effects on field soil loss include both rainfall and temperature. Length of growing season influences the nature and quality of vegetative cover available to protect the soil surface. Thaw and melting snow may cause serious rill and microchannel erosion from unprotected sloping fields, especially when an impervious layer of frozen soil beneath a shallow-thawed surface impedes infiltration. However, the climatic feature most significantly related to field soil loss is rainfall. Data from plots tilled similarly to corn or cotton but kept free of vegetation were analyzed to identify and evaluate rainstorm characteristics most closely correlated with field soil loss in the absence of protective cover. The results, summarized by Wischmeier et d. (1958), show that, even for specific storms, soil loss was poorly correlated with rain amount. The correlation of soil loss with maximum 5-, 1 5 , or 30-

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DWIGHT D. SMITH Ah7) WALTER H. WISCHMEIER

minute intensities was also generally poor. Good correlation with maximum 30-minute intensity was found only on steep slopes or sandy loam. At every location for which fallow-plot data were available, both runoff and soil loss were more highly correlated with rainfall energy than with rain amount or any short-period maximum intensity. Momentum rated second, but was well below energy as a predictor of soil loss from fallow. In further regression analyses of the data, Wischmeier (1959) found that the rainstorm parameter most highly correlated with soil loss from fallow was a product term, kinetic energy of the storm times maximum 30-minute intensity. He called this product the “rainfall-erosion index.” Maximum 30-minute intensity was defined as twice the greatest amount of rain falling in any 30-minute period. A break between storms was defined as a period of 6 consecutive hours with less than 0.05 inches of rainfall. This index was selected as the most appropriate rainfall parameter for use in the soil loss prediction equation. The rainfall-erosion index thus defined explained from 72 to 97 per cent of the variation in individual-storm erosion from tilled continuous fallow on each of six widely scattered soils. The percentage of the soilloss variance explained by the index was greater than that explained by any other of 42 factors investigated and greater than that explained by rain amount and maximum 5, 15, and 30-minute intensities, all combined in a multiple regression equation. The erosion index evaluates the interacting effect of total storm energy and maximum sustained intensity. Thus it is an approximation of the combined effects of impact energy and rate and turbulence of runoff. Rainfall energy is a function of the specific combination of drop velocities and rain amount. The maximum 30-minute intensity is an indication of the excessive rainfall available for runoff. The product terms-rain amount times 30-minute intensity, and momentum times 30-minute intensity-were also more precise estimators of soil loss than was energy alone, although less accurate than the energyintensity product. This supports the conclusion that the erosive potential of a rainstorm is primarily a function of the interacting effects of drop velocity, rain amount and maximum sustained intensity. In assembled plot data, maximum 30-minute intensity was more effective than maximum 15- or 60-minute intensity as the second element of the interaction term. The relationship of soil loss to the storm energy-intensity products is linear. Therefore, the location erosion-index value for a year can be computed by summing the storm energy-intensity products. The assembled plot data showed that when all factors other than rainfall were constant, specific-year soil losses from cultivated areas were directly proportional

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to the yearly values of the index. Yearly or monthly values of the erosion index can be computed on a locality basis from recording rain gauge records. Return-period values can also be computed. The rainfall-erosion index provides valid estimates of the effects of rainfall patterns over long time periods. For prediction of losses from specific storms, precision was improved by combining with the erosion index, the parameters: rainfall energy, an antecedent moisture index, and antecedent energy since cultivation ( Wischmeier and Smith, 1958).

B. SOILERODIBILITY Some soils erode more readily than others. Soil properties that influence soil erodibility by water may be grouped into two types: (1) those properties that effect the infiltration rate and permeability; and ( 2 ) those properties that resist the dispersion, splashing, abrasion, and transporting forces of the rainfall and runoff. Middleton (1930) was one of the first to try to obtain an index of soil erodibility based on the physical properties of the soil. He considered the dispersion ratio and what he called the erosion ratio to be the most significant soil characteristics influencing soil erodibility. His erosion ratio was the dispersion ratio divided by the ratio of colloid to moisture equivalent. The dispersion ratio, expressed as a percentage, was the ratio of the apparent total weight of silt and clay in the nondispersed sample to the total silt and clay in the dispersed sample. He also suggested that organic matter, the silica: sesquioxide ratio, and the total exchangeable bases influenced the erosional behavior of soils. Middleton et al. (1932, 1934) grouped soils of the original ten erosion stations according to the above criteria. Peele et al. (1945) modzed Middleton’s criteria in an analysis of four major soils of South Carolina. Voznesensky and Artsruui (1940) developed a formula for an index of erodibility based on dispersion, waterretaining capacity, and aggregation. O’Neal (1952) attempted to develop a key for evaluating soil permeability on the basis of certain field conditions. The first step in this procedure was to determine the type of structure. Then the class of permeability was estimated from four principal factors and one or more of eight secondary factors. Principal factors used were: relative dimensions (horizontally and vertically) of structural aggregates, amount and direction of overlap aggregates, number of visible pores, and texture. Important secondary factors were: compaction, direction of natural breakage, silt content, cementation, type of clay minerals, character of coatings in aggregates, degree of mottling, and certain features of climate.

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Parr and Bertrand (1960) presented a comprehensive review of past studies of water infiltration into soils. Instrumentation, procedures, and results were briefly summarized. TOdescribe what they considered to be the most important soil properties affecting erodibility, Adams et al. (1958b) made the following field and laboratory measurement on Iowa soils: runoff, infiltration, wash erosion, splash erosion, water-stable aggregates < 0.10 mm., dispersion ratio, per cent silt and clay, bulk density, pores drained by 60-cm. water tension, and air permeability at field capacity. For soils containing relatively large amounts of swelling type clay that crack upon drying, the erodibility has been considered to be significantly influenced by moisture content (Smith et al., 1953; Adams et al., 1958a). Browning et al. (1947) developed a conservation guide for soils mapped in Iowa, to be used in computing field soil loss. The soils were divided into seven groups on the basis of what was considered to be their relative erodibility. This procedure was extended to other soils of the North Central and Northeastern States (U. S. Soil Conservation Service, 1956; Lloyd and Eley, 1952). Many soil properties appear to influence erodibility. Their effects may be interrelated. Some of them are influenced by cropping history, past erosion and management practices. Subjective classifications of soil into erodibility classes by relating observed erosion to soil survey data have often been biased by confounding soil effects with those of rainfall and management. The automatic confounding of rainfall and soil effects has also complicated efforts to evaluate soil erodibility empirically from field plots under natural rainfall. As a means of isolating soil effects If'ischmeier et al. (1958) proposed use of the rainfall-erosion index to adjust measured soil losses for differences in rainstorm characteristics and use of slope factors to adjust for differences to topography. For soil-loss prediction purposes, Wischmeier and Smith (1961) defined soil erodibility as soil loss in tons per acre per unit of rainfallerosion index, measured from tilled continuous fallow with length and per cent of slope at specified values. The soil erodibility factor thus becomes a quantitative factor. Empirical measurements for its evaluation include combinations of all primary and interacting factor effects.

C. TOPOGRAPHY Field soil loss is affected by the degree of slope, the length of slope, and the curvature of the slope. Studies of the first two effects have been conducted under both natural and simulated rainfall, but little has been done to evaluate the third.

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1. Per Cent Slope The effect of per cent slope was studied on small plots under sprinklers by Duley and Hays ( 1932), Neal (1938), Borst and Woodburn ( 1940), and Zingg (1940). Water applied by sprinklers in these studies did not simulate natural rainfall in drop size distribution, drop velocity, or energy. Duley and Hays found that the increase in soil loss with each unit increase in per cent slope became greater as the slope became steeper. In a comparison of a silty clay loam with a sandy soil, the former gave greater erosion loss on the flatter slopes and the latter on the steeper slopes. Borst and Woodburn, using artificial rainfall at Zanesville, Ohio, found soil loss proportional to P 3 0 , where S is per cent of slope. Neal, working with Putnam soil, found soil loss proportional to So.7Z1.2,where Z is intensity in inches per hour. The first comprehensive study of the effect of slope on soil loss was published by Zingg (1940). He concluded that soil loss varies as the 1.4 power of the per cent slope. He used data by Duley and Hays (1932), Diseker and Yoder (1936) and from a series of studies he conducted. For better description of the relationship on the flatter slopes of Midwest claypan soils, and using data of Neal (1938), Smith and Whitt (1947) proposed the equation:

R = 0.10

+ 0.21 s

4/3

(5)

where R is relative soil loss in relation to unity loss from a 3 per cent slope and S is per cent of slope. The authors (Smith and Wischmeier, 1957) evaluated the per cent slope-soil loss relationship on the basis of plot data under natural rainfall secured by several investigators. 0. E. Hays, at the Upper Mississippi Valley Conservation Experiment Station, Lacrosse, Wisconsin, obtained data that covered 17 years of soil loss measurements from slopes of 3, 8, 13, and 18 per cent on Fayette soil. The plots were cropped to continuous barley for five years, followed by twelve years of corn-oatsmeadow rotation with across-slope tillage. A second-degree polynomial gave a better least-squares fit to these data than did the logarithmic relationship suggested by the earlier investigators. The constants of parabolic equations derived from Hays’ data and from Zingg’s rainfall simulator data were nearly identical when the latter were adjusted for cropping effect. Data of Van Doren and Gard (1950) and Borst et al. (1945), each comparing two slopes, if adjusted to conform with conditions at Lacrosse, also fit the equation derived from Hays’ data. The combined data for the four studies gave a very good least-squares fit to the equation:

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DWIGHT D. SMITH AND WALTER H. WISCHMELER

A = 0.43

+ 0.30s + 0.043S2

(6)

in which A is soil loss and S is per cent slope. In both the Hays and Zingg studies, runoff increased significantly with increase in per cent slope although the two soils were quite different, one being a deep loess that sealed under raindrop impact and the other a loam over a clay subsoil. 2. Length of Slope Zingg (1940) concluded that total soil loss varied as the 1.6 power of slope length; and the loss per unit area, as the 0.6 power. His conclusion was based on data from Bethany, Missouri; Guthrie, Oklahoma; Clarinda, Iowa; Lacrosse, Wisconsin; and, Tyler, Texas. A group study in 1946 under Musgrave (1947) proposed 0.35 as the average value for the slope length exponent for soil loss per unit area. In 1956, the results of statistical analysis of data for 532 plot years, involving simultaneous measurements on two or more lengths of slope under natural rainfall from 15 plot-study locations in 12 States, were published by Wischmeier ef nl. (1958). The analysis showed that the relationship of soil loss to slope length often varied more from year to year on the same plot than it varied among locations. The magnitude of the slope-length exponent appeared to be influenced by soil characteristics, rainfall pattern, steepness of slope, cover, and residue management. However, the data were not adequate to provide quantitative evaluations of the factor-interaction efTects. Average values of the slope length exponent for the different locations varied from 0 to 0.9. Magnitude of the exponent appeared definitely to be related to the effect of slope length on runoff. At Hays, Kansas, and Temple, Texas, runoff decreased significantly with slope length. From these data the over-all average value of the exponent did not differ significantly from zero. However, in the final 7-year period of the 15year study at Temple, soil loss was proportional to LO3. At Guthrie, Oklahoma, and for corn following bluegrass sod at Bethany, Missouri, where runoff showed a significant increase with increased slope length, soil loss varied as Lo.’ and Lo.9, respectively. At the other 11 locations studied, slope length had no significant effect on runoff. In these studies, the magnitude of the slope length exponent ranged from 0.27 to 0.60. In seven of the studies cropping was continuous corn or cotton. In the other four it was rotational, including row crops. Average values of the length exponent computed for ten locations in the Corn Belt and the Northeastern States did not differ significantly at the 10 per cent level. The mean of the ten exponents was 0.46. A group

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study at Purdue University in 1956, which included the authors (Smith and Wischmeier, 1957), concluded that for field use the value of the length exponent should be 0.5 2 0.1. D. COVERAND MANAGEMENT The greatest deterrent to soil erosion is cover. Cover and management influence both the infiltration rate and the susceptibility of the soil to erosion. The most effective vegetative cover is a well-managed, dense sod. Fields easily eroded are usually those in poorly managed, cultivated crops. The severest erosion occurs when erosive rainstorms coincide with periods in the rotation when the soil surface is essentially bare. Sodbased rotations have played a predominant role in runoff and erosion control. Also, many new tillage practices have been very effective. Baver (1956) classified the major effects of vegetation on runoff and erosion into five distinct categories: ( 1 ) interception of rainfall by the vegetative cover; ( 2 ) decrease in the velocity of runoff and the cutting action of the water; ( 3 ) root effects in increasing granulation and porosity; ( 4) biological activities associated with vegetative growth and their influence on soil porosity; and, ( 5 ) the transpiration of water leading to subsequent drying out of the soil. Bertoni et al. (1958) observed that final infiltration rates of a soil varied with season of the year. They suggested that the higher infiltration rates during July were due to increased vegetal cover which protected the soil surface against sealing, to lowered surface moisture, and to high soil and water temperatures. Higher infiltration rates during the summer months than during other seasons also were observed by Beutner et al. ( 1940), Horner and Lloyd ( 1940), and Borst et al. ( 1945). Woodward (1943) also found a linear relation between vegetal cover and infiltration rate. In a study of the relation of plant cover to infiltration and erosion in Ponderosa Pine forests of Colorado, Dortignac and Love (1960) concluded that large pore space of the upper 2 inches of soil and the quantity of dead organic materials were the two properties that accounted for most of the variation in infiltration rates among cover types. The case of soil dislodgement by rainfall impact varied with types of soil cover, but soil origin and the amount of exposed bare soil were the main factors affecting erosion. In 15 years of soil-loss measurements, Horner (1960) found that the kind and amount of cover provided during the winter season was the dominant factor affecting runoff and erosion on Palouse silt loam at Pullman, Washington, where large summer storms with high intensities

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DWIGHT D. SMITH AXD WALTER H. WISCHMEIER

were few. Land seeded to winter wheat was more vulnerable to erosion than any other winter cover condition common to the area. Sod-based rotations provided more effective erosion control and soil organic matter maintenance than did cropping systems without the meadow. Summer fallowing caused the largest erosion losses and the most rapid depletion of organic matter. Melting snow on frozen soil contributed significantly to erosion hazards in this area. Exceedingly high soil losses from a gentle rain falling on soil thawed to a depth of 4 inches were also reported by Bay et nl. (1952) in \%‘isconsin. Taylor and Hays (1960) found that a heavy mulch of chopped cornstalks and manure provided excellent erosion control on corn following corn on Fayette silt loam of 16 per cent land slope. Whitaker et nl. (1961) found that fertilization adequate to produce high crop yields and large quantities of plant residues greatly reduced the formerly serious soil and water losses from sloping claypan soils. Seedbed preparation by subtillage, which left shredded cornstalks on or near the surface, significantly reduced erosion losses even from very high intensity storms. Shredding the cornstalks increases their wintertime erosion control value. In studies on Warsaw loam and Russell silt loam with about 4 per cent slope, hleyer and Mannering (1961b) found that soil loss associated with shredded cornstalks was slightly less than half that from cornstalks as left by mechanical pickers when rainfall was applied artificially at 2.4 inches per hour for 60 minutes. However, one trip over the shredded stalks with a disk significantly increased the soil content of the runoff. The importance of crop residues, cover crops, and sod-based rotations in control of runoff and erosion in the Southern Piedmont soil area has been shown by studies at Watkinsville, Georgia (Carreker and Barnett, 1949; Barnett, 1959). Analyzing data from the blackland prairie of central Texas, Adams et al. (1958a) found that both runoff and soil loss from corn managed by subsurface tillage methods were significantly less when the corn followed fescue than when it followed another year of corn. Krall et al. (1958) found that stubble mulch fallow provided better erosion control than did other methods of summer fallowing in the semiarid areas. In Wyoming, Barnes and Bohmont (1955) found that land in grass as commonly left after haying operations absorbed water at a rate 25 per cent lower than did land with “trashy” fallow. Both conditions absorbed from 30 to 75 per cent more water in an hour than did bare fallow land. Raking and baling loose straw from a stubble field reduced water intake rate by more than 30 per cent. Burning the

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residue reduced water absorption by nearly 50 per cent. Duley (1960) reported that in a three-year grain rotation, plowed land lost 2.6 times as much water and 4.8times as much soil in runoff as did stubble mulched land over a 20-year period. The amount of water stored in the soil during summer fallow depended on the amount of residue present on the soil. In a study by Mannering and Meyer ( 1961), 6% inches of simulated rain were applied at 2% inches per hour on various quantities of straw mulch spread over freshly plowed and disked wheat stubble on Wea silt loam with 5 per cent slope. With no mulch, soil loss was 12 tons per acre, but with 2 tons mulch per acre no runoff and erosion occurred. With 1 ton of mulch per acre, soil loss was reduced to % ton per acre; and with ton of mulch, to about 1ton per acre. Minimum tillage practices in which the corn-planting operation coincides with or immediately follows plowing with moldboard plows, and fitting operations with disk and harrow are omitted, have gained in popularity in recent years. Minimum tillage provides erosion and runoff control because of larger aggregate or clod size and decreased compaction (Free, 1960a). Idltration is increased and erosion is reduced during the highly vulnerable seedbed and crop establishment periods. Quantitative data on the magnitude of erosion-control benefits from minimumtillage practices are too limited to permit evaluating the interaction effects of soils, slope, cover, and row direction. Meyer and Mannering (1961a) compared plow-plant as a single operation with planting on a seedbed fitted by two diskings with a trailing harrow on Russell silt loam of 5 per cent slope that previously had been in meadow. Runoff and soil losses were measured from three 5.2-inch applications of simulated rain at 2.6 inches per hour. In a test about 2 weeks after seeding, losses from the minimum-tillage plots averaged 63 per cent of the runoff and 52 per cent of the soil loss from the fitted seedbeds. When the corn in both treatments was cultivated to prevent surface crusting, reductions in both runoff and soil loss by the minimum-tillage practice were still apparent even after corn harvest, but the magnitude of the benefits decreased significantly with successive cultivations and increased vegetative growth. When corn cultivations were omitted on the minimum-tillage areas, both runoff and erosion were greatly increased as a result of surface crusting. Soil loss from this treatment was greater than that from the corn planted on fitted seedbed and cultivated after emergence. Swamy Rao et al. (1960) found that minimum tillage for corn resulted in a higher rate of infiltration, less soil resistance to penetration, lower bulk density, and less soil compaction due to tractor and implement

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DWIGHT D. SMITH AND WALTER H. WISMMEIER

traffic. The data for fallow periods in rotations measured in other studies (Wischmeier, 1960) show that the magnitude of the benefits attainable with minimum-tillage practices may be expected to depend upon crop sequence, quality of meadows in the rotation, and quantity of residues plowed under. Hays (1961) reduced total soil and water losses from rotations by spacing corn rows 60 inches apart and interseeding legumes in the corn after the second cultivation. Rains soon after the interseeding caused increased soil and water losses due to the smoothing and packing of the soil by the seeder, but after the seeding became established losses were significantly reduced. The quality of meadows established by this procedure was good. Effects of specific cover, sod crop sequences, tillage practices, and residue managements on field soil loss have been investigated in cooperative USDA and State Agricultural Experiment Station plot studies under natural rainfall at more than 45 locations in 23 States and Puerto Rico. The data have generally been analyzed and reported by the study locations. Cover and management data have usually been reported either on a crop-year basis or as rotation averages. Crop-year losses reflect the effect of different crops or crop sequences, but do not show the cause of favorable or unfavorable results. These can be more readily discerned if the individual-storm data are analyzed on the basis of different stages of crop growth. To study the relations of cover, crop sequence, productivity level, and residue management to soil loss, Wischmeier divided each crop row into five crop stages, defined for relative uniformity of cover and residue effects as follows: (1) rough falIow-turnplowing to seedbed preparation; ( 2 ) seedbed--first month after crop seeding; ( 3 ) establishment -second month after crop seeding; (4)growing cover-from 2 months after seeding until harvest; ( 5 ) stubble or residue-harvest to plowing or new seedbed. On these bases, soil losses from the cropped plots were compared with losses from tilled continuous fallow under identical rainfall, soil, and topographic conditions. Ratios of these losses, expressed as percentages, were published in the form of a ready-reference table (Wischmeier, 19600).About a quarter million individual-storm soil loss measurements were available for this study. Highly sigdicant inverse correlations between crop yields and erosion losses were apparent in the data. In gcncral, crop yields appeared to provide a fair indication of the combined ef€ects of such variables as

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density of canopy, rate of water use by the growing crop, and quantity of crop residues. Specific-year erosion losses from corn after meadow ranged from 14 to 68 per cent of corresponding losses from adjacent continuous corn. The effectiveness of grass and legume meadow sod plowed under before corn in reducing soil loss from the corn was, in general, directly proportional to meadow yields. Its erosion-control effectiveness was greatest during the fallow and corn-seedbed periods. The residual effect of grass and legume mixtures was greater than that of legumes alone. Direct comparisons of corn foIIowing first, second, and third years of meadow, though limited, indicated that second-year meadow, when allowed to deteriorate, was less effective than one year of meadow. When succeeding meadows were more productive than first-year, they were usually more effective in reducing erosion from corn in the following year. When the corn residues were removed at harvest time, soil losses from corn after corn were high and yields were ususally low. Soil losses from the growing corn under these conditions were from 35 to 50 per cent of those from adjacent continuous fallow. Leaving the cornstalks and plowing them under in spring significantly decreased erosion during the following corn year as well as during the winter period. Effectiveness of the corn residues turned under was directly related to the quantity of residues available and was greatest during the fallow and seedbed periods. Erosion from clean-tilled cotton during the growing period appeared to be about 50 per cent more than from corn under similar management, soil, and rainfall. Soybean data were too limited to reveal a significant difference in average annual erosion from beans in 42-inch rows as compared with corn in comparable sequence. Erodibility of fallow soil occurring for brief periods in crop rotations was influenced more by crop sequence and the nature and quantity of residues turned under than by the inherent characteristics of the soil itself. The erosion control effectiveness of winter cover seedings depended upon time and method of seeding, time of plowing, rainfall distribution, type of cover seeded, and density of cover produced. Covers such as vetch and ryegrass seeded early enough to attain good fall growth and turned in April were effective in reducing erosion not only in the winter months, but also in the following crop year (Uhland, 1958). Small grain alone seeded in corn or cotton residues and plowed under in the spring showed no residual erosion-reducing effect after the next year’s corn or cotton planting.

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DWIGHT D. SMITH AXD WALTER H. WISCKMEIER

E. EROSION-COKTROL PRACTICES Contour tillage and planting, strip cropping, terracing, waterways, and gully control structures are generally included under erosion control practices. Sometimes they are referred to as supporting practices. Tillage practices, sod-based rotations, fertility treatments, and other cropping-management practices discussed in the preceding section are not included in this group, although it is recognized that they contribute materially to erosion control and frequently provide the major control in the farmer’s field. This discussion will be confined to the first three of the listed practices. Contour planting of crops has been in general an effective practice. It functions, however, only to control runoff or scour erosion and then only for those storms that are low to moderate in extent or until the capacity of the rows to hold or conduct runoff is exceeded. In field practice, key rows are either level or have a grade toward a waterway. Because of land slope irregularities, row breakage is frequent with the larger runoff storms. When this occurs losses may equal or exceed those from up- and downslope planting (Smith et al., 1945; Moldenhauer and Wischmeier, 1960). The effectiveness of contour planting and tillage in erosion control has varied with slope, crop, and soil (Smith and Whitt, 1947; Van Doren et al., 1950; Van Doren and Bartelli, 1956; Tower and Gardner, 1953). Its maximum effectiveness in relation to up- and downhill rows is on medium slopes and on deep, permeable soils that are protected from sealing. The relative effectiveness decreases as the land slopes become either very flat or very steep. Row shapes as secured with listing increase the channel capacity and , therefore, increase the average annual effectiveness of farming on the contour. However, when row breakage occurs, the results are disastrous because of the sudden release of large quantities of impounded water. The ratio of soil loss with contouring to that from up- and downhill rows is generally considered to be 0.5 for slopes of from 2 to 7 per cent, 0.6 for slopes down to 1 per cent and up to 12 per cent, 0.8 for 12 to 18 per cent, and 0.9 for 18 to 24 per cent. With the development of land-forming machinery and techniques, controlled row grades and shapes, designed to handle those storms causing the bulk of runoff and erosion, became possible for those soils amendable by management practices applied after reshaping. Strip cropping-a practice in which contour strips of sod alternate with strips of row crops-has proved to be a more effective practice than contouring alone. The sod acts as filter strips when row breakage

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occurs. Quality sod strips have been reported to filter as much as 80 per cent of soil from the runoff from the cultivated strip (Hays et uZ., 1949). Strip crop systems generally use a four-year rotation-two years of meadow, one of row crop, and one of small grain in which the meadow is established. This system reduces soil loss to about 50 per cent of that from contouring (Hays et al., 1949). Three-year rotation systems with one year of sod were slightly less effective (Borst et al., 1945; Smith et aZ., 1945). Alternate strips of row crop and small grain were effective on relatively flat slopes in Texas (Hill et al., 1944). Terracing is one of the oldest practices used to control erosion. It supports contouring by acting as a safety measure to prevent serious field gullies when contour rows break. As used in America, it is a positive means for reducing length of slope. With bench terraces, as used with permanent walls in China, the steepness of slope is reduced to nearly level grades. Terracing with contour farming is generally considexed more effective as an erosion control practice than strip cropping, since it divides the slope into segments equal to the horizontal terrace spacing and about equal to strip width in strip cropping. With both practices, soil loss measurements include only soil movement from the field. With strip cropping, the saved soil is largely that deposited in the sod strip. With terracing, the deposit is in the terrace channel and may equal 90 per cent of the soil moved to the channel (Zingg, 1942). Erosion control between terraces depends upon the crop rotation and other management practices. With strip cropping the situation is somewhat similar, although an effective sod-based rotation is built into the system. Improvement of terrace alignment for efficient operation of modem farming equipment began in the United States after World War I1 (Smith and Coyle, 1960). They describe land forming equipment and its use in reshaping land for parallel terracing and uniform width strip cropping. Wojta (1950) developed a terrace drainage system for soils with poor surface and internal drainage and with land slopes of 4 per cent or less. In this system flat V channels with ridges from 0 to 3 inches above the original ground surface were constructed at grades varying from 0.1 to 1.0 per cent as a means of improving channel alignment. Soil from the channel was used to correct surface drainage between channels. The fields were farmed with the fence lines, and the terraces were crossed without difficulty. Yields were materially improved. By using a cut and fill technique, Smith (1956) developed a parallel terrace system for the Midwest claypan soils. In this system point-row

136

DWIGHT D. SMJTH A h 9 WALTER H. WISCHMEIER

areas were reduced from 30 per cent of the field area to 9 per cent. This reduced farming time by W per cent. An improved layout and construction procedure (Beasley and Meyer, 1957; Beasley, 195s) for parallel terraces was developed in which one man with tractor and scraper could construct and check the cut and fill terraces at costs little above those for conventional terrace construction. Development and application of parallel terrace systems has also occurred in other sections of the United States (Smith et al., 1959; Worley, 1960). Stable-channel design is of great importance in farm water conveyance, whether the channels are those of parallel terraces, for irrigation water supply or for surface drainage. Smerdon and Beasley (1959) considered that the design for cohesive soils could be logically approached on the basis of tractive force theory. This concept presented a theoretical means of evaluating shear at the interface between the flowing water and the soil in the channel. It is the force that initiates soil movement by the flowing water. Of eleven soils tested, the critical tractive force was best correlated with plasticity index and dispersion ratio. The per cent clay, from which these soils derived their cohesive properties, also correlated very well with critical tractive force. However, the authors believed this might not hold for all clays. The clay of the soils tested was all of the same origin. This work was followed by similar tests using compacted soil (Laflen and Beasley, 1960).They concluded that critical tractive force xfaried directly with the degree of compaction based on the linear relationship they secured between the voids ratio and critical tractive force. This work was continued by a graduate student of Professor Beasley ( McCool, 1961), who applied the spatially varied flow and tractive force theories to terrace channel design. His results showed, for the assumed design conditions, that higher grades, even exceeding 2 per cent, were permissible in the extreme upper end of the terrace channels and that grades a little over 0.5 were satisfactory at a length of loo0 feet. Other terrace developments have been made to meet special conditions ( Smith and Coyle, 1960). The Zingg conservation bench terrace was developed for both erosion control and moisture conservation. It utilizes a contour level bench 80 to 150 feet wide with ridge to retain runoff for crop use from an area above about twice the width of the bench ( Zingg and Hauser, 1959; Hauser, 1961) . The basin terrace, a crescent-shaped impounding structure, was developed for the deep loess soils of western Iowa to protect flatter and lower areas in cultivated crops from damage by runoff from pasture areas above.

RAINFALL EROSION

137

IV. Soil Loss Prediction

Guidelines are required for any complex operation. Full knowledge enables expression of the factors involved in theoretical mathematical relationships. For many complex operations, only rational or empirical equations of factor relationship are practical or possible, particularly in early stages of development. This is the situation in the field of soil loss prediction. Equations currently in use, though greatly improved over those first developed twenty years ago, remain empirical. This final section of the rainfall erosion chapter sketches the historical development of soil loss prediction and briefly discusses the prediction equation currently in use in the United States. A similar type of equation is being developed for subtropical areas of Africa (Hudson, 1961b).

A. HISTORICAL Development of equations for calculating field soil loss originated about 1940. The first system, developed in the Corn Belt States, was known as the slope-practice equation. Later it was adapted for use in farm planning in the Northeastern States and for computation of gross erosion from watersheds in flood abatement programs. In this latter use it is generally known as the Musgrave equation. Zingg (1940) published an equation relating length and per cent of slope to relative soil loss based on all published, or otherwise available, data in the United States to 1939. Using this equation as a base, Smith (1941) added crop and conservation practice factors and the limiting annual soil loss concept to develop a graphical method for applying conservation practices to the Shelby and associated soils of the Midwest. Browing et al. (1947) developed the system for use throughout Iowa. They added soil erodibility and management factors and prepared a set of tables to facilitate field use of the method. Conservationists from the Regional Office of the Soil Conservation Service, in Milwaukee, Wisconsin, recognized the value of the soil loss equation for farm planning and teamed with the research workers of the region to develop a system for regional application. This was used so successfully in determining soil deterioration rates for the region that a nationwide workshop was held in Cincinnati during the summer of 1946, under the direction of G. W. Musgrave. This group reviewed all soil loss data in the United States to 1946, re-evaluated factors previously used, and added a rainfall factor (Musgrave, 1947). Smith and Whitt (1947) developed a prediction method for the Midwest claypan soils and later adapted it for the principal soils of

138

DWIGHT D. SMITH AND WALTER H. WISCHMEIER

Missouri (Smith and IVhitt, 1948; Smith ct al., 1948). Van Doren and Bartelli (1956) developed a similar system oriented toward Illinois conditions. They introduced a slope length-erosion relation in which soil loss increased at a greater rate for slopes above 200 feet than for shorter lengths. Lloyd and Eley (1952) developed a graphical solution of the equation model developed at the 1946 workshop under Musgrave’s leadership for use by the Soil Conservation Service in the Northeastern States. The Soil Conservation Service included tables and instructions for field use of the system developed in the Corn Belt in their Ready References for conservation farm planning used in that area (Blakeley et a t , 1957; Pierre and Wischmeier, 1960). A slide rule was developed by Pierre to facilitate field use of the system.

B. THE UNIVERSAL RALVFALL-EROSION EQUATION An improved soil loss prediction equation was developed in the late 1950’s at the Runoff and Soil Loss Data Laboratory of the Agricultural Research Service (Wischmeier and Smith, 1961; U. S. Agricultural Research Service, 1961). This equation was designed to be geographically universal in applicability and to provide major improvements in localized soil loss prediction with minimum changes in basic concepts and the application procedures developed in the past decade. Some features of both the Corn Belt system and the soil loss nomogram of the Northeastern States were retained. Two recent developments, which contributed to major improvements in the field of soil loss prediction procedure, were the rainfall-erosion index (Wischmeier, 1959) and the method of evaluating the cropping-management factor on the basis of local climatic and crop cultural conditions ( Wischmeier, 1960). The soil loss equation is: A=RKLSCP

(7)

where A is computed average annual soil loss in tons per acre from a specific field under a specific rainfall pattern, cropping-management plan, and applied conservation practice. The rainfall factor, R, is a measure of the erosive potential of average annual rainfall in the locality. Its value is that of the rainfall-erosion index (Section 111, A ) . For most of the continental United States, the source of local rainfall-erosion index or R-values is the iso-erodent map reproduced in Fig. 4. Iso-erodents are the lines shown on the map along which the expected annual values of the rainfall-erosion index are equal. To facilitate the reading of local R-values for use in soil loss prediction, the map is modified to include county outlines. The map was developed from recording rain-gauge data for the

RAINFALL EROSION

139

22-year period, 1936-1957. Sources of data and procedures used in development of the iso-erodents were reported by Wischmeier at the 1961 Annual Meeting of the American Society of Agricultural Engineers as Paper No. 61-228. The approximate value of the erosion index was computed for each of about 2000 locations, and the iso-erodents were plotted using these point values as guides to locate the map lines. If soil

FIG.4. Mean annual values of the rainfall erosion index.

properties, topographic factors, and tillage could be held identical throughout the 37-State area, average annual soil loss from continuous clean-tilled fallow over a period of 20 or more years would be expected to vary in direct proportion to the erosion index values shown in Fig. 4. The soil-erodibility factor, K , is the average soil loss in tons per acre per unit of erosion index, from a particular soil in cultivated continuous fallow, with a standard plot length and per cent slope arbitrarily selected as 73 feet and 9 per cent, respectively. Pertinent values of the erodibility factor for each of a series of benchmark soils are obtained by direct

140

DWIGHT D. SMITH AND WALTER H. WISCHMELER

soil loss measurement. Correlation of these values with a series of physical and chemical soil properties is expected to provide empirical equations for approximations of K values for closely associated soils. Erodibility factor values have been approximated for 20 benchmark soils in the United States. Values range from 0.02 to 0.50 tons per acre per rainfall erosion index unit for the standard plot with a 9 per cent slope 73 feet long. This is the only factor on the right side of the equation that has a dimension. L, S, C , and P are dimensionless ratios. Topographic factors, L and S, adjust the soil loss estimate to the specific slope length and per cent existing on the field. Wischmeier et al. (1958) presented a graph showing the combined effect of length and degree of slope in various combinations. The graphical solution assumes the relationships : L = (length in feet/72.6)O (8) and S = (0.52 0.36s 0.0529)/6.613 (9)

+

+

where s is field slope expressed as per cent. These are average relation ratios of field slope dimensions to those of the standard plot. Precision of these factors may be improved by new studies designed for evaluating interaction effects of topographic factors with soil properties, rainstorm parameters, cover, and productivity (Section 111, C). Slope length is defined as the distance from the point of origin of overland flow to either of the following, whichever is limiting for the major portion of the area under consideration: (1) the point where the slope decreases to the extent that deposition begins; or ( 2 ) the the point where runoff water enters a well-defined channel. A channel is defined as a part of the drainage network of a size that is not readily obliterated by cultivation and is usually suitable for stabilization with grass. It may be a constructed channel such as a terrace or diversion. On a field that has terraces or diversions, the slope length is the horizontal spacing of the structures. The cropping-management factor, C , is the expected ratio of soil loss from land cropped under specified conditions to soil loss from cleantilled fallow on identical soil and slope and under the same rainfall. This item reflects the combined effect of cover, crop sequence, productivity level, length of growing season, tillage practices, residue management, and the expected time distribution of erosive rainstorms with respect to seeding and harvest dates in the locality. Data on the expected monthly distribution of erosive rainfall in each locality is provided in convenient form by a series of monthly erosion-

RAINFALL EROSION

141

index distribution curves derived from 22-year rainfall records. In the 37-State area of Fig. 4, 33 different distributions were found. Four of these curves are shown in Figs. 5 and 6. A U. S. Department of Agriculture Handbook on soil loss prediction, prepared by the authors for

CI 2-1 3-1 4-1 5-1 6-1 7-1 8-1 9-110-1 11-1 12-1 12-31

DATE FIG. 5. Time distribution of the rainfall erosion index: Curve A, in northwestern Iowa, northern Nebraska and southeastern South Dakota; curve B , in northern Missouri and central Illinois, Indiana, and Ohio. I00

90 -I

a

2

80 70 60 50

I-

40

$

30 a 20 W n 10 1-1 2-1 3-1 4-1 5-1 6-1 7-1 8-1 9-1 10-1 11-1 I24 12-31

DATE

FIG. 6. Time distribution of the rainfall erosion index: Curve C, in Louisiana, Mississippi, western Tennessee, and eastern Arkansas; curve D, in Atlantic coastal plains of Georgia and the Carolinas.

publication in 1962, includes all of the 33 distribution curves with a key map delineating the area in which each is applicable. Ratios of soil loss from crops to corresponding loss from continuous fallow were computed by Wischmeier (1960) for each crop-stage period

142

DWIGHT D. SMITH A S D WALTER H. WISCHMEIER

(Section 111, D ) for each of 100 combinations of cover, crop sequence, productivity level, and residue management. An excerpt from this tabulation is shown in Table 111. The cited publication explains in detail the procedure for combining the table values with data from the appliTABLE 111 Ratio of Soil Loss from Crops to Corresponding Loss from Continuous Fallow5 Crop yields

Crop-stage periodb

MeadCowr, sequence, and ow management (tons) l\t->r. corn 'after meadow, RdLC 1 lbt-yr, corn after meadow, RdL 2 lst-?r. corn after meadow, RdL 3 2nd-yr. corn after meadow, RdL 3 2nd-) r. corn after 3 meadow, RdRd 3rd-or-more year corn, RdL 3rd-or-more year corn, RdR 1st-yr. cotton after meadow, RdL 2 2nd-yr. cotton after meadow, RdL 2 Small grain w/meadow seeding: a. In disked corn residues After 1st-corn after meadow 2 After 2nd corn after meadow 2 b. On disked corn stubble, RdR After 1st corn after meadow 2 ,Ifter 2nd corn after meadow 2 Established grass and legume meadow

4 (%)

40

23

40

38

25

35

80

15

30

27

15

22

70

10

28

19

12

18

70

3.2

51

41

22

26

70

60

65

51

24

65

70

36

63

50

26

30

60

80

85

60

30

70

hl

15

34

45

35

30

M

35

65

68

46

42

60

-

30

18

3

2

60

-

40

24

5

3

-

-

50

40

5

3

-

-

80

50

7

3

3

-

-

0.4

-

-

Portion of 100-line published table ( Wischmeier, 1960). b Crop-stage periods are defined in Section 111, D. '' RdL, crop residues left and incorporated by plowing. d RdR, crop residues removed. (I

3 (%)

RAINFALL EROSION

143

cable erosion-index distribution curve to derive values of the factor C, which are based upon the specific set of conditions existing on a given field. The erosion-control practice factor, P , takes into account the erosioncontrol benefits gained by farming on the contour, by strip cropping, or by combining terraces with contouring. The values of P are the ratios discussed in Section 111, E, for contouring and strip cropping. Strip crop practice factors are usually used for terracing, although use of the contour factors results in higher levels of control.

1. Application of the Equation. Every field presents a given combination of basic conditions that influence the rate of soil erosion. These include topography, soil characteristics, prior erosion, land use history, growing season, and rainfall pattern. Upon this set of basic conditions, the farmer may apply a selected combination of cropping system, management, and conservation practices. The respective combinations of the latter factor provide various degrees of erosion control. The first four terms in the right side of the erosion equation utilize knowledge of the basic conditions as they exist on a particular field to estimate a base soil loss rate for the field. The other two terms apply the information gained from three decades of erosion-control research to predict how much the estimated base soil loss would be reduced by respective combinations of elective cropping, management, and erosion control practices. Thus, solutions of the equation provide a list of alternative farm plans with which average annual soil loss from the specific field can be held within the limits that can be tolerated under the soil topographic conditions involved. Also, for any given cropping system and per cent slope, critical limits of slope length can be computed. 2. Soil Loss Tolerance The utility of the soil loss equation for conservation farm planning is increased by establishing for specific soil areas upper limits for physical removal of soil, These limits are referred to as soil loss tolerance values. In this use of the equation, soil loss tolerance values are substituted in the equation for the A term and the equation is solved for C P values. Any combination of C P values equal to or less than the computed value will provide satisfactory control of erosion on the field in question. Both physical and economic factors are considered in establishing soil loss tolerance values. The concept is to limit soil loss to levels that will allow economical maintenance of soil productivity. One concept used for Midwest soils of the United States was to limit soil loss to an average

144

DWIGHT D. SMITH AND WALTER H. WISCHMEIER

rate associated with maintenance of organic matter content. Establishment of tolerance values, however, has been largely a matter of judgment. The subject has received little research attention.

REFERENCES Adams, J. E., Henderson, R. C., and Smith, R. M. 1958a. Soil Sci. 87, 232-237. Adams, J. E., Kirkham, D., and Scholtes, W.H. 1958b. Iowa State Coil. 1. Sci. 33, 485-540. Anderson, L. J. 1948. Bull. Am. Meteorol. SOC. 29, 362-366. Atlas, D., and Plank, V. G. 1953. J. Meteorol. 10, 291-295. Barnes, 0. K., and Bohmont, D. W. 1958. Wyoming Agr. Expt. Sta. Bull. 358. Bamett, A. P. 1959. Soil Consem. 24, 245-247. Baver, L. D. 1939. Soil Sci. SOC.Am. Proc. 3, 330-333. Baver, L. D. 1956. “Soil Physics,” 3rd ed. Wiley, New York. Bay, C. E., Wunnecke, G. W.,and Hays, 0. E. 1952. Trans. Am. Geophys. Union 33, 541-546. Beasley, R. P. 1958. Missouri Univ. Agr. Expt. Stu. Bull. 699. Beasley, R. P., and Meyer, L. D. 1957. Agr. Eng. 38, 32-36. Bennett, H. H. 1939. “Soil Conservation.” McCraw-Hill, Xew York. Bennett, H. H., Bell, F. G., and Robinson, B. D. 1951. U . S . Dept. Agr. Circ. 896. Bentley, W. 1904. Monthly Weather Rev. 32, 450-456. Bertoni, J., Larson, W.E., and Shrader, W. D. 1958. Soil Sci. SOC. Am. Proc. a2, 571-574. Bertrand, A. R., and Pam, J. F. 1961. Purdue Univ. Research BuU. 723. Best, A. C. 1950. Quart. 1. Roy. Meteotol. SOC. 76, 16-36. Beutner, E. L., Gaebe, R. R., and Horton, R. E. 1940. Trans. Am. Geophys. Union 21, 550-558. Bisal, F. 1950. Agr. Eng. 31, 621-622. Bisal, F. 1960. Can. J . Soil Sci. 40, 242-245. Blakeley, B. D., Coyle, J. J., and Steele, J. G. 1957. In “Soil the Yearbook of Agriculture” (A. Stefferud, ed.), pp. 290-360. U.S. Govt. Printing Office, Washington, D. C. Blanchard, D. C. 1949. Occasional Heport 17, Project CIRRUS, General Electric Research Laboratory, Schenectady, New York. Blanchard, D. C. 1950. Trans. Am. Geophys. Union 31, 836-842. Blanchard, D. C. 1953. 1. Meteorol. 10, 457-473. Blanchard, D. C., and Spencer, A. T. 1957, Tellus 9, 541-552. Borst, H. L., and Woodbum, R. 1940. U S . Dept. Agr. SCS-TP-36. Borst, H. L., and Woodbum, R. 1942. Agr. Eng. 23, 19-22. Borst, H. L., McCall, A. G., and Bell, F. G. 1945. U.S . Dept. Agr. Tech. BUZZ. 888 Browning, G. M., Parish, C . L., and Class, J. A. 1947. 1. Am. SOC. Agron. 39, 65-73. Carreker, J. R., and Bamett, A. P. 1949. Agr. Eng. 30, 173-176. Cook, H. L. 1936. Soil Sci. SOC. Am. Proc. 1,487-494. Diseker, E. G., and Yoder, R. E. 1936. Ahbarno Polytech. Inst. Agr. Erpt. Sta. Bull. 245. Dortignac, E. J. 1951. U . S . Dept. Agr. Rocky M . Forest and Range Expt. Sta. F07t Collinr, cob. Paper 5. Dortignac, E. J., and Love, L. D. 1960. Trans. Am. SOC. Agr. Eng. 3, 58-61.

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Duley, F. L. 1960. Nebraska Univ. Agr. Expt. Sta. Research Bull. 190. Duley, F. L., and Hays, 0. E. 1932. I. Agr. Research 46, 349-360. Duley, F. L., and Kelly, L. L. 1939. Nebraska Univ. Agr. Expt. Sta. Research Bull. 112. Ekem, P. C. 1951. Soil Sci. SOC. Am. Proc. 16, 7-10. Ekern, P.C. 1954. Soil Sci. SOC. Am. Proc. 18, 212-216. Ekem, P. C., and Muckenhim, R. J. 1947. Soil Sci. SOC. Am. Proc. 12, 441, 444. Ellison, W. D. 1944a. Agr. Eng. 26, 131-136, 181, 182. Ellison, W, D. 194413. Agr. Eng. 26, 306. Ellison, W. D. 1947a. Agr. Eng. 28, 145-146. Ellison, W. D. 1947b. Agr. Eng. 28, 197-201. Ellison, W. D. 1947c. Agr. Eng. 28, 245-248. Ellison, W. D. 1947d. Agr. Eng. 28, 349-351. Ellison, W. D., and Pomerene, W. H. 1944. Agr. Eng. 26, 220. Free, G. R. 1952. Agr. Eng. 33, 491-494, 496. Free, G. R. 1960a. Agr. Eng. 41, 96-99. Free, G. R. 1960b. Agr. Eng. 41, 447-449, 455. Gunn, R. 1949. Rev. Sci. Instr. 20, 291-296. Gunn, R., and Kinzer, G. 1949. 1. Meteorol. 6, 243-248. Harrold, L. L., and Krimgold, D. B. 1948. U. S . Dept. Agr. SCS-TP-61. Hauser, V. I. 1961. Soil and Water 12, 13. Hays, 0. E. 1961. J. Soil and Water Conseru. 16, 172-175. Hays, 0. E., McCall, A. G., and Bell, F. G. 1949. U. S. Dept. Agr. Tech. Bull. 973. Hendrickson, B. H. 1934. Trans. Am. Geophys. Union 16, 500-505. Hill, H. O., Peevy, W. J., McCall, A. G., and Bell, F. G. 1944. U . S. Dept. Agr. Tech. Bull. 859. Homer, G M. 1960. Agron. 3. 52, 342-344. Homer, W. W., and Lloyd, L. C. 1940. Trans. Am. Geophys. Union 21, 522-541. Hudson, N. W. 1961a. Rept. and PTOC. 1st Inter-African Hydrol. Conference Nairobi Paper No. 96. Hudson, N. W. 1961b. Proc. Trans. Rhodesian Sci. Assoc. 29, Pt. I. Izzard, C. F., and Augustine, M. T. 1943. Trans. Am. Geophys. Union 24, 500-511. Jones, D. M. A. 1956. Illinois State Water Survey Research Rept. No. 6. Jones, D. M. A. 1959. Illinois State Water Suwey Circ. 77. Krall, J. L., Power, J. F., and Massee, T. W. 1958. Montana State Coll. Agr. Expt. Sta. BuB. 640. Kuron, H., and Steinmetz, H. J. 1958. AssembZee Gen. Toronto 1, 115-121. Laflen, J. M., and Beasley, R. P. 1960. Missouri Univ. Agr. Expt. Sta. Research Bull. 749. Laws, J. 0. 1940. Agr. Eng. 21, 431-433. Laws, J. 0. 1941. Trans. Am. Geophys. Union 22, 709-721. Laws, J. O., and Parsons, D. A. 1943. Trans. Am. Geophys. Union 24, 452-460. Levine, G. 1952. Agr. Eng. 33, 559-560. Little, J. M. 1940. “Erosion Topography and Erosion.” A. Carlisle, San Francisco, California. Lloyd, C. H., and Eley, G. W. 1952. 3. Soil and Water Comerv. 7, 189-191. Lowdennilk, W. C. 1930. 1. Forestry 28, 474-491. Lutz, J. F., and Hargrove, B. D. 1944. N . Carolina State Coll. Agr. Expt. Sta. Tech. Bull. 78.

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Magono, C. 1954. J . Meteorol. 11, 77-79. McCool, D. K. 1961. Design of Terrace Channels Using Spatially Vaned Flow and Tractive Force Theories. M.S. Thesis, University of Missouri, Columbia, Mk50Un. McDonald, J. E. 1954. J . Meteorol. 11, 478-494. McIntyre, D. S. 1958. Soil Sci. 86, 185-189. Mannering, J. V., and Meyer, L.D. 1961. Agr. Research U. S. 10, 7. Marshall, J. S., and Palmer, W. McK. 1948. J. Meterol. 6, 165-166. Mason, B. J., and Andrews, J. B. 1960. Quart. J . Roy. Meteorol. SOC. 86, 346-353. Meyer, L. D. 1960. Soil Sci. SOC.Am. Proc. 24,319-322. Meyer, L. D., and McCune, 1).L. 1958. Agr. Eng. 39, 644-648. Meyer, L. D., and Mannering, J. V. 1961a. Agr. Eng 42, 72-75. Meyer, L. D., and Mannering, J. V. 1961b. Crops and Soils 13, 24-25. Middleton, H. E. 1930. U . S . Dept. Agr. Tech. Bull. 178. Middleton, H. E., Slater, C. S., and Byers, H. G . 1932. U . S . Dept. Agr. Tech. Bull. 316. Middleton, H. E., Slater, C. S., and Byers, H. G . 1934. U . S . Dept. Agr. Tech. Bull. 490. Mihara, Y. 1952. “Raindrops and Soil Erosion.” Natl. Inst. Agri. Sci., Tokyo, Japan. Miller, M. F. 1936. Missouri Unio. Agr. Expt. Sta. Bull. 366. Moldenhauer, W. C., and Wischmeier, W. H. 1960. Soil Sci. SOC. Am. Proc. 24, 409-413.

Musgrave, G. W. 1947. J . Soil and Water Conseru. 2, 133-138. Neal, J. H. 1938. Missouri Unio. Agr. Expt. Sta. Research Bull. 280. Neal, J. H., and Baver, L. D. 1937. J . Am. SOC. Agron. 29, 708-709. Nelson, L. B. 1958. Soil Sci. SOC. Am. Proc. 22, 355-358. Nichols, hl. L., and Sexton, H. D. 1932. Agr. Eng. 13, 101-103. ONeal, A. M. 1952. Soil Sci. SOC. Am. Proc. 16, 312-315. Packer, P. E. 1957. 17. S. Dept. Agr. Intermountain Forest and Range Expt. Sta. Ogden, Utah Misc. Puhl. 14. Pan, J. F., and Bertrand, A. R. 1960. Adoances in Agron. 12, 311-355. Parsons, D. A. 1943. Trans. Am. Geophys. Union 24, 485-487. Parsons, D. A. 1954. ZJ. S. Dept. Agr. SCS-TP-124. Parsons, D. A. 1955. U . S. Dept. Agr. ARS-41-2. Passerini, G. 1957. “Esperienze a Regime Pluviale Commandato” Instituto Sperimentale per lo Studio e la Difesa del Suolo, Tipografia Bruno Coppini, Firenze. Peele, T. C., Latham, E. E., and Beale, 0. W. 1945. S . Carolina Agr. Expt. Stu. Bull. 367. Pierre, J. J., and Wischmeier, \V. H. 1960. County Agent and Vo-Ag Tazcher 16, 28-29, 32.

Rose, C. W. 1960. Soil Sci. 89, 28-35. Rose, C. W. 1961. Soil Sci. 91, 49-54. Rowe, P. B. 1940. U . S . Dept. Agr. Calif. Forest and Range Expt. Sta. M i x . Publ. 1. Sharp, A. L., and Holtan, H. N. 1940. Tram. Am. Geophys. Union 21, 557-570. Smerdon, E. T., and Beasley, R. P. 1959. Missouri Unio. Agr. Expt. Sta. Research Bull. 716. Smith, D. D. 1941. Agr. Eng. 22, 173-175. Smith, D. D. 1956. Agr. Eng. 37, 342-345.

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Smith, D. D., and Coyle, J. J. 1960. In “Power to Produce the Yearbook of Agriculture” (A. Stefferud, ed.), pp. 107-113. U. S. Govt. Printing Office, Washington, D.C. Smith, D. D., and Whitt, D. M. 1947. Soil Sci. SOC. Am. PTOC. 12, 485-490. Smith, D. D., and Whitt, D. M. 1948. Agr. Eng. 29, 394-396. Smith, D. D., and Wischmeier, W. H. 1957. Trans. Am. Geophys. Union 38, 889-896. Smith, D. D., Whitt, D. M., Zingg, A. W., McCall, A. G., and Bell, F. G. 1945. U . S. Dept. Agr. Tech. Bull. 883. Smith, D. D., Whitt, D. M., and Miller, M. F. 1948. Missouri Uniu. Agr. Expt. Sta. Bull. 618. Smith, R. L., Henderson, R. C., and Tippit, 0. J. 1953. Texas Agr. Expt. Sta. Bull. 781. Smith, R. M., Bennett, A. C., and Henderson, R. C. 1959. Soil and Water 8, 7, 17, 19. Soil Conservation Society of America. 1952. J. Soil and Water Conserv. 7, 144-156. Spilhaus, A. F. 1948a. J . Meteorol. 6, 108-110. Spilhaus, A. F. 1948b. 1. Meteorol. 5, 161-164. Stallings, J. H. 1957. “Soil Conservation.” Prentice Hall, Englewood Cliffs, New Jersey. Swanson, N. P. 1960. U . S. Dept. Agr. ARS-41-43,90-102. Swamy Rao, A. A., Hay, R. C., and Bateman, H. P. 1960. Trans. Am. SOC. Agr. Eng. 3, No. 1, pp. 8-10. Taylor, R. E., and Hays, 0. E. 1960. Agron. Abstr. Am. SOC. Agron. p. 41. Tower, H. E., and Gardner, H. H. 1953. U.S. Dept. Agr. Farmers’ Bull. 1981. Uhland, R. E. 1958. J. Soil and Water Conserv. 13, 207-214. U. S. Agricultural Research Service 1961. U. S. Dept. Agr. ARS-22-66. U . S. Soil Conservation Service. 1956. U. S. Dept. Agr. Ready References for Conservation Farm Planning. Van Doren, C. A., and Bartelli, L. J. 1956. Agr. Eng. 37, 335-341. Van Doren, C. A., and Card, L. E. 1950. Illinois Univ. Coll. Agr. Circ. 667. Van Doren, C. A., Stauffer, R. S., and Kidder, E. H. 1950. Soil Sci. Soc. Am. Proc. 15, 413-417. Voznesensky, A. S., and Artsruui, A. B. 1940. Soils and Fertilizers Commonwealth Bur. Soil 10, 289, 1947. Whitaker, F. D., Jamison, V. C., and Thornton, J. F. 1961. Soil Sci. SOC. Am. PTOC.25, 401-403. Wilm, H. G. 1941. Trans. Am. Geophys. Union 22, 678-686. Wilm, H. G. 1943. Trans. Am. Geophys. Union 24, 480-484. Wischmeier, W. H. 1955. Agr. Eng. 36, 664-666. Wischmeier, W. H. 1959. Soil Sci. SOC. Am. Proc. 23, 246-249. Wischmeier, W. H. 1960. Soil Sci. SOC. Am. Proc. 24, 322-326. Wischmeier, W. H., and Smith, D. D. 1958. Trans. Am. Geophys. Union 39, 285291. Wischmeier, W. H., and Smith, D. D. 1961. Trans. Intern. Cong. Soil Sci. 7th Congr. VI, 2. Madison, Wisconsin, 1960. Wischmeier, W. H., Smith, D. D., and Uhland, R. E. 1958. Agr. Eng. 39, 458-462. Wojta, A. J. 1950. Agr. Eng. 31, 227-229, 231, 233. Woodbum, R. 1948. Agr. Eng. 29, 154-156.

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DWIGHT D. SMITH AND WALTER H. WISCHMEIER

Woodburn, R., and Kozachjn, J. 1956. Trans. Am. Geophys. Union 37, 749-753. Woodruff, C. M. 1947. Soil Sci. SOC. Am. Proc. 12, 415-478. Woodward, L. 1943. Trans. Am. Geophys. Union 24, 468-474. Worley, L. D. 1960. Trans. Am. SOC. Agr. Eng. 3, 68-70, 72. Zingg, A. W. 1940. Agr. Eng. 21, 59-64. Zingg, A. W. 1942. Agr. Eng. 23, 93-94. Zingg, A. W., and Hauser, V. I. 1959. Agron. J . 51, 289-292.

SOYBEAN GENETICS AND BREEDING

.

Herbert W Johnson and Richard L . Bernard United States Department of Agriculture. Beltsville. Maryland. and United States Regional Soybean laboratory. Urbana. Illinois

I . Introduction .................................................... A. Taxonomy ......................................... B . Origin and Distribution ............................. I1. Reproduction ................................................... A. The Flower ............................................... B . Flowering and Seed Formation .............................. C . Crossing ................................................... I11. Genetics of Qualitative Characters ................................ A . Pigments ................................................... B. Plant Characters ............................................ C . Disease Resistance .......................................... D. Physiological Traits .......................................... E. Deficiencies ................................................ F. Linkage ................................................... IV . Genetics of Quantitative Characters ................................ A. Linkage of Genes Conditioning Quantitative Characters . . . . . . . . . . . . B. Type of Genetic Variability .................................. C. Heritability of Characters .................................... D . Correlations Among Characters ................................ E . Selection Indices ............................................ F. Miscellaneous Characteristics ................................ V . Breeding ...................................................... A . General Objectives .......................................... B. Considerations in Genotype Evaluation ........................ C . Breeding Methods .......................................... D. Species Hybrids ............................................ E . Induction of Mutations ...................................... F. General Considerations of Soybean Breeding . . . . . . . . . . . . . References .....................................................

.

1

Page 149

152 152 153 153 157 157 166 170 170 171 171 172 172 174 179 185 192 195 199 199 200 203 207 209 218

Introduction

The rapid increase in the acreage devoted to the soybean (Glycine m u x (L.) Merrill) is unique among the current major crops in the United

States. The soybean was considered a hay crop in the early years of its production. and as late as 1940 the acreage harvested for hay (4.8 mil149

150

HERBERT W. JOHNSOX AND RICHARD L. BERNARD

lion) was equal to that harvested for beans. The acreage harvested for hay decreased steadily after 1940 and only about one-half million acres have been harvested for hay annually since 1956. In contrast, the acreage harvested for beans was nearly 14 million in 1950 and nearly 24 million in 1960. An estimated 27 million acres were harvested for beans in 1961. Soybeans are produced primarily for oil and protein. Seeds of varieties produced in the United States average about 21 per cent oil and 40 per cent protein on a dry weight basis. Nearly 90 per cent of the soybean oil used in the United States is in human foods, primarily shortening and margarine, and over 95 per cent of the protein used domestically is in animal feeds. The soybean has been used little as a research tool in genetics and breeding research. However, research workers interested in improving the species for economic use have made substantial contributions to the literature on the genetics and breeding of soybeans. Notable breeding progress has been made in the United States and Canada in the past 25 years and this progress played a major role in the expansion of soybean acreage in the two countries. Breeding progress is becoming increasingly difiicult, however, because of an increased number of breeding objectives, notably resistance to diseases, and because the gross gains in the breeding of the introduced crop for a new production area have been made. Future gains will be more difiicult than those of the past and will require more refined techniques or procedures. The purpose of this review is to summarize and present information on the genetics and breeding of soybeans, particularly information relative to improving breeding procedures. In the attempt to make the review as complete as possible, permission to use unpublished information was obtained from several individuals. This was done when the unpublished information was considered to be especially pertinent in rounding out the available information on a given subject or in a few instances when the information was pertinent to a given subject and would not be published elsewhere. The authors express their sincere thanks to all the individuals contributing unpublished information. A. TAXONOMY The soybean belongs to the family Leguminosae, subfamily Papilionoideae, and the genus Glycinc L. The botanical classification of the cultivated form has been controversial and the multiplicity of names applied to it has created confusion as to its correct designation. However, Ricker and Morse (1948) contend that according to international botanical rules the correct name of the soybean is Glycine max (L.) Merrill, a viewpoint shared by most taxonomists.

SOYBEAN GENETICS AND BREEDING

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The literature on the species situation within the genus Glycine has been a grossly confused issue, and this confusion has complicated the investigation of species in the genus. A recent concentrated attempt to obtain seed of species of Glycine has met with some sucoess and a taxonomic investigation by F. J. Hermann has greatly simplified the species situation. He concluded that in addition to G. inux the genus was made up of the following species: G. clandestina Wendl.; G. falcata Benth.; G. ktrobeana (Meissn.) Benth.; G. tabacina (Labill.) Benth.; G. tomentella Hayata; G. petitiana (A. Rich.) Schweinf.; G. javanica L.; G. ussuriensis Regel & Maack; and a tenth species, G. sericea Benth. not Willd., for which a new name is proposed in Dr. Hermann’s publication. Seven subspecies or varieties are listed for G. javanica and one for G. ckndest5na.l G. max and G. ussuriensis are known to have 40 chromosomes and both behave as diploids. They are cross fertile and their hybrids usually have normal fertility. Ramanathan (1950) listed G. javanica as having 20 pairs of chromosomes, the basic number of the genus being 10. H. L. Weaver (personal communication) observed 20 pairs of chromosomes in some types of G. javanica and 10 in others. He also observed 20 pairs in G. falcata. Limited attempts in the United States to cross some of the species (other than G. ussuriensis) listed above to G. mux have failed. A type referred to frequently in the literature as G. gracilis was considered to belong to G. max in Hermann’s investigation of the genus. B. ORIGINAND DISTRIBUTION The origin of the cultivated form of the soybean is unknown. “The soybean is native to eastern Asia” is a statement frequently transferred from one publication to another. Nagata (1960a) recently reviewed the literature on the subject and considered the distribution of soybeans and other ethnobotanical principles in an interesting study of the origin of the cultivated type. Although he concluded that the origin of soybean culture still remains obscure, his results indicated that the origin was in China proper, especially in north and central China. He based his conclusions in part on the distribution of G. ussuriensis, which he considers to be the progenitor of the cultivated form. According to Morse (1950) there is little doubt that G. max was derived from G. ussuriensis since apparently no other wild plant found can possibly be its ancestor. Nagata postulated that the cultivated form was introduced into Japan ~

1 Information from a manuscript entitled “A Revision of the Genus Glycine and Its Immediate Allies” prepared by F. J. Hermann for publications as U . 5’. Dept. Agr. Tech. Bull. 1268. Sincere appreciation is extended to Dr. Hermann for permission to use the information prior to publication.

152

HERBERT W. JOHNSOS AND RICHARD L. BERNARD

via Korea and presented information to suggest that it was introduced into Korea directly from north China sometime during the period 200 B.C. to the third century. Hamada (1955) described preserved types stored in the Shosoin Treasury since about the seventh century (along with medicinal herbs introduced from China) that resembled the shortseason types currently grown in Kyushu and Loochoo Provinces of Japan. Nagata (1960a) interpreted this to indicate that the short-season types of Japan may have been introduced directly from central China to south Japan. Additional detail on the ancient history of the soybean may be obtained from Morse (1950). According to Bening (1951) the first news of the soybean was brought to the Western Hemisphere in the writings of Engelbert Kaempfer in 1712. Morse (1950) presented a detailed account of the modem history of the soybean and recorded that the first published account of the plant in the United States appeared in 1804. According to him not more than eight varieties of soybeans were grown in the United States prior to the numerous introductions by the U. S. Department of Agriculture beginning in 1898. Introductions from Manchuria, China proper, Korea, and Japan have played a predominant role in the soybean industry in the United States. The early varieties and the germ plasm used in soybean breeding in this country came from these introductions (see Section V, F). II. Reproduction

A. THEFLOWER Soybean flowers are normally about 6 to 7 mm. in length, and their smallness imposes a limitation on the ease with which controlled pollinations can be made. Guard (1931) described the soybean flower as having a tubular calyx terminating in five unequal lobes. The largest of these is anterior, the next two lateral, and the smallest two, obliquely posterior. The calyx is persistent, being intact on the ripe fruit, but rapidly deteriorates and only fragments may be found on pods that have been exposed to weather for a considerable length of time. The corolla consists of five separate petals. The largest (standard) is posterior, the two next in size (wings) lateral, and the two keel petals anterior. There is no fusion of the keel petals as in some other legumes. The ten stamens are separate at first, but shortly before anthesis the filaments of nine of them are elevated as a single structure by the development of a basal region, leaving the posterior stamen separate. Miksche ( 1961 ) recently reviewed the literature on morphological studies with soybeans and presented the results of an interesting study on the developmental anatomy of the plant. He studied organ and tissue

SOYBEAN GENETICS AND BREEDING

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organization from dormant seed to floral initiation. Although this area of work is beyond the scope of this review, Miksche’s results and the literature cited by him are valuable for research workers interested in soybeans. B. FLOWERING AND SEEDFORMATION The time of flowering of soybean plants depends largely on the number of hours of darkness they receive each day. Other factors such as temperature, nutrition, and light intensity and quality may influence the response of soybeans to dark periods suitable for flowering; but in the field the length of the dark period is usually the primary influence in the induction of flowering. Plants of many varieties are completely incapable of flowering unless they receive 10 or more hours of darkness daily and plants of all varieties flower more quickly with daily dark periods of 14 to 16 hours than with shorter ones (Borthwick and Parker, 1939; Parker and Borthwick, 1951). Since the length of the daily dark period is a function of latitude, soybean varieties are adapted as a fullseason crop to narrow belts of latitude. The effect of changes in natural photoperiods on the maturity of soybeans occurs primarily prior to flowering. Rates of development in subsequent stages also are influenced by photoperiod but the periods from about seed set or end of flowering to maturity are similar for all soybean varieties regardless of maturity. Natural photoperiods over a wide range of latitude also are similar during the latter stages of development of the soybean. When the natural photoperiods are manipulated to create substantial differences, differences among varieties in their response to photoperiod in stages of development after flowering are readily observed (Nagata, 1960b; Johnson et al., 1960). This is discussed in greater detail in the review by Cartter and Hartwig in this volume. Soybean plants normally produce many more flowers than pods that finally mature. Shedding of 75 per cent or more of the flowers is not uncommon, and even under the most favorable conditions the loss of a substantial portion of the flowers can be expected. Flower and pod shedding apparently are not due to a lack of viable pollen (Van Schaik and Probst, 1958b) or to lack of fertilization (Kato .et d.,1955). C. CROSSING 1. Natural Soybeans are completely self-fertile and the amount of outcrossing under natural conditions is about 0.5 per cent for plants in adjacent rows and 1 per cent for plants grown in close contact (Weber and Hanson, 1961).

154

HERBERT W. JOHSSOS ASD RICHARD L. BERNARD

Soybean breeders and geneticists have become increasingly concerned in recent years with techniques for increasing the amount of “natural” outcrossing in soybeans. A search for male-sterile, female-fertile types has been unsuccessful, and various other approaches have been investigated. chemical referred to in the literature as FW-450 and two apparently related chemicals ivere evaluated by Casas (1961) as selective gametocicles. Although pollen viability was reduced by the chemicals, the flowers failed to open properly and an actual decrease in outcrossing resulted. Similar results were obtained with F\17-450by Hanson (personal communication ) . Casas obtained 5.2 per cent outcrossing of normal plants in cages containing honey bees compared to only 0.6 per cent for plants outside the cages. \\‘eber and Hanson (1961) obtained a four- to sixfold increase in outcrossing of plants from seed irradiated with different dosages of X-rays and thermal neutrons. Outcrossing of the untreated checks was approximCitely 1 per cent. From a theoretical consideration of the amount of outcrossing required to have practical utility in intermating populations, they concluded that the figure should be higher than the approximately 4 to 6 per cent which they obtained. Athow (personal communication) has observed as much as 16 per cent outcrossing of plants infected with tobacco ringspot virus. A small percentage of the seed from infected plants normally give rise to virusfree normal plants (-4thow and Bancroft, 1959) and the possibility of utilizing the normal plants as pollen parents in a virus-infected population caged with bees is intriguing. The virus-free plants could be used to establish noinial lines after the desired number of generations of in termating. 2. Artificial Crossing soybeans is tedious. The procedure followed in emasculating is standard. but the time of emasculation, collection of pollen, and pollination varies greatly. This variability depcnds to a large extent on environment and to a lesser extent on the personal preference or needs of indib idual workers. The small size and fragileness of soybean flowers make it necessary to use extreme care in emasculating. The only instrument used is a small pair of forceps. The inner surfaces should be flat without corrugations and the spring end should have light tension. Flowers which would normally open the morning following the day the cross is made are used as females. These are in the bud stage with the color of the petals readily visible. The lobes of the calyx are removed by grasping them

SOYBEAN GENETICS AND BREEDING

155

individually with the forceps and pulling downward. The corolla is then grasped with the forceps at a right angle to the axis and removed with a slow pull, working the forceps gently from side to side during the process. The corolla and all the anthers may be removed in one motion; however, the keel and/or anthers often are not removed with the first pull. The keel can be removed easily and the points of the forceps can be used to remove the anthers. However, removal of the anthers in this manner frequently results in injury to the remaining parts of the flower and poor success in crossing. When a genetic marker can be used to distinguish F1 plants from plants of the female parent, most experienced operators make no attempt to remove the anthers with the points of the forceps. Local environmental conditions, including weather and insects, determine the time of day when pollen is collected and crossing is most successful. Generally, in the central and northern parts of the United States, flowers that have opened the day the cross is to be made can be used to furnish pollen and crossing can be done successfully throughout the day. However, in the southern States it is extremely difficult to get viable pollen from open flowers. In this area pollen flowers are collected early in the morning an hour or two before they would have opened and stored in a cool, dry place for use later in the day. A desiccator is usually used to ensure dry storage conditions. Pollinations in mid to late afternoon are usually the most successful. In the southern States the percentage of success from pollinations made in the forenoon is extremely low. When the flowers are ready for pollinating, the tips of the forceps are inserted in the back of the keel of the pollen flowers and the pistil and column of anthers removed. This is used as a brush to deposit pollen on the exposed stigma of the emasculated flowers. Generally three or four pollinations can be made with a single flower. From one to three flowers at a node may be in the right stage for crossing and all other flowers and buds should be removed. The flowers or node should be tagged for identscation of the cross. The flowers should be checked about a week after the pollinations are made and all newly developed flower buds removed. Soybean flowers may continue to develop after a cross has been made and when harvesting the crossed seed it is sometimes difficult to determine whether the pod developed from the emasculated flower or from one that developed later. Pods resulting from a cross can readily be distinguished when small by the absence of the calyx lobes on the base, and this distinction can usually be made when the pods are mature. Some published information indicates that flowers should be emascu-

156

HERBERT W. JOHNSON AND RICHARD L. BERNARD

lated one day and pollinated the next and that a leaf should be pinned around each crossed flower for protection. Most agronomists in the United States use no type of covering or protection for crossed flowers. They emasculate the flowers desired for one cross and pollinate them immediately. Even when the best available techniques are employed by experienced operators, the percentage of successful crosses varies greatly from time to time. Too much or too little moisture, low night temperatures, insects, manipulation of the photoperiod, and various other factors have been observed to influence the success of crosses. The ideal environment for crossing soybeans is unknown, but the success of the crossing program often can be increased greatly by a well-timed application of an insecticide or supplemental irrigation. Although the standard procedure of crossing soybeans has been used for some large undertakings in recent years, much time and effort are required in the actual crossing operation and in obtaining the desired flowering plants over a sufficient length of time. Techniques for storing pollen and means for speeding large-scale crossing operations would therefore facilitate current research programs. Hanson (personal communication ) materially increased the number of pollinations that could be done per day by doing some of the operations in the laboratory in the morning when pollinations are least successful. Flowers were collected at the appropriate time in the morning and the anthers and pistil were separated from the floral parts in the laboratory. Up to 30 anther rings were stored in a 00 gelatin capsule by sticking them around the edges of the capsule. The capsules were stored over a mixture of 25 ml. of concentrated sulfuric acid and 75 ml. of water in a refrigerator to maintain the desired humidity. Kuehl (1961) recently obtained useful data on a number of questions of importance in crossing soybeans: (1) Germination of pollen in a 30 per cent sucrose solution containing 120 p.p.m. of boric acid was found to be a good indicator of the germination or effectiveness of pollen in crosses; ( 2 ) pollen was stored successfully in a calcium chloride desiccator at 3.3"C. for approximately 1 month. Storage at -20" was less successful. The stored flowers were dry and brittle but storage for about 30 minutes over water in a closed container restored moisture to the tissues and induced the anthers to dehisce. ( 3 ) Pollen first became viable about 10 hours prior to natural anthesis; and (4) the stigma of emasculated flowers was most receptive to pollen on the day preceding the morning of normal anthesis and remained receptive for 2 days after anthesis. In a report of a detailed study of the time required for various

SOYBEAN GENETICS AND BREEDING

157

stages of development from flower bud differentiation in the plant to differentiation of the embryo in the seed, Kato et ul. (1954) presented photographs interpreted to indicate fertilization on the day of flowering. Because flowers normally open in the early daylight hours and the exact time of collection of flowers was not given, the results can be interpreted to indicate that fertilization takes place in about 10 hours or less after natural pollination. If this same time sequence prevails in flowers used in crosses 15 to 20 hours before anthesis would have occurred, there would seem to be little likelihood of self-pollination even in nonemasculated flowers if viable pollen is used since the pollen of the crossed flower would not be viable until 5 to 10 hours after the cross was made (Kuehl, 1961). 111. Genetics of Qualitative Characters

Although the soybean has never been the subject for extensive research by geneticists, the mode of inheritance of simply inherited characters has been studied from time to time by agricultural workers interested in the soybean as a crop. Reviews which include the results of much of this work have been published by Owen (1928a), Woodworth (1932, 1933), Matsuura (1933), Morse and Cartter (1937), Weiss (1949), Williams (1950), and Johnson (1961). Genes reported for the soybean and the traits that they influence are given in Table I. The list includes all genes reported except those presented as only tentative suggestions and those apparently based on expressions of previously reported gene pairs. Accordingly, in this review, del (Stewart and Wentz, 1930) is considered to be the same as t; de2 (Woodworth and Williams, 1938) to be p2; f ( Takahashi, 1934) to be nu; h (Ting, 1946) to be t; lo (Domingo, 1945) to be o (oval leaflet); 90 (Stewart, 1930) to be o (reddish-brown seed coat); and y2 (Morse and Cartter, 1937) to be g. Table I includes the gene symbols (where duplicate symbols have been assigned the ones used in the more recent review papers are presented here), a brief descriptive phrase for the contrasting traits, and the major reference( s ) identifying the gene pair and assigning the symbols. A soybean strain carrying the specified gene(s) is listed for each of the more uncommon traits. A few genes such as df, st, and yl have been lost whereas others such as A, BZ, E, e, L, 1, S, s, sh, sp, and w2 are probably present in available germ plasm but have not yet been reidentified.

A. PIGMENTS The variation among soybean varieties in pigmentation of various parts of the plant, especially the seed, has provided the material for

TABLE I of the I’h(wotylw.: and tlw 1iefrrcwc.c.s 13st:il)lishing the h l o d ~of 1nlicrit;inc.c nncl Assigninq thv S ~ m l m l s

‘4 1,ist of Cerics 13t*imrt(dfor thc Soy1)cwi Ilic.111diiig;i &scription

_.____l_._l_

Symbol -.

~ . . I _ . .

A a Ah nh

Bl 4 B , h , or h , o r 12, El hl

c , c,

c , or c2

cs CS

D,

dl Df df Dt

at E e F

f Fe fe

OT

a,

D,

-~

~

..

1)cscription ....

~.

~

.

Ilctfcrcmx ~

Appww’cI p ~ i l w s ~ x * n ~ ~ ~ ~ Erwt p r i l ~ w c . ~ w x 1,c:nf abscission :it matririty Dc4aycd nlwission ( T206)a Bloom on srcd coat ( T 4 ) No bloom Slrarp prilwsccncc: tip Blurit pii1)c~sc.cnc.ctip Cr;ick(d s ( d coat ( T217) Entirc sccd coat 1hist;incc. to frogoy lcafspot Snscepti1)ility Yollow cotyicdons in srecl Green cotylcdons (T38) Normal plant Dwarf plant Indeterminate stem (T10) Determinate stem ( T 6 ) Early maturity Late maturity Normal stem Fasciated stem (T173) Normal iron utilization Inefficient iron utilization (T203)

. .-

-..

..-

.-~ ~- --.

-

~

~---

Kurasn\vn, 1036; Morsr and C:irtt(,r, 1937

l’rolnt, 1950 Woodworth, 10.32, 1933

Ting, 1946 Napii, 19%; hf;itsurira, 1933

a Athow and Probst, 19S2 Woodworth, 1921; Owm, 1 9 2 7 ~ ; Veatch und Woodworth, 1930 Stewart, 1927; Wooclworth, 1932, 1933 Woodworth, 1932, 1933 Owen, 1927b Nagai, 1926; Takngi, 1929; Woodworth, 1932, 1933 Weiss, 1943

‘%

U

TABLE I (Continued) Symbol

Fl

P G g I ii ik

i

K k L

I M m Mi,, Mi,, MiR mi,, mi,, miR N n Na na

No fl0

0 0

Description Brown flecks on black seed coat (T85) Self black seed coat Green seed coat (T164) Yellow seed coat Light hilum color Dark hilum color Dark saddle pattern (T19) Self dark seed coat (T152) Dark color on hilum only Dark saddle pattern (T153) Dark pod Light brown pod Black stripes on brown seed coat (T125) Self brown seed coat Resistance to certain races of downy mildew Susceptibility Normal hilum abscission Lack of abscission at hilum (T25) Broad leaflet Narrow leaflet, high number of seeds per pod (T41) Nodulating plant Nonnodulating plant (T201) Brown seed coat (T25) Reddish-brown seed coat (T27)

Reference Morse and Cartter, 1937 Terao, 1918; Takahashi and Fukuyama, 1919; Nagai, 1921; Woodworth, 1921 Nagai, 1921; Nagai and Saito, 1923; Owen, 1928a; Woodworth, 1932, 1933; Mahmud and Probst, 1953

2E

Nagai and Saito, 1923; Takagi, 1929 Woodworth, 1923 Nagai and Saito, 1923 Geeseman, 1950 Owen, 1928a Takahashi and Fukuyama, 1919; Nagai, 1926; Woodworth, 1932, 1933; Takahashi, 1934; Domingo, 1945 . Williams and Lynch, 1954 Nagai, 1921

w

s

TABLE I (Continued)

Symbol

Description

0

Oviitc leaflet

o

Oval leafiet, few secds per pod (T122) Glabrous plant (T43) Pubescent plant Normal pubescence hlinute pubescence ( T31) Normal plant Pseudo-mosaic (dwarf plant, crinkled leaves, sterilc) (T211) Resistance to Phytophthora rot Susceptibility Black seed Brown seed Susceptibility to cyst nematodc Resistance ( T 6 ) Tall, late plant Short, early plant Peduncnlate inflorescence ( T208 ) Subsessile inflorescence ( T109) Nonshattering pods Shattering pods Shattering pods Nonshattering pods Long, spreading branches Short, erect branches

PI PI

p,

P, Pm Pm

Hdercnce

E

Domitigo, I015

Nagai and Saito, 1923; Stcwart and Wentz, l9ZG Stewart and Wcntz, 1926 Probst, 1950

m

B

3 +

8x

I3crnard st ul., 1957

0"2

Nagai, 1921; Woodworth, 1921; Stcwort, 1930; Williams, 1952

3

CaldwclI et al., 1960

2!

Woodworth, 1923

8

?-

F

r Van Schaik and Probst, 1958a Morse and Cartter, 1937 Nagai, 1926; Morse and Carttcr, 1937 Nagai, 1926; Matsuura, 1933

TABLE I (Continued) Symbol St

st

T t

Description

Reference

Normal seed production Sterility

Owen, 1928b; Woodworth, 1932, 1933

Brown pubescence, black or brown seed coat Gray pubescence, imperfect Mack or bufl seed coat

Nagai, 1921; Woodworth, 1921

Normal chlorophyll Chlorophyll variegation in leaf ( T93)

Woodworth, 1932, 1933

Purple flower White flower

Tikahashi and Fukuyama, 1919; Woodworth, 1923

Pale piirple flower with W, Purplish-red flower with W,

Takahashi and Fukuvama, 1919; Matsuura, 1933

Dark purple flower with W ,

Hartwig and Hinson, 1962

X

Yl Yf

YS Ys

y, y4

8 ti

0

Dilute purple flower with W, Purple flower with W, Very dilute purple or near white flower with W,

X

B

M

Five leaflets per leaf (T143) Three leaflets per leaf

Takahashi and Fukuyama, 1919; Nagai, 1926; Woodworth, 1932, 1933

Normal green plant Greenish-yellow, weak plant

Nagai, 1926; Morse and Cartter, 1937

Normal green plant Green seedling becoming yellow (with g) (T139) Normal green plant Greenish-yellow leaves, weak plant (T102)

Nagai, 1926, Takagi, 1929; Terao and Nakatomi, 1929; Morse and Cartter, 1937 Morse and Cartter, 1937; Woodworth and Williams, 1938 (as y 5 )

3

w

a w

TAB1.E I (Continued)

Symbol

F Q,

Ocscription

Reference

Normd grcrn plant Ycllow-gwn leavra, low vigor ( 1’1 16 ) Norm.11 green plmt Pde grccw plant (T136)

hlorsc: m t l C:irtter, 1037; Woodworth and Williams, 1938 (as y 4 ) Morse and Cartter, 1937; Woodworth and Williams, 1938

Normal grwn plant Yellow-green Icaveq in young plant, becoming grcm (T138) Normal green pliint C r e e n i h - y c h v 1eavc.s ( T135) Norm.11 grwn plmt Yellow-gt ccn recdling heroming green (T161) Normal green pLnt Lethal yellow plant, Y,, Y , ~ is yellow green (T219) Susceptibility to hactcrial pu\tule Resistance ( recessive) Normal plant ( dominant ) Dwarf plant, mutation from colchicinc-treatcd seedling (T210) Yellow cotyledons in seed ( cytoplasmically inherited ) Green cotyledons (T-14) (cytoplasmically inherited)

Morse and Cartter, 1937 (as y H ) ; Williams, 1950

t\3

Bg

+I

Prohst, 1950

9

* 0

m

I’rol)st, I950

3

0

z

Wcbcr

:id

Weiss, 1959

Feaster, 1951; Hartwig and Lehman, 1951

Ei

E

r Porter and Weiss, 1948

m

Terao, 1918; Owen, 1927a; Veatch and Woodworth, 1930

E

5 The T numbers designate soybean strains carrying the specified gene( s ) . These strains are maintained by the U. S. Regional Soybean Laboratory, Urbana, Illinois, and are available to research workers upon request.

N

SOYBEAN GENETICS AND BREEDING

163

much of the qualitative genetic work with soybeans. The genetic basis for most of the common pigment variants is now well established after considerable divergence of opinion among earlier investigators. However, many less common pigment types still await genetic investigation, and the genetic interactions of many of the genes presented in Table I have not been studied. 1. Flower Color The flower color of most soybean varieties is either white or purple. Takahashi and Fukuyama (1919) and Woodworth (1923) established that this difference is due to a single gene pair with purple ( W 1) dominant to white ( w l ) .A pleiotropic effect of this gene pair on seed color is described in Section 111, A, 4. In addition the anthocyanin pigment conditioned by the W 1allele appears in other parts of the plant, notably in the hypocotyl, frequently in the pulvinules, and during ripening in the pod wall, petiole, and stem where exposed to strong sunlight. The degree of this expression varies considerably, but the purple pigment is always absent in plants of the w1 (white flower) genotype. Small variations in intensity and in shade of purple are frequently observed among varieties, and two reports concerning the genetic basis of these variations have been published. Takahashi and Fukuyama (1919) reported a ratio of 9 purple to 3 purplish blue to 4 white. Matsuura (1933) assigned the symbols W zw2 to this effect. Nagai (1926) similarly reported ratios of 9 purple to 3 purplish red to 4 white. It is not possible at present to relate these shades of purple to those of any known soybean varieties and the more recent genetic studies of flower color have not included such distinctions. Bicolored flowers occur in a few varieties, such as LAREDO and TANNER. In these flowers the purple color is restricted to an area near the base of the standard and the remainder of the corolla is white. Hartwig and Hinson (1962) reported that inheritance of this dilute purple type involves two loci (W3 w3 and W4 w4) which influence the intensity of purple. Apparently w3 W4 is the most prevalent genotype, and it produces the typical purple flower color. With 1473 W4 the flowers are dark purple; with W 3 w4 the flowers are dilute purple; and with w3 w4 the flowers are white or nearly white. With w1 the flowers are always white, and the effects of W 3w3and 147, w4 are observable only in W1 genotypes. Since there is no known source of W2 wz, its relationship to the other modifying genes could not be tested.

2. Pubescence Color Pubescence is abundant on the leaf, stem, and pods of most varieties, and its color gives a distinctive appearance to the plant. Except for

lf34

HERBERT W. JOHNSON AND RICHARD L. BERNARD

occasional intermediate types, the pubescence of soybean varieties appears either tawny, due to the presence of a brown pigment in most of the hairs, or gray, due to the absence of the brown pigment from most of the hairs. The brown pigment is not present in the seedling but usually becomes apparent by the time the plant has produced several trifoliolate leaves. Varieties of both color types are commonly grown in the United States. The difference in pubescence odor was found by Woodworth (1921) to be due to a single gene pair with tawny pubescence ( T ) dominant to gray ( t ) .The important effect of this gene pair on seed color is discussed in Section 111, A, 4. A few varieties, such as GRANT, carry the T allele but have an intermediate or light tawny pubescence color, and others, such as KINGWA, have T with a completely gray pubescence color. These types when used in crosses have given rise to anomalous ratios and difficulties in classification. Probst (1950) reviewed the previous reports of unusual segregation for pubescence color and presented additional data indicating that pubescence color is conditioned by several gene pairs. Apparently genes at other loci modify or suppress the effect of T on pubescence color, but no generally acceptable genetic hypothesis for this has been published. 3. Pod Color Three main pod colors are found in soybeans (Nagai, 1926; Williams, 1950), black, brown, and light straw-yellow. The color of the pubescence on the pod influences its general appearance but the actual pod wall color is not greatly affected. Takahashi and Fukuyama (1919), apparently working with brown versus light pod color, reported that segregation could be explained by a single gene pair with the darker color dominant. F’1700dworth (1923) reported dark pods dominant to light and assigned the symbols L 1. Apparently Woodworth‘s “dark brown” pod was the type here designated black. Most commercial varieties in the United States have the brown pod color but quite a few, such as ADAMS, WABASH, DORMAN, and LEE, have light pods. None of the varieties presently grown is black podded. 4. Seed Color

Soybeans eshihit a great variety of colors and patterns in the seed. Common seed colors are yellow, green (presumably chlorophyll), black (intense anthocyanin), and several shades of brown. Various patterns of black .or brown may occur on yellow or green seed coats. The differences among most varieties in the black and brown colors

SOYBEAN GENETICS AND BREEDING

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are explainable in terms of two loci, T t and R T , with modification of certain gene combinations by 0 o and W1 w1. The differences in the pattern or degree of development of these pigments are controlled by the allelic series 1, ii, ik,i, with dominance in the order listed. The seed colors produced by combinations of these genes are presented in Table 11, based on the results published by Nagai (192l), Woodworth (1921), Nagai and Saito (1923), Owen (1928a), Stewart (1930), Will'lams ( 1952), and Mahmud and Probst ( 1953). TABLE I1 The Genetic Basis of the Dark-Colored Pigments in Soybean Seeds Genotype

TR TrO Tro tRW, tRw, tr

I

i

Phenotype

Seed color Black (with brown pubescence) Brown (with brown pubescence) Reddish brown (with brown pubescence) Imperfect black (with gray pubescence and purple flowers) Buff (with gray pubescence and white flowers) Buff (with gray pubescence) Seed color pattern Light hilum-dark pigment reduced in intensity, to a gray hilum (with black genotypes) or completely absent (with brown or buff genotypes) Dark hilum-dark color present in hilum only S a d d l d a r k color present in a saddle-shaped pattern extending from the hilum over about half of the seed coat Self-dark color present over entire seed coat

With genotypes I or ii the seed coat still may develop considerable black or brown pigment. This is called mottling and usually appears in irregular patterns varying greatly from seed to seed. The color of the mottling is determined by the same genes that control seed color as given in the top half of Table 11. Mottling was studied by Woodworth and Cole (1924) and Owen (1927c), and its development was found to be much greater under certain environments and to be more common with certain varieties, but the specific environmental and genetic factors were not ascertained. Three other genes which affect the black and brown pigment in seeds have been reported. The gene k (Nagai and Saito, 1923; Takagi, 1929) produces a saddle pattern similar to that of ik.The gene M (Nagai and Saito, 1923) produces black bands on a brown seed coat. The gene FZ (Morse and Cartter, 1937) produces brown flecks on a black seed coat.

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HERBERT W. JOHSSOS A N I RICHARD L. BERNARD

In all commercial varieties in the United States, except a few blackor brown-seeded hay varieties, the dark pigment is restricted to the hilum or completely absent, and these varieties have been selected for low tendency to mottle. Almost all hilum colors listed in Table I1 are represented in commercial soybean varieties in the United States. -Among the more common varieties, CHIPPEWA, CLARK, SHELBY, FORD, and LEE have black hilums ( 2 T R ) ; HILL has a brown hilum (i' T T ) ; OGDEN and HAWKEYE have imperfect black hilums (ii t R Wl); and ADAMS and BLACKHAWK have buff hilums of the ii t R w1 genotype while L L N D A ~ , also with a buff hilum, is of the i* t T genotype, Among the light-colored hilum types RICHLASD with a gray hilum is 1 t R WI, EARLYANA with a V ~ O W hilum is 1 T I, and XQXMRIX and HAROSOY with yellow hilums are 1 t 1'. \Yhen the black and bron-n pigments do not occur in part or all of the seed coat, its color will be either light yellow or light green depending on the gene pair G g. Terao (1918) first showed that green seed coat ( G ) was dominant to yellow ( g ) and was monogenic. Most grain varieties are yellow-seeded, but a few such as OCDEN carry G and have a light green seed coat. This latter type is considered undesirable for certain commercial uses of the soybean. The cowledons of the soybean are green before maturity but turn yellow in most varieties as the plants mature. However, the cotyledons of a fen. strains remain green in the ripe seed. The seed coat also remains green, and the leaves, stems, and pods do not turn yellow on ripening as the\. do in most soybean varieties. The work of Terao (1918), Woodworth. (1921), and Owen (1997a) showed that two types of green cotyledons occur, one due to the complementary action of two recessive genes ( d l d 2 ) and one controlled by cytoplasmic factors. The green cotyledons give some soybean products an undesirable appearance, and soybeans of this type are not grown as grain varieties in the United States.

B. PLANTCHARACTERS 1. Pubescence Type Considerable variation exists among soybeans in the amount, size, orientation, and distribution of the pubescence, and several types are distinct enough to permit genetic analysis. Most varieties in the United States have erect pubescence; but a few varieties grown in southern United States such as CNS (Grabe, 1957) and a large number of Asian varieties, especially those from Japan, have appressed pubescence. In this type the hairs lie nearly flat against the leaf blade surface except for a few erect ones over the major veins.

SOYBEAN GENETICS AND BREEDING

167

On the rest of the plant the hairs are only slightly less erect than normal. Appressed pubescence has been reported to be due to a single dominant gene ( A ) by Karasawa (1936), working with an appressed strain of wild soybeans, and by Ting (1946), working with an appressed variety of cultivated soybeans. A number of varieties grown in Japan are glabrous. Nagai and Saito (1923), working with an apparent mutation to this type, reported that inheritance was controlled by a single gene pair (PI pl) with glabrousness dominant. This type of inheritance was also found by Stewart and Wentz (1926) working with a glabrous Japanese variety. In addition, Stewart and Wentz reported a recessive mutant ( p z ) which produced glabrous plants. However, this type might better be described as puberulent or minutely pubescent as it has many short, stubby hairs. The glabrous type has been reported as resistant to a pod borer (Laspeyresia glycinivorella) (Williams, 1950). Glabrous varieties have never been grown commercially in this country, and they are very susceptible to attack by the potato leafhopper (Empoasca fabae). In another distinctive type, the pubescence is flat and curled and is deciduous at maturity. Williams (1950) stated that inheritance of this type (referred to as “appressed) was monogenic, the heterozygote being intermediate in appearance. This type occurs in a number of varieties from Japan and Korea. In parts of the United States such varieties suffer severely from feeding by the potato leafhopper. Ting (1946) found that the inheritance of the shape of the pubescence tip was monogenic with sharp (BZ)dominant to blunt ( b l ) .Grabe (1957) reported that most United States varieties have a blunt pubescence tip but that a few, such as KINGWA, LAREDO, and MIDWEST, have a sharp tip. 2. Leaf Shape A few varieties from Japan have a high proportion of leaves with five leaflets. The extra leaflets occur at the base of the lateral leaflets. Takahashi and Fukuyama (1919) reported that the fiveleaflet condition is determined by a single dominant gene (X). A number of varieties from Asia have a much narrower and longer leaflet than normal. In crosses, Takahashi and Fukuyama (1919) found Fz ratios of 1 broad leaflet to 2 intermediate to 1 narrow leaflet, and Woodworth (1932, 1933) assigned the gene symbols N a nu. A high number of seeds per pod seems to be associated with the narrow leaflet trait, and frequently many four-seeded pods occur on narrow-leafleted plants. Takahashi (1934) assumed that the leaf effect and the pod effect were due to different genes and presented evidence of their close linkage.

168

HERBERT W. JOHNSON AND RICHARD L. BERNARD

However, he did not attempt to prove the existence of two gene pairs, and high number of seeds per pod and narrow leaflet probably are pleiotropic effects of the same gene. A unique type with oval leaflets has been found. The trait was reported to be a monogenic recessive by Domingo (1945) and the gene was designated o (not the same as the o controlling reddish brown seed coat color). Here also the leaf shape character was associated with the number of seeds per pod, this time with a low number of seeds per pod. Domingo presented evidence of close linkage between the leaf and pod traits but did not consider whether the traits were due to the same gene. Therefore, these leaf and pod effects should be conconsidered to be due to the pleiotropic action of one gene pair unless future evidence should prove the existence of two separable gene pairs.

3. Stem Type and n4aturity Varieties which have a determinate, or abruptly terminating, mainstem are common in southern United States ( HILL, HOOD, LEE, and OGDEN, for example) and in Japan, whereas varieties from Manchuria and northern United States usually have an indeterminate, or tapering, mainstem. \Voodworth (1932, 1933) reported that this difference in a cross between the determinate variety PEKING and the indeterminate variety ILLINI was controlled by a single gene pair, Dt dt, with the indeterminate type dominant. Another trait appearing in many varieties grown in southem United States is that of having a long inflorescence stalk or peduncle, whereas pods of most northern varieties are sessile or nearly so. Van Schaik and Probst (1958a) reported that this trait is controlled by a major gene, Se, dominant to subsessile, se. They also concluded that peduncle length was much affected by modifying genes and by environment. Nagai (1926) reported that a difference in branching type, long, spreading branches versus short, erect branches, was due to a single gene difference with long branches dominant. Matsuura ( 1933) assigned the gene symbols, S p sp, to this. An abnormal type of soybean with fasciated stems is occasionally grown in Japan. Nagai (1936) and Takagi (1929) reported that fasciation is due to a single recessive gene, which was later designated f by T!’oodworth ( 1932, 1933). Woodworth (1923) found a population segregating for two fairly distinct types. A tall, luxuriant, and late-maturing type was reported to be conditioned by a dominant gene ( S ) and a short, compact, and early type by the recessive allele ( 8 ) . Owen (192%) found that maturity tended to be linked with pubes-

SOYBEAN GENETICS AND BREEDING

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cence color and assigned the symbols E e to this maturity factor. The possibility exists that this is the same as Woodworth's S s factor pair. Singh and Anderson (1949) studied the maturity of segregating progenies from several crosses and found evidence for a few major genes and a number of minor ones. They reported evidence for the dominance of earliness in some crosses and for the dominance of lateness or the absence of dominance in others.

4. Other Plant and Seed Traits A few varieties retain the leaves on the plant for a longer period than normal, and the leaves become dead and dry before falling. This delayed abscission is considered to be a desirable trait in a hay variety as it may reduce the loss of leafy material. The hay varieties KINGWA and WISCONSIN BLACK exhibit this trait. Probst (1950) found that delayed abscission was due to a single recessive gene which he designated ub. Soybean varieties differ widely in the tendency for their pods to shatter. Most currently grown varieties have been selected for a high degree of shattering resistance. Two factor pairs have been reported to have a major effect on shattering. Nagai (1926) reported the shattering (Sh,) of wild soybeans to be dominant to the nonshattering (&) of KURADAIZU, a Japanese variety. Morse and Cartter (1937) reported that the nonshattering (Sh,) of P.I. 22876 was dominant to the shattering (Sh,) Of MEDIUM GREEN. A single recessive gene ( n ) was reported by Owen (1928a) to cause a failure in development of the abscission layer in the hilum. In this type, part of the seed stalk remains attached to the seed when the seeds are separated from the pods. Seeds with this appearance have been found frequently in varieties received from Europe but are otherwise of rare occurrence. The wild soybean and a few black- or brown-seeded hay varieties have a thick layer of bloom on the seed coat. Woodworth (1932, 1933) reported that three dominant genes ( B1Bz B 3 ) were necessary for the development of this trait. A few black- or brown-seeded soybean varieties have a characteristic pattern of checking or cracking of the outer seed coat on the sides of the seed. This is quite distinct from the occasional seed coat cracking which occurs under adverse environment or the cracking which almost always occurs on self buff or imperfect black seed coats. Nagai (1926) reported that this cracking is due to two complementary genes, C1 and C,. These are probably the same genes reported by Liu (1949) as de3 de4. The reversal of dominance may be due to different classification standards or different growing conditions.

170

HERBERT W. JOHNSON Ah?) RICHARD L. BERNARD

c.

DISEASE ~SISTAVCE

Differences in variety reaction to many soybean diseases are known to euist. Hen-ever, the genetic basis of resistance versus susceptibility has been reported for only five diseases. The reaction of segregating populations of soybeans to three races of the organism causing downy mildew ( Peronosporn manslzurica) was studied by Geeseman ( 1950), and a system of three gene pairs (Mi,mi,, Aii, mi2, and Mi, mi,) was proposed to explain the rather complex results which he obtained. The inheritance of the resistance to bacterial pustule leafspot (Xanthomonos phnseoli var. sojensis) found in the CKS variety was reported to be due to a single recessive gene by Hartwig and Lehman (1951) and Feaster (1951). This gene has been used widely in breeding programs and is now present in most commercial varieties grown in southern United States, where it is of considerable economic value in preventing yield losses from bacterial pustule. Atliow and Probst (1952) and Probst and Athow (1958) found that a single dominant gene (Cs) produced resistance to frogeye leafspot ( Cercospora sojina ). This gene also bas been used extensively in breeding. Varieties with a high degree of resistance to Phytophthora rot (Phtjfophthora mcgaspermu var. sojne ) are rather common and include BLACKHAWK, NUKDES, ILLLKI, and ARKSOY. Most commercial varieties are susceptible and range from those highly susceptible to those with a high level of field tolerance. Bernard et al. (1957) and Smith and Schmitthenner (1959) reported that the high level of resistance is due to a single major dominant gene ( P s ) . The resistance to the soybean cyst nematode (Heterodera glycitzcs) of the PEKLUC variety was reported by Caldwell et al. (1960) to be due to the complementary action of three recessive genes (rhg, r h g rhg3). D. PHYSIOLOGICAL TRAITS

A mutant soybean resistant to infection by the nodule-forming bacteria (Rliizobiunz japonicum) was found by Williams and Lynch (1954). They reported that failure to nodulate was inherited as a simple recessive, which they designated no. A strain which became very chlorotic when the level of available iron was moderately reduced was found by Weiss (1943). This trait was found to be due to a singIe recessive gene, which Weiss designated fe.

SOYBEAN GENETICS AND BREEDING

171

E. DEFICIENCIES 1. Chlorophyll Deficiencies A series of eleven different loci affecting chlorophyll development have been reported and assigned the symbols yl to yll. The genes yl, y4, y5, tJ6, ys, and ylo are all recessive, apparently mutant types which produce chlorophyll deficiencies in varying degrees and patterns and at various stages in the plant’s life cycle. These deficiencies affect plant vigor in varying degrees, but none causes death or sterility. The chlorophyll deficiencies described by Nagai ( 1926), Takagi (1929), and Terao and Nakatomi (1929) as appearing in 1/16 of the plants in segregating populations are apparently all the same. Seedlings appear normal, but the plants become more and more yellow as they grow. Morse and Cartter (1937) assigned the symbols y2 y3 to this type, but since Terao and Nakatomi (1929) furnished evidence that g was one of the two genes involved, the symbol g is used in place of y2 in Table I. Apparently y3 is a fairly common gene in green-seeded varieties since it has been reported frequently. One other instance of a chlorophyll deficiency due to two complementary genes (y7y8) was reported by Williams (1950). In this case the seedlings were chlorophyll deficient but the plants became green at an early stage. The origin of this type is unknown. The mutant gene yll reported by Weber and Weiss (1959) is lethal in the homozygous state, but the heterozygote produces a yellow-green plant. Thus with one locus three distinct seedling phenotypes are produced in a 1:2:1 ratio. The viable types, Yll Yll and Yll yll, continue to be distinguishable until the plants mature. A mutant gene (vl) was reported by Woodworth (1932, 1933) to produce a variegated pattern (very light green and green) on the leaf. The variegated pattern is most apparent at about the third trifoliolate leaf stage and decreases in intensity thereafter.

2. Dwarfs and Sterilcs Three recessive gene mutations have been reported in this category. The gene df reported by Stewart (1927) produced a very small lightgreen plant with few pods, and the gene st reported by Owen (192813) produced a sterile but normal-sized plant. The gene prn reported by Probst (1950) produces a very small sterile plant with crinkled leaves.

F. LINKAGE The linkage relationships that have been established for soybeans are listed in Table 111, together with the source of the data. Linkage

172

HERBERT W. JOHXSOS AZ;D RICHARD L. BERNARD

group 4 was assigned by W o o d ~ o r t h and Williams (1938) to an apparent linkage between minute pubescence ( p 2 ) and defective seed coat, but more recent evidence indicates the existence of only a single gene with pleiotropic effects (Williams, personal communication). LinkTABLE I11 Linkage Relationships in Soybeans Linkage group number Genes linked 1 2 3 .3

3 3 5

T t and E e

P, p , and bf m PI p 1 and R r D , d, and G g D , d, and G g D , d, and G g Dt dt and L 1

Crossover percentage

Reference

6 18 12 13 13 13 36

Owen, 1927b Nagai and Saito, 1923 Owen, 1927b Woodworth, 1921 Owen, 1927a Woodworth and Williams, 1938 Ting, 1946

ages were reported for T t and D2 (1, and for M m and 0 o by Woodworth and Williams (1938), but additional data indicate that in both cases the genes are independent ( Weiss, personal communication ) , IV. Genetics of Quantitative Characters

An understanding of the genetic variability in important characters of a species is necessary to the intelligent choice and efficient use of breeding procedures. Recent research on the soybean has been directed toward characterizing the kind and amount of genetic variability in important characters. A. LINKAGE OF GENESCO~~ITIOSIKG QUANTITATIVE C H A R A ~ S Little, if anything, is known about the distribution of genes condition-

ing quantitative characters on the chromosomes of the soybean. If, however, the genes conditioning a character are assumed to be distributed randomly among 20 pairs of chromosomes, one might reasonably question whether the genetic variability arising from intrachromosomal recombination contributes appreciably to total genetic variability. The limitation to recombination of genes on a given chromosome imposed by linkage is not in question; but rather, do the number of genes conditioning a given character and the distribution of the genes over many chromosomes indicate that interchromosomal recombination would be the overriding consideration? Hanson ( personal communication ) has recently shown that for a species such as soybeans the average recombination per total map length is essentially inversely proportional to the number of chromosomes. He concluded that for soybeans the effect of linkage on estimates

SOYBEAN GENETICS AND BREEDING

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of genetic variance was negligible but that it could be appreciable for species with few chromosomes, especially in the presence of extreme linkage disequilibrium. It should be noted, however, that this conclusion does not necessarily follow for specific combinations. Estimates of genetic variance are based on averages, and even if they are little affected by linkage, obtaining a desired or necessary specific recombination may be made very difficult by linkage. Hanson (1959a) demonstrated that the undisturbed gene blocks following a meiotic division are of considerable size and pointed out that such blocks are discouragingly large when one crosses two genotypes with the objective of recombining the desirable genes from each source. In a breeding program most crosses are made to obtain recombinations of the genes contributed by both parents and linkage is obviously a limitation to complete recombination. The usual procedure of crossing two genotypes of a self-fertilized species such as soybeans and the selection of progenies at various stages of s e h g insures that the limitations to genetic recombination imposed by linkage will be maximum. In a theoretical consideration of the effects of intermating on the breakup of linkage blocks, Hanson ( 1 9 5 9 ~ demonstrated ) that the principal breakup can be expected in the first four or five intermatings. He also considered the effect of mating system on the breakup of initial linkage blocks (Hanson, 1959d) and demonstrated that the size of blocks at a given stage of intermating is reduced as the number of original parental types is increased. He concluded that a breeding program for a selfpollinated species should include at least one generation of intermating, or preferably three or four, and that at least four parents should be used to synthesize the intermating population. Hanson’s work (1959a, b, c, d ) has clearly shown that procedures employed in the conduct of quantitative genetics and breeding research in soybeans have provided little opportunity for the breakup of parental linkage blocks. Gates et al. (1960) employed variances and covariances among the Fz to F7generation progenies of the soybean cross ADAMS X HAWKEYE to detect and estimate linkages affecting eight quantitative characters. They interpreted their results to indicate that linkage was of sigpificant importance for flowering time, height, and yield but not for maturity, time of flowering to maturity, seed weight, oil percentage, or lodging. Linkage in components related in form to additive genetic variances was found in all three characters whereas linkage related in form to dominance variance was demonstrated only for plant height. However, their conclusions could have been influenced by the seasonal pattern unique to the study and the observed generation and year effects were confounded. The data on the limitations imposed by linkage in soybeans are not

174

HERBERT W. JOHNSOS All?) RICHARD L. BERNARD

in complete agreement. The existing data are, however, pertinent to the selection of a breeding procedure. Regardless of whether linkage is a minor or a major limitation, the intermating of a population involving more than two original parent genotypes, as suggested by Hanson (1959d), has merit and appears to be a fruitful approach to soybean breeding which is being explored. Using more than two parental types in the cross would obviously increase the diversity of types in the offspring, and intermating one or more generations before homozygous lines are extracted would increase the opportunity for chromosomal recombinations and the breakup of parental linkage blocks. Regardless of whether or not such a breakup is a necessary part of the system, its advantages could certainly be expected to be greater than its disadvantages. The importance of carefully choosing the crosses to be used, evaluating large populations, and streamlining evaluation procedures in such a system is obvious.

B. TYPEOF GESTTICVARIABILITY 1 . Data from the F 1 Generation of Crosses The performance of F1hybrids in comparison with that of the parents provides the first opportunity in the sequence of events in hybrid populations to obtain information on gene action. The additive effect of genes is so defined that the superiority of an F1 over the high parent must be due to some type of gene action other than additive. In view of the complex interrelations among characters, and especially between maturity and other characters in soybeans, this may be an oversimplification. Nevertheless, superiority of the F1 over the high parent is usually interpreted to indicate some type of gene action in addition to additive. Data on F, and parent performance in soybeans are not extensive. \Yentz and Stewart (1924), Veatch (1930), and Woodworth (1933) obtained considerable superiority of the F1 over the high parent in seed yield and plant height; however, only occasional hybrid vigor for seed size was observed. Eleven of the fifteen Fl’s studied by Veatch flowered as late as the late parent or later. tVeiss et al. (1947) evaluated 17 crosses in the greenhouse and in the field. The yield of 10 and 9 of the F 1s ’ was significantly higher than that of their high-yielding parent in the greenhouse and field, respectively. There was, however, distinct disagreement between the performances of the F,’s under the two conditions. Yields of the Fl’s averaged 32 and 14 per cent above that of their high parent in the greenhouse and field, respectively. Maturity of the F,’s was intermediate between the maturities of parents. Kalton (1948) compared four Fl’s with their respective parents in each of two years and observed heterosis in all four crosses in one year and in two crosses in the

SOYBEAN GENETICS AND BREEDING

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other year. The Fl’s varied in maturity and height from intermediate between the two parents to later and taller than the latest and tallest parent. Weber (personal communication ) has evaluated numerous Fl’s in comparison with their respective parents. Seed yield, the only character exhibiting heterosis, has averaged 14 per cent higher for the Fl’s than for the high parent and ranged from 39 per cent below to 90 per cent above the high parent. Leffel and Weiss (1958) studied all 45 possible Fl’s from diallel crosses among 10 varieties of soybeans and evaluated the relationships between parents and F;s by conventional analysis of variance and by combining-ability and diallel-crosses analyses. They concluded that all analyses were similar in detecting lines giving unexpected performance in the F1 generation. Their results indicated mean complete dominance to overdominance for yield and height, mean partial dominance for oil content and seed size, and no or slight partial dominance for flowering, maturity, flowering to maturity, protein content, and seed quality. They pointed out, however, that no analysis used could be considered conclusive in distinguishing between allelic and nonallelic interactions. Yield and height were the only characters studied by Leffel and Weiss (1958) which exhibited a considerable degree of heterosis (superiority of F1 over high parent). Lines designated 1, 2, and 3 were the earliest, shortest, and lowest-yielding of the 10 parental lines. Although no significant heterosis for maturity was observed, lines 1, 2, and 3 were involved in 13 of the 14 Fl’s exhibiting significant heterosis for yield and in 7 of the 10 F;s exhibiting significant heterosis for height. The highestyielding line was not involved in any of the Fl’s exhibiting heterosis for yield, and the tallest line was involved in only one F1 exhibiting heterosis for height. Estimates of general combining ability were low for the three lowest-yielding lines and those for specific combining ability were high. Conversely, for the two highest-yielding lines estimates were high for general and low for specific combining ability. Although the maturity of Fl’s is in general intermediate between maturities of the parents, maturity appears to be involved in the expression of heterosis for yield and height. All three characters are positively intercorrelated and complications due to variability in maturity are especially difficult in genetics and breeding studies conducted under environmental conditions favoring the performance of either early or late segregates. Environmental conditions could and probably do at times favor the performance of the intermediate F1 from an early x late cross, especially if the spread in maturity between the two parents is appreciable.

176

HERBERT W. JOKVSON AND RICHARD L. BERNARD

F1 soybean seeds are not easily obtainable, and all the studies of F1 performance previously discussed involved spaced plantings in one environment. Brim and Cockerham (1961) recently conducted a unique experiment as far as F1 performance is concerned. They evaluated the F;s from two crosses in drilled seedings at two locations over two years. The mean Fl performance of both crosses was significantly higher than the mean of the parents in maturity (lateness), unthreshed weight (total weight of plants just prior to threshing), seed weight, and yield, and for one of the crosses in height. Fl performance was significantly greater than that of the high parent for yield, height, and unthreshed weight in one population and for yield only in the other. It is interesting that the significant deviation of the F1 performance from the mean of the parents in maturity and height was maintained through the F5 generation of one cross and for maturity through the F, generation of the other. F1 performance in soybean crosses deviates from that of the mean of the parents or the high parent with sufficient consistence, especially in height and yield, that some type of gene action in addition to strict additivity seems to be indicated. However, in addition to the possible effects of maturity, three genes-S s, Dt dt, and Se se (see Section 111)are known to affect height. The S s gene apparently acts through its effect on maturity. The three genes are known to interact in certain ways and other apparent interactions have not been worked out genetically. Also, the general absence of heterosis in characters whose expression is relatively uninfluenced by maturity (seed size, oil content, and protein content) would seem to indicate further that maturity may be a factor in the observations of heterosis in height and yield reported to date. 2. Data from Segregating Generations of Crosses Horner and Weber (1956) utilized the data on maturity from the Fz through F7 generations of a soybean cross ( ADAMS X HAWKEYE) in evaluating the relationship of sample variance and covariance components to genotypic variances and covariances. They found that a completely additive model explained 96 per cent of the variation among sample covariance and variance components. Gates et al. (1960) observed additive genetic variance for seven additional characters ( flowering time, period from flowering to maturity, height, lodging, seed yield, seed weight, and oil percentage) of the same genetic materials and pointed out that evidence is mounting for the prevalence of additive genetic variance in quantitatively inherited characters in soybeans. The 45 diallel crosses reported by Leffel and Weiss (1958) were evaluated in the F, and F3 generations by Leffel and Hanson (1961). Both generations were evaluated in bulk at two or three locations in two

SOYBEAN GENETICS AND BREEDING

177

years and the F3 generation was further evaluated on the basis of the performance of the progeny of 80 randomly selected F2 plants (Fa lines). The F3 lines were evaluated at two locations in two years with a subsample of 20 lines from each cross being evaluated in each locationyear combination. Average effects of parents in crosses were especially prominent for seed yield, seed size, and maturity while specific effects were relatively large for maturity and plant height only. In the diallelcrosses analysis, a significant degree of dominance was indicated for seed yield, size, and quality and plant maturity, height, and lodging, with the estimate for the dominance component of variance frequently exceeding that for the additive component. Significant nonallelic interactions were indicated for oil and protein percentages. The authors suggested that unequal contributions of individuals in the heterogeneous populations or biases in estimating maturity, height, or lodging may have inflated the estimates of dominance. Two features of the data of Leffel and Hanson (1961) should be noted. First, the specific effects for maturity and height in the F3 lines traced to crosses that were not identified by the analyses of the F1 data (Leffel and Weiss, 1958). Secondly, maturity was significantly correlated with height in all generations and with yield in all except the F1. Brim and Cockerham (1961) evaluated at two locations in two years the F1 through F5 generations of two soybean crosses and progenies resulting from the crossing of randomly selected pairs of F3 lines within each cross. They obtained 10 estimates of progeny components of variance and covariance for each of 9 characters (fruiting period, maturity, height, lodging resistance, unthreshed weight, seed weight in grams per 200 seed, yield, percentage of protein, and percentage of oil) and fitted the estimates to 6 different genetic models involving additive, dominance, and additive X additive epistatic effects individually and in combinations. Their results were similar to what would be expected if there were no dominance or additive X additive epistatic variance, and they concluded that additive variance was the principal component of variance for the nine characters. They pointed out that when genetic variance is all additive mean performance of the population should be the same in various generations of inbreeding. However, mean performance in some of the characters did vary with generation, in some instances increasing with the advance of generations and in others decreasing. The changes were neither large nor consistent, but they do provide some indication of nonadditive gene action. Hanson and Weber (1961) listed a number of difficulties involved in the partitioning of genetic variability of self-pollinated species and outlined a procedure based on the performance of homozygous lines.

178

HERBERT W. JOHXSON A X D RICHARD L. BERNARD

Utilizing the same source material as Horner and Weber (1956) and Gates et 01. (1960), they obtained estimates of significant additive components for maturity, height, seed weight, percentage of oil (0.10 level), and lodging, characters that breeders have found to be quite responsive to selection in comparison with yield. A significant estimate of the additive x additive component for per cent oil and an indication that epistatic \miability could be important in yield were obtained. They listed what appear to be four sound arguments for their homozygous-line analyses: (11 the approach extends the concept for the partition of the total genetic variability to the analysis of the genetic variability of selfed progenies and permits a generalization for the partition of genetic variability without restricting gene frequency to 0.5; ( 2 ) the genetic parameters reflect the genetic variability characteristic of self-pollinated species; ( 3 ) locus contribution is described in terms of the effect of substituting the liomozygous pair of alleles into a population which yields estimates of genetic effects and variances having biological interpretation: ( 4) the partition requires the use of advanced single-plant progenies which are not subject to within-plot competition, a condition which clouds the interpretation of the majority of quantitative genetic data available in self-pollinated species. The available estimates of the type of genetic variability in soybeans obviously are not in good agreement. The differences could be due to differences in analytical methods, genetic materials, environments, sampling proceclures, and combinations of these and other factors. The situation is accurately summarized by Hanson and Weber (1961) as follows: “Only when adequate information on the genetic variability for soybean characters has been accumulated can the final conclusions concerning thc partition of genetic variability be made.” For the present, however, the importance of additive genetic variance in soybeans appears to be well established, the main question at issue being the relative importance of nonadclitive effects. Further work in characterizing the type of gene action in soybeans is needed before conclusions can be final. However, the data on additive gene action indicate that at least one cycle of combining or intermating selections from crosses 11ould be a fruitful substitute for intensive selection among progenies of the initial cross. Such recombinations should increase the chances of accumulating genes with additive effects in the desired selections. Utilization of a wider genetic base in crosses also seems to be indicated by the suggestion that epistatic variance may bc important in some instances and by the need for utilizing greater genetic diversity even if gene action is primarily additive. The frequent expressions of heterosis in the F1 generation and some estimates of a sig-

SOYBEAN GENETICS AND BREEDING

179

nificant amount of dominance variance suggest that dominance in some instances may complicate the evaluation of early-generation segregates.

C. HERITABILITY OF CHARACTERS Heritability has been variously defined, but the statistics reported as heritability in soybeans have been based on the ratio of an estimate of genetic to phenotypic variance or on parent-offspring regressions. Such estimates were made from data on various generations of segregating populations and under varying environmental conditions. A meaningful comparison of the published estimates therefore involves a careful study of the materials and methods employed. Hanson (1962) recently considered various aspects of heritability in detail and discussed the difficulties resulting from differences in mode of reproduction and selection units employed in applying to plants the concept of heritability originally developed for animals. Sound reasons for redefining heritability were presented, and the following definition was given: “Heritability is defined as the fraction of the phenotypic variability for a defined reference unit expected to be transmitted to the progeny (or propagules), or in terms of selection concepts, the fraction of the selection differential expected to be gained when selection is practiced on a defined reference unit.” To be meaningful, heritability statements based on this definition must still be accompanied by a statement of the material and selection unit upon which the estimates were based. However, such a statement would create little difficulty for breeders working on a given crop. For example, most soybean breeders would accept and understand two replications within two environments as a standard reference basis for selection in soybean work. Some of the published estimates of heritability in soybeans are summarized in Table IV. Selection units, method of analysis, and source of data are indicated in the table and only a brief description of the various procedures followed will be given. Estimation of heritability of F2 plant differences consisted of equating variances among plants of the Fl’s and/or parents to environmental variance and subtracting it from variance among F2 plants to obtain estimates of genotypic variance. Heritability was estimated as the ratio of genotypic variance to Fz variance. In estimating heritability of F2 plant differences by the regression method the F3 lines were evaluated in replicated tests at the same location at which the F2 plants had been evaluated the preceding year. The procedures followed in estimating heritability of differences in plot means varied greatly. Mahmud and Kramer’s (1951) regression estimates involved the evaluation of the F3 and F4 generations in sub-

TABLE IV Estimates of Heritability in Per Cent Selection unit, method, and authority

-

Character Yic4d

Hright

Maturity

Days fl. to Time of maturity flowering

Sced wt.

I ,otlS;lng

%

m

Oil

Protcin

-

-

F , plants Variances Cross 1 Cross 2 Cross 3 Varianceb Variance6 Average 10 crosses Range Varianced Average 3 crosses Rangc Regression F, on F20 Regression F, on F,e Regression F, on F,f Cross 4 Cross 5 Cross 6 Regression F, on F2d Average 3 crosses Ranee

F , plants Regression F, progeny means on F, plantsf Cross 4 Cross 5 Cross 6

60 -78 13 43

50 60 76 41

-

-

52 41-71

84 72-93

92 87-96

-

54

-

68 64-76 6

67 73 86

72 53 42

-

35

-

-

19 19 16

87 57 35

6 4-9

-

13 14 5

64

77 56

95 77

62 114 94

-

-

-

-

-

-

-

-

-

TABLE IV Selection unit, method, and authority Regression F, family means on F, plantsf Cross 4 Cross 5 Cross 6 Plot means Regression F, on F3b Cross 45 Cross 5t Cross 61 Variance, familiesb Variance, F, linesa Variance, F, linesg Cross 7 Cross 8 Variance, cross l h F, lines F, lines Estimated from actual gain& F, plants F, lines F, lines 0

0

d

Weber and Moorthy (1952). Mahmud and Kramer (1951). Yoshino et al. ( 1955). Nagata (1960~).

Continued)

Character Yield

Height

17 21 8

83 71 66

77 48

91

Days A. to Time of Maturity maturity flowering

Seed wt.

%

%

Lodging

Oil

Protein

-

-

-

-

-

-

-

-

-

-

-

17 38 75

-

62 112 92

53

64 55

33 76 67

68 89 74

101 62 104 111 93 92

25 40

61 81

40 53 12 22 40

-

-

-

-

-

-

-

-

-

71 71

77 43

84 89

68 92

73 34

68 78

39 83

73 82

79 86

60 71

80 87

46 58

45 57

63 74

-

35 59 71

32 59 77

-

-

35 46

-

34 61 62

-

-

-

56

Weber (1950). 5 Bartley and Weber (1952). g Johnson et al. ( 1955a). h Hanson and Weber (1962). e

-

-

-

-

-

-

-

2 PZ 8

3

2

Q

*

3

#

E3 2

cl

182

HERBERT W. JOHh3OS AKD RICHARD L. BERNARD

pIots of the main plots in three replications of an experiment conducted at one location in one year. They attempted to avoid genetic shifts by bulking equal quantities of seed from each of seven F3 plants to provide the bulk F4 generation. They concluded from the data that there appears to be no important genetic reason why F3 lines should not provide good estimates of average yield potentialities of their segregates when genetic shifts and interactions with environmental factors are controlled. Bartley and IVeber (1952) evaluated the F3 and F4 generations in two replications at one location in succeeding years as would be done in the conduct of a breeding program, and their estimates were notably smaller than those of Mahmud and Kramer. Procedures followed in estimating heritability of differences in plot means by the variance method differed primarily in that Mahmud and Kramer (1951) evaluated their materials at only one location in one year whereas Johnson et al. (1955a) evaluated theirs over locations and years. Hanson and IVeber (1962) obtained an estimate of genotype X environment interaction from the performance of generations over years. Their estimates basecl on actual gain were obtained as the fraction of the selection differential in the indicated generations actually obtained in the Fj, F6, and Fi generations. Consideration of the data in Table IF7 makes apparent the difficulties in applying the concept of heritability to plants. For eyample, what does heritability of F2 plant differences mean in comparison with the heritability of differences among F3 plot means? Obviously there is no simple basis for comparing the various estimates. In spite of this, however, the data do provide meaningful information. For example, they demonstrate that heritability of yield differences is smaller and less consistent than that for other important characters considered in soybean breeding. The estimates for the important characters height, maturity, oil percentage, and protein percentage are more consistent than those for yield and substantial in magnitude in all instances. These differences between yield and other important characters have been encountered in breeding programs in that the identification of genotypic differences with respect to yield is much morc difficult than that for other important characters. Reported estimates for miscellaneous characters are summarized in Table V. The estimates of Van Schaik and Probst (1958a) and Yoshino et al. (1955) are for F r plant differences estimated by the variance method whereas those of Johnson et al. (19.55) are for differences among F3 lines. Estimates of heritability have utility in estimating genetic advance expected from selection; however, few of the reported estimates have been so used. Johnson et al. (1955a) and Hanson and Weber (1964)

183

SOYBEAN GENETICS AND BREEDING

estimated the advance expected from selecting the top 5 per cent of the lines in the sample. Their results are summarized in Table VI. One of the most important aspects of heritability is that accurate estimates of heritability provide a quantitative estimate of the relative extent to which various characters respond to selection under various conditions. Most experienced plant breeders have a good idea of the relative ease TABLE V Heritability of Miscellaneous Characters 10 Crossesb Character No. branches No. nodes No. fruiting nodes Length of internodes No. pods per plant 1-Seeded pods "/o 2-Seeded pods "/o 3-Seeded pods "/o Av. no. seed per pod Undeveloped ovules % No. seed per plant No. seed per node No. ovules per plant Days to fruiting Specific gravity of seed Peduncle length Flowers per node Flower shedding "/o a b c

Cross 7a Cross 8a 38 69 52 74 22 35 38 69 60 40 19 49 21

-

73 64 19 72 50 53 65 69 59 40

55 64 56

-

6 Crossesc

Av.

Range

Av.

Range

67

-

-

86

33-83 78-93

55

36-83

-

-

74 47

-

-

-

-

60-86 -7-97

-

-

-

-

-

-

-

-

-

-

72 76 59

10-98 29-93 21-80

Johnson et al. ( 1955a) . Yoshino et al. (1955). Van Schaik and Probst (1958a).

with which various characters respond to selection in the absence of, or in spite of, estimates of heritability. For example, experience quickly teaches the futility of selecting for yield among Fa soybean plants in spite of some of the data in Table IV. Estimates of heritability of F P plant differences are influenced by numerous difficulties involved in evaluating the genotype of individual plants. Hinson and Hanson (1962) demonstrated that with four test genotypes there was no spacing (up to 32 inches apart) at which the compensation for distance between plants was equal for all genotypes and that in spacings up to 32 inches between plants competition among the four genotypes was unequal. Some characters were affected more than others by both competition and compensation for space, the varia-

TABLE VI Genetic Advmce Expected from Selecting the Top 5 Per Cent of the Lines in the Sample a n d h'frall\ for Various Characters Based upon Two Replications in Two Environments Cross

Character Yield (bu. per acre) Height (inches) Maturity (days from Scpt. 1) Days flowering to maturity Days to flowering Seed weight (grams per 100) Lodging (score 1-5) Oil ( % ) Protein ( % ) 0

b c d

Expected advance F, F4 2.7 4.3 5.5 3.5 5.4 0.7 0.3 0.6

-

4.0 5.9 7.4 4.9 7.3 1.1 0.4 0.8 -

la

Crow 7r

Approximate meanb

Expected advance

49.0 41.7 24.7 76.8 39.9* 17.4 2.0 21.4 -

1.3 4.3 3.2 5.9 6.4 1.2 1.1 0.7 0.6

Hanson and Weber (1962). Mean for F, through F, generations. Johnson et al. (1955a). Data recorded as days after May 31 rather than from planting.

C r o s 80

Mean

- ____ 28.1 45.3 59.7 85.3 69.7 14.0 3.2 19.7 42.2

Expected advance

Mean

2.0 5.6 3.9 2.4 4.5 4.0 0.3 1.0 1.9

27.7 40.1 61.8 83.4 67.1 20.2 2.7 19.5 43.7

185

SOYBEAN GENETICS AND BREEDING

bility for yield being the most severely affected in both cases. Estimates of heritability for yield were demonstrated to be grossly affected by competition and spacing, and the results were interpreted to indicate that estimates of genetic parameters for yield based on performance of individual plants in heterogeneous populations may be extremely biased. They concluded that yield data on individual plants selected from mixed populations at any spacing would have limited application to row plantings. The data in Table VII are presented to provide information on the heritability of various characters based on data and the observations of several experienced soybean breeders and geneticists. The estimates TABLE VII Expected Heritability of Various Characters of Soybeans When Selection Units are F, Plants and Means of F, or Later Generation Lines in Two Replications in Two Environments Selection unit

Character Yield Height Maturity Flowering to maturity Days to flowering Seed weight Resistance to lodging % Oil % Protein

F, plant 5 45 55 40 60 40

Mean of F, or later generation lines in two replications within two environments 38 75 78 65 84

68

10

54

30 25

67 63

should be regarded as “expected heritability” and taken to indicate the fraction of the selection differential expected to be gained when selection is practiced on the indicated selection unit. All possible ramifications of the effects of generation, etc., on the estimates cannot conveniently be accounted for, but the values should provide a reasonably accurate indication of the relative ease with which various characters can be influenced by selection.

D. CORRELATIONS AMONG CHARACTERS The extent to which various characters are correlated has been studied by a number of investigators. The correlations between many different pairs of characters have been estimated, but it seems appropriate to give special attention here to correlations involving the three im-

186

HERBERT W. JOHNSON AND RICHARD L. BERNARD

portant and expensively measured characters yield, percentage of oil, and percentage of protein.

1 . Yield and Other Characters The expression of many characters in soybeans is influenced less by environment than is yield, and such characters would be useful indicators of yield if they were consistently associated with it. Thus far, however, there has been little use of other characters as indicators of yield in soybean breeding. Woodworth (1933) reported that 26 varieties differed greatly in average number of nodes per plant, pods per node, seeds per pod, percentage of aborted seed, unit seed weight, and yield per plant. In general, the characters were independent of each other and only low percentage of aborted seed and high unit seed weight were appreciably associated with yield. Vl’eatherspoon and Wentz (1934) found that 237 strains differed in plant height and the characters considered by Woodworth. Plant height, nodes per plant, pods per plant, and pods per node were significantly correlated with yield. Multiple correlations between yield and combinations of characters were estimated and a correlation of 0.56 was obtained between yield and the combination of height, nodes per plant, pods per plant, aborted seed, and seed size. They concluded that number of pods per plant, height, and seed size were by far the most important characters and that the others gave little additional information concerning yield. In two experiments involving 16 and 24 varieties studied in successive years, Ma (1946) found that number of pods per plant, percentage of aborted seeds, seed size, number of seeds per pod, and percentage of developed pods differed significantly. Associations among characters were not consistent although the experiments had 10 varieties in common. In the first experiment yield was associated only with percentage of developed pods and number of seeds per pod, whereas in the second, yield was associated only with seed size Yoshino et al. (1955) estimated the correlations between yield and various characters on F:! plants of 11 crosses. The magnitude of the correlations varied from cross to cross, the number of branches, number of pods, plant height, and flowering time being variously associated with yield. In all the studies previously mentioned, no distinction was made between the environmental and genetic contributions to the correlations. \\’eber and Moorthy (1952) utilized F, plants of three crosses to estimate genotypic and phenotypic correlations between all pairs of the characters flowering time, period from flowering to maturity, maturity, height, oil percentage, seed weight, and yield. In general, the genotypic

SOYBEAN GENETICS AND BREEDING

187

correlations were higher than the phenotypic. Substantial positive correlations between yield and maturity, height, and seed weight were obtained. Johnson et al. (1955b) estimated all possible genotypic and phenotypic correlations between pairs of 24 characters. The selections involved were in the F4 generation and traced through bulked seed to individual Fz plants of two crosses. The genotypic correlations were in general higher than the phenotypic. There was, however, general agreement in both sign and magnitude between estimates of genotypic and phenotypic correlations. Genotypic correlations between yield and long period from flowering to maturity, lateness, heavy seed, and resistance to shattering and lodging were appreciable in magnitude. They estimated that selection based on maturity, seed weight, or resistance to shattering would be approximately 50 per cent as effective in improving yield in the two populations as selection based on yield itself. Some of the published correlations between yield and other characters are summarized in Table VIII. The experimental units involved in the correlations differed greatly and the extent to which the results are comparable depends largely on the extent to which the expression of the various characters was influenced by the environment of the different experimental units. The data do indicate, however, that few characters in soybeans can be considered to be reliable indicators of yield. Widely spaced plants develop branches more profusely than do plants spaced at the usual rate (Lehman and Lambert, 1960), and plants of most soybean genotypes have a capacity to utilize much more space than is allotted in commercial production and most research programs. Correlations based on spaced plants and involving characters greatly influenced by plant spacing may therefore be misleading insofar as their utility in practical breeding is concerned. 2. Oil, Protein, and Other Characters

A negative correlation between per cent oil and per cent protein has been observed by numerous investigators. Weiss et at. (19552) summarized reported correlations between oil and protein ranging from -0.26 to -0.74. They obtained a correlation of -0.92 among varieties which did not differ appreciably among locations or years and a correlation of -0.46 among dates of planting. Johnson et al. (1955b) reported genotypic correlations between oil and protein percentages of -0.48 and -0.70 and phenotypic correlations of -0.48 and -0.69 for two segregating populations. Hanson et aE. (1961a) obtained estimates of -0.47, -0.54, and 4 . 3 9 for the phenotypic, genotypic, and error correlations, respectively, between oil and protein percentages. The estimates were

188 HERBERT W. JOHNSON AND RICHARD L. BERNARD m

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-!

4

c!

I

5

I

I l S l l I

11811

I

I

I

3

I

H?ll I

I

I I ~ I II

I

I

1

0

112jII I

m I

0

I

I

I

I

9

I I I I tCN1

SOYBEAN GENETICS AND BREEDING

189

based on 80 F3 lines from each of 45 crosses among 10 varieties with a subsample of 20 lines from each cross evaluated in 1 of 4 environments. Genotypic correlations ranged from -0.41 to -0.76 over the four environments. The reported correlations clearly indicate that the correlation between oil and protein percentages is negative and substantial. Since oil and protein together account for approximately 60 per cent of the dry weight of the seed of most soybean genotypes, a negative correlation between the two components is not surprising. It is, however, interesting to speculate on when and how the genetic correlation arises. How the respective genes for oil and protein operate in the control of energy finally stored in the two fractions would be difficult to ascertain. The observed end product provides little information on whether the genes operated from the beginning of the production of simple sugars destined for the oil or the protein fraction or operated in the distribution of or competition for energy, with the efficiency of energy accumulation being under the control of a different set of genes. Hanson et aE. (1961a) reported an interesting approach to the study of the genetic potential to produce seed energy and the genetic control of the distribution of energy between seed fractions. Starting with the premise that initial sugar carbons are not associated with a particular ultimate energy fraction, they considered the energy finally stored in the oil, protein, and residual (total organic material less oil and protein) portions of the seed in terms of carbon equivalents required. This involved a consideration of the “work” carbons required in the conversion of simple sugar carbons to the more complex forms. When oiI and protein were expressed in terms of carbon equivalents of stored and “work” energy, the genetic correlation between them was -0.85 as compared with - 0 . 5 4 when they were expressed in percentages. In contrast, the correlation between protein and residual portions of the seed was -0.65 a Data based on means of 10 plants per plot in each of 10 replications in 1 environment. b Data based on means of 2 plants per plot in each of 10 replications in 1 environment. Two hundred thirty-seven F, lines tracing to a single F, plant were evaluated. ** significantly different from 0 at the 0.01 level of probability. 0 Data based on approximately 50 F , plants in each of 11 crosses. They represent the range over crosses from the lowest to the highest correlation. d Data based on approximately 300 F, plants in each of 3 crosses; 0.11 and 0.15 required for significance at the 0.05 and 0.01 levels of probability, respectively. e Data based on 89 lines in cross 7 and 64 in cross 8, evaluated in 2 replications at 2 or 3 locations in 1 year. Plot values for characters measured on individual plants were means of 10 plants per plot.

190

HERBERT W. JOHNSON AND RICHARD L. BERNARD

on a percentage basis and - 0 . 2 9 on a carbon-equivalent basis. For oil and residual, the correlations were 4 . 2 9 and 4 . 2 6 , respectively. They concluded that the genetic controls of the distribution of energy into protein and oil fractions of the seed are very closely interrelated. Weiss (1949) summarized the limited estimates of correlations of oil and protein with various other characters reported before 1949. One investigator reported that seed size or weight had a small positive association with oil percentage and a small negative association with protein, and another reported the exact opposite. A third investigator reported a notable positive association between seed weight and percentage of oil in the F, and F3 generations of an interspecific cross and a small negative association between seed weight and percentage of protein in the F3 generation. Genotypic and phenotypic correlations between oil and various other characters reported by Weber and Moorthy (1952) and Johnson et al. (195%) are summarized in Table IX. As with yield, the correlations vary TABLE IX Estimates of Genotypic and Phenotypic Correlations of Oil with Other Characters of Soybeansa Per cent oil with... Flowering time Flowering to maturity Maturity Height Seed weight

Cross 1

Cross 2

Cross 3

Cross 7

Cross 8

-.47 (-.36) ?J .32 (.02) -.32 (-.39)

-.36 (-.30) -.09 (-.18) -.45

-.28 (-.24) .13 t.04) -.24 (-.20) -.30 (-.22) -.46 (-.01)

-.27 (-.24) -22

-.32 (-.31) .30 (.23) -.27 (-28)

-54 (-.13) -.32 (--.05)

(-4 1 -.50 (-.25) -.28 (-.23)

(.I91 -.22 (-.16)

.oo

-.09

(.05) .12

(-.W)

.I8

(-12)

(.I51 Data for crosses 1, 2, and 3 from Weber and Moorthy (1952) and for crosses 7 and 8 from Johnson et nl. (1955b). See footnotes to Table VIII for information on methods. b Phenotypic correlations are in parentheses.

appreciably but do indicate that earliness of flowering and earliness of mattuity are associated with high oil. Time of flowering and time of maturity also are positively correlated. Published correlations between protein and characters other than oil are even more limited in scope than those for oil. Of the correlations involving protein and the characters listed in Table IX, Johnson et al (195%) found only the period from flowering to maturity to be con-

SOYBEAN GENETICS AND BREEDING

191

sistently correlated with protein. The correlations were negative and almost identical in magnitude to the positive correlations between the character and oil (Table IX). Nitta (1952) reported that the oil content and specific gravity of seed were negatively correlated. Collins and co-workers (personal communication) also observed and investigated a negative association between specific gravity of the seed and percentage of oil and a positive association between specific gravity and percentage of protein. Hartwig and Collins (1962) have utilized density separates of seed of individual plants to select for high oil and for high protein. They interpreted their results to indicate that progress in both characteristics can be achieved through the economical procedure of separating seed into the desired density classes. Other investigators have made similar observations, but whether the technique will work as well for protein as for oil appears to be in question. The utility of the technique is known to be limited to good-quality seed and other limitations appear to exist; however, the technique does show enough promise to make the correlations between specific gravity and other characters reported by Yoshino et ab (1955) of interest. They estimated correlations from data on F2 plants of 11 crosses. The ranges in the correlations between specific gravity of the seed and indicated characters were as follows: flowering time, -0.05 to 0.54; maturity date, 0.14 to 0.78; days to flowering, 0.13 to 0.29; height, -0.08 to 0.56; number of branches, 0.04 to 0.54;number of pods per plant, -0.06 to 0.25; and yield, 0.01 to 0.43. Oil percentage was apparently not measured in the study.

3. Miscellaneous Characters Published correlations among miscellaneous characters are highly variable. However, the following are consistent in sign and substantial in magnitude: height and earliness, negative; height and resistance to lodging, negative; early flowering and early maturity, positive; and early flowering and length of fruiting period, positive. Correlations between the number of flowers per node and three other characters were reported for the F2 generation of six crosses by Van Schaik and Probst (1958a). The correlations between flowers per node and peduncle length ranged from 0.81 to 0.92; between flowers per node and pods per node, -0.04 to 0.56; and between flowers per node and per cent shedding, 0.33 to 0.89. The reported correlations appear to offer no consistent limitations to breeding progress except for the serious limitations imposed by the negative association between protein and oil. In northern latitudes where early maturity is a necessity, the negative correlation between height

192

HERBERT W. JOHNSON AND RICHARD L. BERNARD

and earliness and the positive correlation between height and 1-ield indicate definite limitations which have been encountered in practice. The negative correlation between height and lodging is more serious in southern latitudes than in northern latitudes where height is limited by early maturity. Except for the correlations between oil and protein, the reported correlations are inadequate to provide a reliable indication of the associations among characters. Most of the reported correlations were based upon individual F2 plants or means of early-generation lines, whereas selection among F, or later generation lines is by far the most important type of selection in soybean breeding. Estimates of correlations among important characters obtained from advanced lines of diverse genetic origin would provide information more applicable to the selection process than that currently available. Such estimates are currently being obtained in research programs in the United States; but in the absence of such information, expected values are presented in Table X. The data in the table are based upon the available published data and unpublished data of several investigators. They represent the best estimates available, but they probably are much less precise than similar data for heritability presented in Table VII. E. SELECTIONINDICES The multiple correlation of Weatherspoon and Wentz ( 1934) (Section IV, D, 1) was apparently the first attempt at a selection-index approach to soybean breeding, and relatively little work along this line has been done since. Johnson et al. (1955b) estimated the effectiveness of a selection index for yield. Some representative estimates of the effectiveness of various combinations expressed as a percentage of the progress expected from selection for yield alone are as follows: fruiting period( 1) seed weight( 2), 95 per cent; (1) (2) lodging( 3 ) , 108 per cent; (1) (2) (3) protein per cent(4), 136 per cent; (1) + ( 2 ) (3) (4) oil per cent(5), 136 per cent; and (1) (2) (3) (4) (5) + yield(6), 141 per cent. The most effective index for oil was only 7 2 per cent as effective as selection based on oil percentage itself. Brim et aE. (1959) evaluated indices involving the six characters yield of oil, yield of protein, yield of seed, resistance to lodging, seed weight, and fruiting period. Variable economic weights were assigned to oil and protein in the ratios of 1:1, 1:0.6, and 1:0.2 with the 1:0.6 ratio approximating the average ratio of the price of oil and protein per pound over about 25 years. Advance from selection based on various indices was estimated for oil and protein and for oil equivalent, or the value of the

+

+ +

+

+ +

+

+

+

+

+

+ +

TABLE X Expected Genotypic Correlations among Various Characters of Soybeans When Selection Units are F4 or Later Generation Lines in Two Replications in Two Environments

Character

Yield

Yield Height Maturity Flowering to maturity Days to flowering Seed weight Resistance to lodging % Oil

-

Height

Flowering to Maturity maturity

Resistance Daysto flowering

Seed weight

.3

.4

.2

.o

.2

-

.4

.o

.5 .5

.o

-

.2

-

-.6

-

.1 .2

.o -

to

lodging

%

Oil

.o

.O

.o .o

-.2 1

-.2

.1 -.2

.1

.o

.o

-

.O

-

.O .O -.6

E

‘2

3

-.2

.o

-.4 -.2

-.

.1

%

Protein

3

m

2 8

b

E

H

E

5

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HERBERT W. JOHNSON AND RICHARD L. BERNARD

advance in both oil and protein expressed in terms of pounds of oil. The advance in pounds of oil or protein was little affected by changes in price ratio in one population but was greatly affected in the other. Selection advance in oil equivalent from an index composed of yield of oil and seed weight was 101 and 113 per cent of that based on an index composed of oil, protein, and yield in the two populations. Estimates of advance expected from selection based on various indices and the 1:0.6 price ratio presented by Brim et al. (1959) are summarized in Table XI. TABLE XI Expected Advance in Pounds per Acre for Oil, Protein, and Oil Equivalent@ Population l a Characters in index

Oil

Oil Protein equivalent

Yield of oil( 1) 31.3 39.0 54.7 Yield of protein( 2 ) 21.6 34.3 42.2 Yield of seed(3) 26.3 37.8 49.0 (1) ( 2 ) 35.8 38.0 58.6 (1) ( 3 ) 34.5 37.5 57.0 ( 2 ) (3) 32.0 39.0 55.4 (1) ( 2 ) (3) 35.7 38.4 58.8 Resistance to lodging( 4 ) + fruitingperiod(6) 25.1 42.0 50.3 Seed weight( 5) + 6 24.3 41.7 49.4 (4) (5) (6) 26.8 47.3 55.2 ( 3 ) ( 4 ) (5) ( 6 ) 31.8 51.5 62.7 (3) ( 5 ) ( 6) 31.0 48.6 60.2 (1) ( 3 ) ( 5 ) ( 6 ) 37.0 47.4 65.5 (1) ( 2 ) (4) 70.1 51.2 ( 5 ) (6) 39.4 (1) ( 2 ) (3) 52.2 70.7 39.4 (4) (5) (6) a Brim et al. (1959). b Populations 1 and 2 are the same as crosses 7 and

+ + + + +

+ + + + + + + + + + + + + + + + + + +

Population 2 Oil

Oil Protein equivalent

29.4 16.4 22.5 24.5 25.5 21.5 24.7

36.5 56.9 49.6 48.5 45.3 51.2 48.6

51.2 50.6 52.2 53.6 52.7 52.3 53.9

15.9 29.5 29.5 31.2 31.1 30.7

16.9 51.1 51.1 59.0 58.6 59.4

26.0 60.1 60.1 66.6 66.3 66.3

30.2

61.1

66.8

30.2

61.0

66.8

8 in Table IV.

Hanson and Johnson (1957) presented a method for calculating and evaluating a general selection index and employed the same basic data as Brim et aE. (1959> in a numerical example for soybeans. Utilizing an index composed of yield of oil, yield of protein, and yield of seed, they estimated that a general index based on the data from two populations was 92 and 97 per cent as effective in the two populations as a specific index calculated for each population. Utilizing the index developed specifically for one population for selection in the other was 89 per cent as effective as the specific index in one population but only 53 per cent as effective in the other. When the characters were weighted according

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to their economic worth only, the resulting indices were 83 and 99 per cent as effective as the calculated indices in the two populations. In spite of the promising results of the limited amount of data on selection indices in soybeans, the true merits of the selection index approach to soybean breeding are unknown. Hanson and Johnson (1957) and Brim et al. (1959) emphasized the importance of correct or adequate estimates of genotypic and phenotypic variances and covariances and of economic values or worth in the use of selection indices. However, they also recognized that alternative procedures, such as independent culling levels, visual appraisals, or mental thumb rules, do not overcome any of the objections to indices. The accumulation of additional genetic data in the evaluation of selection indices and additional theoretical consideration in their use appear to be fruitful areas of research in soybean genetics and breeding, but the general use of indices in practical breeding is not recommended on the basis of available data.

F. MISCELLANEOUS CHARACTERISTICS Reports of differential varietal responses to various environmental factors or treatments are frequent in the literature and no attempt will be made to summarize them here. Information on a number of characteristics not discussed in the preceding section is, however, of interest in various aspects of soybean research. Differences in the response of soybean genotypes to the compounds 2,4-dichlorophenoxyaceticacid (2,4-D ), 2,4,5-trichlorophenoxyaceticacid (2,4,5-T), and isopropyl N-phenylcarbamate ( IPC) have been observed (Williams, 1953; Fribourg and Johnson, 1955). Although the observed differences were large, the most resistant or tolerant genotypes were damaged significantly. Williams ( 1953) reported a correlation of 0.76 between varietal responses to 2,4-D and 2,4,5-T, but zero correlations between the responses to IPC and the other two compounds. Although oil and protein contents receive major consideration in soybean breeding, the quality of oil and protein has not as yet become an objective in breeding. Quality factors, however, have received attention in research designed to explore the feasibility of effecting changes in quality through breeding as well as the possibility that changes in quality might occur as a result of selection for other characteristics.

1. Fatty Acids of Soybean Oil Considerable interest in lowering the linolenic acid in soybean oil through breeding has been indicated because linolenic acid may be at least partly responsible for an undesirable flavor reversion in refined

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HERBERT W. JOHINSON ASm RICHARD L. BERNARD

soybean oil. White et al. (1961) reviewed published reports on the amount of linolenic acid in soybean oil ranging from 0.5 to 12.5 per cent and pointed out that recent studies have not revealed percentages of linolenic acid approaching the lowest values reported by earlier workers. The differences apparently are due to improvement in analytical techniques. Percentages of linoleic acid ranged from 43 to 59. White et aZ. (1961) obtained a range in linolenic acid of 4.89 to 9.28 per cent and in linoleic, 35.8 to 53.4 per cent among 251 plant introductions grown in single plots. The range in 1119 samples from F1 and F2 plants was 3.35 to 11.0 per cent for linolenic and 22.6 to 62.1 per cent for linoleic. Linolenic acid percentages for F1 plants were approximately intermediate between those of the parents, and those of F2 plants formed an essentially continuous range from small to large values. Linolenic acid percentages indicating transgressive segregation to low amounts of the acid were obtained for two F2 populations in particular. In general, poor agreement between linolenic acid percentages for F, plants and those for F3 rows was obtained and the environment markedly affected the quantity of the acid. Variability in linolenic acid due to locations has been found to be essentially equal to variability due to genotypes of commercial varieties (Collins and Howell, 1957; Howell and Collins, 1957). Howell and Collins (1957) obtained a significant negative correlation between maximum temperatures and both linolenic and linoleic acids and a positive correlation between percentages of the two acids of 0.74 by locations and 0.76 by varieties. Similar correlations between the two acids were reported by White et aZ. (1961). Linoleic acid is a valuable component of soybean oil, and reduction in the percentage of this acid would be undesirable. Reported correlations between linolenic and linoleic acids indicate that significant reductions in linolenic can be expected to result in reductions in linoleic. This situation plus the strong influence of the environment on the amount of linolenic acid and the uncertainties with respect to the genetics involved discourage breeding emphasis on linolenic acid. Until clear-cut chemical data are available to indicate the merits of a reasonable decrease in linolenic acid in soybean oil, accompanied by a decrease in linoleic, there is little apparent justification for making low linolenic acid an objective of breeding programs. 2. Amino Acids of the Protein The nutritional value of soybean protein is apparently limited by a deficiency in methionine (Krober, 1956), and prior to the advent of an economical source of synthetic methionine, considerable interest in in-

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creasing methionine through breeding prevailed. Krober ( 1956) studied the methionine content of the protein of 14 varieties grown at 12 to 18 locations in 3 years. Substantial variability associated with locations and years was observed, but differences among varieties were large enough and consistent enough to lead to the conclusion that the development of soybean varieties high in methionine through breeding should be possible. Kuiken and Lyman (1949) studied the amino acid composition of 20 varieties and selections representing a wide range of genotypes with respect to origin and area of adaptation. The results were amazingly uniform considering the genetic and environmental diversity involved in the samples analyzed. Maximum variability was observed for methionine with the highest value for methionine being only 19 per cent greater than the lowest. Similar data (unpublished) were obtained in 1955-1956 in a cooperative study conducted by a group of individuals engaged in various aspects of soybean research. In contrast to the samples of Kuiken and Lyman (1949), the 12 samples involved differed greatly in protein content attributable to both environmental and genetic causes. The available data on the amino acid composition of soybean protein indicate that breeding for high total protein should not adversely affect the amino acid balance of the protein. Krober and Cartter (1962) investigated the changes in various nonprotein constituents of the soybean seed associated with a change in protein. In high-protein samples the decrease in other constituents corresponding to the increase in protein was about one-third sugars, onethird oil, and one-third holocelluloses and pentosans. The suggestion from these samples that three units of protein could be gained for the loss of only one unit of oil is a little more optimistic than has been observed in the past. Soybean research workers generally assume a 2 :l relation between protein and oil whether the changes involved are environmental or genetic. Hanson et al. (1961a) obtained statistical estimates essentially equal to a 2:l ratio for changes in protein and oil due to environment and on the basis of energy involved. On the genetic scale, however, the ratio of change was only 1.3 to 1. 3. Interactions between Soybean and Other Genotypes

The inheritance of resistance to diseases is discussed in Section 111,

C,but for some diseases genetic variability in the pathogen also has been observed. This is best exemplified by the downy mildew organism, in which a number of physiologic races have been reported (Grabe and Dunleavy, 1959). Genetic variabilities in the pod and stem blight fungus

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HERBERT W. JOHNSON AND RICXARD L. BERNARD

( Diaporthe phaseoEorurn var. s q a e ) (Dunleavy, 1957) and in the fungus causing the destructive Phytophthora rot (Hildebrand, 1959) are additional examples in species other than the soybean that affect the soybean breeder. Interactions between genotypes of nodulating bacteria and genotypes of soybeans (Johnson and Means, 1960) and between genotypes of the bacteria for nodule sites (Means et al., 1961) have been reported. The results are interpreted to indicate the possibility of selecting or developing a strain of nodule bacteria especially suited to a given soybean genotype.

4. Response to Nutrients In addition to differences in efficiencyof iron utilization (Section 111,

D ) which has been used extensively as a tool in research on iron nutrition of plants, genetic variability in the response of soybeans to other nutrients has been reported. Allen (1943) observed differences in the response of two varieties to varying levels of potassium and magnesium and to a less extent to nitrogen, phosphorus, and calcium. Some varieties will tolerate much more zinc in the nutrient solution than others, and the resistant varieties continue to thrive at levels of zinc extremely toxic or lethal to others (Earley, 1943). Tops of plants of the LEE variety have been observed to have an essentially constant chloride content under conditions giving rise to variable amounts of chloride in other varieties and species ( McCollum, 1960). A rather extensive evaluation of the response of 44 soybean varieties to high levels of phosphorus in the nutrient solution was recently reported by Howell and Bernard (1961). The varieties were classed in five classes ranging from tolerant to very sensitive. Most of the currently important varieties of the southern States were classed in the tolerant group, whereas most of the important varieties of the northern States were classed in intermediate, sensitive, or very sensitive classes. Differences in the response of tolerant and very sensitive varieties to high levels of phosphorus in the field have been observed (Howell, personal communication), and the inheritance of the extreme difference in response appears to be relatively simple. The varietal differences in phosphorus response provide an unusual tool for genetic and biochemical studies of phosphorus nutrition of soybeans which shows promise of extensive use in the future. The large differences in the response of varieties to chloride and zinc also appear to merit attention similar to that given to the nonnodulating phenomenon by Weber (personal communication) and by Clark (1957).

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V. Breeding

A. GENERAL OBJECTIVES The main objective of soybean breeding in the United States is the development of disease-resistant varieties that produce high yields of beans high in oil and protein. Diseases of soybeans are numerous, and the specific diseases considered in breeding vary from location to location. Yield is the important consideration in all programs, and chemical composition also receives major consideration. Breeding for high oil content has been emphasized to the near exclusion of protein in most soybean breeding in this country, but in recent years the emphasis on protein has been increased greatly. Because of the negative correlation between oil and protein (Section IV, D, 2), two separate breeding programs are required to make maximum advance in each of the two important components. Resistance to lodging and resistance to shattering also are important objectives in soybean breeding, as is improved seed quality as indicated by various physical characteristics of the seed. Color of the seed is an important consideration, yellow seed coats and yellow cotyledons being considered essential characteristics of new varieties. Hilum color has not been considered in most breeding programs although some specialized uses of the beans and the tendency of dark pigments in the hilum to spread over relatively large areas of the seed coat under certain environmental conditions have caused some increase in emphasis on hilum color in recent years. Leffel (1961) questioned the merits of intensive selection for resistance to lodging. He concluded that the interaction of lodging with seed yield was so complex as to suggest relaxing of selection for lodging resistance in soybeans and a concentration of selection for combineharvestable yield. This conclusion was based upon the results of artificial lodging and staking of soybeans. Prevention of natural lodging by staking did not increase yields and the effects of artificial lodging varied with date of lodging, variety, and environment. Essentially opposite results were obtained by Weber (personal communication) in a comparison of the performances of staked to unstaked plants utilizing distinctly different genotypes and environments from those employed by Leffel. A small amount of effort is devoted to breeding soybeans for vegetable purposes in the United States, and in such breeding large seed and various other seed characteristics are important considerations.

200

HERBERT W. JOHNSON AND RICHARD L. BERNARD

B. CONSIDERATIONS m GENOTYPE EVALUATION

1. Maturity Groups Soybean research workers in the United States and Canada have divided soybean varieties and selections into 10 maturity classes from 00 to VIII. Selections in Group 00 maturity are adapted to northern United States and southern Canada and those of Group VIII, to the Gulf Coast area. Groups 0 and 00 were added to the original numbering scheme with the development of increasingly earlier varieties and the movement of soybean production northward. Research workers in the soybean-producing States of the United States and provinces of Canada cooperate in evaluating superior selections from individual programs in uniform tests. The first such evaluations are in preliminary tests conducted at only a few locations in an area to which a given maturity group is adapted. The best selections are advanced from the preliminary tests to the regional uniform tests conducted at 10 to 30 or even more locations depending upon the maturity group. The number of years selections should be tested in the uniform tests before they are discarded or released as varieties is difficult to determine. Sampling of environments by locations over a region should substitute in part for sampling of environments over years within a given state. However, little information relating specifically to the question of what constitutes adequate testing in uniform tests is available. This is due primarily to inherent difficulties in defining what constitutes superior performance of a selection, in part to the natural reluctance of an individual to utilize regional data in lieu of local data, and in part to failure to utilize all the information provided by the uniform tests. In an attempt to obtain information on the length of time a selection should be tested and on how the efficiency of uniform testing procedures might be improved, the entries in the uniform tests in 1954 were retained and tested in 1955 and 1956. The data (unpublished) indicated that an L.S.D. for yield at the 5 per cent level of signscance of about 3 bushels per acre can be expected for the means of strains tested at 20 locations in 1 year. Increasing the number of locations beyond 20 would give very little additional precision, and to reduce the 5 per cent L.S.D. from 3 bushels to 1 would require that testing be continued for 6 years at 20 locations, Testing for three years would reduce the L.S.D. from three bushels to two. The number of replications used would have little effect on precision of estimating strain means if 20 or more test environments were involved, and more than 2 replications would be of little value un-

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less there were very few test environments. The data indicated that testing that would give adequate estimates for yield differences would give more than adequate estimates for other traits investigated-maturity, lodging, height, seed quality and weight, and protein and oil percentages. The data clearly indicate that from the standpoint of regional objectives testing procedures can be simplified considerably. Recommendations on exactly how this can best be done must await completion of the analyses of the data. However, a reduction in the number of replications in the evaluation of yield and a reduction in amount of data taken on characters other than yield are indicated and would materially reduce the amount of effort required in the conduct of the tests. Conversely, such reductions would reduce the precision of the data provided by the tests in local situations. 2. Plot Size, Shape, and Cost

Weber and Horner (1957) utilized uniformity data from basic units of single-row plots 8 feet long to estimate cost and optimum plot size and shape for measuring yield and chemical characters of soybeans. Although long, narrow plots were more variable in yield than short, wide plots, various designs and other considerations made them more desirable. A plot size of about 24 feet appeared to be optimum but not greatly more efficient than the usual 18- to 20-foot plot with 16 feet harvested. For the same precision, yield required 33 times more replication than oil and 50 times more replication than protein. Utilizing an index of soil heterogeneity developed from data from soybean variety trials, Brim and Mason (1959) estimated optimum plot size for measuring yield in bordered plots to be 3.6 times the basic unit of 8 feet. The currently used %row plot with the center 16 feet harvested, or 4row plot with the center 16 feet of the two middle rows harvested, was not greatly different in efficiency from the optimum. In both studies mentioned above, the relative cost of various operations in man-hours was considered in arriving at an optimum plot size, and harvesting was by far the most time-consuming operation. In one case (Weber and Homer, 1957), the work was done in an area where bordered plots are not generally used, and in the other (Brim and Mason, 1959) the work was done in an area where bordered plots are standard. In both instances the current practice of harvesting the center 16 feet of an 18-foot row was not seriously different from optimum. Although larger plots may increase precision, the serious limitation imposed by the time required at harvest makes a plot size larger than an 18-foot single- or multiple-row plot of questionable merit.

202

HERBERT W. JOHIVSON AND RICHARD

L. BERNARD

3. Plant Spacing and Competition The differential response of soybean genotypes to space and the differential competition among genotypes are important considerations in genotype evaluation and have been demonstrated by a number of investigators (Wiggans, 1939; Probst, 1943, 1945; Hartwig et al., 1951; Yamada and Horiuchi, 1953; Lehman and Lambert, 1960; Hanson et al., 1961b; and Hinson and Hanson, 1962). The data on competition between genotypes in adjacent rows have been interpreted to indicate the necessity of multiple-row plots in the southern portion of the soybean-producing area of the United States ( Hartwig et al., 1951) and the adequacy of single-row plots in the northem portion (Probst, 1943). Hanson et al. (1961b) developed a design and analysis for competition studies which permitted the estimation of both average and specific competition effects. Their results confirmed those of Hartwig et al. (1951) and clearly demonstrated the need for bordered plots in the southern area and suggested the need for bordered plots in the northern area, at least where Group N maturity selections are adapted. Results involving F3 lines suggested that, in genetic studies involving similar genetic material, a 2-row nonbordered plot would sufficiently minimize the effects of competition. With more diverse material, 2-row plots bordered by a single row of a common variety were suggested. The latter suggestion is supported by the observation that the competing system tended to follow a simple additive model in which interacting effects could be ignored. Percentage of oil and percentage of protein were little affected by competition, and the evaluation of soybean genotypes for oil and protein can apparently be done effectively in nonbordered plots. Mumaw and Weber (1957) evaluated the differential competition among genotypes in varietal blends simulating bulk populations. Marked changes occurred in the percentage of various varieties in each of three blends. High yielding capacity of a variety in pure stands was not an assurance that it would dominate or even maintain its original percentage in the blend. Several superior varieties in pure stands were greatly reduced in successive years of the blends, and this was interpreted to indicate a limitation to the bulk hybrid method of plant breeding. Conversely, Weber (1957) obtained essentially equal results from selecting single plants superior for yield from bulk hybrid populations at drilled, 4-inch, and &inch plant spacings. Hinson and Hanson (1962) demonstrated that the relative yields per plant of specific varieties in mixtures had little similarity to the

SOYBEAN GENETICS AND BREEDING

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relative yields of the varieties in pure stands. Varieties differed greatly in their capacity to compete and to compensate for space, and the relative yields of plants in mixtures and heterogeneous populations were therefore considered to be essentially useless as indicators of the relative yields of their progenies in drilled seedings. Conversely, the average yield per plant in mixtures with plants spaced from 2 to 32 inches apart differed little from the average yield per plant of the component varieties in pure stands at the same spacings. The results indicated that the reduced yield of plants at a competitive disadvantage in mixtures or heterogeneous populations was essentially equal to the increased yield of plants with a competitive advantage. The available data on soybeans indicate no way of overcoming the nongenetic plant-to-plant variability of yield in soybeans through plant spacing. If the plants of a heterogeneous population are spaced too closely, variability will be increased by competition; if spaced too far apart, variability will be increased by differential capacity of genotypes to utilize space. Hinson and Hanson (1962) found that percentage of oil, percentage of protein, seed weight, plant height, number of branches, date of flowering, maturity, days from flowering to maturity, and seed quality were also affected by plant spacing and/or competition but much less than yield. Percentage of oil tended to increase while percentage of protein tended to decrease with spacing, but there were notable varietal exceptions. In general, the variability due to spacing and competition indicated that selection for either oil or protein on an individual plant basis would meet with little success.

C. BREEDING METHODS 1. Backcrossing Although several improved varieties have resulted in recent years from selections from the cross of one of the two parents to their F1, varieties changed by the addition of one or more specific characteristics through backcrossing have not been released. In recent years, however, resistance to a number of diseases has been successfully added to popular varieties by backcrossing. The first such varieties are scheduled for release in 1963. The backcross technique has been used rather extensively by soybean research workers in recent years in the transfer of specific characteristics to a common genetic background for genetic analysis. The technique also is used in the development of closely related lines differing in simply inherited characteristics, such as resistance versus susceptibility to various diseases, nodulating versus nonnodulating, and efficient versus in-

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HERBERT W. JOHNSON AND RICHARD L. BERNARD

efficient utilization of iron, for use in estimating losses from diseases and in physiological and agronomic studies.

2. Hybridization and Selection Selections from introduced varieties were the foundation for early soybean production in the United States, but virtually all selection in modem breeding programs is done in segregating populations resulting from artificial crossing of genotypes selected as parents for specific purposes. Methods of handling the segregating generations following hybridization are by no means standardized, but are in general similar to those used for other self-fertilized annuals. The bulk and pedigree systems and combinations of the two, as well as the backcross system, are employed. Procedures vary with the objectives and parentage of various crosses, but are similar with respect to the emphasis placed on selection for various characters in early generations. In general, major emphasis is placed on selection for height, maturity, seed quality, and resistance to shattering and diseases in the F2 and F3 generations with minor emphasis on yield or chemical composition before the F4 generation. A number of studies designed to estimate the utility of performance data obtained in various generations following a cross and the relative effectiveness of different breeding procedures have been reported. a. Sebction among crosses. Johnson (1961) reviewed the work of Weiss et al. (1947) and Kalton (1948) involving segregating populations and of Probst (1957) and Mumaw and Weber (1957) involving variety blends, which generally indicated that the relative yields of bulk populations of soybeans did not accurately predict the relative yields of pure lines derived from the crosses. He questioned whether the experimental techniques employed had given the merits of the relative yields of bulk crosses as indicators of the relative yields of segregates from the crosses an accurate test. Torrie ( 1958b) demonstrated the importance of experimental technique and adequate testing in a study of 11 unselected bulk soybean crosses in the F3 through F7 generations. His results indicated that (1) the relative seed yields of the same generation of crosses grown in different years were inconsistent and resulted in significant cross x year interaction, and ( 2 ) that the average yield of all crosses evaluated with 1-year-old seed was significantly higher than with 2- or 3-year-old seed. In contrast, relative yields of crosses within a given year and age of seed were consistent and the cross x generation interaction was nonsignificant. If the relative yields of bulk crosses are to be indicative of the relative yields of segregates from them, the yields of both crosses and

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segregates must be evaluated with a high degree of precision. Leffel and Hanson (1961) evaluated 45 diallel crosses among 10 soybean varieties in the bulk Fz and F3 generations in 4 or more location-year environments and a subsample of 20 F3 lines from each cross in 4 location-year environments. These evaluations provided a much more extensive evaluation of the mean performance of bulk crosses and lines than has been reported previously. The results too were different from those reported previously and indicated that the mean performance of parents or their crosses in early bulk generations were reliable predictors of the mean performance of F3 lines obtained from the crosses. They pointed out that the performance of bulk crosses would be especially valuable in an intermated population or in other situations in which the performance of the parents could not be evaluated conveniently. The results of Hinson and Hanson (Section V, B, 3 ) indicating that the average yield of plants in mixtures was essentially the same as the average yield per plant of the component varieties in pure stands is further indication that the mean yields of bulk crosses should be indicative of the mean yields of the lines extracted from the crosses. b. Selection within crosses. Investigations of Weiss et al. (1947) indicated little association between the relative yields of F3 or Fq lines at one location in one year and those of F4 or F5progenies at the same location the following year, Kalton (1948) obtained similar results for the F3 and F4 generations. Raeber and Weber (1953) compared the effectiveness of the bulk and pedigree systems of breeding with four crosses. The results suggested to them that replicated tests and unreplicated spaced plantings should be evaluated simultaneously in F3 and subsequent generations. Superior lines would be identified by the replicated tests, and individual plants of these lines would be selected on the basis of maturity and height from spaced plantings. Seed from selected plants would be used for simultaneous progeny testing and selection in the next generation. They found an appreciable degree of genic fixation for yield in the F4 generation and a fairly high degree of homozygosity in the FS. In contrast, Mahmud and Kramer (1951) estimated significant segregation for yield through the F5 or F6 generations. Torrie (1958a) compared the bulk and pedigree methods of breeding utilizing F6 lines in six crosses. He found that the lines developed by the bulk method averaged about 2 days later than those developed by the pedigree method but that the two methods gave essentially equal results for yield, height, lodging, and oil and protein contents of the seed. Weber (1957) evaluated progenies of plants superior for yield selected from bulk hybrid populations in drilled ( 1-inch) , 4-inch, and

206

HERBERT W. JOHNSON AND RICHARD L. BERNARD

8-inch spacings and found that the spacing of the bulk population had no appreciable effect on the effectiveness of selection for yield. He concluded that the quantity of seed desired by the breeder from individual plants in segregating populations should determine optimum spacing. Voigt and Weber (1960) utilized five crosses in a comparison of a method of early-generation testing and concurrent phenotypic spacedplant selection for seed yield with the standard bulk and pedigree methods of breeding. The three selection procedures were applied to the same germ plasm base established by dividing the seed from 75 Fz plants per cross into three portions, one for each procedure. In general, the method of early-generation testing and concurrent phenotypic spacedpiant selection resulted in more improvement in yield than the other methods. As pointed out by the authors, however, the relative efficiencies of the methods per unit of time or cost were not compared. The pedigree and bulk methods were equally effective in selection for yield, but the pedigree was superior to the bulk for maturity, height, and lodging. The results were interpreted to indicate that little progress should be expected from phenotypic selection for seed yield in early generations of a cross. Visual selection for various characteristics is practiced extensively in soybean breeding. The effectiveness of such selection for height, maturity, resistance to diseases, and resistance to lodging and shattering has been demonstrated experimentally and through experience. The effectiveness of such selection for yield, however, has received relatively little attention experimentally. Hanson et al. (1962) reported the results of visual descriptions of the phenotypic capacities of plots for seed yield made by three experienced soybean breeders. Although the selections were classed in only three simple classes, the three observers were primarily effective in classifying the extremely poor yielders in a “discard group.” The observers differed considerably in the identification of lines as being in the top one-fifth or second one-fifth for yield. All three individuals correctly identified only 79 of the 360 plots in the top onefifth but 648 of the 1080 plots in the bottom three-fifths. Lodging was apparently the principal factor involved in the visual concept of seed yield although the correlation between lodging and yield was only -0.23, Hanson et al. (1962) concluded that visual discrimination in material representing a wide range in types has merit. A simple visual scheme, such as identifying discards and a selection intensity of about 50 per cent for yield or a proportion depending upon the proportion of poor phenotypes obvious visually, was suggested. Brim and Cockerham (1961) estimated that progress expected from

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selecting among Fa, F4, and F5 progenies was approximately 30, 44, and 50 per cent greater, respectively, than that expected from selection among F2 progenies. Hanson and Weber (1961) presented estimates of genetic variance for F4 through F7 progenies suggesting similar results. Although the results were not translated into specific recommendations for handling segregating populations, they have resulted in the use of a system of advancing generations through a single seed from each plant of the preceding generation (personal communication). Large numbers of plants from Fz or intermated populations are used, and the populations are advanced with a minimum of effort. Selection for easily measured characters may be practiced at any stage of inbreeding but is not recommended prior to F4 or F5 generations. The suggested system appears to have merit from the standpoint of both the genetics and cost involved. Results published to date appear to warrant the following conclusions pertaining to selection within segregating populations of soybeans: ( 1 ) Rigorous selection for height, maturity, and resistance to shattering and diseases on an individual plant basis is justified. ( 2 ) Selection for the same characters plus lodging on an individual row basis should be effective. Selection for large differences in oil and protein contents should be effective on a single-row basis. ( 3 ) In the preliminary evaluation of genotypes, effective selection for all characters may be based on a replicated test at one well-chosen location. The effectiveness of such selection will vary among characters, and final evaluations should be made concurrently with the evaluation for yield. (4) Final selection for yield must be based on tests conducted in an adequate sample of the environment for which the varieties are being selected. (5) If the relative yields of F3 or later generation lines are accurately estimated, they are indicative of the relative yields of the bulk Fq or later generation progenies. (6) If the relative yields of bulk generations of crosses are accurately estimated, they are indicative of the mean yield of lines extracted from them. (7) Advancing a large number of plants from an Fz or intermated population through single-seed descent to about the F5generation before extracting lines has merit as a breeding procedure, and use of this procedure in soybean breeding should increase.

D. SPECIESHYBRIDS Most reported studies on hybridization of species involve crosses between G. max and G. ussuriensis. These two species and a type identified as G. gracilis were found by Fukuda (1933) to be very similar in number and size of chromosomes and size of pollen grains. Variation in seed size within species was great enough to result in continuous

208

HERBJIRT W. JOHNSON AND RICHARD L. BERNARD

variation in the genus. Karasawa (1936) found the F1 hybrids of G. m a x and G. ussuriensis to be intermediate to the parents in most characters. However, they resembled the G. ussuriensis parent in the twining habit of growth and appressed pubescence. No cytological abnormalities were observed during sporogenesis of the F1, and fertility was normal. Ting (1946) studied a cross between the same two species and found fertility of the F1 to be normal. The mode of inheritance of 10 simply inherited characters was found to be identical with that of crosses within G. nwx. Height of plants and size and shape of seed were quantitatively inherited. The Fl plants approached the wild parent in height and seed shape and the geometric mean of the two parents in seed size. The F2 distribution of height was skewed toward the cultivated parent whereas that of seed size and shape was skewed toward the wild parent. IYilliams (1948) reported 50 per cent pollen and ovule abortion in F, plants from most of 15 crosses of G. max and G. ussuriensis; however, F, plants from some of the crosses had normal fertility. Seed size was found to be geometric in inheritance and neither parental type was recovered in over 4000 F, plants or in the first backcross generation to G. nzux. Seed size of the cultivated parent was recovered in the BCIS1 and BC, generations when G. max was used as the recurrent parent. Oil content of the F1 was intermediate between the two parents, but the oil content of neither parent was recovered in the F-. Dominance of the high protein content of G. ussuriensis was exhibited in the F1, and transgressive segregation for high protein occurred in the F2. Maturity of the F1 was likewise intermediate and transgressive segregation occurred in the F-. The prostrate growth of G. ussuriensis predominated in the F2 and Fa, and the erectness of G. max was not recovered in the first backcross to the erect parent. N'eber (1950) observed reciprocal F;s of a cross between G. max and G. ussuriensis to be identical in maturity, seed size, oil content, protein content, and iodine number of oil. The F2 and F3 distributions for seed size were skewed toward the small parent with a notable failure to recover the large parent. Gene action was found to be geometric, and a large number of genes conditioning seed size were postulated. Lack of dominance, complex inheritance, and a mixture of geometric and additive gene action were indicated for oil content and iodine number. High protein exhibited partial dominance, and evidence of additive action of a relatively few genes conditioning protein was found. For maturity, there were transgressive segregation in F2 and Fa, additive gene action, a low number of genes involved, and little evidence of dominance. Reported estimates of heritabilie for the characters are presented in Table IV. Among F, plants

SOYBEAN GENETICS AND BREEDING

209

and FS lines, large seed size was notably associated with high oil content and low iodine number and slightly associated with low protein. Oil content was negatively correlated with protein content and iodine number. In general, species crosses have been of little value in practical breeding; however, currently there is considerable interest in the high protein content of G. ussuriensis. Evidence that this species may have resistance to stem canker has been obtained, and an attempt is being made to transfer the resistance to cultivated types. A thorough study of the species in the Glycine genus is needed and this type of work has increased in recent years.

E. INDUCTION OF MUTATIONS Humphrey (1951) studied plants from soybean seed that had been subjected to irradiation for different periods of time and identified the treatments as “1000, 1500, 2500, and 3500 roentgen units of neutron irradiation.” No effect from the l00Or and 1500r treatments was observed in the first generation following irradiation. Plants from seed subjected to the 2500 r and 3500 r treatments were markedly different from normal plants. Young plants had a grayish appearance, venation of the leaves was very coarse, and the leaves were somewhat rugose. The plants later turned very dark green, but the leaves remained rugose. Maturity was delayed appreciably. The effects of the higher irradiation treatment were notably greater than those of the 2500r treatment. Only a few mutations were observed in the second generation after the two lower irradiation treatments, but 228 mutant plants of 4200 were observed in the second generation following the two higher treatments. No indication of how these were distributed between the two treatments was given. The mutations involved leaf color, stem size, flower color, internode length, sterility, maturity, and leaf texture, shape, and pubescence. Seven plants showing a marked increase in vigor over normal plants were also observed. In subsequent generations Humphrey ( 1954) found promising variants for the following: resistance to shattering, seed size, oil content, maturity, plant vigor, and yield. Most of the mutant types reported in 1951 were found to breed true for the mutation concerned. Zacharias (1956) reviewed the results of the few irradiation experiments with soybeans reported prior to 1956 and reported the results of a study dealing with the HEIMKRAFT I variety and various X-ray irradiation dosages from 4000 to 60,000r. He concluded from greenhouse experiments that dosages of 6,000 to 12,000r appeared to be best suited for field trials and used dosages of 6,000, 8,000, 10,000, and 12,000r in

210

HERBERT W. JOHNSON AND RICHARD L. BERNARD

field experiments. He reported results from the XI, X2, and X3 generations. Zacharias (1956) obtained various mutant forms including the following: more densely branched, early maturing, large seed, increased pod set, various chlorophyll types, leaf form and size, and pubescence and seed coat color. He also obtained some interesting indications that variability in seed germination at low temperatures was induced. Seed from some X2 plants germinated better at 4.5%. than that from control plants although the extent to which the observed differences are heritable had not been demonstrated at the time of his report. Although he pointed out that additional trials would be required to confirm some of the mutants which appeared to be superior in yield, he concluded that further work could be expected to give rise to types that mature earlier and yield more than the initial material. Humphrey (1959) published essentially the same information as that published previously, but the final outcome of his promising variants of 1954 apparently still was in doubt. A rather careful search of the literature failed to reveal publications by Zacharias following up his 1956 report. However, Stubbe (1959) reported further on Zacharias' work to the effect that tests carried out in 1957 confirmed the supposition that the mutant strains were superior in yield and that some of them were 5 to 7 days earlier than the original variety. He also reported that from the seeds that germinated at 4.5"C. lines have been produced that germinate about 6 days earlier than the original variety and have an advantage in growth that persists to maturity. Stubbe concluded that through experimentally induced mutations the two most important breeding objectives, increase of yield and reliability of yield, had been attained. He pointed out, however, that the absolute values were not always a sufficient basis for extending the cultivation of soybeans in Germany but that some of the mutants were valuable for crossing with early, high-yielding introductions. The results of irradiating seed of two varieties, ADAMS and HAWKEYE, have been rather extensively studied in the United States. Rawlings et al. (1958) employed five dosages of X-rays 7500 to 20,000r and seven dosages of thermal neutrons (Nth) from 5.6 X 1OI2 Nth/cm.2 to 2.4 x 10'3 Nth/cm.2 in estimating the effects of ionizing radiations on the genetic variance of plant height, maturity, seed weight, and yield in the Rz generation. Estimates of genetic variance in the irradiated populations averaged five times as large as those in the controls. However, the increased variability in yield was largely in the negative direction and led to the conclusion that higher-yielding genotypes cannot be easily extracted directly from irradiated material. Williams and Hanway (1961) reported on oil and protein in the

SOYBEAN GENETICS AND BREEDING

211

populations evaluated by Rawlings et al. (1958). Estimates of genetic variance for oil and protein percentages were six or more times greater in the irradiated populations than in the controls. They concluded that since heritabilities and predicted gains were relatively high in irradiated populations and similar in magnitude to those observed in segregating populations derived from hybridization, selection for oil and protein percentages in irradiated populations should result in genetic progress. Papa et a,?. (1961) used the progenies from the experiment of Rawlings et al. (1958) to compare predicted and actual gains for yield, seed weight, maturity, protein content, and oil content. In general, significant gains from selection were made, but they were considerably less than the predicted. Although a selection advantage existed for yield in the irradiated populations, the amount gained was only enough to compensate for the lower initial yield of the irradiated populations. When selection was practiced for both high and low protein and oil, no gain for high oil was made, but considerably more gain was made for high than for low protein. The results were interpreted to indicate that radiations have possibilities in the improvement of quantitative characters in soybeans. Dunleavy (1959) utilized X-ray dosages of 2500, 5000, 7500, 10,000, and 15,000 r on HAWKEYE soybean seed in an attempt to induce mutations for resistance to the diseases bacterial blight ( Pseudomonm glycinea) and stem canker. Apparent resistance to both diseases was observed in early generations following irradiation, but that to stem canker was completely lost in subsequent generations. Although the progenies of plants resistant to bacterial blight became increasingly susceptible with succeeding generations, some plants in the seventh generation following irradiation were less susceptible than others. In 1954 several soybean research workers in the United States initiated a cooperative study of the effects of X-ray and neutron irradiation on soybeans. Because much of this work may not be published, a brief summary of the results is given here. Specific data and details are omitted in deference to the individual investigators, but it does seem appropriate to give the general outcome of a cooperative effort of this magnitude. The primary objective of the work was to evaluate the utility of irradiations in inducing mutations conditioning fairly simple characters, primarily disease resistance. Each investigator used a variety or varieties adapted to his area and characters which were of special interest to him. Although many genetic variants were observed, no investigator obtained the specific mutation or mutations in which he was particularly interested. However, in some instances results were similar to those of Dun-

212

HERBERT W. JOHNSON Ah?)

RICHARD L. BERNARD

leavy (1959) in that types less susceptible than the parent variety were isolated. In general the specific objectives were not attained. X-Ray and neutron irradiations of soybean seed can increase estimated genetic variability in quantitative characters in soybeans and give rise to mutations affecting qualitative characters. There is, however, considerable doubt as to the merits of the quantitative variability created bv irradiation in comparison with that created by hybridization. Much more genetic variability exists in soybeans than is being utilized in breeding, and the use of irradiations to create variability in a pure line of soybeans appears to be an expensive and difficult alternative to using variability already available. The possible utility of irradiation in inducing mutations for disease resistance and similar characteristics is not questioned even though the limited amount of such efforts with soybeans has not been encouraging. Similarly, the possible merits of irradiation in creating Variability that is not already available, such as Zacharias’ (1956) work with germination at low temperatures, are not questioned. Even in such instances, however, an adequate survey of available genotypes for the characteristic in question would seem to be indicated before resorting to irradiation. With the objectives of the current breeding programs in the United States, utilization of the available extensive variability in soybeans appears more deserving of the available research effort than attempts to create additional variability by irradiation. This conclusion detracts in no way from the merits of basic research into the effects of irradiation on the species. Colchicine-induced autotetraploid soybean plants have been reported by a number of investigators (Tang and Loo, 1940; Sonnenschein, 1941; Andres, 1944; Porter and Weiss, 1948; Oinuma, 1952; and Sen and Vidyabhusan, 1960). The results are in general agreement and indicate that the tetraploid plants have larger, rougher, and thicker leaves, larger stomata, larger cells, larger nuclei, thicker stems, larger seed and pollen, later maturity, and much lower seed yield than diploid plants. Fertility of tetraploids has been observed to be greatly reduced (Porter and Weiss, 1948; Oinuma, 1952), and tetraploids were cross-incompatible with diploids in the work of Porter and Weiss (1948). The results of cytological examination of the tetraploid plants were recorded by Oinuma (195%) and Sen and Vidyabhusan (1960), Oinuma attributing the lowered fertility of tetraploid plants to observed meiotic irregularities. Conversely, Sen and Vidyabhusan considered the frequency of meiotic irregularities to be low7. Gobs-Sonnenschein ( 1943) reported that diploids, triploids, tetraploids, and octoploids were produced by treatments with mixtures of colchicine with veratrine and atropine. Successful crosses of tetraploids

SOYBEAN GENETICS AND BREEDING

213

with diploids that produced only diploid progeny and among tetraploids that produced many tetraploid sets of chromosomes also were reported. Treated plants ffowered earlier and were shorter than nontreated. From the practical standpoint of soybean production, tetraploid soybeans are generally inferior to their diploid counterparts and tetraploids apparently have been little used in basic genetic or physiological studies. However, the results of Gobs-Sonnenschein (1943) appear to indicate some advantage of tetraploids; and Oinuma (1952) observed markedly increased nodulation and nitrogen fixation of tetraploid plants that led to the conclusion that they might be of commercial value in Japan for their capacity for nitrogen fixation. However, prolonged activity of nodules and increased nitrogen fixation normally accompany delayed maturity caused by reduced pod set of diploid soybeans. The advantages of ploidy per se on nodulation and nitrogen fixation could best be measured in early stages of plant development. Limited observations of autotetraploid and diploid types of different varieties prior to flowering, made by the authors, have indicated no striking differences in nodulation pattern or nodule numbers. Franzke (personal communication) has observed variants in the progeny of colchicine-treated plants similar to those reported for sorghum (Franzke and Ross, 1952; and subsequent papers). A cooperative endeavor in 1959 ( Weber, Hartwig, and Johnson-unpublished) in which some eighty varieties of soybeans were treated with colchicine in the early seedling stage resulted in no observed mutant types except for numerous autotetraploids. Mortality of treated seedlings was high and the negative results could well have been due to faulty technique. In view of additional information relative to technique (Sanders et al., 1959; Franzke et aZ., 1960), additional research on the utility of colchicine in the induction of mutations in soybeans seems to be justified.

F. GENERAL CONSIDERATIONS OF SOYBEAN BREEDING The intensive cooperative program of soybean breeding in the United States began with the establishment of the U. S. Regional Soybean Laboratory in 1936, and 24 improved commercial varieties have been released from the program. Improved disease-resistant versions of some of these varieties, resulting from backcrossing, are scheduled for release in 1963. One or more of the 24 improved varieties are adapted to each of the soybean production areas in the United States, Ten improved varieties have been released in Canada and several have come from private programs in a similar period of time. In comparison with the introduced varieties previously grown in the United States, the new varieties are much superior in yield, oil content, resistance to lodg-

TABLE XI1 (kwn Pl;ism Soiirc.c, o f Commcrcid Soybean Varieties in the United Stutcs, Maturity Groups 00-1Va .

.

Maturity groiip and commercial vnricty ___ Group 00

A . K.

-

AChlE FLAMBEAU

CAPITAI. CHANT hiANDAnIN

Group I BLACKIIAWK CHIPPEWA MONROE

X%.ANCIIII

( Manchiirh )

__---

Anccistr:il v;iric.tirs ; 1 i d coiintry of origin _____ .._ h1UKI)EN

(Mallchurin )

(Manchurin )

HICHLANI)

(Man-

-

X

-

-

-

-

-

-

-

-

X

-

-

-

X X X

-

-

No. 171 (Manchuria)

-

-

X

X

FLAMBEAU

-

-

-

-

X

-

X X

-

-

X

X X

-

X

-

-

X

-

Iirinosoy HAWKEYE LINDAAIN

Croup I11 X

-

-

-

-

-

-

-

-

X X

-

SHELBY

X X X

X

LINCOLN

-

-

X

FORD

Other

churia) __

Croup I1

ADAhlS

~

I _ _ _ _

MANILWIN

-

-

-

( Ott:l\Va)

(h4anchuri;~ ~

-

NORCIIIEIZ

~

l)lJNF1131.1~

( Manchriria )

Group 0 COMET

..

X

-

X

MANITOBA BROWN

Introduced from

(unknown)

U.S.S.R.

(unknown) (Manchuria)

MANITOHA BROWN

SENECA

(U.S.S.R.)

E

TABLE XI1 (Continued) Ancestral varieties and country of origin Maturitygroup and commercial variety GrouD

Iv

CLARK

PERRY

scorn WABASH

A. K.

DUNFIELD

MANCHU

MANDARIN

(Manchuria)

(Manchuria)

(Manchuria)

(Manchuria)

churia)

-

-

X

-

X

-

x

X

X

-

-

X

-

X

-

MUKDEN

(Man-

RICHLAND

(Manchuria)

Other

X

-

-

PATOKA

-

X

CNS

-

(Manchuria) and L37-1355 (unknown) (China)

All known ancestral varieties are included for each commercial variety. For example, CLARK is a selection from LINCOLN ( 2 ) Since LINCOLN’S parents were MANCHU and MANDARJN, the three introduced varieties RICHLAND, MANCHU, and MANDARIN are checked for CLARK. 0

x

RICHLAND.

2 8

8

216 HERBERT W. JOHNSON AND RICHARD

C

h

B

a

A

5 -

0

2

F

X I X

X I X

xx I

I l l

I l l

1x1

L. BERNARD

SOYBEAN GENETICS AND BREEDING

217

ing, shattering, and some diseases, seed quality and color, and adaptability to mechanical harvesting and are specifically adapted in maturity to various production areas. It is impossible to estimate accurately the increased returns from these new varieties, but even the most conservative estimates indicate that they are surprisingly great. Soybean improvement in the United States followed the usual route of introduction, selection, and hybridization with selections that have been grown commercially at some time in the past forming an almost exclusive group of ancestral varieties (Tables XI1 and XIII). In the northern production area six varieties of Manchurian origin form the exclusive background for varieties currently grown on approximately 95 per cent of the total acreage and are a part of the background of the varieties grown on virtually all the remaining acreage. The predominance of Manchurian germ plasm in northern varieties (Table XII) is partially understandable because such a large number of the early introductions adapted in maturity to the northern States were from Manchuria. However, many early introductions from other countries, especially Korea, Japan, and China proper, also were of the desired maturity. The ancestors of the varieties grown in the southern production area of the United States are of more diverse origin than those of the northern area with the germ plasm originally from Manchuria, China, and Korea playing a predominant role. One of the six selections of Manchurian origin, A. K., is in the background of varieties currently grown on approximately 60 per cent of the acreage in the southern area and only one selection from Japan is involved. It should be noted that in a few instances the only information available on the origin of a given variety is that it is a selection from another variety. In these instances the variety in question may have originated from the same source as the original, from hybridization, or from mixtures. The origin of the original variety has been used in this discussion. Also, the information in Tables XI1 and XI11 indicates only ancestral varieties, not pedigrees of current commercial varieties. Pedigrees of many current and old varieties were given by Poehlman (1959). The diversity of the ancestral varieties of current commercial varieties does not necessarily indicate the diversity of germ plasm involved in the breeding program. Most of the crosses that have been made did not give rise to commercial varieties. Hence, the ancestral varieties could in one sense be considered a highly selected source of germ plasm particularly suited to the conditions of the United States. However, the small number of ancestral varieties is partly due to the intensive effort devoted to crosses involving these varieties and selections resulting from them.

218

HERBERT W. JOHiiSON AND RICHARD L. BERNARD

R’hether more breeding progress could have been made had a larger group or a group more diverse in origin been used is academic. The important consideration is that soybean breeders have recognized the desirability of increasing genetic diversity and of evaluating the breeding potential of selections of diverse origin in the germ plasm collection and are currently doing so. REFERENCES Allen, D. I. 1943. Missouri Univ. Agr. Expt. Sta. Research Bull. 361. Andres, J. hl. 1944. Ruenos Aires Univ. lnst. de Gene‘t. [P.] 2, 95-102. Athow, K. L., and Bancroft, J. B. 1959. Phyfopathology 49, 697-701. Athow, K., and Probst, A. H. 1952. Phytopathology 42, 660-662. Bartley, B. G., and Weber, C. R. 1952. Agron. 1. 44, 487-493. Bening, W. 1951. Soybean Dig. 11(5), 20-22. Bernard, R. L., Smith, P. E., Kaufmnnn, M. J., and Schmitthenner, A. F. 1957. Agron. 3. 49, 391. Borthwick, H. A., and Parker, hf. W. 1939. Botan. Gaz. 101, 341-365. Brim, C. A,, and Cockerham, C. C. 1961. Crop Sci. 1, 187-190. Brim, C. A., and Mason, D. D. 1959. Agron. J. 61, 331-334. Brim, C. A., Johnson, H. W.,and Cockerham, C. C. 1959. Agron. J . 61, 42-46. Caldwell, B. E., Brim, C. A., and Ross, J. P. 1960. Agron. J. 62, 635-636. Casas, E. 1961. M.S. Thesis, North Carolina State College, Raleigh, North Carolina. Clark, F. E. 1957. Can. J. Microbid. 3, 113-123. Collins, F. I.. and Howell, R. W. 1957. J . Am. Oil Chemists’ Soc. 34, 491-493. Domingo, W.E. 1945. J. Agr. Research 70, 251-268. Dunleavy. J. 1957. lowa Stnte Coll. J . Sci. 32, 105-109. Dunleavy, J. 1959. Proc. Zowa Acad. Sci. 66, 113-122. Earley, E. B. 1943. J. Am. Soc. Agron. 36, 1012-1023. Feaster, C. V. 1951. Missouri Univ. Agr. Expt. Sta. Research Bull. 487. Franzke, C. J., and Ross, J. G. 1952. J. Heredity 43, 107-115. Franzke, C. J., Sanders, hl. E., and ROSS,J. G. 1960. Nature 188, 242-243. Fribourg, H. A., and Johnson, I. J. 1955. Agron. 1. 47, 171-174. Fukuda, Y. 1933. Japan. 3. Botany 6, 489-506. Gates, C. E., JVeber, C. R., and Horner, T. W. 1960. Agron. 1. 62, 45-49. Geeseman, G . E. 1950. Agron. J . 42, 608-613. Gobs-Sonnenschein, C. 1943. Ziichter 16, 62-68. Grabe, D. F. 1957. Proc. Assoc. Ofic. Seed Analysts 47, 105-119. Grabe, D. F., and Dunleavy, J. 1959. Phytopathology 49, 791-793. Guard, A. T. 1931. Botan. Gnz. 91, 97-102. Hamada, H. 1955. Crop Sci. Soc. Japan Proc. 23, 276. Hanson, W. D. 1959a. Genetics 44, 197-209. Hanson, JV. D. 1959b. Genetics 44, 833-837. Hanson, \V. D. 1959c. Genetics 44, 839-846. Hanson, W. D. 1959d. Genetics 44, 857-868. Hanson, W.D. 1962. In “Statistical Genetics and Plant Breeding, Proceedings of a Symposium.” NAS-NRC Pub. 891A (in press). Hanson, W.D., and Johnson, H. \V. 1957. Genetics 42, 421-432. Hanson, \V, D., and Weber, C . R. 1961. Genetics 46, 1425-1434.

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Hanson, W. D., and Weber, C. R. 1962. Crop Sci. 2, 63-67. Hanson, W. D., Leffel, R. C., and Howell, R. W. 1961a. Crop Sci. 1, 121-126. Hanson, W. D., Brim, C.A., and Hinson, K. 1961b. Crop Sci. 1, 255-258. Hanson, W. D., Leffel, R. C., and Johnson, H. W. 1962. Crop Sci. 2, 93-96. Harhvig, E. E., and Collins, F. I. 1962. Crop Sci. 2, 159-162. Hartwig, E. E., and Hinson, K. 1962. Crop Sci. 2, 152-153. Hartwig, E. E., and Lehman, S. G. 1951. Agron. J. 43, 226-229. Harhvig, E. E., Johnson, H. W., and Cam, R. B. 1951. Agron. J. 43, 443-445. Hildebrand, A. A. 1959. Can. J. Botany 37, 927-957. Hinson, K., and Hanson, W. D. 1962. Crop Sci. 2, 117-123. Homer, T. W., and Weber, C. R. 1953. Bwmetrics 12, 404-414. Howell, R. W., and Bernard, R. L. 1961. Crop Sci. 1, 311-313. Howell, R. W., and Collins, F. I. 1957. Agron. J. 49, 593-597. Humphrey, L. M. 1951. Soybean Dig. 12(2), 11-12. Humphrey, L. M. 1954. Soybean Dig. 14(7), 18-19. Humphrey, L. M. 1959. J. Sci. Club (Calcutta) 13, 83-85. Johnson, H. W. 1961. In “Handbuch der Pflanzenziichtnng” (H. Kappert and W. Rudorf, eds.), 2nd ed., Vol. 5, pp. 67-88. Paul Parey, Berlin and Hamburg. Johnson, H. W., and Means, U. M. 1960. Agron. J. 62, 651-654. Johnson, H. W., Robinson, H. F., and Comstock, R. E. 1955a. Agron. J. 47, 314-318. Johnson, H. W., Robinson, H. F., and Comstock, R. E. 195%. Agron. J. 47, 477-483. Johnson, H. W., Borthwick, H. A., and Leffel, R. C. 1960. Botan. Gaz. 122, 77-95. Kalton, R. R. 1948. Iowa State Coll. Agr. Expt. Sta. Research Bull. 368, 671-732. Karasawa, K. 1936. Japan. J. Botany 8, 113-118. Kato, I., Sakaguchi, S., and Naito, Y. 1954. TBkai-Kinki Natl. Agr. Expt. Sta. (Japan) Bull. No. 1, 96-114. Kato, I., Sakaguchi, S., and Naito, Y. 1955. TBkai-Kinki Natl. Agr. Expt. Sta. (Jupun) Bull. No. 2, 159-168. Krober, 0. A. 1956. 1. Agr. Food Chem. 4, 254-257. Krober, 0. A., and Cartter, J. L. 1962. Crop Sci. 2, 171-172. Kuehl, R. 0. 1961. M.S. Thesis, North Carolina State College, Raleigh, North Carolina. Kuiken, K. A., and Lyman, C. M. 1949. J . Biol. Chem. 177, 29-36. Leffel, R. C. 1961. Crop Sci. 1, 346-349. Leffel, R. C., and Hanson, W. D. 1961. Crop Sci. 1, 169-174. Leffel, R. C., and Weiss, M. G. 1958. Agron. J. 60,528-534. Lehman, W. F., and Lamhert, J. W. 1960. Agron. J. 52, 84-86. Liu, H.-L. 1949. 1. Heredity 40, 317-322. Ma, R. H. 1946. M.S. Thesis, University of Illinois, Urbana, Illinois. McCollum, R. E. 1960. Agron. Abstr. p. 21. Mahmud, I., and Kramer, H. H. 1951. Agron. J. 43, 605-608. Mahmud, I., and Probst, A. H. 1953. Agron. J. 46, 59-61. Matsuura, H. 1933. “A Bibliographical Monograph on Plant Genetics,” 2nd ed., pp. 100-110. Hokkaido Imperial Univ., Sapporo. Means, U.M., Johnson, H. W., and Erdman, L. W. 1961. Soil Sci. SOC. Am. PTOC. 26, 105-108. Miksche, J. P. 1961. Agron. J. 63, 121-128.

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Morse, W. J. 1950. In “Soybeans and Soybean Products” (K. S. Markley, ed.), pp. 3-59. Interscience, New York. Morse, W.J., and Cartter, J. L. 1937. Yearbook Agr. U . S. Dept. Agr. pp. 11541189. Mumaw, C. R., and Weber, C. R. 1957. Agron. J. 49, 154-160. Nagai, I. 1921. Tokyo Uniu.Coll. Agr. I. 8, 1-92. Nagai, I. 1926. Agr. and Hort. (Tokyo) 1, 1-14, 107-118. Nagai, I., and Saito, S. 1923. Japan. J. Botany 1, 121-136. Nagata, T. 196Oa. Mem. Hyogo Uniu. Agr. 3, 63-102. Nagata, T. 1960b. Sci. Repts. Ser. Agt. Hyogo Uniu. Agr. 4, 101-104. Nagata, T. 1960c. Sci. R e p . Ser. Agr. Hyogo Uniu. Agr. 4, 108-112. Nitta, K. 1952. Hokkaido Natl. Agr. Erpt. Sta. Bull. No. 63, 64-69. Oinuma, T. 1952. Japan. J. Breeding 2, 7-13. Owen, F. V. 1927a. Genetics 12, 441-448. Owen, F. V. 1927b. Genetics 12, 519-529. Owen, F. V. 1927c. J. Agr. Research 34, 559-587. Owen, F. V. 192% Genetics 13, 50-79. Owen, F. V. 1928b. Plant Physiol. 3, 223-226. Papa, K. E., Williams, J. H., and Hanway, D. G. 1961. Crop Sci. 1, 87-90. Parker, M. W., and Borthwick, H. A. 1951. Soybean Dig. 11( l l ) , 26-28, 30. Poehlman, J. M. 1959. “Breeding Field Crops,” pp. 220-239. Holt, New York. Porter, K. B., and Weiss, M. G. 1948. J. Am. SOC. Agron. 40, 710-724. Probst, A. H. 1943. 1. Am. SOC. Agron. 36, 662-666. Probst, A. H. 1945. J. Am. SOC. Agron. 37, 549-554. Probst, A. H. 1950. Agron. J. 42, 35-45. Probst, A. H. 1957. Agron. J. 49, 148-150. Probst, A. H., and Athow, K. L. 1958. Phytopathology 48, 414-416. Raeber, J. G., and Weber, C. R. 1953. Agron. 3. 45, 362-366. Ramanathan, K. 1950. Current Sci. (India) 5, 155. Rawlings, J. O., Hanway, D. G., and Gardner, C. 0. 1958. Agron. J. 50, 524-528. Ricker, P. L., and Morse, W. J. 1948. J. Am. SOC. Agron. 40, 190-191. Sanders, 9.1. E., Franzke, C. J., and Ross, J. G. 1959. Am. J. Botany 46, 119-125. Sen, N. K., and Vidyabhusan, R. V. 1960. Euphytica 9, 317-322. Singh, hf. P., and Anderson, J. C. 1949. Agron. J. 41, 477-482. Smith, P. E., and Schmitthenner, A. F. 1959. Agron. J. 51, 321-323. Sonnenschein, C. 1941. Forschungsdienst 12, 532-537. Stewart, R. T. 1927. J. Heredity 18, 281-284. Stewart, 1%. T. 1930. J. Agr. Research 40, 829-854. Stewart, R. T., and Wentz, J. B. 1996. J. Am. SOC. Agron. 18, 997-1009. Stewart, R. T., and Wentz, J. B. 1930. J. Am. SOC. Agron. 22, 658-662. Stubbe, H. 1959. Proc. 10th Intern. Congr. Genet. (Montreal, 1959) 1, 247-260. Takagi, F. 1929. Sci. Repts. TBhoku Uniu. Fourth Ser. 4, 577-589. Takahashi, N. 1934. Japan. J. Genet. 9, 208-225. Takahashi, T.,and Fukuyama, J. 1919. Hokhido Agr. Expt. Sta. Rept. No. 10. Tang, P. S., and Loo, W. S. 1940. Science 91, 222. Terao, H. 1918. Am. Naturalist 52, 51-56. Terao, H., and Nakatomi, S. 1929. Japan. J. Genet. 4, 64-80. Ting, C. L. 1946. J. Am. SOC.Agron. 38, 381-393. Tome, J. H. 1958a. Agron. J. 50, 198-200. Torrie, J. H. 195% Agron. J. 60, 265-267.

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Van Schaik, P. H., and Probst, A. H. 1958a. Agron. J. 50, 98-102. Van Schaik, P. H., and Probst, A. H. 195813. Agron. J. 50, 192-197. Veatch, C. 1930. J. Am. SOC. Agron. 22, 289-310. Veatch, C., and Woodworth, C. M. 1930. J: Am. SOC. Agron. 22, 700-702. Voigt, R. L., and Weber, C. R. 1960. Agron. J. 52, 527-530. Weatherspoon, J, H., and Wentz, J. B. 1934. J. Am. SOC. Agron. 26, 524-531. Weber, C. R. 1950. lowa State Coll. Agr. Expt. Sta. Research Bull. 374, 767-816. Weber, C. R. 1957. Agron. J. 49, 547-548. Weber, C. R., and Hanson, W. D. 1961. Crop Sci. 1, 389-392. Weber, C. R., and Homer, T. W. 1957. Agron. J. 49, 444-449. Weber, C. R., and Moorthy, B. R. 1952. Agron. J. 44, 202-209. Weber, C. R., and Weiss, M. G. 1959. J. Heredity 50, 53-54. Weiss, M. G. 1943. Genetics 28, 253-268. Weiss, M. G. 1949. Advances in Agron. 1, 77-157. Weiss, M. G., Weber, C. R., and Kalton, R. R. 1947. J. Am. SOC. Agron. 39, 791-811. Weiss, M. G., Weber, C. R., Williams, L. F., and Probst, A. H. 1952. Agron. J. 44, 289-297. Wentz, J. B., and Stewart, R. T. 1924. J . Am. SOC. Agron. 16, 534-540. White, H. B., Quackenbush, F. W., and Probst, A. H. 1961. J. Am. Oil Chemists’ SOC. 38, 113-117. Wiggans, R. G. 1939. J. Am. SOC. Agron. 31, 314-321. Williams, J. H. 1953. Agron. J. 45, 293-297. Williams, J. H., and Hanway, D. G . 1961. Crop Sci. 1, 34-36. Williams, L. F. 1948. Genetics 33, 131-132. Williams, L F. 1950. In “Soybeans and Soybean Products” (K. S. Markley, ed.), pp. 111-156. Interscience, New York. Williams, L. F. 1952. Genetics 37, 208-215. Williams, L. F., and Lynch, D. L. 1954. Agron. J. 46, 28-29. Woodworth, C. M. 1921. Genetics 6, 487-553. Woodworth, C. M. 1923. J. Am. SOC. Agron. 15, 481-495. Woodworth, C. M. 1932. Illinois Univ. Agr. Expt. Sta. Bull. 384, 297-404. Woodworth, C. M. 1933. J. Am. SOC. Agron. 25, 36-51. Woodworth, C. M., and Cole, L. J. 1924. J. Heredity 15, 349-354. Woodworth, C. M., and Williams, L. F. 1938. J. Am. SOC. Agron. 30, 125-129. Yamada, T., and Horiuchi, S. 1953. Japan. J. Breeding 3, 9-16. Yoshino, Y., Ozaki, K., and Saito, M. 1955. Hokkaido Natl. Agr. Expt. Sta. Research Bull. No. 68, 15-24. Zacharias, M. 1956. Zuchter 26, 321-338.

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FERTILIZERS AND THE EFFICIENT USE O F WATER Frank G. Viets. Jr

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United States Department of Agriculture. Fort Collins. Colorado

I . Introduction ................................................. A Importance of Efficient Use of Water ...................... B. Some Popular Viewpoints .................................. C. Early Studies ............................................ D Purpose of This Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Definition of the Problem ...................................... A. Definition of Terms ...................................... B. Significance of the Ratio in Water-Use Efficiency . . . . . . . . . . . . . . I11. Validity of Evapotranspiration Data ............................ A . Precautions That Must Be Observed and Scarcity of Data . . . . . . B. Oasis and Clothesline Effects .............................. C. Net Radiation and Fertilizer Effects ........................ D . The Significance of Advection .............................. IV . The Effects of Fertilizers on the Relationship of Evapotranspiration and Yield .................................................... A. ET and Y Increase Linearly; No ET When Y Is Zero .......... B. ET and Y Increase Linearly; ET Is Appreciable When Y Is Zero C. ET Is Independent of Y .................................... D . ET Is Independent of Y after Reasonably Complete Cover IS Attained ................................................ E . ET Decreases as Y Increases ............................... F. ET Increases Faster Than Y Increases ...................... G . Experiments That Cannot Be Classified ...................... H . Recapitulation ........................................... V. Fertilizers and Water-Use Efficiency in Terms of Applied Water .... VI . Fertilization and Water-Use Efficiency with Limited Moisture Supply . VII . Fertilization and Moisture Extraction by Roots .................... VIII . Fertilizers and the Infiltration of Water .......................... IX. Fertilization. Crop Maturity, and Water Use ...................... X. Other Practices for Increasing Water-Use Efficiency ................ XI . Is Maximum Water-Use Efficiency Desirable? ...................... XI1. Conclusions .................................................. References ..................................................

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Introduction

A. IMPORTANCE OF EFFICIENT USEOF WATER Agriculture is by far the largest consumptive user of water in the United States. As an average for the nation. about 21 inches of the 30 223

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inches of annual precipitation is evaporated or transpired from land and water surfaces. Practically all this evapotranspiration is from land in farm crops, grass, or timber. As urban and industrial demands for water grow, agriculture will be pressed to make more efficient use of its decreasing allotment. This pressure will, of course, be greatest in desert and semiarid areas where the competition for water is keenest. Man must have food and fiber. All food and natural fibers require water for their production. Agriculture, faced with increasing competition for water and an increasing demand for its products, must become more efficient in its use of water. Using water more efficiently is a problem not confined to agriculture in the United States. Growing populations all over the world point up sharply the need for better use of limited water resources, especially in semiarid and arid regions, and for more research information on how this may be achieved. Fertilizers, when needed, are among the best tools for increasing production of food and fiber, but what happens to water demands when fertilizers are used? B. SOMEPOPULAR VIEWPOINTS Many farmers and irrigation technicians have the impression that, when fertilizers are used, water requirements are greatly increased because the crop is larger. The following are some excerpts from a recent report on irrigation requirements : “The estimated water requirements of small grains are 12, 15, and 18 acre-inches per ton of yield on soils of land classes 1, 2, and 3, respectively. . . . The water requirement of potatoes has been estimated for the purpose of this report as 2.0 and 2.5 acre-inches per ton of yield . . . sugar beets will require 1.5 and 2.0 acre-inches of water per ton of yield on class 1 and 2 lands . . .” The important part of these statements is not that water is required on poor land of class 3 compared to the best land of class 1,which is undoubtedly true, but rather that within a land class the water requirement is proportional to yield. Since proper use of fertilizers on a land class is one way to increase yields, it follows from this statement that fertilizers will increase water demands. Here is another statement, made by a county agent at a dinner honoring winners in a corn-growing contest, connecting higher yields with higher irrigation requirements: “The high plant populations used today require six to eight irrigations instead of the two that growers used before.” The view that yields twice as large require twice as much water is one extreme point of view. The other extreme may be that yield of crop has no effect on water requirements. Empirical and aerodynamic methods of estimating evapotranspiration do not allow for fertilization and size of the crop. The Blaney-Criddle (1950) method, widely used in

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and and semiarid regions to estimate consumptive use of water, uses only mean monthly temperature, monthly percentage of annual daytime hours, and an empirical coefficient, characteristic of the crop, to estimate consumptive use for the season. In the Penman (1948, 195613) formula the variables are sunshine duration, temperature, wind velocity, and relative humidity. Only air temperature is used in the Thornthwaite (1948) formula. This viewpoint is based on the fact that evapotranspiration is a physical process and that when water is nonlimiting, evapotranspiration is almost entirely a function of meteorological conditions and is dependent “scarcely at all on the nature of the vegetation as long as it is green (i.e., in a stage of vegetative growth) and effectively covers the soil” (Schofield, 1952). What constitutes complete coverage of the soil, however, is now being questioned. Makkink (1957) noted that the discrepancy between evapotranspiration of grass in lysimeters and calculations of it by the Penman formula depended on grass length. Tanner et al. (1960) showed that “fully grown corn does not provide sufficient interception of radiation to assure that the potential evapotranspiration condition of a fully covered surface is realized.” They worked with corn in 40-inch rows, oriented either east-west or north-south, and stand densities of 13 and 22 kiloplants per acre. Likewise, Denmead and Shaw (1959) found that corn in Iowa approached the condition of a “green crop” for only 2 to 3 weeks from shortly before silking to about 16 days after silking.

C. EARLYSTUDIES Research on water requirements and water-use efficiency can be roughly divided into two periods. The early period began with the work of Sir John Lawes at Rothamsted before 1850 and culminated in the classic studies of Briggs and Shantz in the second decade of this century when agriculture was pushing into drier and drier regions of the Great Plains of North America. Parallel developments were taking place in Europe, particularly in Russia (Maximov, 1929). The second period is the present, in which new concepts of energy exchange and radiation efficiency in CO2 assimilation are being introduced. Richards and Wadleigh (1952) reported little current activity in this field. Briggs and Shantz (1913b) reviewed the work of 23 separate investigations on water requirements of plants (water transpired per unit of dry weight) as affected by fertilization of soils in containers, beginning with the studies of Lawes on the effects of manures on water requirements of five different kinds of crops. Briggs and Shantz concluded: “Almost without exception the experiments herein cited show a

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reduction in the water requirement accompanying the use of fertilizers. In highly productive soils this reduction amounts to only a small percentage. In poor soils the water requirement may be reduced one-half or even two-thirds by the addition of fertilizers. Often the high water requirement is due to a deficiency of a single plant-food element. As the supply of such an element approaches exhaustion, the rate of growth as measured by the assimilation of carbon dioxide is greatly reduced but no corresponding change occurs in the transpiration. The result is inevitably a high water requirement.”

D. PURPOSE OF THISh v m w The purpose of this review is to survey the current status of knowledge on the effects of fertilizers on the evapotranspiration of plants and the efficiency with which that water is used in dry matter and salable crop production. Attention must be given both to conditions in which water is nonlimiting and to conditions in which it is limiting for evapotranspiration. Also, the effects of fertilizers on root extraction of water and on infiltration and runoff cannot be laid aside in any discussion of the efficient use of water. The effects of water on efficient use of fertilizer and nutrient uptake of plants are not reviewed here. The science of evapotranspiration in relation to meteorological factors is now under such active investigation that there is high probability that anything written at this time will soon be outdated. The brief reviews of fertilizers and efficient water use given by Kelley ( 1954), Haise and Viets (1957), and Haise et al. (1960) show that fertilizers can increase the efficiency of water substantially. However, Richards and WadIeigh (1952) stated: “Present data seem to indicate that large decreases in growth and yield resulting from deficiencies of nutrients and moisture may cause only nominal increases in water requirement.” And further, “Adequate data are not available for a quantitative evalution, but the foregoing evidence indicates that the effect of nutrient deficiencies and soil moisture stress on the water requirements of crops is small.” II. Definition of the Problem

A . DEFIXITIOS OF TEAhfS

In the broadest and popular sense, efficiency in the use of water for crop production means growing as much crop as possible for the “water used,” whether that water is actual evapotranspiration, irrigation water applied, or rainfall. Each of these has sigdcance in specific situations. However, much confusion can and does arise unless the

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“water u s e d is defined. For example, water applied may include variable losses by runoff and/or deep percolation. Amount of rainfall does not give information on how much water is lost by runoff or leaching. Measured evapotranspiration is the only basis having broad application for making comparisons of the effects of fertilizers on water use and on the efficiency of that water use. The terminology used in this review is similar to that employed by Haise and Viets (1957); it is as follows: Water requirement: The ratio of weight of water absorbed by the plant during the growing season to the weight of dry matter produced by the plant during that time. This definition ignores the quantity of water metabolically used by the plant, which is relatively small. Briggs and Shantz used water requirement and transpiration ratio interchangeably. See Haise and Viets (1957) for a discussion of the value of this term. Evapotranspiration or consumptive use: The sum of the volumes of water used by plant growth in transpiration and evaporation from soil or intercepted precipitation on an area in any specsed time, divided by that area. Usually expressed as acre-feet or acre-inches per acre, or more simply, feet or inches. Water-use eficiency: The weight of dry matter or marketable crop produced per unit volume of water used in evapotranspiration ( E T ) . As used here for comparative purposes, the acre is usually the unit of area and the inch is the unit of depth. Water-use efficiency =

dry weight/acre

ET in acre-inches/acre

- dry

weight

ET in acre-inches

Metric units are often preferable. If both the dry matter and ET are expressed in units of mass, then water-use efficiency becomes a true ratio. In mass units, water-use efficiency is similar to the term “efficiency of transpiration,” which Maximov (1929) says was first used by L. A. Ivanov in a series of lectures at St. Petersburg in 1913. Effciency of transpiration was defined as the grams of dry matter produced per kilogram of water transpired. The advantage of the term water-use efficiency is that the emphasis is put on the water, which we are interested in using most efficiently. Comparison of fertilizer effects on production per unit volume of water and even comparisons on alternative use of water are easily made, i.e., alfalfa vs. corn, or even tons of sugar from sugar beets vs. tons of steel from a steel mill. The term has at least one disadvantage unless care is used in making comparisons. Increasing water-use efficiency may

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not always be desirable because, as will be shown later, crops grown on dryland frequently use water more efficiently than well-watered crops, but at much lower levels of production.

B. SIGNIFICANCE OF THE RATIOIN WATER-USE EFFICIENCY Maximizing the ratio of dry weight production (net assimilation) to evapotranspiration, increases water-use efficiency. Any inquiry into the effects of fertilizers on water use and efficiency of water use must examine the effects which they have on the actual water use per unit area (the denominator), which in itself is important; the effects on dry weight (the numerator); and, finally, their relative rates of change with fertilizer application, which is water-use efficiency. For dry weight per acre, units of marketable crops can be substituted. For basic studies the best expression of yield is the net assimilation rate (NAR) of CO,. Watson (1952), among many, has pointed out that net assimilation is the product of leaf area and the net assimilation rate per unit of leaf area. He claims that the major effect of fertilization is on total leaf area and that the effects on photosynthetic efficiency per unit of leaf area are small. Whether the latter conclusion is correct is beyond the scope of this review. The available data, in the author’s opinion, do not justify generalization to situations where deficiency symptoms are present. 111. Validity of Evapotranspiration Data

A. PRECAUTIONS THATMUSTBE OBSERVED AND SCARCITY OF DATA

Valid data on evapotranspiration are dBcult and costly to obtain in the field because of the large number of soil moisture measurements needed and because of possible losses by deep percolation and runoff. Also there is considerable uncertainty as to what should be done with precipitation, particularly from showers in semiarid and desert climates. Robins and Haise (1961) discuss the precautions needed in getting good consumptive use data and the problems of interpretation. Many thousands of fertilizer experiments have been conducted in which no attempt was made to measure evapotranspiration. In fact, in most of the more costly factorial experiments in which irrigation regimes and fertility rates are studied in all combinations, the imgated plots are main plots for practical reasons and fertility treatments are subplots of the split plot design. All fertility treatments in an irrigation treatment are irrigated alike, and in most cases no attempt is made to measure consumptive use on each plot. Actually there are few field experiments that have produced information on the fertilizer-yield-consumptive use complex, and most of these are from studies involving irrigation or from dryland

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studies in which deep percolation does not complicate the measurements of ET. B. OASISAND CLOTHESLINE EFFECTS Most of the data available on fertilizers in relation to evapotranspiration come from small-container experiments, lysimeters, or small-plot field experiments, which vary in their extent of control of border effects. Most container experiments are conducted without the benefit of a surrounding crop so that the plant is not confined over the area of the soil in the container. Opportunities to intercept heat and radiation from the surroundings are great, and the yields and water-use data cannot be projected to an area basis in the field. Lysimeters may be subject to this error unless precautions are taken, and even these may not be sufficient. Van Bavel (1961) gives a good discussion of lysimeter techniques for obtaining data on evapotranspiration. Even small plots, differentially treated, are exposed to different opportunities for evapotranspiration. This effect of advection of energy is, or may be, a serious consideration in experimental data because the prime effect of fertilizer response is an increase in crop yield, which may be apparent in plant height, leaf area, and sometimes in leaf or crop color. This problem can best be understood through the concept of the heat budget and its significance to evapotranspiration (see Penman, 1948; Tanner, l957,1960a, b ) . The heat budget or energy balance approach is based on the fact that the net radiation (difference between the incoming radiation and the radiation losses from soil and crop surfaces) can be partitioned into components that either evaporate water; heat the air, the soil, or the plant; or can be used in photosynthesis. The following simplified expression of the heat budget equation, in which all terms represent vertical heat flux, is taken from Peters (1960):

R,=S+A+

E+P where R, znet radiation; S = soil heat; A = sensible heat (air); E = evaporation; P =photosynthesis. A term C for heat storage in the crop should be added. Over a growing season, energy going into S and C is small. Energy going into P is often ignored, but recent data show that it may be higher than commonly supposed. For short time periods, in per cent of total radiation it can be 4 per cent for sugar beets (Gaastra, 1958; Blackman and Black, 1959), 5 per cent for corn (Lemon, 1960), and 5.5 per cent for barley (Kamel, 1959). In the data available on fertility-evapotranspiration relationships, interest centers on how crop size and color affect net radiation and sensible heat transfer. On small plots and lysimeters, energy available for evapotranspiration can be increased by the turbulent transfer of sensible

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heat downward from surrounding drier areas. Lemon et al. (1957) call this the oasis effect, and Halstead and Covey (1957) call it turbulent transfer. Small plots, inadequately guarded lysimeters, and containers are subject to another source of upward bias of evapotranspiration-the horizontal movement of sensible heat moving through the crop by wind movement from hotter and drier areas. This is called the “clothesline” effect by Tanner (1957), who states that on the upwind sides of fields this horizontal transfer is much greater than the turbulent exchange from above. He further states that all of a small test plot is subject to the clothesline effect, All investigators agree that these effects are greater in hot and climates than in cool humid ones and that they are greater the hotter and drier is the day. Some of the extreme effects of advection may be illustrated as follows: Lemon et al. (1957), working with a continuous cover of wellwatered cotton 5 days after a general rain of 3 inches, found that after 2 P.M. energy for evapotranspiration greatly in excess of net radiation was gained from a drier area upwind even though identical cotton was growing in that direction for 10 miles. This is the oasis effect, and Lemon et al. state, “It is of interest that such a marked effect can be found in an oasis of this large size.” Of particular interest to the problem of fertilizer effects on plant height are observations of Lemon et al. (1957) on water loss of a continuous cover of cotton on plots on which the cotton grew to different heights influenced by preceding moisture treatments. Plant height did not affect net radiation. When the mean integrated soil moisture tension was in the range of 0 to 2 atmospheres, the relative evapotranspiration rates for cotton 40, 25, and 18 inches high were 1, and about 0.8 and 0.7, respectively. They state: “This feature must have been due to horizontal heat transfer brought about by a plant characteristic, namely, plant height and/or leaf area difference.” Tanner (1957,1960b) shows that on an unusually hot, dry day in Wisconsin with north wind sweeping over a dry area, heat derived from the air contributed as much as 25 per cent to the total evapotranspiration of an alfalfa-bromegrass-ladino clover hay field. For alfalfa the same day, evapotranspiration was 1.1 times the net rediation. Lemon et al. (1957) studied evapotranspiration and net radiation on five large blocks (1150 by 120 feet oriented in the direction of the prevailing wind) at College Station, Texas. Three of the blocks were irrigated at 1-, 12-, and 16-day intervals, respectively, and two were nonimgated. When measurements were made, the plots had a continuous cover of cotton and plant heights varied from 40 to 23 inches. The net radiation was essentially the same over all five blocks regardless of moisture tension

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or plant height. A dryland block ( N 1 ) was badly wilted and evapotranspiration was essentially zero, but on the block with a moisture tension of almost zero the evapotranspiration was more than 2.5 times the net radiation. Lysimeters may be especially subject to advective heat influences, particularly of the “clothesline” type. Tanner (1957) cites some data of Moldenhauer (1952) showing that the evapotranspiration of reed canarygrass in 2.4-foot lysimeters surrounded by mowed grass exceeded the net rediation by 2.5 times for a 2-week period in August.

C. NET RADIATION

FERTILIZER EFFECTS Fertilizer does affect plant size, total leaf area, and often the color of the foliage. As mentioned above, Lemon et al. (1957) found no effect of moisture tension at which plants were grown and their heights on net radiation. His measurements were made when plants varied from full turgor to wilted, so there should have been differences in leaf temperature. Chlorophyll content and color of plants vary with nutrient supply. In fact, reflectance of red radiation at 625 mp has been proposed as a reliable measure of the chlorophyll content of the leaf (Benedict and Swidler, 1961). Energy in the visible spectrum amounts to about 40 per cent of the total solar radiation. Possibly, the visible differences in color and reflectance observed are compensated by differences in longwave radiation because of color of the radiating leaf surface. Aubertin and Peters (1961) studied the effect of 20- and 40-inch row spacings of corn, each at stand densities of 15,600 and 31,300 plants grown on a dark Brunizem soil and found differences in net radiation in midsummer. Net radiation was greatest when more soil could “see” the sky. On light-colored, dry soils net radiation, because of increased reflection, could be lower, the lower the soil cover. Bahrani and Taylor (1961) reported a marked decrease in net radiation from about 400 to 240 cal. cm.-2 day-1 when alfalfa was harvested, accompanied by an increase in mean soil temperature at the 4-inch depth from 24 to 33” C. After irrigation soil temperature dropped and net radiation rose to 320 cal. cm.-2 day-l. This removal of vegetative cover and irrigation of a noncovered soil surface is far more drastic than that fertilizers would bring about, except as fertilizer aided in establishment of vegetative cover. The extent to which fertilizers may affect net radiation of a crop must be considered to be an unsettled question at this time. AND

D. THESIGNIFICANCE OF ADVECTION The problem posed by advected heat, whether it be by horizontal or vertical transfer, is the extent to which container or small plot experiments

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VIES, JR.

can be relied on to predict the evapotranspiration and the evapotranspiration-yield relations of a large field. Tanner (196Ob) states, “Many of the evapotranspiration data that have been gathered from greenhouse pots and small plots are not suitable for field interpretation.” In an earlier paper Tanner (1957) stated: “When small plots, lysimeters, etc., are employed, they should be ‘guarded’ for some distance by a crop of identical size, maturity and moisture conditions. When moisture or fertility variables are inserted in an experimental field, they often introduce different crop heights on the different areas, and the ‘clothesline effect’ will cause different amounts of evapotranspiration on these areas. The usual small plot and lysimeter layouts, and also experimental strips across a large field, cannot be used to obtain evapotranspiration data representative of a large field.” Robins and Haise (1961) caution: “Plant height differences between small plots due to fertilizer or other treatments appreciably alter the turbulent air flow pattern and thus evapotranspiration rate, especially where advected energy is a problem. To the author’s knowledge, no suitable procedure and, in fact, no general understanding of the magnitude of such effects is now available. Some difference in evapotranspiration is inevitable, however, and this factor makes comparisons of such treatments in field experiments difficult.” Under conditions of low advection in humid regions, fertilizers may have much smaller or no effects on evapotranspiration. Van Bavel ( 1961) summarized data from Coshocton, Ohio; Seabrook, New Jersey; and Raleigh, North Carolina, showing that such diverse crops as corn and meadow had about the same evapotranspiration rates when grown adjacently in lysimeters. De \Vit (1958) has reinterpreted some of the data of Briggs and Shantz (1913a, 1914), Dillman (1931), and Miller (1916, 1923) and has developed the relation P = mW/E,, in which P is production per container, \V is kilograms water transpired, E , is evaporation in millimeters from a sunken U. S. Bureau of Plant Industry pan, and m is a constant characteristics of a species and holds for the bright sunshine conditions of the Great Plains from Dalhart, Texas, to Mandan, North Dakota. He says m is independent of weather, nutrient supply (if not too low), and water availability (if not too high). It is implicit that some variations in production ( P ) must come from differences in fertility status. The ratio W / E e may be a measure of advection. From actual alfalfa hay yields obtained in five western USA irrigation experiments, De \\’it calculated the evapotranspiration by the use of the rn values from the container experiments and found that they agreed closely with the water used. He states, “From the agreement between measured and calculated slope, it may be concluded that these experimental plots

FERTILIZERS AND THE EFFICIENT USE OF WATER

233

of half an hectare of less obtained considerable amounts of heat by advection.” De Wit points out that the transpiration rate of alfalfa in the field would have to be twice the free-water surface evaporation rate if m from containers was applied. So he concludes that “on fields, su%ciently supplied with water, the relation between transpiration and production can only be the same as in containers, if the fields are so small that sufficient energy is obtained by advection.” De Wit (1958)also examined the data available from northen Europe on dry matter production of oats, peas, and beets in relation to transpiration and found the relation P = nW, where P is dry weight, W is water transpired, and n is a constant for a species. The constant n derived from containers could not be used to predict evapotranspiration rates of large fields with high yields. Evapotranspiration of large fields was always lower because of lack of advected energy. This discussion of the heat budget approach and heat advection is not redundant to a discussion of fertilizers, crop yields, and evapotranspiration. The data now available come from experiments in which this iduence was either ignored or minimized, and in all cases, not evaluated quantitatively. At present no one has designed an experiment that meets what appear to be the requirements for an unequivocal answer to the effects of fertilizers on evapotranspiration. On the other hand, it must be remembered that in the natural situation in the field the crop is subject to advective transfer of heat. In arid and semiarid climates it is universally conceded that this influence is greater than in humid climates. Furthermore, the effects of advection can be great for short periods even in humid climates and yet be of little quantitative significance over a longer period of time such as the entire growing season of a crop. The best the author can do in reviewing the data available is to convey the pertinent details as given by the investigators reporting. IV. The Effects of Fertilizers on the Relationship of Evapotranspiration and Yield

At least six possible situations may exist for evapotranspiration ( E T ) and yield ( Y ) as Y is changed by fertilization which lead to six possible relations between water-use efficiency (Y/ET) and Y. Diagrammatic models of these are presented in Fig. l. For some cases no data are available, and therefore only hypothetical curves and lines can be given, The yield increase produced by fertilization usually follows a decreasing increment function of some sort (see Mason, 1956, for examples). This means that each succeeding equal increment a level of

2.34

FRiKK G . VIETS, JR.

nutrient availability is reached at which yield increases become infinitesimal. Therefore, the points derived from fertilizer experimentation on a plot of ET vs. 2’ draw closer together as 2’ increases if fertilizers were applied in equal increments of a nutrient. Overfertilization may produce yield decreases that are ignored in the models. In these models it is presumed that water is available for evapotranspiration and that the water conductivity of the soil and the capacity of the root system to absorb water are not seriously limiting the ability

FIG.1. Six possible models of the relation between evapotranspiration (E T ) and yield of dry matter (Y) (top part of each diagram), and water-use efficiency (Y/ET) reiative to yield (lower part). These models presume that water is nonlimiting for yield or evapotranspiration and that fertilizer is applied in equal increments resulting in declining increments of yield.

of the soil-plant system to meet the evaporative demand. Data presented in this section are chosen with this criterion in mind, but there is always some question as to the extent to which it is fulfilled as the soil dries and the moisture tension increases. Increase in moisture tension affects not only the capillary flow of water to the soil surface for evaporation, but also the flow to the root surface (see Kelley (1954), Veihmeyer and Hendrickson (1950), Hagan (1955), and Russell (1959) for recent reviews on the plant availability of soil water at various tensions, and contents at equal tensions). Tanner (1960a, b ) drew attention to difficulties of interpreting field experiments relating plant growth to soil

FERTlLIZERS AND THE EFFICIENT USE OF WATER

235

moisture tension because of the confounding effects of soil moisture on net radiation and the distribution of net radiation between crop and soil.

A. ET AND Y INCREASE LINEARLY;No ET WHENY Is ZERO Model A in Fig. 1illustrates these relationships. This case arises only in containers where evaporation is blocked by a vapor proof seal. Briggs and Shantz (1913a, 1914), Montgomery and Kiesselbach (1912), and Miller (1916) used this technique. ET becomes transpiration ( T ) only. T (or ET) is usually a linear function of Y and the regression line goes through the origin for both total water use and total yield. De Wit (19%) discusses these experiments in detail. The data of Ballard (1933) working with barley and Sudangrass in solution culture in which nitrate was varied, data of Lawes in 1850 cited by Briggs and Shantz (1913b) for wheat, barley, and clover, and data of Thom and Holtz (1917) for wheat in culture solutions fit this concept. Note that in this model there is no increase in water-use efficiency with increase in yield through fertilization. Such a situation could exist only in commercial hydroponics with extreme advection or in the field if plastic or rock mulches were used and all water was added below the mulch so that no evaporation could occur. B. E T AND Y INCREASE LINEARLY; E T Is APPRECIABLE WHENY Is ZERO See model B in Fig. 1. Figure 2 illustrates these functional relationships of timothy grown in lysimeters 56.42 cm. in. diameter at Gunnison, Colorado, with rates of nitrogen application of 0, 200, 400, and 800 pounds per acre. The data are the sums of two cuttings made on June 30 and September 1, 1960, respectively. The lysimeters were surrounded with timothy for a few feet but were subject to both vertical and horizontal advection from the surrounding dry mountains. The extrapolation of ET to zero yield gives an intercept of about 100 kg. or 40cm. of water. The actual evaporation from a bare soil in a lysimeter, irrigated like the grass grown in the others, when the tension reached 500 to 600 cm. of water at the 8-cm. depth was 56.7 kg. or 22.6 cm. for the same period. Linear ET vs. Y functions with an intercept give hyperbolic functions of Y/ET vs. Y in which Y/ET approaches a limit as ( E , T)/ET approaches 1, where E , is evaporation from the soil surface and T is transpiration, In other words evaporation becomes a smaller proportion of evapotranspiration. Such ET vs. Y functions give increasing water-use efficiency as yield increases simply because water evaporated when plant growth is zero is wasted. Another container experiment is that of Cassady ( 1957), who grew PLAINSMAN milo in paraffin-coated, 10-gallon garbage cans filled with

+

236

FRANK G . V E T S , JR.

Dalhart fine sandy loam that was fertilized well with phosphorus and potash. The cans, 14.16 inches in diameter and 16.5 inches deep, were buried to a depth of 13.5 inches on a rectangular plan with 45-inch centers. Bare, dry soil surrounded the cans. The experiment was conducted outdoors at Las Cruces, New Mexico. Seed was planted May 21, and the total yield of grain, tops, and roots was determined after

I

0

/

/ 04 0

I

i I

I

1

200

400

600

Yield in grams

FIG.2. A. Relationship of evapotranspiration (ET) to yield ( Y ) of timothy grown in lysimeters. B. Relationship of water-use efficiency to yield for the same lysimeters. The yields in each figure from left to right were produced with 0, 200, 400, or 800 pounds of nitrogen per acre, respectively. (Unpublished data of H. K. Rouse, F. Willhite, and A. R. Grable, Soil and Water Conservation Research Division, Agricultural Research Service, U.S.D.A., and the Colorado Agricultural Experiment Station cooperating. )

harvest 123 days later. Forty-eight amounts of nitrogen were applied in solution as NH4N03 at planting or at thinning to give a series from 0 to 940 pounds per acre. Plants were thinned to three per can after they reached a height of 6 inches. Water was added when tensiometers placed at either 6- or 14-inch depths showed a tension of 750cm. of water. Record was kept of all irrigation water applied and rainfall received. Drainage was prevented. Only a portion of the data is presented in

237

FERTILIZERS AND THE EFFICIENT USE OF WATER

Table I. ET is a linear function of the total top weight. Extrapolation of this line to zero yield gives an evaporation of 28 inches. The actual evaporation from a bare soil similarly irrigated was 12.54 inches. Wateruse efficiency is a hyberbolic function of yield. So the data conform in all respects to model €3. TABLE I Total Top Yield, Evapotranspiration, and Water-Use Efficiency of PLAINSMAN Milo Grown in Containers at Las Cruces, New Mexico, as Affected by Nitrogen Supplp Nitrogen applied (lb./acre) 0 0

100 200 300 400 500 600 700 800 900 940 a

b C

Y

ETb (in./can) 12.540 30.68 36.70 42.10 51.53 60.28 65.68 69.71 74.96 78.49 78.49 78.49

( g./can 1

No crop 22.4 62.0 102.5 157.5 208.9 240.4 310.2 325.9 353.9 355.2 351.9

Y/ET (g./in./can)

-

0.73 1.69 2.43 3.06 3.46 3.66 4.45 4.35 4.51 4.52 4.48

From Cassady (1957). Includes 0.88 inch precipitation. Private communication from C. F. Cassady, Jr.

Other container experiments fit this model: e.g., Scofield (1945) at Riverside, California, grew alfalfa in garbage cans filled with 87kg. of soil and measured ET and Y for plants on unfertilized soil and on soil irrigated with nutrient solutions (pertinent data are shown in Table 11.) The shape of the ET vs. Y function cannot be deduced from only two points, but fertilization did increase both ET and Y, and the extrapolated intercept of E T at Y is zero is positive. Fertilization increased water-use efficiency slightly. Allison et al. (19%) showed that the TABLE I1 Yield and Evapotranspiration of Alfalfa Grown in Containers at Two Nutrient Levelsa Number of cuttings 6

7 a

ET Growth period

Treatment

(kg.)

Jan. 6 to Oct. 16, 1943 Jan. 8 to Nov. 22, 1944

None Fertilized None Fertilized

605 682 608 756

From Scofield (1945).

Y (g.) 754 861 860 1143

Y/ET

1.25 1.26 1.42 1.52

238

FRANK G . V E T S , JR.

evapotranspiration of a broad array of crops variously fertilized and grown in 63-inch-diameter lysimeters filled with Lakeland sand was a linear function of crop yield. Extrapolation of E T to zero yield gave ET of 18.7 inches. They showed that the dry matter produced per inch of evapotranspiration was a linear function of yield but admit that a “slightly-curved” line would fit the data better. Since fertilizers increased yields of some of the crops markedly, use of fertilizers on this infertile sand increased efficiency with which the precipitation and irrigation water, applied in drought periods, was used. These lysimeters were unguarded and surrounded with sand. Hence, advective flow of energy was undoubtedly high. Some experiments with adequate water for evapotranspiration in the field show that fertilizers increased E T under the experimental conditions. Stanberry ( 1959) presents graphically the effects of nitrogen fertilization (none to 240 pounds per acre) on the yield and water use by barley grown on Superstition fine sand at Yuma, Arizona. The plots were small, and advected energy in this desert environment could be large. Fertilization increased yield from 15 to SO bushels per acre in a typical Mitscherlich-type curve, increased water use from 1s to a maximum of 24 inches (with 160 pounds of nitrogen an acre) and increased Y/ET from 0.83 to 3.7 bushels per acre-inch of water with a Y/ET vs. Y function that can be fitted with either a curve the shape of that shown in Fig. 1, model B, or a straight line with positive slope. Table I11 gives a portion of the data of Jensen and Sletten, who have published some of their results on the relationships of irrigation regime, yield, and evapotranspiration of grain sorghum (Jensen and Sletten, 1957; Jensen and Musick, 1960). Attention here is called to the data for the wet TABLE 111 Effect of Sitrogen Fertilizer on Yields and Evapotranspiration of Hybrid Grain Sorghum (RS-610) at Bushland, Texas, in 1958a Moisture treatment Nitrogen applied 1.’ (Ib./acre) (Ib./acre)

0 120

240

2979 2626 2430

M.2

M,b ET (in.) 14.8 15.4 14.8

Y/ET Y (lb./acre-in.) (Ib./acre) 201 171 164

3442 6964 7232

ET Y/ET (in.) (Ib./acre-in.) 21.0 22.3 22.9

164 312 316

a Jensen, h4. E., and Sletten, W. H. Unpublished data, southwestern Great Plains Field Station, USDA, Bushland, Texas, in cooperation with the Texas Agricultural Experiment Station. b Preplanting irrigation only. Rainfall from planting to harvest was 11.30 inches. c Preplanting irrigation plus 4-inch irrigations on August 12 and 30.

239

FERTILIZERS AND THE EFFICIENT USE OF WATER

treatment M4, in which nitrogen fertilizer almost doubled yields and increased ET sigdcantly by almost 2 inches. Water-use efficiency was almost doubled. A plot of ET vs. Y appears to be linear with a low positive slope, which makes the Y/ET vs. Y curve a very flat hyperbola. Fertilizer subplots in this experiment were 15 by 50 feet. On August 12 plants were in the late boot stage on the unfertilized plots and were headed and blooming on the plots getting 120 pounds of nitrogen. The average plant heights were 3.73, 4.35, and 4.05 feet for the 0-, 120-, and 240-pound rates of nitrogen application, respectively. Mean integrated moisture tensions to a depth of 4 feet were always higher on the plots getting 240 pounds of nitrogen than on those getting 60 except soon after irrigation. This difference in available moisture could have reduced the differences in ET due to fertilization. Winter wheat Y vs. ET relationships as affected by nitrogen application with adequate moisture ( M 4 ) for a dry year (1956) and a normal year (1957) as given by Jensen and Sletten are shown in Table IV. Some aspects of this work have been published (Jensen, 1956; Jensen and Musick, 1960). Nitrogen fertilization significantly increased both yields and evapotranspiration rates. Water-use efficiency was increased by TABLE IV Effect of Nitrogen Fertilization on Yields, Consumptive Use, and Water-Use Efficiencv of Conch0 Winter Wheat at Bushland. Texa9 Moisture treatment M4c

Year

1956 (dry year)

1957a (normal year)

N Y (lb.,/ (bu./ acre) acre)

Y (bu./ acre 1

ET Y/ET (in.) (bu./acrein.

0 80 120

16.9 18.1 17.5

19.4 19.7 20.3

0.87 0.92 0.85

33.6 45.9 52.4

23.6 30.4 30.2

1.42 1.51 1.74

0

27.1 42.3 44.4 20.2 33.9 26.7

17.1 18.1 17.3 18.4 19.4 19.1

1.58 2.34 2.57 1.10 1.75 1.40

28.8 52.5 48.9 26.3 49.8 38.2

24.1 28.4 27.4 26.6 27.0 26.2

1.20 1.85 1.78 0.99 1.84 1.46

120 180 1958 (wet year)

Y/ET ET (in.) (bu./acrein. )

0

120 180

Jensen, M. E., and Sletten, W. H. Unpublished data, Southwestern Great Plains Field Station, USDA, Bushland, Texas, in cooperation with the Texas Agricultural Experiment Station. b Preplanting irrigation only. 0 Preplanting irrigation plus 4-inch irrigations in April and May. Yields were adjusted for hail damage by counting broken culms per unit length of row on each treatment. Q

*

2440

FRASK G. VIETS, JR.

fertilization. In the wet year (1958) fertilization increased yields but had little effect on ET. In both 1957 and 1958, use of 180 pounds of nitrogen, as compared to the 120-pound rate, decreased yields and wateruse efficiency because of lodging. The fertilized subplots in this experiment were 12 by 65 feet. In the normal and dry years when advection would be expected to increase the ET of taller wheat in small plots, yield and ET were closely associated, but this was not the case in the wet year 1958. In all three years, nitrogen fertilization increased the efficiency of water use. C. ET Is INDEPENDENT OF Y This situation could exist where three conditions obtain: ( 1 ) there is no advective heat; ( 2 ) soil surface is continuously moist; and ( 3 ) net radiation of bare moist surface is the same as that of area with complete vegetative cover. The ET-Y relationship and the Y/ET-Y relationship are shown in model C of Fig. 1. This case gives a linear increase in water-use eEciency with increasing yield. The author is not aware of data with land plants that fit this model. This situation might occur with submerged or free-floating aquatics if their presence did not change the net radiation.

D. ET IS

IhmEPENDENT OF

Y AFTER REASONABLY COMPLETE COVER Is A-ITAINED

This relationship is shown in Fig. 1 as model D. A portion of the data of Holmen et al. (1961) is plotted in Fig. 3 for illustration. In Fig. 3A, ET is plotted against bromegrass hay yields for the 1955 and 1956 seasons at Upham, in central North Dakota. The high-moisture plots were irrigated when 40 per cent of the available water was depleted to a depth of 4 feet, and the medium-moisture plots when 70 per cent was depleted. Rainfall was 9.8 inches each year. In 1955, three cuttings were taken in a 144-day season, the data are for plots getting 80, 160, or 200 pounds of nitrogen per acre. In 1956, two cuttings were taken in a 133day season; data are from plots getting 0,40,80, or 160 pounds of nitrogen per acre. Yields were lower in 1956 because of severe winterkilling the previous winter. ET for medium moisture plots was slightly lower in both years than for high moisture plots. In each year and in each moisture regime fertilization markedly increased yields, but had on significant effect on ET. In Fig. 3B water-use efficiency is shown to be a linear function of yield whether yield is changed by season, irrigation regime, or fertilizer application. Only the nonirrigated plots to be discussed later do not conform to this model. As part of the same study, Holmen et aZ. (1961), using the same treatments but cutting with twice the frequency to simulate pasturing, found that again ET was unaffected by fertilizer

FERTILIZERS AND THE EFFICIENT USE OF WATER

241

application even though pasture yields were increased two- to threefold. Water-use efficiency again was linearly related to yield. Weaver and Pearson (1956) measured ET and Y of Sudangrass from 2-inch height to soft dough stage (July 3-30) at Auburn, Alabama. Treatments were two moisture levels, two nitrogen levels (0 or 100 pounds per acre) and three stand densities in all combinations. All subplots were 14 by 4 2 / 3 feet contained in a bin of Lloyd clay loam

30t

400 .

.-

c

r

A

=;

*+

. ++.

i

B

-

I

0

2

4 Tons per acre

I

6

Fig. 3. A. Evapotranspiration ( ET) in relation to yield ( Y ) of smooth bromegrass in central North Dakota. B. Water-use efficiency (Y/ET) of smooth bromegrass as a function of yield. Filled circles ( 0 ) denote results for a high moisture level and crosses ( + ) for a medium moisture level, both levels being attained with irrigation. The open circles ( 0 ) denote results for nonirrigated land. (From Holmen et al., 1961.)

that had a concrete wall extending 4 inches above the soil surface. Opportunities for advection would appear to be high. On the highmoisture treatment ( irrigation at %-atmosphere tension at 8-inch depth) mean of the three populations for no nitrogen were: Y = 1791 pounds/ acre, ET = 4.91 in. and Y/ET = 365 pounds/acre-inch. For the fertilized plots the means were Y = 3089 pounds/acre, ET = 5.10 inches, and Y/ ET = 606 pounds/acre-inch. Fertilization had no significant effect on ET but water-use efficiency was markedly increased because Y was

242

FRANK G . VIETS,

JR.

increased. On the low-moisture plots (irrigated to prevent wilting) ET and Y were lower but fertilization still increased Y, had no effect on ET, and increased water-me efficiency. Highest water-use efficiency was obtained with high moisture and N application. Carlson et al. (1959) obtained marked increases in water-use efficiency with irrigated corn by the use of 120 pounds of nitrogen per acre in central North Dakota, in both 1956 and 1957 as shown in Table V. Two stand densities were used, and plots were irrigated so that available moisture in the 0- to 4-foot depth did not drop below 40 per TABLE V Forage Yield, Evapotranspiration, and Water-Use Efficiency of Corn for Two Levels Each of Nitrogen Fertilizer, Moisture, and Plant Densitya Nonirrigated

Irrigated

Treatment

Y (Ib./acre)

ET (in.)

A B C D

6560 7330 7170 7240

10.82 11.11 10.57 10.97

1956 606 660 678 660

5870 9330 6870 10630

13.91 14.81 15.56 13.77

422 630 442 772

A B

5060 5950 5950 5860

10.03 10.39 10.14 9.82

1957 504 573 587 597

6780 9110 8300 10900

16.89 18.38 19.40 16.53

401 496 428 659

C D

Y/ET (lb./acre-in.)

Y (Ib./acre)

ET Y/ET (in.) (Ib./acre-in.)

Adapted from Carlson et ul. (1959). A: No nitrogen; 14,000 plants per acre 1956, 10,000 in 1957. B: 120 lb. nitrogen; 14,000 plants per acre 1956, 10,000 in 1957. C: no nitrogen; 23,000 plants per acre 1956, 20,000 in 1957. D: 120 Ib. nitrogen; 23,000 plants per acre 1956, 20,000 in 1957. a b

cent of storage capacity. Nitrogen fertilization did not affect seasonal evapotranspiration. Data of Jensen and Sletten for the M 4 moisture treatment reported in Table IV showed no effect of fertilization on evapotranspiration of winter wheat in 1958 at Bushland, Texas, even though yields and water-use efficiency were almost doubled. Penman (1956a) plotted the cumulative growth of orchardgrass (Dactylis gZomeruta) from May, 1954, to November, 1955, against the potential evapotranspiration on plots where water was nonlimiting at Woburn, England. Grass was cut when 8 to 9 inches in height. Two approximately straight lines were obtained; one for a plot getting 0.15 cwt. nitrogen per acre after each cutting gave less yield than for a plot getting 0.30 cwt. nitrogen per acre. The potential transpiration was

FERTILIZERS AND THE EFFICIENT USE OF WATER

243

calculated from meteorological data; evapotranspiration was not actually measured. If the actual evapotranspiration was identical with potential evaporation and not influenced by fertilization, then fertilization produced a marked increase in water-use efficiency.Schofield (1952) stated that evapotranspiration of grass in lysimters at Rothamsted was measurably the same over a threefold range of dry matter production produced by the application of fertilizer. The water-use efficiency would therefore be tripled. Bryan and Brown (1961) found that nitrogen fertilization of cotton, both irrigated and nonirrigated, had no consistent, measurable influence on the evapotranspiration rate in studies on Sharkey clay and Grenada silt loam in eastern Arkansas. AS Y INCREASES E. ET DECREASES

This model is shown as E of Fig. 1. Since ET is so dependent on total net radiation and Y so dependent on the shortwave (visible) portion of the total net radiation, it is quite unlikely that ET could decrease as Y increases. The author is not aware of data fitting this model. If means are found to increase the efficiency of photosynthesis, thus converting more radiant energy into chemical energy and leaving less energy for heat, then ET could conceivably decrease as L increases.

F. ET INCREASES FASTER THANY INCREASES No data are known to the author which fit this model ( F in Fig. 1 ) . ET is strictly a physical phenomenon dependent on the energy available for evaporation, provided that water is available for evaporation and that transpiration is not limited by the capacity of the plant to transmit water through root damage or plant senesence. Y on the other hand is a complex phenomenon depending to a large extent on soil and crop management practices, such as fertilization, plant spacing, and temperature, that affect the production of leaf area and its assimilation rate. Therefore, it is reasonable to assume that ET is always at or near the radiation potential when water is adequate, whereas Y is not. So ET cannot increase faster than dry matter production. Instances could probably be found in which overfertilization decreases the salable product or cuts the production of grain while total dry matter production and ET are increasing. Such instances would fit this model.

THATCANNOTBE CLASSZFIED G. EXPERIMENTS The reports cited below contain worth-while information that cannot be classified in the six models because of insufficient points to establish an ET vs. Y function with certainty, too little information, or ET was not measured separately for each fertility treatment.

244

FRANK G. VIETS, JFt.

Stanberry ct al. (1955) conducted a 3-year experiment with alfalfa, under probably the greatest conditions of advection possible in the field, on small plots surrounded by desert on the Yuma Mesa, Arizona. The main plots were three irrigation regimes as follows: (1) “dry”-irrigate ( 5 or 6 inches of water) when plants show darkening of foliage and top-droop that precedes wilting; ( 2 ) “medium”-apply 4 inches of water whenever tensions at 12 or 18 inches approximate 600cm. H2O tension; and ( 3 ) “wet”-add a 2-inch irrigation when tension approximates 200cm. H,O at 12-inch depth. The quantities of water used on these main plots were 215, 223, and 255 inches for the 3-year period and include the meager rainfall. Each irrigation plot contained four subplots (15 by 15 feet) getting 1, 2,4, and 6 cwt. PzOj per acre banded before seeding. Stanberry et al. stated that there was no deep percolation, and they assumed that water applied plus rainfall represented the evapotranspiration. No measurements were made of evapotranspiration on individually fertilized subplots. This means that if fertilizers did increase ET after irrigations, the effects were compensated by decreased ET due to decreased moisture availability before the next irrigation. The significant data are shown in Table VI. The authors concluded that the irrigation regimes had no significant effect on water-use efficiency, TABLE VI Total Yields, Evapotranspiration, and Water-Use Efficiency for Alfalfa for 1950-1952 at Yuma, Arizona, as Affected by Irrigation Treatment and Phosphate Fertilizationa Irrigation treatment

P20, Applied (lb./acre) 100 200 400 600 ET (in.) a b

Medium

Dry

Y

Y/ET

Y

Y/ET

Wet

Y

Y/ET

( T.b/acre)(T./acre-in. ) ( T./acre) ( T./acre-in.) (T./acre) ( T./acre-in.)

25.77 28.62 34.52 34.94

0.120 0.133 0.160 0.162 215.3

28.74 30.73 37.65 40.57

0.129 0.138 0.169 0.182 223.3

33.29 37.72 44.79 47.20

0.130 0.148 0.175 0.185 255.3

After Stanberry et al. (1955). Ton (2000 pounds).

but that the use of superphosphate increased it. However, Stanberry (1959) showed for all fertility treatments in the experiment that wateruse efficiency on the medium treatment was about 90 per cent of the value for the wet treatment and about 80 per cent of the value for the dry treatment. Keller (1954), in a study of greenhouse techniques for measuring

FERTILIZERS AND THE EFFICIENT USE OF WATER

245

water requirements of grasses as a tool for use in breeding programs for grasses with lower water requirements, found difference in water requirements of orchardgrass whether grown with a high or a low supply of nitrogen. Grass yields and consumptive use were also not significantly affected. Trumble and Walker (1952) stated that the transpiration ratio of first-cutting Wimmera ryegrass in Australia was reduced from 626 to 481 by application of nitrogen. For three cuttings the ratio was reduced from 567 to 480. Rosanow (1959) reported that the water transpired per gram of ryegrass produced was 3 5 times greater when the yield was low because of nitrogen deficiency than when it was high with optimal nitrogen fertilization. Increased rates of nitrogen producing higher dry weights did not always increase total evapotranspiration. Hanks and Tanner (1952) determined the water-use efficiency of corn, oats, and cucumbers grown with adequate soil moisture on Plainfield sand in Wisconsin and found that the bushels of corn produced per acre-inch of water for 1949 and 1950 averaged 3.69 and 2.63 for the high and low nitrogen treatments, respectively. For oats in 1949 the figures were 4.37 for high nitrogen and 2.12 for low nitrogen, respectively. For cucumbers in 1949, 507 pounds were obtained per acre-inch of evapotranspiration when 72 pounds of nitrogen, 48 pounds of Pz05, and 96 pounds of K 2 0 were used per acre. With 6 pounds of nitrogen, 32 pounds of P205, and 48 pounds of KzO, only 398 pounds of cucumbers were obtained per acre-inch of evapotranspiration. Acre yields and evapotranspiration data are not given.

H. RECAPITULATION Six possible models of evapotranspiration-yield relationships have been presented for conditions where ET is not limited by water availability. Only two of these models ( B and D of Fig. 1 ) appear to exist in the field. If fertilizer increases yield, evapotranspiration either increases or remains the same. If evapotranspiration increases as yield increases (model B ) , then water-use efficiency is a decreasing-increment function of yield and is asymptotic to a limit that has no physical sigdcance. If evapotranspiration is independent of yield ( remains the same), then water-use e5ciency increases Iinearly with yield (model D). The important point is that if fertilizers increase yield they increase automatically the efficiency of the consumptively used water. The consumptive use may or may not increase, depending on ( 1 ) the interaction of changed plant size, cover, and height with available advected energy, and ( 2 ) the effect of changes in plant cover and color on net radiation. Insufficient information on these points and on the magnitude of advection in various climatic and crop situations is now available.

!?A6

FRANK G.

VETS, JR.

V. Fertilizers and Water-Use EfFiciency in Terms of Applied Water

Many experiments are conducted in which no attempt is made to measure consumptive use because of cost or because the site or soil characteristics pose unsurmountable difficulties in getting meaningful measurements. However, the irrigation water put on the area and the runoff are accurately measured and precipitation is accurately gaged. Such experiments also show that water can be used more efficiently if fertilizers increase yield. Scarsbrook et al. (1959) reported that increased efficiency in use of applied irrigation water by cotton was attained through nitrogen fertilization in Alabama. With 12 inches of irrigation water in 1956 and 13 inches in 1957, the mean 2-year production of lint per acre-inch of irrigation water was 29, 40, 45, and 57 pounds for 0, 60, 120, and 240 pounds of nitrogen per acre, respectively. Irrigation and nitrogen fertilization delayed maturity. Haddock ( 1953, 1959) and Kelley and Haddock (1954) give voluminous data on the favorable effects of nitrogen, phosphorus, and manure fertilization on sugar beet yields and total sugar produced in various moisture regimes wherein the amount and timing of irrigation were varied in relation to moisture tension or depletion of available soil moisture. For some experiments the amount of irrigation water applied is given. No consumptive use data are reported. Since fertilizers increased yields of roots and sugar regardless of the irrigation regime, it follows that fertilization increased production per inch of water applied. In the production of mountain meadow hay at Hayden, Colorado, 'IVillhite et aZ. (1956) found for a 3-year period that 8.7 acre-inches of water from irrigation and rainfall were required without fertilizer to produce a ton of hay, 7.9 acre-inches with adequate phosphorus, 5.6 acre-inches with an average of 180 pounds of nitrogen per year, and 5.3 acre-inches with nitrogen and phosphorus together. The corresponding hay yields were 2.6, 2.9, 4.0, and 4.3 tons per acre. VI. Fertilization and Water-Use Efficiency with limited Moisture Supply

Under semiarid conditions, crops and range grasses are subjected to unpredictable periods of high moisture stress between rains or to increasing moisture stresses as the water accumulated over winter or in a preceding weedless fallow period is extracted. All too frequently, the plant has a finite amount of water on which it can draw and then whether it withers and dies or survives depends on its innate capacity for drought resistance. If the plant has produced its crop, there are no serious consequences of this soil moisture depletion. Magnitude of the stresses to which wheat may be subjected was shown by Haise ct al. (1955) in their finding that the moisture content of the soil in which roots were

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well-disseminated was greater than 26 atmospheres when the soil had reached its “minimum point” of water content. Kmoch et al. (1957) and Ramig (1959) also show significant moisture depletion below the 15atmosphere percentage by wheat in Nebraska. Fifteen-atmosphere tension is usually regarded as the lower limit of available moisture for most practical purposes. Therefore, one of the problems of dryland agriculture and range management is to determine whether there are favorable moisture interludes in the cycle of plant development when the soil cannot supply sufficient nutrients for maximum growth. If, in these periods, fertilizers can increase the net assimilation rate or growth without exhausting water at a faster rate, then total yield and water-use efficiency can be increased. If fertilizers accelerate the rates of both growth and water use, the yield and water-use efficiency will depend on the total supply of water and the status of the crop when the moisture supply becomes exhausted. Thus accelerated water use through fertilization can be disastrous for grain crops if the soil moisture supply is exhausted and rains do not come before the grains are filled. This timing of moisture use, total moisture supply, and plant development is much less critical for crops that are grown for their vegetative parts and need not complete their life cycle through seed production. Forage grasses. Holmen et al. (1961) studied the Y vs. ET relationships of smooth bromegrass grown without irrigation and simultaneously studied relationships in irrigated bromegrass, as shown in Fig. 3. Nitrogen was applied at rates of 0, 40, or 80 pounds per acre. Fertilization increased hay yields and had no significant effect on total evapotranspiration; water-use efficiency increased linearly with yield. Similar results were obtained when grass was cut twice as often to simulate pasturing. Whether comparisons are made on the basis of hay or pasture yields, the water-use efficiency was higher on dryland than on irrigated land for a comparable rate of nitrogen application. Burton et al. (1957) found marked reductions in the water required to produce a pound of dry matter by three varieties of Bermudagrass, Pensacola Bahiagrass, and Pangolagrass when 50, 100, or 200 pounds of nitrogen per acre was applied in each of 2 years. These studies were conducted on a deep sand at Tifton, Georgia. The authors assumed that all precipitation from April 1 through October 31 was consumptively used and none was lost by percolation beyond the root zone or by runoff. Moisture depletion in the profile between start and end of the season was assumed to be 4 inches. This was added to rainfall. In 1953 water use was then stated to be 49.66 inches and in 1954,17.68inches. Even in dry 1954, fertilization about doubled water-use efficiency for all grasses except Pangola.

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Studies were conducted under more droughty conditions than those noted above: Sneva et al. (1958) found that fertilization with NH4N03 reduced the pounds of water required to produce a pound of crested wheatgrass hay in dry central Oregon where precipitation comes mainly in the winter. With 0, 10, 20, 30, or 40 pounds of nitrogen applied per acre to different plots in each of 3 years, the respective average “water requirements” for the year of application were 2800, 2400, 2000, 1900, and 1900, respectively. Mean annual hay yields increased from 711 to 1176 pounds per acre. Water used was taken as precipitation from November 1 to June 1 and was not actual consumptive use. Fertilization increased the percentage of the total yield produced before June 1. This more rapid growth depleted the soil moisture at the 6- and 15-inch depths more rapidly the greater the quantity of fertilizer used. The fertilized grass cured earlier as a result of the earlier exhaustion of soil moisture, which appeared to be complete at those depths by early July. Plots were 15 by 15 feet in an area where advection is probably great. Thomas and Osenbrug ( 1959) reported marked increases in efficiency of use of seasonal precipitation through nitrogen fertilization of smooth brome-crested wheatgrass grown for hay on Pierre clay in western South Dakota. Replicated plots 8 by 35 feet were fertilized annually for 4 years. Hay yields were taken each year and for a subsequent 4 years. Seasonal precipitation was rainfall between March 30 and June 22, these dates corresponding to initiation of annual growth and harvest, respectively. Seasonal precipitation vaned from a low of 3.06 to a high of 9.82 inches, the mean being 5.72 inches for the period of study. Fertilization with 255 pounds of nitrogen an acre more than doubled mean yields and efficiency of rainfall use, as shown in Table VII. Lower rates of nitrogen produced smaller, but still highly significant, increases. The regression coefficients of yield on seasonal precipitation within a fertility level showed a marked increase as fertilizer application was increased. Haise et al. (1960) reported unpublished data of J. R. Thomas which show marked increases in water-use efficiency of a smooth brome-crested wheatgrass mixture from fertilization of Pierre clay in both 1956 and 1957 in western South Dakota. Pounds of hay per acre-inch of water consumptively used increased linearly with yield, fertilization having little or no effect on evapotranspiration under these dryland conditions. Similar unpublished data of J. J. Bond obtained with blue gramagrass (Bouteloua gracilis) and Sudangrass in the Texas Panhandle are also cited. Haise et al. (1960) plotted water-use efficiency against yield for all these data. Within a grass species for a particular year, these functions were linear, but the regression lines of different species and of different

249

FERTILIZERS AND THE EFFICIENT USE OF WATER

years for a species had different slopes. Nevertheless all water-use efficiency vs. yield functions were linear and can be extrapolated to the origins of both axes, thus conforming to model D in Fig. 1. Wheat. Koehler (1960) measured the effect of nitrogen fertilization on the consumptive use (April 29-July 23) and yield of Omar winter wheat on summer fallow in the 15-inch rainfall area of Washington. Growth was accelerated by nitrogen applied at each of five rates from 0 to 160 pounds per acre. Plants getting 80 or 160 pounds reached maximum growth considerably earlier than the others. Difference in TABLE VII The Effect of Fertilizer on Yields and Efficiency of Use of Seasonal Precipitation for an 8-Year Period by Smooth Bromegrass-Western Wheatgrass in Western South Dakotaa Total fertilizer applied per acre None 32 Tons of manure 32 Tons of manure and 85 pounds of N 85 Pounds of N 170 Pounds of N 255 Pounds of N 170 Pounds of N and 255 pounds of P,O, a

Linear regression coefficient (lb./acre-in.)

Mean annual hay yield (lb./acre)

Yield vs. seasona1 precipitation (lb./acre-in. )

490 828 999

86 145 175

36.2 117.2

661 844 1067 872

116 148 187 152

57.0 74.6 103.2

-

From Thomas and Osenbrug ( 1959).

total consumptive use between the unfertilized wheat and that getting 160 pounds of nitrogen per acre was 0.7 inch, the maximum use being about 12.5 inches. All the difference in consumptive use was due to greater moisture extraction in the 6-, 7-, and 8-foot depths. The top 5 feet had no available water when the wheat matured. Koehler used the statistical procedure of Leggett (1959), which predicts that 4 inches of water is needed just to produce a wheat plant, and then calculated that 6.4 bushels of wheat was produced per acre-inch of water without fertilizer and 7.2 bushels was produced per acre-inch with 80 pounds of nitrogen. This gain in efficiency is not outstanding, but then fertilization increased yield only from 58 to 68 bushels on this Walla Walla silt loam summer-fallowed the previous year. Power et aZ. (1960) found that RESCUE spring wheat in northeastern Montana produced more dry matter and grain per inch of soil moisture and precipitation used when the wheat was fertilized with phosphorus.

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VIETS, JR.

Dry matter was increased by phosphate application at seeding at each of the following stages: tillering, heading, dough stage, and harvest; yield was raised about 4 bushels, or 16 per cent. Phosphorus fertilization and the resultant accelerated growth did not increase total evapotranspiration or the rate of soil moisture depletion on the four moisture regimes employed (see Section VII). The plots were 8 by 20 feet. Ramig (1959) measured the effects of stored soil moisture at planting and nitrogen fertilization on the yields and evapotranspiration of Cheyenne winter wheat on plots 64 inches by 13 feet on Holdrege silt loam at North Platte, Nebraska. Soil was wet to about 0, 2, 4, and 6 feet by preplanting irrigation. Evapotranspiration was taken at rainfall plus change in soil moisture content to the 6-foot depth between planting and harvest. No runoff occurred. Average data for 2 years and the two moisture extremes as shown in Table VIII. When the soil was dry at TABLE VIII Average Evapotranspiration and Water-Use Efficiency of Cheyenne Winter Wheat Grown after Wheat at North Platte, Nebraska, for 1954 and 1955a Approximate inches of available water in 6-foot profile at seeding time Nitrogen applied (Ib./acre)

0 20 0 ' 40 40 80 60 b

Fb

s F

s

F S

8.1

0 ET

Y/ET

ET

( in. )

( bu./acre-in. )

(in.)

Y/ET (bu./acre-in. )

13.7 13.6 13.2 13.8 13.8 13.7 13.4

0.62 0.90 0.76 0.78 0.67 0.62 0.61

20.3 20.7 21.2 21.3 21.3 21.4 21.6

1.12 1.66 1.44 1.84 1.74 2.04 1.84

From Ramig (1959). F and S are fall and spring applications, respectively.

planting, nitrogen fertilization had no effect on yield, evapotranspiration, or water-use efficiency. When the soil was wet to field capacity 6 feet deep ( approximately 8.1 inches of available moisture) nitrogen fertilization increased yield and total water used. but the water-use efficiency increased from 1.12 bushels per acre-inch to a maximum of 2.04 bushels. On the plots with a full profile at seeding, nitrogen fertilization increased the use of water at tensions above 15 atmospheres. This amounted to as much as 2 inches for the profile. On the dry soil, extraction at tensions above 15 atmospheres could not contribute to total water use because these plots were drier than the 15-atmosphere tension at time of seeding. Near Amarillo, Texas, Jensen and Sletten got significant increases

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in yield and water-use efficiency by nitrogen fertilization of Concho winter wheat in normal 1957 and wet 1958,but not in dry 1956. Data are shown in Table IV as the MI moisture treatment that got only a preplanting irrigation. The yields and consumptive-use data show that this treatment was definitely moisture-limited compared to the M4 treatment. The M1,however, was not typical of nonirrigated land. Corn. Carlson et al. (1959) studied the effect of nitrogen fertilization at two stand densities of corn on nonirrigated plots concurrent with their studies on irrigated corn. The data are shown in Table V. On dryland, nitrogen fertilization did not affect yields, water use, or water-use efficiencies significantly in either year. Forage produced per acre-inch of water was higher without irrigation than with irrigation unless a high nitrogen rate and thick stand were used together. As mentioned earlier, this points up the fact that water-use efficiency is frequently greater on dryland than on irrigated land. Further, the interaction of nitrogen and stand shown in these data demonstrates the need for the best combination of soil and crop management practices if water is to be used efficiently. Grain sorghum. Jensen and Sletten (see Table 111, M1 moisture treatment ) got no significant differences in yield, evapotranspiration, or water-use efficiency of hybrid grain sorghum getting one preplanting irrigation only when nitrogen fertilization varied from none to 240 pounds per acre. Comparison of yields and evapotranspiration data of the M 1 vs. M 4 moisture treatments shows that water was certainly limiting on the M1treatments. Cereal crops in Nebraska. Olson et al. (1960 with additions from personal communication) reported that nitrogen fertilization increased yields, soil moisture extraction, and water-use efficiency of four nonirrigated crops grown at many locations in Nebraska. In 29 experiments with wheat, nitrogen increased average yields from 31 to 37 bushels an acre, water use by 0.9 inch, and water-use efficiency by 12 per cent. In 16 experiments with oats, yields were increased from 46 to 66 bushels, water extraction by 0.8 inch, and water-use efficiency by 38 per cent. With corn in 12 experiments, yields were raised from 70 to 112 bushels an acre, water use went up 1.3 inches, and water-use efficiency increased by 44 per cent. Grain sorghum in nine experiments showed a yield increase of 15 bushels an acre over a check yield of 60 bushels. Water use was up 1.4 inches and water-use efficiency by 11per cent. Olson also states that even under very dry conditions water-use efficiency was increased by fertilization and that it was higher under dry conditions than when moisture supply was more plentiful. He also reports reductions in water-use efficiency when nitrogen was used on wheat grown on fallowed land that produced high yields without fertilizers even though

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FRANK G. VIETS, JFL

adequate water was left in the profile at harvest. Consumptive use was taken as the difference in soil moisture content to a depth of 6 feet between planting and harvest plus 90 per cent of the precipitation, the other 10 per cent assumed to have run off. Differences in consumptive use induced by fertilization were due to differences in moisture extraction in the upper 4 feet of profile by oats and wheat and throughout the 6-foot profile for corn and sorghum. VII. Fertilization and Moisture Extraction by Roots

The favorable effects of fertilizers, when nutrients are deficient, on the mass and distribution of roots have been reported many times and are generally well known. In general, fertilization promotes top growth faster than root growth, leading to an increased top : root ratio. However, there are only a few reports on the concurrent effects on total water extraction and on vertical and horizontal changes in the extraction pattern. Whether these would be expected to change would depend, at least in part, on whether the evaporative demand of the tops were changed. Changes in depth of rooting or ramification of roots in the soil would be of particular importance under conditions of limited water supply where full exploitation of soil water would be of greatest significance. Two studies with wheat show no effect of fertilization on soil moisture extraction. Perhaps many studies with similar results have been conducted but have not been published because of the negative findings. Zubriski and Nonim (1955) in 12 North Dakota trials showed that there was only 7.8 inches of total water, not available water, remaining in the soil profile to a depth of 5 feet whether the wheat without fertilizer yielded 13.1 bushels per acre or with fertilizer yielded 17.9 bushels per acre. Power et al. (1961) found that fertilization of dryland spring wheat with phosphorus on Williams silt loam in northeastern Montana had no consistent effect on cumulative soil moisture use or total moisture use from seeding to tillering, to heading, to dough stage, or to harvest, respectively. Phosphorus application increased dry weight production compared to nonfertilization at all growth stages and raised average grain yields about 4 bushels per acre, or 16 per cent. This experiment had two levels of preplant moisture ( 2 and 4 feet of wet soil) and two levels of seasonal moisture attained by irrigation or by covering some plots during rains (see Section VI). Several investigators have reported that fertilization does affect the rate of depletion and/or total extraction of soil moisture. Koehler (1960) found no differences in soil moisture content to a depth of 4 feet of a deep Walla Walla silt loam at harvest due to fertilization of wheat, but

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he found that the differences in total water use of 0.7 inch between unfertilized plots and those fertilized with 160 pounds of nitrogen per acre occurred in the 5-, 6-, and 7-foot depths. Kmoch et al. (1957) studied the distribution and weight of wheat roots, and available soil moisture content at three dates on plots adjusted to four levels of soil moisture by irrigation before planting, with and without the application of 80 pounds of nitrogen per acre. The soil was a deep Holdrege very fine sandy loam at North Platte, Nebraska. Samples taken in June, 4 weeks before harvest, showed about 50 per cent greater root weight due to nitrogen fertilization regardless of the depth to which the soil was wet at planting. Soil wet to zero or 2 feet at planting contained less roots than that wet to 4 or 6 feet. Soil samples showed that the nitrogen fertilizer had increased the moisture extraction by 1.5, 0.9, 1.2, and 5.0 inches for the 10-foot profile on the plots having 0, 2, 4, and 6 feet of wet soil, respectively, as compared to the unfertilized plots. Extraction of moisture at tension values over 15 atmospheres occurred. Thus nitrogen fertilization increased total root weight and soil moisture depletion, but appeared to have little effect on rooting depth. As part of this study, Ramig (1959) showed that nitrogen fertilization resulted in less available water in the 6-foot profile at all times of sampling from April 11 to harvest on July 12, 1956, where the profile at seeding had been wet to 6 feet. With 80 pounds of fall-applied nitrogen, water extraction of as much as 1.08 inches in excess of the 15-atmosphere percentage for the 6-foot profile had occurred. Since there was no runoff, these effects of fertilizers were effects on evapotranspiration, not on runoff. Where the profile was dry at seeding, nitrogen fertilization had no effect on soil moisture content as the profile was always dry and the soil moisture content for the 6-foot profile was always less than that for the 15atmosphere percentage. Thus the wheat yields of about 5 bushels per acre obtained on this soil moisture regime were due to quick interception and use of surface moisture from rains. Sneva et al. (1958) found that nitrogen-fertilized crested wheatgrass in semiarid central Oregon depleted the soil moisture at the 6- and l5inch depths more rapidly than unfertilized grass. This earlier depletion of soil moisture caused the grass to cure earlier, but the hay yields were also increased. Smika et al. (1961) showed that fertilization of mixed native grass at Mandan, North Dakota, with 30 or 90 pounds of nitrogen per acre annually had increased the moisture extraction to a depth of 6 feet compared with unfertilized grass in the 3 years between the start of the experiment and the inception of soil moisture sampling in 1954 and that these differences in soil moisture in the subsoil persisted through 1958. The plots were 5 feet wide and 20 feet long.

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Nitrogen fertilization increased profile moisture extraction in a series of experiments reported by Olson et al. (1960 plus personal communications). In 29 wheat experiments and 16 oat experiments without irrigation in Nebraska, moisture use from the 4-foot profile was increased 0.9 and 0.8 inch, respectively. Little water use from the 5- and &foot depths occurred. In 12 experiments with corn and 9 with grain sorghum, nitrogen fertilization increased profile moisture use 1.3 and 1.4 inches, respectively. This additional moisture came from throughout the 6-foot profile. Detailed studies in 1960 and 1961 showed that fertilization always resulted in less water in the soil profile through the growing season. Fertilization does sometimes permit deeper penetration of the soil by roots, and thus the amount of water available for extraction is increased. Under dryland conditions, the deeper subsoil frequently contains no available water for exploitation, but under more humid conditions this subsoil moisture is a reserve that can be tapped. Koehler (1960) noted greater use of moisture below the 5-foot depth in eastern Washington by nitrogen-fertilized wheat. Pesek et al. (1955) indicated that unfertilized corn in Davis County, Iowa, in 1953 took soil moisture from less than 5 feet of soil whereas well-fertilized corn removed water to a depth of 7 feet, as indicated by late fall moisture sampling. The amount of extra water used was around 4 inches. The fertilized corn produced more bushels per acre for each inch of water used than did the unfertilized corn. Nelson and Stanford (1958) cite studies of G. E. Smith showing that corn on a Missouri clay pan soil could not effectively use subsoil moisture unless fertilized. On August 17 unfertilized corn had 4.5 inches of available water remaining to the 42-inch depth and well-fertilized corn had only 1.04 inches. Without fertilizer only 18 bushels per acre was produced, requiring 21,000 gallons of water per bushel. Adequate fertilizer produced 79 bushels per acre with a water requirement of only 5600 gallons per bushel. Although Carlson et aE. (1959) reported no effect of nitrogen fertilization on the total evapotranspiration of corn (see Section IV, D and Table V ) , they stated that it increased the proportion of the total water taken from the upper 2 feet of the profile. From a 4-fOOt profile 90 to 95 per cent was taken from the top 3 feet, 75 to 85 per cent from the top 2 feet, and 45 to 55 per cent from the surface foot at various sampling intervals. Vill. Fertilizers and the Infiltration of Water

Since fertilizers frequently increase the amount of vegetative cover and retard runoff, infiltration may be increased. This increased infiltration and greater soil moisture storage is significant in areas where water is

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in short supply. Fertilizers thus indirectly may increase the efficiency of precipitation. Information given here deals specifically with the chemical effects of fertilizers directly on infiltration of water into soils under field conditions. However, the chemical effects are not easily separable from the effects on plant cover and mass of roots. Pillsbury and Richards (1954) observed an interaction between nitrogen source and amount of organic matter addition on the infiltration rate of irrigation water into Ramona sandy loam in southern California. With no organic matter addition, the nitrogen source had little effect. With organic matter additions, ( NH4 ) 2 S 0 4 produced higher infiltration than did urea when both were added annually at the rate of 2 pounds of nitrogen per tree (i.e., 180 pounds nitrogen per acre). These conclusions were based on mean infiltration rates for a 7-year period. Previous studies by Huberty and Pillsbury (1944) showed that Ca(N03)2 increased infiltration and ( NH4)2S04depressed it markedly on this same soil owing to formation of dispersed surface crust high in adsorbed ammonium. Gifford and Strickling (1958), on the other hand, reported that anhydrous ammonia increased water-stable aggregation in Maryland soils at 23 locations. Fox et al. (1952) noted deleterious effects on water intake of eroded Sharpsburg silty clay at Lincoln, Nebraska, when NH4N03 was broadcast on the surface. The dispersion of soil and reduction in infiltration rate increased with the rate of application and were related to the persistence of the adsorbed ammonium ion in the soil surface. These effects lasted from November, when the fertilizer was applied, through July of the following year. Fox et al. caution that surface application of ammonium fertilizers on fine-textured, sloping lands low in organic matter can lead to reduced moisture storage and increased runoff and erosion. Aldrich et al. (1945) reported marked effects of fertilizers applied over a 16-year period on infiltration rates of disturbed samples of Ramona sandy loam. NaN03 and (NH4)2S04 produced much lower infiltration rates than Ca( N03)2. The dispersive effects of sodium could be partially offset with gypsum application, and of ammonium with lime. These observations fit in with their measurements of macrospore space and with observations of poor citrus tree vigor on plots having lower infiltration rates. Mazurak et al. (1960) found that using 40 pounds of nitrogen per acre annually on smooth bromegrass and intermediate wheatgrass did not increase the rate of water infiltration as measured by cylinder infiltrometers on a deep chernozem soil in central Nebraska, even though fertilization increased grass yields and total nitrogen content of the soil. Use of barnyard manure in irrigated rotations on a chestnut soil in western Nebraska increased water infiltration rates, according to Mazurak

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VZETS,

JR.

et al. (1955). They also found that manure doubled the infiltration rate, measured at the end of 2 hours, of plots continuously in potatoes, corn, or barley, but had a much smaller effect on plots continuously in alfalfa. IX. Fertilization, Crop Maturity, and Water Use

On nutrient-deficient soils, fertilizers hasten the development of some crops like corn and grain sorghums and delay the growth cycle of others. Some crops do not appear to be affected. If a crop is harvested as soon as it matures, as judged by its water content, or as soon as it is marketable in the case of vegetables, then the delayed or hastened maturity may have considerable effect on the total evapotranspiration by affecting the number of days that the crop is exposed to evapotranspiration. A time saving of 7 days at the end of the season, if the end occurred in midsummer, might mean a saving of 2 inches of water if the daily evapotranspiration is 0.3 inch per day. Conversely, a delay of 7 days might mean a loss of 2 inches of water. Whether these would be significant gains or losses depends on whether this water is lost subsequently by evaporation. In the autumn when daily evapotranspiration is low, a change of 7 days in evapotranspiration would have little effect on the total evapotranspiration. The effects of the change in total evapotranspiration on water-use efficiency would depend on the yieldevapotranspiration relationships. To illustrate, harvesting sugar beets 2 weeks earlier would save water through shortening of the growing period, but if the yield or sugar produced suffered, then the water-use efficiency might actually be decreased. Although considerable attention has been paid to the effects of fertilizers on maturity of crops, little attention has been paid to how much water could be saved by earlier maturity, or how much lost by delayed maturity. Under dryland conditions much of the favorable effect of fertilization on increasing water-use efficiency is through hastened maturity and a reduction in the time the crop is exposed to evapotranspiration of the limited water supply. Adequate nutrients, particularly phosphate, shorten the development period and hasten maturity. Nitrogen is variable in its effects, depending on the kind of crop and the amount applied. Only a few examples will be given. Black (1957) presents data for corn showing that nitrogen application affects maturity and grain moisture content at harvest: the greater the nitrogen deficiency and the yield response to added nitrogen, the greater the reduction in grain moisture content at harvest. If the nitrogen supply was so high that yields were reduced, then maturity was delayed and the water content of the shelled grain was increased.

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Black also reports a positive nitrogen times phosphorus interaction in the yield of oats and water content of the plants 1 week before harvest, the higher the yield the lower the water content. Clore and Viets (1949) showed that leguminous winter cover crops and nitrogen fertilization hastened the development of sweet corn and increased the percentage of usable ears. Painter and Learner (1953) reported that irrigated PLAINSMAN grain sorghum headed almost 2 weeks earlier with adequate nitrogen than without in northeastern New Mexico. Ware (1938) showed that phosphorus application increased the yield of snap beans from 8 to 157 hampers per acre and the yield of early beans in per cent of the total from 25 to 48 per cent. When phosphorus is deficient, phosphorus application generally hastens development and maturation of the crop. In Black's (1957) review of the effects of potassium deficiency, he cites evidence that maturity of oats, soybeans, grapes, and corn is delayed, but that cotton may be speeded up and achieve premature maturity. Zinc fertilization of zinc-deficient soils hastens maturity of beans (Viets, 1951) and corn (Viets et al., 1953). Nitrogen fertilization of the small-grain cereals is frequently reported to delay maturity. Russell and Russell (1950) discuss the problem in detail. Undoubtedly all crops are hastened in maturity by fertilization if the deficiency is severe. However, if the fertility level is adequate or nearly so, then additions of an element in excess, such as nitrogen that is adsorbed readily, can delay maturity. Indeterminate varieties of crops such as cotton and tomatoes and all commercial strains of sugar beets are particularly susceptible to delayed maturity by excess nitrogen application. Attention was previously directed (Section IV, B and Table IV) to the effects of excess nitrogen on lodging of winter wheat and the resulting reduction in yield and water-use efficiency. X. Other Practices for Increasing Water-Use Efficiency

Although the theme of this review is the role of fertilizers in improving the efficiency with which water is used, water use and efficiency have been discussed in terms of yield, not in terms of fertilizer rates, sources, or methods of application. It is apparent then that any practice that promotes plant growth and the more efficient use of sunlight in photosynthesis will likewise increase water-use efficiency. This is true because evapotranspiration is so dependent on the heat available for vaporization and is limited only by the capacity of the soil and the plant to transport the water to the sites of vaporization. Such a list of practices would include all cultural practices leading to early and rapid production of leaf area, including date of planting,

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FRANK G . V E T S , JR.

seeding rate, row widths-in fact, almost all the objectives of agronomic research. Included would be stand density, plant orientation (as in corn), optimum leaf area and leaf area index. Also included would be many of the objectives of plant breeding and of plant disease control. Burton (1959) gives a good discussion of these practices after restating,the concept that evapotranspiration is dependent on radiation and attendant meteorological conditions, not on the kind of crop. To quote Burton: “If this generally accepted, broad relationship holds, then plant breeders and agronomists, as they have increased the yields of crops in a given region, have automatically and often unconsciously improved the efficiency of water use. Hundreds of examples exist to prove that crop specialists, through breeding and management, can make a substantial contribution to solving the problems of dwindling water resources.” Insect and disease control are also needed for high water-use efficiency not only for production of new growth and dry matter accumulation by the plants, but also for the preservation of that already attained. Thus the relation dry matter production

= water-use efficiency

water used

becomes one of the most fundamental in agricultural research and practice. Any practice that increases dry matter production will lead to increased efficiency in use of water unless water greatly in excess of consumptive-use demands is essential in attaining that production. Examples of the latter are frequent irrigations after planting to establish small-seeded crops and deliberate leaching with irrigation water to remove soluble salts. Another approach to improvement in water-use efficiency is reduction of evapotranspiration, the denominator of the equation, at a rate faster than dry matter production is reduced. This has long been the goal of dryland agriculture. The objective has been attained at least partially by using lower plant densities on dryland than on irrigated or humid area land so that the rate of evapotranspiration is considerably less than the potential. Under semiarid conditions the natural drying of the surface soil reduces the evaporation component of evapotranspiration considerably below that for a continuously moist soil. For these reasons water-use efficiency is frequently higher on dryland than on betterwatered land-a subject that will be discussed later. Finally, the reduction of evaporation from the soil through use of chemicals and artificial mulches, by saving water for transpiration offers definite possibilities for increasing water-use efficiency.

FERTILIZERS AND THE EFFICIENT USE OF WATER

259

XI. Is Maximum Water-Use Efficiency Desirable?

As pointed out in the introduction, greater emphasis will be placed in future years on more efficient use of water in food and fiber production. The evidence shows that crop yield is an important determinant of how efficientlywater is used. Average crop yields are now below the economic optima because the best fertilizer and cultural practices, singly and in combination, developed by agricultural research are used by only a minority of growers possessing the knowledge and skill to apply them. So agriculture on the average has a long way yet to go in using its land and water resources most efficiently. Does this mean that maximum water-use efficiency, or a minimum water requirement, should be our goal? The answer is no, and can only be no, for two very practical reasons. One deals with the economics of obtaining high yields; the other deals with utilization of a limited natural moisture supply under dryland conditions. Many data have been given showing that maximum water-use efficiency is contingent on maximum yield under a given situation with respect to water supply. This is true whether evapotranspiration is increased or not. However, the cost of the practices necessary to achieve these higher yields must be related to their marginal returns in monetary terms. Yield increase from fertilizers usually follows some kind of decreasing increment function such that each successive unit of fertilizer produces less profit than its predecessor. Thus the most profitable agriculture must stop short of maximum per acre production. As yield stops short of this maximum production, so must water-use efficiency. The only adjustment needed in our present thinking on fertilizer recommendations based on sound economics is consideration of how much water could be saved in the production of a given commodity, i.e., 1000 bushels of corn, and whether this water could be used elsewhere and at what net return. This concept applies mainly to water used for irrigation that could be used elsewhere, legal and political conditions permitting. Under semiarid and humid conditions such savings would mean little until methods of "harvesting" water and protecting it from evapotranspiration are developed and a market for the water appears. The second reason for nonmaximum water-use efficiency stems from the availability of water for evapotranspiration. Under dryland conditions all cultural operations are directed at reducing the rate of evapotranspiration as much as possible to stretch the available water for production of as much salable or usable product as possible. Water-use efficiency is frequently higher under dryland conditions than when

260

FRANK G .

VIETS,

JFl.

irrigation is used. This higher efficiency, of course, is attained at a much lower, and sometimes disastrous, level of production. Jensen and Sletten (Table 111) obtained greater water-use efficiency by unfertilized grain sorghum with limited water than with ample water. With fertilizer, water-use efficiency was higher with more available water. Instances of higher water-use efficiency by wheat with limited water supply compared to ample supply are shown in Table IV. Holmen et al. ( 1961) (see Fig. 3 ) obtained higher moisture efficiency in bromegrass hay production under dryland conditions than with irrigation at comparable yields in 1956. Carlson et al. (1959) got higher water-use efficiency in production of corn forage without irrigation than with irrigation unless a high rate of nitrogen and a thick stand were used (Table F J ) . On the other hand, Dreibelbis and Harrold ( 1958) increased water-use efficiency by irrigation of corn and meadow in the weighing lysimeters at Coshocton, Ohio. Thus water-use efficiency cannot be considered apart from the moisture and yield situations under which it was obtained. Unless these precautions are taken, water-use efficiency becomes, as Penman and Schofield (1951) said of the transpiration ratio, “normally a useless concept.” XII. Conclusions

Increasing demands for water by industry, a growing population, and increasing food and fiber needs will require agriculture to become more efficient in its use of water. To accomplish this better use of water production of dry matter or salable crop must be increased per unit of water used in evapotranspiration or for irrigation in semiarid and desert areas. Evapotranspiration is essentially dependent on the heat available from net radiation and advection. Both of these are to a very large extent set by the climate and site conditions. Dry matter production or net assimilation of COZ, on the other hand, is largely determined by crop and soil management practices that lead to as complete utilization of visible radiation as possible for the period of evapotranspiration. Fertilizers, if needed, play a major role in the early establishment of sufficient leaf area capable of photosynthesis. Fertilizers thus affect the size and height of the plant, which may, in turn, affect the opportunity for transfer of water from the soil to the air. Whether fertilizers affect evapotranspiration depends, it appears, on whether the larger plant has the opportunity to intercept advected heat. In small containers, unguarded lysimeters, and some small-plot experiments, advected heat is great and fertilizers do increase evapotranspiration. Some field experiments, on the other hand, show no effect of fertilizers and of increased growth on evapotranspiration. Since agriculture in desert and semiarid

FERTILIZERS AND THE EFFICIENT USE OF WATER

261

areas is subject to advection and even crops in humid areas for short periods of time are similarly subject to turbulent heat transfer, no one can categorically say that fertilization and a larger crop may not use more water. On the other hand, it is safe to conclude that in the field a crop twice as large does not require twice as much water, and that its consumptive use is about the same or is only slightly increased. Whether fertilizers increase consumptive use not at all or only slightly, all evidence indicates that water-use efficiency, or dry matter produced per unit of water used, can be greatly increased if fertilizers increase yield. So fertilization for the adequate nutrition of all crops plays a major role in the efficient use and conservation of water resources. Fertilizers may also increase root development within the soil so that soil water is used to higher tensions and at greater depths. This effect is important in dryland agriculture and even in farming in humid areas during periods of drought. REFERENCES Aldrich, D. G., Jr., Parker, E. R., and Chapman, H. D. 1945. Soil Sci. 59, 299312. Allison, F. E., Roller, E. M., and Raney, W. A. 1958. Agron. J . 50, 506-510. Aubertin, G. M., and Peters, D. B. 1981. Agron. J . 63, 269-272. Bahrani, B., and Taylor, S. A. 1961. Agron. J . 53, 233-237. Ballard, L. A. T. 1933. Australian J . Exptl. Biol. Med. Sci. 12, 161-176. Benedict, H. M., and Swidler, R. 1961. Science 133, 2015-2016. Black, C. A. 1957. “Soil-Plant Relationships.” Wiley, New York. Blackman, G. E., and Black, J. N. 1959. Ann. Botany (London) [N.S.] 23, 131-145. Blaney, H. F., and Criddle, W. D. 1950. U. S . Dept. Agr. Soil Conserv. Serv. SCS-TP-96. Briggs, L. J., and Shantz, H. L. 1913a. U. S. Dept. Agr. Bur. Plant Ind. Bull. 284. Briggs, L. J., and Shantz, H. L. 1913b. U . S. Dept. Agr. Bur. Plant I n d . Bull. 285. Briggs, L. J., and Shantz, H. L. 1914. I. Agr. Research 3, 1-65. Bryan, B. B., and Brown, D. A. 1961. Arkansas Univ. (Fayetteville) Agr. Expt. Sta. Bull. No. 647. Burton, G. W. 1959. Advances in Agron. 11, 104-109. Burton, G. W., Prine, G. M., and Jackson, J. E. 1957. Agron. J . 49, 498-503. Carlson, C. W., Alessi, J,, and Mickelson, R. H. 1959. Soil Sci. SOC. Am. Proc. 23, 242-245. Cassady, C. F., Jr. 1957. M. S . Thesis, New Mexico College of Agriculture and Mechanic Arts, University Park, New Mexico. Clore, W. J., and Viets, F. G., Jr. 1949. Proc. Am. SOC Hod. Sci. 64, 378-384. Denmead, 0. T., and Shaw, R. H. 1959. Agron. J . 51, 725-726. De Wit, C. T. 1958. Verslag. Landbouwk. Onderzoek. 64.6, 1-87. Dillman, A. C. 1931. J. Agr. Research 42, 187-238. Dreibelbis, F. R., and Harrold, L. L. 1958. Agron. J . 50, 500-503. Fox, R. L., Olson, R. A., and Mazurak, A. P. 1952. Agron. J. 44, 509-513. Gaastra, P. 1958. Mededel. Landbouiohogeschool Wageningen 68, 1-12.

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GiEord, R. O., and Strickling, E. 1958. Soil Sci. SOC. Am. Proc. 22, 209-212. Haddock, J. L. 1953. Utah State Agr. Coll. Erpt. Sta. Bull. 362. Haddock, J. L. 1959. J . Am. SOC.Sugar Beet Technologists 10, 344-355. Hagan, R. M. 1955. Rept. 14th Intern. Hort. Congr. Netherlands Symposium 2, 82-102. Haise, H. R., and Viets, F. G., Jr. 1957. Trans. 3rd Congr. Intern. Comm. Irrigation and Drainage, Sun Francisco, California, R25, 8.497-8.508. Haise, H . R., Haas, H. J., and Jensen, L. R. 1955. Soil Sci. SOC.Am. Proc. 19, 20-2.5. Haise, H. R., Viets, F. C., Jr., and Robins, J. S. 1960. Trans. Intern. Congr. Soil Sci. 7 f h Congr. Madison, Wisconsin, 1960 1, 663-671. Halstead, M. H., and Covey, W. 1957. Soil Sci. SOC. Am. Proc. 21, 461-464. Hanks, R. J., and Tanner, C. B. 1952. Agron. J . 44, 98-100. Holmen, H., Carlson, C. W., Lorenz, R. J., and Jensen, M. E. 1961. Trans. Am. SOC. Agr. Engrs. 4, 41-44. Huberty, M. R., and Pillsbury, A. F. 1944. Trans. Am. Geophys. Union 26, 896-899. Jenscn, hl. E. 1956. Soil and Water 5( l l ) , 22-23. Jensen, hi. E., and Musick, J. T. 1960. Tram. Intern. Congr. Soil Sci. 7th Congr. Madison, Wiscom*n, 1960 1, 386-393. Jensen, M.E., and Sletten, W. H. 1957. Soil and Water 6 ( 7 ) , 8-9. Kamel, M. S. 1959. Mededel. Landbouwhogeschool Wageningen 59( 5 ) , 1-101. Keller, W. 1954. Agron. J. 46, 495-499. Kelley, 0. J. 1954. Adcances in Agron. 6, 67-94. Kelley, 0. J., and Haddock, J. L. 1954. Proc. Am. SOC. Sugar Beet Technologists 8, 344-356. Kmoch, H. G., Ramig, R. E., Fox, R. L., and Koehler, F. E. 1957. Agron. J . 49, 20-25. Koehler, F. E. 1960. Proc. 11th Ann. Pacific Northwest Fertilizer Conf. Salt Lake City, Utah, 1960, pp. 141-146. Leggett, G. E. 1959. Wash. State Coll. Agr. Erpt. Sta. Bull. 609. Lemon, E. R. 1960. Agron. J. 52, 697-703. Lemon, E.R., Glasser, A. H., and Sattenvhite, L. E. 1957. Soil Sci. SOC.Am. Proc. 21,464-468. hlakkink, G . F. 1957. J. Inst. Water Engrs. 11, 277-288. Mason, D. D. 1956. In “Methodological Procedures in the Economic Analysis of Fertilizer Use Data” (E. L. Baum, E. 0. Heady, and J. Blackmore, eds), pp. 76-98. Iowa State Coll. Press, Ames, Iowa. Maximov, N. A. 1929. “The Plant in Relation to Water” (Translated from Russian by R. H. Yapp). George Allen & Unwin, London. Mazurak, A. P., Cospter, H. R., and Rhoades, H. F. 1955. Agron. J . 47, 490-493. Mazurak, A. P., Kriz, W., and Ramig, R. E. 1960. Agron. J . 62, 35-37. Miller, E. C. 1916. J . Agr. Research 6, 473-484. Miller, E. C. 1923. Kansas Agr. Erpt. Sta. Tech. Bull. 12. Moldenhauer, R. E. 1952. M. S. Thesis, University of Wisconsin, Madison, Wisconsin. Montgomery, E. G., and Kiesselbach, T. A. 1912. Nebraska Univ. Agr. Erpt. Sta. Bull. l28. Nelson, W. L., and Stanford, G. 1958. Aduances in Agron. 10, 67-141.

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Olson, R. A., Hanway, D. G., and Dreier, A. F. 1960. Nebraska Agr. Ezpt. Sta. Q U U T7~(.3 ) : 7-9; 16. Painter, C. G., and Learner, R. W. 1953. Agron. J . 46, 261-264. Penman, H. L. 1948. Proc. Roy. SOC. A19S, 120-145. Penman, H. L. 1956a. In “The Growth of Leaves” (F. L. Milthorpe, ed.), pp. 170-177. Butterworths, London. Penman, H. L. 1956b. Trans. Am. Geophys. Union 37,43-46. Penman, H. L., and Schofield, R. K. 1951. Symposia SOC.ExptZ. B i d . 6, 115-129. Pesek, J. T., Nicholson, R. P., and Spies, C. 1955. Iowa Farm Sci. 9( lo), 3-6. Peters, D. B. 1960. Agron. J. 62, 536-538. Pillsbury, A. F., and Richards, S. J. 1954. Soil Sci. 78, 211-217. Power, J. F., Grunes, D. L., and Reichman, G. A. 1961. Soil Sci. SOC. Am. Proc. 25, 207-210. Ramig, R. E. 1959. Ph.D. Thesis, University of Nebraska, LincoIn, Nebraska. Richards, L. A., and Wadleigh, C. H. 1952. In “Soil Physical Conditions and Plant Growth” (B. T. Shaw, ed.), pp. 73-251. Academic Press, Inc., New York. Robins, J. S., and Haise, H. R. 1961. Soil Sci. SOC. Am. Proc. 26, 150-154. Rosanow, I. M. 1959. World Congr. Agr. Research 1969, 9. (Abstr. in 1960. Soils and Fertilizers 23, 123). Russell, E. J., and Russell, E. W. 1950. “Soil Conditions and Plant Growth,” 8th ed. Longmans, Green, New York. Russell, M. B. 1959. Advances in Agron. 11, 1-131. Scarsbrook, C. E., Bennett, 0. L., and Pearson, R. W. 1959. Agron. J. 61, 718-721. Schofield, R. K. 1952. Proc. Intern. Grassland Congr. 6th Congr. Univ. Park, Pennu. 1952. 1, 757-762. Scofield, C. S. 1945. U. S. Dept. Agr. Circ. 735. Smika, D. E., Haas, H. J., Rogler, G. A., and Lorenz, R. J. 1961. J. Range Management 14, 213-216. Sneva, F. A., Hyder, D. N., and Cooper, C. W. 1958. Agron. J. 50, 40-44. Stanberry, C. 0. 1959. Proc. 1st Intersoc. Conf. on Irrigation and Drainuge 1957 pp. 59-67. Stanberry, C. O., Converse, C. D., Haise, H. R., and Kelley, 0. J. 1955. Soil Sci. SOC. Am. Proc. 19, 303-310. Tanner, C. B. 1957. 1. Soil and Water Conserv. 12, 221-227. Tanner, C. B. 1960a. In “Water and Agriculture” (R. D. Hockensmith, ed.), pp. 173-195, Publ. 62. Am. Assoc. Advance. Sci., Washington, D. C. Tanner, C. B. 1960b. Soil Sci. SOC. Am. Proc. 24, 1-9. Tanner, C. B., Peterson, A, E., and Love, J. R. 1960. Agron. J. 62, 373-379. Thom, C. C., and Holtz, H. F. 1917. Wash. State Coll. Agr. Expt. Sta. Bull. 146. Thomas, J. R., and Osenbrug, A. 1959. Agron. J . 61, 63-66. Thomthwaite, C. W. 1948. Geograph. Rev. 38, 55-94. Trumble, H. C., and Walker, A. J. K. 1952. Proc. Intern. Grassland Congr. 6th Congr. Univ. Park, Penna. 1952 1, 427-432. van Bavel, C . H. M. 1961. Soil Sci. SOC. Am. Proc. 26, 138-141. Veihmeyer, F. J., and Hendrickson, A. H. 1950. Ann. Reu. Phnt Physiol. 1, 285-304. Viets, F. G.,Jr. 1951. Wash. State Coil. Agr. Expt. Sta. Circ. 143 (revised 1952). Viets, F. G., Jr., Boawn, L. C., Crawford, C. L., and Nelson, C. E. 1953. Agron. J . 46, 559-565. Ware, L. M. 1938. PTOC.Am. SOC. H o e . Sci. (1937) 96, 699-703.

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FRANK G. VETS, JR.

Watson, D. J. 1952. Adoances in Agron. 4, 101-145. Weaver, H. A., and Pearson, R. W. 1956. Soil Sci. 81, 443-451. Willhite, F.hf., Rouse, H. K., and Siemer, E. G. 1956. C o b . Agr. Expt. Sta. Gen. Ser. Paper 614. Zubriski, J. C., and Norum, E. B. 1955. N . Dakota Agr. Expt. Sta. Bimonthly Bull. 17, 126-127.

EVALUATION OF FERTILIZERS BY BIOLOGICAL METHODS G. L. Terman, D. R. Bouldin, and J. R. Webb Tennessee Valley Authority, Muscle Shoals, Alabama, and Iowa State University, Ames, Iowa

I. Introduction . .. .. .. .. .. .. .. .. .. .. . . .. .. .. .. . . .. .. . . . . . . . . . . . . 11. Chemical and Physical Characteristics of Fertilizers . . . . . . . . . . . . . . . . A. Compounds Present in Fertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Dissolution of Fertilizers in Soils . . . . . . . .. . . . . . . . . . . . . . . . . . C. The Nature of Fertilizer Reactions with Soil . . . . . . . . . . . . . . . . . . D. Effects of Granule Size and Solubility . . . . . . . . . . . . . . . . . . . . . . 111. Concepts of Fertilizer Evaluation . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . A. Nature of Response Curves . . . . . .. . . .. .. . . .. .. .. . . .. . . .. .. . B. Techniques for Evaluating Fertilizers with the Same Limiting Yields C. Techniques for Evaluating Fertilizers with Different Limiting Yields D. Plant Response Criteria for Estimation of Fertilizer Effectiveness . . E. Differences among Fertilizers in Relation to Experimental Errors IV. Methods Used in Fertilizer Evaluation Tests . . . . . .. . . . . .. .. .. .. . A. Field Plot Evaluation of Fertilizers . . . . . . . . . . . . . . . . . . . . . . . . . . B. Evaluation of Fertilizers in Greenhouse Pots . . . . . . . . . . . . . . . . . . C. Seedling and Microbiological Methods of Evaluation . . . . . . . . . . D. Effect of Kind of Crop and Climate on Evaluation of Fertilizers . . E. Use of Growth Chambers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Conclusions . .. .. . . . . .. .. . . . . . . . . .. .. . . . . .. .. .. .. . . .. . . .. .. .. . References ..................................................

Page 265 266 267 274 275 278 280 281 288 292 292 293 295 295 306 311 313 315 316 317

1. Introduction

Much effort has been spent in measuring the comparative value of different commercial fertilizers since the introduction of superphosphate more than a century ago. Early evaluation of a small number of such fertilizers consisted largely of comparisons with barnyard manure and other unprocessed natural materials which were available for crop fertilization. As new fertilizer sources were introduced, evaluation became a process of characterizing and determining the effectiveness of these materials relative to the ones in common use at that time. The problem has become much more complex in the last twenty-five years because of the large number of new sources which have appeared on the market and the trend toward more varied and intensive uses of fertilizer. 265

266

G . L. TERMAN, D. R. BOULLXN, AND J. R. WEBB

The influx of new fertilizers in recent years has resulted from simultaneous developments in the fertilizer industry, agriculture, and other related fields. Technological advances in fertilizer chemistry and manufacturing have made possible the use of processes resulting in the production of materials which were not feasible or were unknown in the past. The history and nqture of these developments in the fertilizer industry have recently been described in detail by Jacob (1959) and Sauchelli (1960). The result has been the introduction of an array of new higher analysis fertilizers, as well as an improvement in quality and greater economy in some of the older sources. Nitrogen and phosphorus compounds have predominated among the new sources. However, it should be mentioned that the number of sources available in many parts of the world is still limited for economic reasons. Stimulated by a rapidly growing demand for food and clothing in the world, the annual world consumption of fertilizer has nearly doubled since 1949 and shows every indication of continuing to rise. The everpresent need for greater and more efficient crop production not only has increased the total need for fertilizers, but has created demand for the most efficient sources of fertilizer. Farmers have begun to show an increased interest in the most effective and profitable sources for their particular conditions. As a result of these trends the consumption of some sources has decreased while the use of others has increased manyfold a short time after their introduction. It is significant that these developments in fertilizer production and usage have been accompanied by an expansion in fertilizer evaluation research. The large number of articles in the literature dealing with this subject provide evidence of the increased research activity. Both the fertilizer producer and consumer have indicated the need for such research and have widely used the resultant information for guidance. The producer has sought information regarding the types of fertilizers he should produce, since a variety of products are now technologically and economically feasible. The consumer needs information regarding which fertilizers are best for his crops, soils, and farming practices. The basic approaches which have been used in the experimental evaluation of fertilizers have been aimed at providing these two kinds of information. The following review will discuss some of the problems, concepts, and methods associated with fertilizer evaluation research. II. Chemical and Physical Characteristics of Fertilizers

Over a period of many years, farmers and other users of commercial fertilizers have become aware of the importance of the plant nutrient content of fertilizers, although only the percentages of the three major

BIOLOGICAL EVALUATION OF FERTILIZERS

267

nutrients-N, P, and K-are usually considered. Formerly, purchases were commonly made on the basis of price per ton of fertilizer, frequently without regard to its analysis. A step toward efficient use of commercial fertilizers is to consider their actual composition, not only as to the percentage of plant nutrients, but also as to the chemical compounds making up each fertilizer. Practically all of the countries in the world have regulations governing the fertilizer trade that prescribe the quality of the marketed fertilizer. These regulations usually specify that the producer must express and guarantee the content of major nutrient elements in a fertilizer as determined by standard laboratory procedures. The bases of the guarantee, and hence the laboratory procedures, vary from country to country. The primary purpose of such regulations is to provide consumer protection, but the producer is also offered a certain degree of protection in market competition. Quality control tests must of necessity be relatively simple. Some of the common tests are based on a solubility measurement of the nutrient in question. They are indispensable for the purpose for which they are used, but are not designed to characterize fertilizers completely. They do not precisely reflect the solubility of phosphorus compounds, offer little information regarding the form and assemblage of salts in the fertilizer, and provide little knowledge of physical properties and other features important in determining the effectiveness of fertilizers. Jacob and Hill (1953) presented an excellent review of the principles and methods used in this phase of evaluating phosphate fertilizers. The official procedures used in the United States, Canada, and Mexico are described by the Association of Official Agricultural Chemists ( 1955). The methods used in fourteen countries of western Europe are outlined in a publication of the Organization for European Economic Cooperation (1952). A. COMPOUNDS PRESENT IN FERTILIZERS Inorganic fertilizers are usually composed of one to several solid phases intermixed rather intimately, Generally, the X-ray and petrographic properties of these phases can be related to definite compounds and an identification of the solid phases can be made on this basis. However, in many complex fertilizers, such as ammoniated superphosphates and nitric phosphates, the solid phases are usually mixtures of colloidal to microcrystalline particles. This makes positive identification difficult. An additional factor which should be considered is that the solubility characteristics of the poorly crystallized solid phases may be different from those of well-defined crystals of the various compounds.

TABLE I Compounds Present in Nitrogen, Phosphorus, or Potassium Fertilizer Materials Applied Separately or Used as Components of Mixed Fertilizersn

Fertilizer material

Reprcsentative grades-% N-P,O,-K,O

Water solubility of N, P, or K, ( 70 )

N, P, and K compounds prcscnt Major phases Minor phases

Major accessory compounds present

!E . . 0

r hl

Nitrogen Ammonium chloride Ammonium nitrate Ammonium sulfate Ammonium nitrate-lime Calcium cyanamide Calcium nitrate Sodium nitrate Urea

45-0-0

100 100 100

Phosphorus Ordinary superphosphate

0-20-0

85

0-45-0

87

Ca( H,PO,),*H,O

0-48-0

90

Ca( H,PO,),-H,O

0-54-0

90

Concentrated superphosphate Wet-process H,PO,

Electric furnace H,PO, High analysis superphosphate

26-0-0 33.5-0-0 20.5-0-0 20.5-0-0 22-0-0 15.5-0-0 16-0-0

-

100

Clay conditioner

100 100 100

-

CaCO, CaCO,

-

CaF, CaSO, *2H,O, Silica CaHPO,, Apatite

CaF, CaS0, .2H,O, Silica Silica CaF, Silica

TABLE I (Continued) Water solubility of N, P, or K,

Major accessory compounds present

Fertilizer material

Representative grades-% N-P,0,-K20

Dicalcium phosphate HC1 process Electric furnace H,PO, Calcium metaphosphate

0-40-0 0-48-0 0-62-0

4 3

Fused tricalcium phosphate

0-28-0

<2

Rhenania phosphate

0-33-0

<2

Alpha-Ca,( PO,) in glass matrix Ca silico-phosphates

Serpentine phosphate glass

0-22-0

<2

Ca, Mg slico-phosphates

-

Basic slag Colloidal “clay” phosphate Phosphate “pebble” phosphate ore

0-9-0 0-22-0

<2 <1

Ca, silico-carnotite

-

Apatite

Al phosphates

Ca, Na silicates Ca, Mg silicates Ca silicates Clay minerals

0-32-0


Apatite

-

CaC03

0-0-60 0-0-48 0-0-23

100 100 100

KC1

-

(%I

5

N, P, and K compounds present

Major phases

Minor phases

CaHPO, -2H20 CaHPO, Ca( PO,), glass

CaHPO, CaHPO, .2H20 Beta Ca,P,O,

Silica

-

Quartz ‘Z03

Potassium Muriate of potash Sulfate of potash Sulfomag a

K2S04

K,SO,. MgSO, .6H,O

Beta Ca,(PO,),, Apatite

-

-

This information was provided largely by J. R. Lehr, Fundamental Research Branch, TVA, and other sources.

Quartz R2°3

Type of fertilizer

TABLE I1 Compounds Present in Some Commercial and Expcrimental NP, NK, and NPK Fertilizers0 IienrescntaWater tive solubility N, P, and K compounds prcscnt grades-% of P,O, ...N-P,O,-K,O (%) Major phases Minor phascs

Ammoniated concentrated superphosphate

0

Ammoniated ordinary superphosphate

5-47-0

50

9-48-0

50

10-20-20 (granulated with H,SO,)

50

4-14-0

35

6-12-12

NH,H,PO,, CaHPO, or CaHP04.2€I,0 (NH,)&'O,, ChHPO,

+

30

-

NH,H,PO,, (N€I,),HPO,, CaHPO, NH,C1 KC1 NH,NO, KNO, NH,H,PO,, Basic Ca-phosphate

+

+

8-16-16

65

+

Ammonium phosphate nitrate

30-10-0

100

-

NH,H,PO,, Basic Ca-phosphates, KC1 NH4N0, NH,Cl + KNO, (NH,),HPO,, CaHPO,, Basic Ca-phosphate, KCl +NH,NO, KNO, NH,CI NH,&PO,, NH,NO,

Major accessory compounds present

Basic Ca-phosphate" Unreacted apatite NH,H,PO, Basic Ca-phosphate') Unreacted apatites

SO,, CaF, SiO,, CaF,

(NH4)2S04

SiO,, CaF,

Basic Ca-phosphate Unreacted apatites CaHPO,, ( NH4

Unreacted apatites Ca( NH4 SO, I2.H2O CaHPO,, ( NH4

Unreacted apatites Ca( NH,,K),( SO, ) ,.H,O NH,H,PO, ( NH4

Unreacted apatities Ca ( NH4,K)2( SO, I2.H2O ( NH, $PO,

CaSO, CaSO, .H,O CaS0,-2H20, CaF2, SiO, CaSO4.2H,O, CaF,, SiO, CaS04 CaSO,. 1/2 H,O CaS04-2H,0, CaF,, SiO, CaSO, CaSO,-1/2 H,O

-

‘I’ABLE I1 (Continued)

Type of fertilizer Ammonium phosphate nitrate (continued)

Ammonium phosphate sulfate

ReDresentaWater tive solubility of P,O, grades-% N-P,05-K20 (%I

N, P, and K compounds present Minor phases Major phases

Major accessory compounds present

27-14-0

100

-

18-18-18

100

-

11-48-0

90

13-39-0

90

16-20-0

90

16-48-0

90

13-13-13

90

Basic Ca phosphateb Ca( NH, 1 ( SO, 12 - H20 Basic Ca phosphateb Ca( NH, 12( SO4)2*H20 Basic Ca phosphateb Ca(NH4)2(S04)2.H20 Basic Ca phosphate* Basic Ca phosphateb K,SiF, Colloidal hydrated Al, Fe phosphates

100

Diammonium phosphate

21-53-0

Nitric phosphates HNO, + H3P0, process

12-32-0

40

CaHPO,, NH,H,PO,

14-14-14

20

CaHPO, KCl NH,NO, NH,Cl + KNO,

+

-

Basic Ca phosphateb Unreacted apatites Ca,H( P0,)3-3H,0 NH,H,PO, Basic Ca phosphateb Unreacted anatites

CaSO, * 2H20, CaF2 CaS0,*2H20, CaF, CaS0,-2H20, CaF, CaF,, CaSO, .2H20

-

SiO,, CaF2

SiO,, CaF2

TABLE I1 (Continued)

Type of fertilizer

Ileprcwntative gracles--c/o N-P,O,-K,O

Watrr soluhility of P,O,

20-20-0

30

CaHPO,, NH,l-i21’04, NH,NO,

CaHPO, .2H,O IJnreacted apatites

11-11-11

20

CaHPO, KC1 NH,NO, KNO, NH,Cl Basic Ca phosphates

NH,H2P0, CaHPO, 2H20

NH,H,PO,, CaHPO,, Carbono-apatite

CaSO, 1/2H20 CaSO, .2H,O, CaCO,, CaF, CaSO, .2H20 CaCO,, CaF,

( ’/. )

-.

Nitric phosphates (continued) HNO, H,SO, process

+

to

HNO,

+ CO,

process

+

-

CaHP0,.2H,O NH,NO,

12-12-12

10

Carbono-apatite KCl NH,NO, NH,Cl KNO,

NH,H,PO,,

+

+

-

_I__

-

10

N

Calciuni metaphosphate Hydrolyzed, ammoniated

+

16-14-0

4

Leached zone fertilizer

N, P, arid K compounds present Major phases Minor phases -

CaHPO,

20-20-0

30

Colloidal AlPO, .nH,O NH,NO,

NH,H,PO,, CaHPO,, Basic Ca phosphateb

14-14-14

15

Colloidal AlPO, -nH,O KC1 NH,NO, NH,Cl+ KNO,

NH,H2P0,, CaHPO,, Basic Ca phosphateb

16-33-0

15

Microcrystalline Ca( NH,),P,O,*H,O NH,NO,

NH,H,PO, Vitreous Ca ( PO, ) Basic Ca phosphateb Beta Ca2P,0,

+

,

Major compounds present accessory SiO,, CaF,, CaS0,.2H20 CaSO, .1/2H,O SiO,, CaF, CaSO, 2H20 CaSO, * 1/2H20

-

CaSO,. 2H,O Quartz

TABLE I1 (Continued)

Type of fertilizer

RepresentaWater tive solubility grade-% of P,O, N-PZOE-KZO ( %)

N, P, and K compounds present Major phases

Calcium metaphosphate ( continued )

Partially hydrolyzed

13-13-13

50

+

t o 4

( NH4

w

Partially hydrolyzed

9-18-18

25

-

Vitreous Ca( PO,), NH,NO, + KC1 NH,CI KNO,

-

Ca( NH, 1,P,O,.H,O Vitreous Ca( PO,), NH,NO, KC1 NH,Cl KNO, (NH,),SO, Ca( NH, ),P,O,.H,O

+ +

Minor phases

Major accessory compounds present

NH,H,PO, Beta Ca$,O,

CaSO, .2H20

NH,H2P04 CaHPO, Beta Ca,P,O,

CaS04.2Hz0

Compounds were identified largely by J. R. Lehr, Fundamental Research Branch, TVA. Basic Ca phosphate has the optical and physical properties of microcrystalline collophane and a diffuse apatite X-ray diffraction pattern. a

b

274

G . L. TERMAN, D. R. BOULDIN, A N D J. R. WEBB

Further studies concerning the identification and solubility of such complex fertilizers are needed, since the associated phases may markedly influence the behavior of any given phase when placed in soil and thereby influence plant response. Hence, relating plant response to the several solid phases commonly present in the fertilizer is an extremely complex problem. Compounds commonly occurring in commercial fertilizers are listed in Tables I and 11. Nitrogen and potassium compounds are largely water soluble and comprise a relatively small proportion of the total number of compounds occurring in commercial fertilizers. A large number of phosphorus compounds occur, which vary widely in solubility. Jacob and Hill (1953) have discussed the solubility characteristics and chemical methods for laboratory evaluation of phosphate fertilizers. Various phosphorus fertilizers may be grouped into the following categories of solubility according to AOAC procedures: 1. Water-soluble phosphates. Ammonium phosphates and phosphatenitrates, sodium phosphates, superphosphates, some pyrophosphates and condensed phosphates, and liquids, including phosphoric acid. 2. Citrate-soluble phosphates. ( a ) Those partially soluble in water or hydrolyzing rather rapidly to water-soluble forms-calcium metaphosphate, some pyrophosphates, fused potassium phosphates, condensed phosphorus compounds, etc. ( b ) Those remaining largely insoluble in water4icalcium and tricalcium phosphates, basic slag, Rhenania phosphates, etc. 3. Mixtures of I and 2. Partly hydrolyzed calcium metaphosphate, either ammoniated or not; potassium metaphosphate, ammoniated superphosphates, nitric phosphates, etc. 4. Citmte-insoluble phosphates. Phosphate rocks, "colloidal" phosphates, some precipitated apatites, and complex alkali aluminum-iron phosphates. B. DISSOLUTION OF FERTILIZER^ IN SOILS When fertilizer granules are placed in soil the rate of dissolution is largely controlled by the solubility of the constituent compounds, or perhaps more specifically by the difference in vapor pressure of the solution in and adjacent to the granule and the soil water (Kolaian and Ohlrogge, 1959). With relatively soluble salts such as monocalcium phosphate [ Ca( HrP04)2.H20], potassium chloride ( KCI) and ammonium nitrate ( NH4N03) , essentially saturated solutions are generally formed within the granules, which means that the free energy of water is much lower in the granule than in the surrounding soil. Water moves toward the granule as a result of this free energy gradient and in a relatively short

BIOLOGICAL EVALUATION OF FERTILIZERS

275

time (one to several days) the readily soluble components of the fertilizer have dissolved and moved into the soil. With less-soluble compounds such as anhydrous dicalcium phosphate ( CaHP04) and tricalcium phosphate [Ca3( PO4) 2], saturated solutions may also be formed in the granules, but the saturated solutions are so dilute that no appreciable free energy gradients exist for water movement and these compounds may persist for months and years in the soil. Lawton and Vomocil (1954)studied the influence of granule size, soil moisture level, and other variables on rates of dissolution of concentrated superphosphate. They found that the rate of dissolution increased as the granule size was decreased and as the moisture content of the soil was increased. C. THENATURE OF FERTILIZER REACTIONSWITH SOIL Nitrogen. As ammonium fertilizers dissolve in the soil solution, much of the ammonium is sorbed by the soil cation exchange materials in the vicinity of the granules. This ammonium is held in forms exchangeable with other cations, although some is fixed in nonexchangeable forms. These reactions may be delayed a few days in the case of urea, which must first hydrolyze to the ammonium form. Under many soil conditions the ammonium ions are transformed to nitrate by biological agents over a variable time period. Nitrate nitrogen is not appreciably sorbed by most soils and hence is free to move with the soil water and by diffusion. Both ammonium and nitrate nitrogen are used by soil microorganisms. These various fates of nitrogen, in addition to utilization by crop plants, should be considered in any evaluation of nitrogen sources for crops. Potassium. As potassium salts such as potassium chloride dissolve in the soil, most of the potassium is sorbed by the exchange minerals and held in exchangeable and nonexchangeable forms in the vicinity of the granules until released by some exchange process. As a result, potassium is somewhat less mobile in most soils than nitrogen. The chloride ion reacts only slightly with most soils and presumably remains in solution. Phosphorus. On addition of soluble phosphates to soil, phosphate reacts with soil constituents to form less-soluble Compounds distinctly different from the original fertilizer salts. Lindsay and Stephenson (195%) studied the reactions of monocalcium phosphate with soil and found that considerable amounts of iron, aluminum, manganese, and other elements were dissolved by the acid solution that emerges from granules of superphosphate. Later, Lindsay and associates ( 1962) identified products resulting from.reaction of various fertilizer solutions with soil. Some properties of the fertilizer

276

G . L. TERMAN, D. R. BOULDIN, AM) J. R. UTEBB

solutions and some of the reaction products which have been identiiied are shown in Table 111. The results listed in Table I11 indicate the nature of the reactions and also how complex some of the reaction products may be. A careful study of Lindsay et al. suggests that the reaction products formed by different fertilizers in the same soil are likely to be somewhat different. Differences in the reaction products may result in differential response of plants to various phosphate fertilizers. In most soils the soil solution is undersaturated with respect to dicalcium phosphate, and acid soils are generally undersaturated with respect to octacalcium phosphate and hydroxyapatite. Hence, these compounds in fertilizers applied to soils would be expected to dissolve slowly and the phosphate to react with some soil constituent. Moreno et al. (1960) studied the reactions of dicalcium phosphate dihydrate in acid soils and concluded that probably iron and aluminum compounds reacted with the phosphate in solution. However, these reactions were relatively slow and the soil solution in the vicinity of a granule of dicalcium phosphate would be expected to remain saturated with this compound so long as it is present. Additions of nonphosphatic salts such as NH4N03 and KCl with phosphate fertilizers influence the dissolution of the phosphates and the reactions which occur at the granule site and in the soil surrounding the granule (Starostka and Hill, 1955; Bouldin and Sample, 1958, 1960). Numerous investigators have found much better utilization of phosphorus by crops when nitrogen is applied with the phosphate fertilizer (Olson and Dreier, 1956; Duncan and Ohlrogge, 1958; Miller and Ohlrogge, 1958;Grunes, 1959). Apparently the utilization of fertilizer phosphorus in the presence of nitrogen is influenced by the nature of the reaction products formed and by root proliferation in the presence of nitrogen. The above discussion indicates that when fertilizers are added to soil, the fertilizer salts dissolve in the soil solution and subsequently react at least partially with the adjacent soil body. It illustrates the general principle that, although a fertilizer may be well characterized chemically and physically, the fertilizer does not retain its identity for appreciable periods of time following application to the soil. Thus, the real importance of the chemical and physical properties of the applied fertilizer depends upon how these properties influence subsequent reactions with the soil. Even more important, the economic value of the fertilizer is determined by the capacity of the resulting soil-fertilizer complex to supply nutrients to plants. Thus, it should be recognized that properties of both the fertilizer and the soil are equally important, since the solution emerging from the fertilizer granule or band reacts with the soil. Consequently, proper evaluation of a fertilizer for crop pro-

TABLE I11 Composition of Saturated Solutions of Certain Fertilizer Compounds and Initial Reaction Products Identified in Filtrates from Soilsa Composition of saturated solution at 25°C. Accompanying cation, moles/l.

Initial reaction products identi6edb in filtrates from: M

pH

Moles/l.

Monocalcium phosphate [Ca ( H2P04)2-H201

1.48

3.98

Ca 1.44

Monoammonium phosphate

3.47

2.87

NH, 2.87

3.99

1.69

K 1.69

6.25

6.480

NH, 10.9

None

7.98

3.82

NH, 7.64

MgNH,PO,. 6H20 NH,A12(P0,),0H~8H20

Fertilizer

An acid soil Colloidal ( Fe,A1,X)P0,-nH20 CaHPO, .2H,O K,Al,H,( P0,)8*18H20 ( NH,) ,Al,H,( PO,) s*18H20

( NH4H2P04)

Monopotassium phosphate

K,AI,H, ( PO, ) 18H20

A calcareous soil

5s

8

8

Ca( NH,),P,O,.H,O CaP,0,.4H20 MgNH,PO, 6H20 Ca2(NH4),( HP0,)3.2H20 Collophane

Data from Lindsay and Stephenson (1959) and Lindsay et al. (1962). A large number of additional compounds have been found as a result of reacting fertilizer solutions with soil components and around fertilizer granules in the soil. c Consisted of 3.40 M phosphorus as orthophosphate and 3.08 M phosphorus as polyphosphates. a b

a*

Colloidal P ( FE,Al,X)PO4.nH20 CaHPO, .2H20 HSK(Al,Fe)3(P04)6*6H20 CaHP0,- 2H20 MgNH,PO,. 6H20 Z None

( KH2P04

Ammoniated superphosphoric acid Diammonium phosphate [ ( NH, ) 2HPO41

F0

1

278

G. L. TERMAN, D. R. BOULDIN, AND J. R. WEBB

duction cannot be independent of the soil to which the fertilizer is applied. A second general principle implied in the above discussion is that generally the nutrients added in the fertilizer are not homogeneously distributed throughout the whole soil volume. In the case of potassium and ammonium fertilizers, the exchange capacity of soil in the surface plow layer is vastly greater than the amount of fertilizer ion ordinarily added. The net result is that portions of the soil adjacent to the fertilizer particles are well supplied with exchangeable K+ and NH4+, while large volumes of the soil may not be influenced by the fertilizer K+ and NH4+. Sooner or later, as a result of cultivation and water movement, the distribution will become more homogeneous, but this will likely require one or more cropping seasons. Even much more mobile ions, such as nitrate, are probably not homogeneously distributed in soil immediately after application. Transport by diffusion in the soil is relatively slow, and considerable time will be required for mass flow of water and diffusion to distribute the nitrate homogeneously in the soil mass. With phosphate fertilizers the capacity of the soil to react with the fertilizer is also much greater than the amount added. Again, small volumes of soil adjacent to the fertilizer particles are iduenced by fertilizer, but generally the bulk of the soil volume is not influenced by fertilizer phosphate. The plant is thus dependent for its fertilizer nutrient on the small volumes of soil adjacent to the fertilizer granule or band.

D. EFFECXS OF G R ~ W LSIZE E AND SOLUBILITY Measurable effects of granule size on effectiveness of soluble nitrogen and potassium fertilizers have not been found, and size of granules is apparently of little importance in their evaluation for crop growth. The potassium and ammonium ions are largely held in exchangeable form. Presumably, plants are able to use the exchangeable ions as efficiently whether they are distributed in many small volumes of soil (as with small granules) or in fewer large volumes of soil (as with large granules). The same considerations apply to nitrate. Granule size effects with phosphate fertilizers are rather complex and depend upon the solubility of the phosphate fertilizer being considered. In acid soils it is generally found that large granules of AOAC water-soluble phosphates are somewhat more effective than small granules. However, the reverse situation is generally found with the AOAC water-insoluble phosphates. Figure 1, in which the relative plant response to monoammonium phosphate and dicalcium phosphate is plotted against granule size, illustrates this behavior and also shows that

279

BIOLOGICAL EVALUATION OF FERTILIZERS

as granule size is reduced, differences between these phosphate sources are decreased. The presence of complex mixtures of ammonium, monocalcium and dicalcium phosphates, and apatites in highly ammoniated superphosphates and nitric phosphates further complicates the interpretation of crop response to such fertilizers. However, the conclusions of Bouldin et al. (1960) that effectiveness of a water-soluble phosphate depended on content per granule and that effectiveness of a waterinsoluble phosphate depended on geometric surface area of the granules I

I

I

I

I

I

1

4 W v)

z 0 i 3

-

a I-

z 4

-I P W

2 -

I

5-I w

a

I -

0 0

1.0

2.0 3.0 4.0 5.0 6.0 R E C I P R O C A L OF GRANULE RADIUS (MM. -1)

I -9+14

I -16+20

I

I

-28+35 -35+48 GRANULE SIZE ( T Y L E R )

7.0

8.0

I -48+60

FIG.1. Relative response by oats grown in greenhouse cultures to various granule ,izes of monoammonium and dicalcium phosphates. (From Bouldin et al., 1960.)

ndicate the possibility of predicting relative crop response to mixtures if water-soluble and water-insoluble phosphates. Chemical and physical xoperties of the water-insoluble phosphates are also important ( Cooke, 19%). The dependence of granule size effects on soil properties is illustrated )y the data in Table IV, which show that large granules of water-soluble ihosphates are more effective than small granules in acid Hartsells fine mandy loam whereas the reverse situation is true in calcareous Webster ilty clay loam. Hence, statements about granule size effects with

280

G . L. TERMAN, D. R. BOWLDIN’, AND J. R. WEBB

phosphate fertilizers should consider the properties of the soil to which they are applied. Granule size effects with the water-soluble phosphates depend upon differences in reaction products and volume of soil influenced by the fertilizer phosphorus. Since dissolution depends upon transport of water to the granule, it will take a large granule somewhat longer to dissolve than a small one. As a result, soil immediately adjacent to the large granule will be in contact with concentrated phosphorus solution for TABLE IV Increase in Uptake of Phosphorus by Oats Grown on an Acid and a Calcareous Soil in Greenhouse Pots for 6 Weeks, as Affected by Granule Size of Monoammonium Phosphate Added as the Source of Phosphorusa Increase in P uptake over no applied P

Soil type and pH Hartsells fine sandy loam-5.2 Webster silty clay loam-8.3 a

-6+14 mesh (mg./culture)

--40+60 mesh (mg./culture)

11.9 9.0

5.2 12.5

Data from Stanford and Bouldin (1961).

somewhat longer periods of time than the soil adjacent to small granules. The final result is that more fertilizer phosphorus reacts with the soil adjacent to the large granules than to the small granules. Generally in acid soils the availability of soil-fertilizer phosphorus reaction products increases as the quantity of fertilizer phosphorus per unit of soil increases. Thus, the reaction products adjacent to large granules are more available than those adjacent to small granules. The net result is that generally large granules of water-soluble phosphates are more available than small granules in acid soils. If methods were available for estimating the distribution and availability of soil-phosphorus reaction products about granules of different sizes, it should be possible to predict granule size effects of water-soluble phosphates rather accurately. Such methods have not yet been developed. 111. Concepts of Fertilizer Evaluation

The scientific literature contains literally thousands of papers describing the chemical nature of soil-fertilizer reaction products, and plant behavior as influenced by a variety of soil conditions. However, the plantsoil-fertilizer system is exceedingly complex and our present knowledge (viewed from the standpoint of what we need to know) is so limited that fundamental consideration of this system does not often lead to entirely

BIOLOGICAL EVALUATION OF FERTILIZERS

281

reliable predictions as to how different fertilizers will influence economic yields of plants. Consequently, fertilizer evaluation at the present time is based in part on fundamental considerations and in part on more empirical studies in which plant response to several sources of a given nutrient is measured with little consideration of why the plants respond differently to different fertilizers. The succeeding sections will be concerned primarily with (1)the techniques of evaluating fertilizers on the basis of plant response data, and (2) the relationships between the observed response data and the nature of the plant-soil-fertilizer system where such relationships have been established.

A. NATURJZ OF RESPONSECURVES Numerous examples of response of crops to varying quantities of fertilizers supplying a deficient nutrient are available in the literature. The data Iisted in Table V illustrate a typical example. These data show TABLE V Average Yields of Irrigated Corn on 9 Sites in Eastern Oregon, as Influenced by Additions of Nitrogen as Ammonium Sulfatea

a

Nitrogen (Ib./acre)

Yield (bu./acre)

0 50 100 150

75.6 99.7 109.6 110.3

From Hunter and Yungen (1955).

that the corn responded to each successive increment of nitrogen supplied as ammonium sulfate, but the response to each successive increment decreased and, in fact, the response to the last increment was very small. Apparently, the yields were approaching some limiting value which depended upon factors other than the quantity of ammonium sulfate applied. The data indicate that further additions of ammonium sulfate fertilizer would not have increased yields appreciably, but changing other factors in the environment might have changed the value of the limiting yield appreciably. Evidence from other experiments indicates that decreases in yields may occur if fertilizer additions are increased appreciably beyond the quantities required for the limiting yields. An example of this behavior is listed in Table VI of Munson and Doll ( 1959), where both excessive nitrogen and phosphate additions reduced predicted yields below those predicted for lower rates of nitrogen and phosphate. Presumably, in the example listed in Table V the corn was responding

282

G . L. TERMAN, D. R. BOULDIN, AND J.

R. WEBB

to the nitrogen component of the ammonium sulfate. In terms of fertilizer evaluation, the experimenter is interested in determining how some other source of nitrogen (for example, ammonium nitrate or sodium nitrate) would have influenced plant response. When two or more fertilizer sources of the same nutrient are compared under otherwise similar conditions, two different situations may arise when the results are analyzed. These two situations are discussed as Case I and Case I1 below. Case I. Same limiting yield for all sources: fertilizers differ only in m i e n c y . In this situation the yield with two sources under test approach the same limiting value and the responses obtained with the two sources at lower rates of addition are more or less different, depending upon the relative value of the two sources. Almost invariably the response function fitted to the data for one source will become identical to the response function obtained with the other source if a simple transformation of the nutrient axis is made. For example, if 'y.4 = f ( X A ) (1) where Y.%is the yield obtained with quantity f (X, ) is some arbitrary function, then

XA

of fertilizer A and

Y,=f(bXB) (2) where b is a constant and Ys is the yield obtained when quantity X B of fertilizer B is added. An example of this situation follows. In a greenhouse experiment conducted by TVA at Wilson Dam, Alabama, -6+14 mesh granules of anhydrous dicalcium phosphate ( DCPA ) and concentrated superphosphate (CSP) were compared as sources of phosphorus for oat forage. Amounts of the two fertilizers supplying 30, 60, 120, and 240 mg. of phosphorus were mixed with 3 kg. of Hartsells fine sandy loam ( p H TABLE VI Dry hlatter Yields of Oats as Influenced by Quantities of Phosphorus Supplied as 4 + 1 4 Mesh Anhydrous Dicalcium Phosphate (DCPA) and 4 + 1 4 Mesh Concentrated Superphosphate (CSP) Quantity of P added (mg. P. per 3 kg. soil) 0 30 60 120 240

Yield of dry matter (g. per culture) DCPA

CSP

2.2 7.8 12.5 19.1 25.6

2.2 11.9 18.9 23.8 26.6

283

BIOLOGICAL EVALUATION OF FERTILIZERS

5.2), and oats were grown for 84 days. The above-ground portions of the plants were harvested and were weighed after drying. The results listed in Table VI show that the yields of dry matter were different when the smaller quantities of phosphate were added, but the differences between the yields with the two sources were very small when 240 mg. of phosphorus was added. The data suggest that differences in limiting yields obtained with the two sources would be very small. The hypothesis stated in Eqs. ( 1) and ( 2 ) is illustrated by Fig. 2, where the yields are plotted against the quantity of phosphorus added as concentrated superw

30r

I

I

I

a

a ' W

I

.*o'

I

I

I

I -0

Be'-----

20 /

I-

/

LEG END: 0 CSP P DCPA LL

0

n

I

w 0

30

60

SO

120 150

180 210 240

X C S P OR ~ . ~ ~ X D C P A P QUANTtTY OF (NUTRIENT) ADDED, MG./ 3 KO. OF SOIL FIG.2. Yields of dry matter by oats plotted against quantity of phosphorus added as concentrated superphosphate (XcBp) or 0.50 times that added as anhydrous dicalcium phosphate ( O.56XD,,,),

phosphate and against the quantity of phosphorus supplied as anhydrous dicalcium phosphate multiplied by 0.56 (the procedure used to determine the parameter 0.56 will be discussed below). The results in Fig. 2 indicate that the behavior of the two fertilizers conforms to the situation described by Eqs. (1) and ( 2 ) . This result implies that, if

= 0.56XDCP*

(3) the same yield of dry matter would have been obtained in the experiment described above. Thus, the quantities of phosphorus required to produce any given yield were different for the two fertilizers, but the behavior of the fertilizers was similar in other respects. In essence, they differed XCSP

284

G . L. TERMAN, D. R. BOULDIN, AND J. R. WEBB

only by a factor which will be defined later as an availability coefficient or as an efficiency factor. Extending the above principles to several sources, the situation denoted by Case I may be described by Eq. (4) Yi

= f ( b,X,)

(4)

where Y, is the yield obtained with quantity X of fertilizer i; br is a parameter characterizing fertilizer i; f ( b&) is an arbitrary function selected to describe the results. It is possible that the same limiting yields would be obtained and yet Eq. ( 4 ) would not apply to all sources. This situation will be discussed below. 30 I

L

r

I

I

I

.a. . -... Y

Ar

F'

I

I

i l

L E G END: 0 AS 0

CN

Case 11. Limiting yields differ among sources. In this situation the yields with two or more sources do not approach the same limiting value. An example of this behavior is illustrated in Fig. 3 with data Mulder ( 1956) obtained in a field experiment with spring wheat using ammonium sulfate and calcium nitrate as sources of nitrogen. It is obvious that in this case the limiting yields with the two sources of nitrogen are entirely different and presumably some factor other than nitrogen per se needs to be studied. In this case, Mulder (1956) found that when magnesium was added, the yields with the two sources approached the same limiting value for all practical purposes. Further examples of Case Z and Case 11. Lorenz and Johnson (1953) studied the response of potatoes to ammonium sulfate and calcium

285

BIOLOGICAL EVALUATION OF FERTILIZERS

nitrate on Hesperia fine sandy loam (pH 7.5) in greenhouse experiments. They found that the limiting yields obtained with ammonium sulfate were much higher than with calcium nitrate. From subsequent laboratory and greenhouse experiments they concluded the phosphate level in the soil was rather low and the pH decrease associated with microbiological transformation of NH4+ to NO3- had increased the ability of the cultures treated with ammonium sulfate to supply phosphorus to the plants. When adequate phosphate fertilizer was applied, limiting yields

cn” v)

a w a

0

0

A

o a w w ~ - n : 5 l0um aA a a

5 X

5 -

0

A

A 0 0 0 LEGEND: 0 1953 0 1954

a 5

A 1955 0 0 25 50 75 P E R CENT OF A O A C - A V A I L A B L E PZO, W AT E R-SOLUBL E F O R M

100 IN

FIG.4. hlaximum predicted yield increases of corn to P,O, in several fertilizers plotted against the percentage of AOAC-available P,O, in water-soluble form. (From Pesek and Webb, 1957.)

with the two nitrogen sources were very nearly the same, although perhaps somewhat higher with calcium nitrate than with ammonium sulfate. Webb and Pesek (1958) reported a series of experiments performed in Iowa over a number of years in which several phosphate fertilizers were compared when hill-placed with corn. In further analysis of the data (Pesek and Webb, 1957) it was found that the predicted maximum yield responses to P205varied widely among sources of phosphorus. Results of their analysis of the data are presented in Fig. 4, where the

286

G . L. TERMAN, D. R. BOULDIN, AND J. R. WEBB

maximum predicted yield response to P205 (predicted limiting yield with a given source of PzO, minus the yield with no added phosphorus) is plotted against the percentage of the AOAC-available P205 in watersoluble form. The results listed in this figure demonstrate that indeed the limiting yields may vary among different sources of the same nutrient, and in this case it is obvious that the limiting yields are correlated with the percentage of the AOAC-available P205 in watersoluble form. However, this behavior is not a property of the fertilizer per se, but depends upon the method of placement, as shown in a second series of experiments. Webb and Pesek (1959) compared many of these same fertilizers by broadcasting and plowing under for corn. The results of the experiments indicate very little if any difference among fertilizers in which the AOAC-available P205in water-soluble form varied from 0 to 100 per cent. There was no evidence that limiting yields varied among sources. Prummel (1957) summarized a series of field experiments in the Netherlands in which broadcast and row placements of superphosphate, potassium sulfate, and nitro chalk were compared. He concluded that broadcast and row placements gave the same limiting yields, but with row placement the limiting yields were attained at lower levels of applied nutrient. Hagin ( 1957) compared powdered and granular superphosphate in greenhouse experiments with red clover on several soils. On one soil, powered superphosphate gave much higher limiting yields than granular superphosphate. Presumably, a more careful search of the literature would reveal additional examples of both Case I and Case 11. When Case I prevails, fertilizer evaluation is reduced to a problem of determining relative efficiencies; different quantities of a nutrient supplied in either of two fertilizers will be required to produce a given yield, but if enough of either source is added the same yield can always be produced. If Case I1 prevails, both relative efficiency and yield potentials must be considered and both considerations may be very important economically. Within the range of yields less than the lower of the two limiting yields, relative efficiency is the important consideration; however, the range of yields included between the two limiting yields can be obtained only with the source giving the higher maximum yield. No fundamental explanation for the observed differences in limiting yields is evident in all the examples cited above. Perhaps the really important point to consider is that the phenomena described by Case I1 are worthy of further research work. In many areas of the world, limited arable crop land together with large populations demand that crops be

BIOLOGICAL EVALUATION OF FERTILIZERS

287

fertilized to essentially the limiting yield level. Perhaps the underlying causes of different limiting yields with different sources of nutrients will provide additional clues on how to raise the limiting yields to higher values. At any rate, it is surely unsatisfactory to leave any of these observations unexplained. In some cases, differences in limiting yields between two fertilizers may be the result of differences in accessory nutrients in the two fertilizers. Ordinary superphosphate usually contains about 5 per cent sulfur ( S ) in the form of CaS0,.2H20, whereas fertilizers prepared from electric furnace phosphoric acid usually contain very small amounts of sulfur. Under conditions of sulfur deficiency, differences between ordinary superphosphate and fertilizers prepared from furnace acid may be the result of differences in sulfur supplied. Perhaps equally noteworthy are differences in micronutrient contents. Bingham ( 1959) determined the micronutrient contents of several phosphorus sources in the United States and found that the eastern phosphates usually contained less zinc than western phosphates. Clark and Hill (1958) analyzed several samples of rock phosphate. They found considerable variation in the micronutrient content of phosphates from different deposits and among samples from the same deposit. Phosphates and phosphoric acid prepared from electric furnace phosphorus contain much lower quantities of micronutrient elements than corresponding products prepared from “wet process a c i d (phosphoric acid produced from rock phosphate and sulfuric acid). As illustrated by the data of Lorenz and Johnson (1953), which were quoted above, the fertilizer reactions with the soil may influence the ability of the soil to supply nutrients to plants, particularly when band placements are used. Lindsay and Stephenson (1959a) demonstrated that the solution formed when superphosphate is placed in soil may dissolve considerable amounts of iron, aluminum, and manganese. Presumably, other elements are also influenced by these solutions. Other fertilizer solutions may behave in a similar fashion, although the ability to dissolve the various elements may vary widely among fertilizers. Hence, the fertilizer may change the ability of the soil to supply nutrients to the plant, even though the fertilizer itself does not contain any of the nutrients in question. One other aspect of the situation may be noteworthy. When comparing band placements of phosphates, rather high concentrations of phosphate in the soil solution adjacent to the band may be produced with AOAC water-soluble phosphates ( Bouldin and Sample, 1959b). With AOAC water-insoluble phosphates, the concentration of phosphate in the soil solution may be somewhat lower. Because the seedling plant

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has a rather restricted root system, the higher concentrations of phosphate in the soil solution produced by the water-soluble phosphate may result in a plant much better supplied with phosphorus than one adjacent to a band of AOAC water-insoluble phosphate. This in turn may lead to a generally more vigorous plant in the succeeding growth period. Perhaps a mechanism such as this would explain the different limiting yields observed by Pesek and Webb (1957). A research project to test this hypothesis may be worth while. In the preceding discussion the general occurrence of two situations has been noted, namely, under situations denoted by Case I, the same limiting yields are obtained with different sources of the same nutrient; whereas under the situations denoted by Case 11, different limiting yields are obtained. In the succeeding two sections the techniques of fertilizer evaluation in these two situations will be discussed.

B.

TECHNIQUES

FOR

EVALUATING FERTILIZERS WITH LIMITING YIELDS

THE

SAME

For the most part, the principles of biologicaI assay may be applied to the results in Case I. Black and Scott (1956) and White et al. (1956) have discussed the application of these principles to fertilizer evaluation in a very concise manner. In terms of fertilizer nutrient added

a=yX where a is the availability, y is the availability coefficient, and X is the quantity of nutrient added. Using this definition of availability, fertilizer evaluation is reduced to the problem of determining the availability coefficients for each of several sources of the same nutrient. In general, this procedure is useful only when the quantity of one nutrient is varied, while all others are kept at a constant level. For example, several sources of phosphate may be compared in an experiment with levels of added nitrogen and potassium kept constant. Usually, if levels of other nutrients are vaned, either intentionally or otherwise, different limiting yields will be obtained and the results should be analyzed according to procedures described under Case 11. As pointed out by Black and Scott (1956), absolute values for a cannot be measured. The usual method of analysis is to use some procedure which enables the investigator to derive numbers which are presumed to be related to a through an unknown proportionality constant. Some investigators proceed one step further and express the results relative to some arbitrary standard fertilizer or treatment.

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The theory of this method is illustrated-by the following equations. The function defined by Eq. (5) expresses the yield as a function of a

Y =f(a) (5) where Y is the yield, and f ( a ) is some function of the availability of the nutrient under study. Since no independent, absolute method of measuring a is available, the assumption is usually made that numbers protional to a can be measured. Thus, for practical use Eq. ( 5 ) needs to be rewritten as Eq. ( 6 ) Y=f(ka) (6) Substituting the expression yX for a, Eq. ( 6 ) becomes

Y = f(kyX)

(7)

By proper treatment of data in a well-designed experiment, values of parameters in Eq. ( 7 ) and values of kyX for a variety of fertilizers may be determined. The term “availability coefficient index” will be used to refer to ky. Usually the assumption is that y is independent of X. Black and Scott (1956) suggest that this is not always true. In a practical sense, the data will usually be consistent with any hypothesis designed to test this assumption because experimental errors and the arbitrary nature of function ( 7 ) makes any valid statistical test difficult. Likewise, it is difficult to test the hypothesis that f ( k y X ) is different for any of the fertilizers included in a particular experiment. Thus, for the sake of convenience, the assumption is usually made that y is independent of X and that function ( 7 ) will describe the responses to all fertilizers in a particular experiment, since deviations from this model are usually within experimental error. The usefulness of the availability coefficients is illustrated in the following discussion. Suppose the farmer wishes to know which of several sources of a nutrient to apply under a given situation. According to the above discussion, when ylXl = y2X2 = y3X3, etc., the same plant yields will be obtained. That is,

Y = f ( k y i x i ) = f ( ky,&) = f ( kysX3) If C1, C2, and C3 are the prices of the nutrient per unit of X , respectively, the cost of the yield Cyl, C1’2, Cy3, etc., is cy1= XlCl

= x2c2 = x3c3

c1.2

c p 3

Hence, the farmer would probably pick the fertilizer which would be cheapest. In the example illustrated in Fig. 1, ky for concentrated super-

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phosphate and anhydrous dicalcium phosphate (per unit of P208) is equal to 1.0 and 0.56, respectively. If concentrated superphosphate could be purchased for 10 cents per pound, it would be more profitable to use anhydrous dicalcium phosphate only if the price was less than 5.6 cents per pound of Pz05. If 10 pounds of P20, were applied as concentrated superphosphate, fertilizer costs would be one dollar. However, 18 pounds of P,O, as anhydrous dicalcium phosphate would have to be applied to obtain the same yield. This 18 pounds of P205 would cost more than a dollar unless the price were less than 5.6 cents per pound of PZO;. The same conclusion would be reached regardless of the yield selected. This example illustrates the usefulness of availability coefficients in decisionmaking processes. The economists usually maximize profits; they might perhaps arrive at a slightly different conclusion in the above situation (Munson and Doll, 1959; Pesek and Webb, 1957). Several types of functions have been used to define the response to added nutrients [ Eq. ( 7 ) ] in fertilizer evaluation experiments. The simplest to use is the concurrent straight-line model of White et al. ( 1956 ) . In this model the response to each fertilizer is represented by a straight line, with the added restriction that all lines pass through a common point when X = 0 (that is, the yield in the absence of added nutrient is the same for all sources). Usually, response is not a linear function of added nutrient except in cases where the level of the native soil nutrient and the availability of fertilizer nutrient are low. Furthermore, unless there is other evidence to the contrary, differences in limiting yields may exist. Hence, this model is convenient for its simplicity but is questionable unless data on limiting yields are also available. Several functions which express yield as a nonlinear function of added nutrient have been used in analyses of experiments. One of the most used is Eq. ( 8 ) , commonly called the Mitscherlich equation: y = A[1 - 1 O - ( b 1 9 + 8 ) ] (8) where T is the yield when quantity X of nutrient is applied; A is the limiting yield, that is, the yield approached as X increases indefinitely; k is a parameter defined by Eq. ( 6 ) ; y is the availability coefficient of the fertilizer; and s is the level of nutrient native to the soil. A transformation of this equation, called the Spillman equation, may also be used : Y=A-BCX (9) where B = A ( lo-"), C = lo-". For economic interpretation of yield data, polynomial functions of various forms are commonly used. Munson and Doll ( 1959) have discussed

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these and other equations. Steenbjerg and Jakobsen (1959) and Hagin (19SO) suggest that a sigmoid-type curve may represent the data better when available nutrients are very low. Perhaps the simplest procedure of all is to draw freehand the response curve for a standard fertilizer, and then use graphical interpolation to compare the other fertilizers. Cooke and Widdowson (1959) used this procedure. Cooke (1956) plotted response curves for superphosphate and estimated the efficiency of other phosphorus sources in terms of “superphosphate equivalents.” This method gives results comparable to those obtained by calculation of availability coefficients and is simple to apply. Determining the parameters in the Mitscherlich or Spillman equations by statistical methods for no phosphorus and for each source applied at more than two levels is laborious. White et al. (1956) calculated the sum of squares of deviations from the concurrent Spillman model (same A and B for all sources) for various values of A and B and then picked the values of A and B that resulted in the smallest sum of squares. Values of C [Eq. ( 9 ) ] are also obtained by this procedure. By definition, log C = -kyX. Relative availabilities of the several fertilizers are then calculated from these values of k y X . Electronic computers materially reduce the time required for this procedure. Although laborious, this procedure is a very satisfactory means of analyzing data from experiments in which several sources are applied at two or more quantities. A common experimental design includes two or more quantities of a “standard source, and only one quantity of several “test” sources. With this design, the response data obtained with the “standard” source are used to calculate A in Eqs. ( 8 ) or ( 9 ) . The value of the parameter A may be calculated using a least squares procedure described by Eid et al. (1954) or a graphical procedure used by Bouldin and Sample ( 1959a). The graphical procedure is applied as follows: Rearranging Eq. ( 8 ) and taking logarithms of both sides log ( A -Y ) = log AlO-’

-k y X .

(10) Thus, a plot of the logarithm of ( A -Y ) against X will give a straight line with slope -ky. Several values of A are picked by inspection and the logarithms of ( A - Y ) are plotted against X . The value of A is then selected which gives the best straight line. When X is zero, log ( A -Yo) = log A10-8 where Yo is the check yield. Substituting this value for log A10-6 in Eq. ( l o ) , and rearranging:

ky=-

1

X

log

A-Y A - Yo

___

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TERMAN,

D. R . BOULDIN, AND J. R. WEBB

Using Eq. ( l l ) ,values of ky may be calculated for each value of Y. This procedure may also be used with more than one quantity of the various test fertilizers, as illustrated by Bouldin and Sample (1959a) and Bouldin et al. (1960). However, it is difficult to analyze the data statistically if this procedure is used.

C . TECHNIQUES FOR EVALUATING FERTILIZERS WITH DIFFERENT LIMITINGYIELDS Perhaps the most satisfactory means of evaluating fertilizers in this situation is to use economic yields of plants and base the evaluation on maximum profit considerations. This procedure is illustrated by Pesek and Webb ( 1957) with phosphate fertilizers containing different percentages of AOAC-available P20, in water-soluble form. The average responses to P205 from the various fertilizers (yield with added P205 minus the yield in the absence of added P205) at several locations in a given year were calculated. Multiple regressions of these responses on quantity of AOAC-available Pz05 added and percentage of AOACavailable P205in water-soluble form were calculated. Using this equation, an equation relating maximum profit to added P205 and per cent water solubility was derived. By substituting the value of the crop and the cost of Pn05in these equations, the maximum profit possible with the several sources of phosphorus could be selected. In case some third variable (water-soluble P205 in the example above) is not readily apparent, the economic analysis may be carried out using a response curve for each source.

D. PLANTRESPONSECRITERIA FOR ESTIMATION OF FERTILIZER EFFECTIVENESS In the preceding discussion, no particular attention has been given to what plant attribute should be used as a measure of response to fertilizer. Commonly in field experiments economic yield is used; for example, yields of grain or forage. In greenhouse experiments, yields of above-ground dry-matter and nutrient uptake are commonly used. With the advent of readily available and more or less inexpensive isotopic tracers in the last 15 years, considerable use has been made of isotopiclabeled fertilizers. By use of conventional isotope dilution equations, the amount of nutrient in the plant which was derived from the fertilizer can be calculated on the basis of isotope content of the plant. Actually, any of several plant attributes may be used satisfactorily. Some of the factors which must be considered in selecting the attributes to be measured will now be discussed. In general, the experimenter is interested in the effectiveness of the

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several fertilizers in terms of economic yield, with the provision that economic yield is defined broadly enough to include quality factors as well. However, sometimes measurements of economic yield are not feasible, as in most greenhouse pot experiments, or measurements of economic yield may be much more expensive to make than some other plant response measurement. Another factor to be considered is the sensitivity of the results. For example, if the fertilizers being evaluated give the same limiting yields, and fairly high rates of fertilizer are applied, the economic yields may change very little even though the availability of ,the nutrient is changed considerably. Hence, differences in effectiveness of the fertilizers may be completely overshadowed by experimental errors. In these cases, nutrient content may be more sensitive to changes in availability of the nutrient than economic yield, and isotopic tracers are particularly useful here because generally the isotope content changes considerably with availability of nutrient over a wider range of nutrient availability than economic yields. If it is not practical to make economic yield measurements, it should be established that the yield attribute chosen is correlated with the economic yield. For example, the assumption is commonly made that fertilizer evaluation based on yields of dry matter in greenhouse experiments will be correlated with evaluations based on economic yields in the field. When the nutrient content of plants is measured, the assumption is usually made that the fertilizer which leads to the highest nutrient content will also give the most profitable economic yields in the field. However, these assumptions are not always true, and it is not possible to select a yield attribute which should be used universally. The experimenter must consider the cost, reliability, and sensitivity of the several plant attributes and select the one most practical for each particular situation.

E. DIFFERENCES AMONG FERTILIZERS IN RELATIONTO EXPERIMENTAL ERRORS Procedures for calculating approximate variances for the availability coefficient indexes and availability coefficient ratios have been described by White et al. (1956) for the concurrent straight-line model and the concurrent Mitscherlich (or Spillman model) where all sources are applied at several levels. In the examples they used, the concurrent linear model gave an average value of ky of 1.26 with a standard error of 0.27 for the yield of phosphorus in the above-ground parts of the oats in a field experiment. This standard error was approximately one-fifth as large as the mean value of the parameter ky. With the concurrent

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Mitscherlich model, the ratio of standard error to mean C [Eq. ( 9 ) ] was about 0.12. Terman (1960, 1961) analyzed results from a total of 298 field experiments in which various water-soluble and citrate-soluble phosphate fertilizers were compared with respect to the probability of measuring differences among phosphorus sources. In most of these experiments, superphosphate was applied at several rates and other sources were compared at a common rate. A statistically significant yield response (at the 5 per cent probability level) was obtained in all these experiments. Limiting yields were estimated for all experiments by means of the hlitscherlich response function or by inspection of response curves. The limiting yield was defined as the highest yield obtained as the quantity of phosphorus was increased indefinitely at a given level of other growth factors. The least significant difference for each experiment was compared with the difference between the yield given by a rate of concentrated superphosphate and the limiting yield or the control plot yield. These comparisons provided estimates of whether or not differences between concentrated superphosphate and the other sources could be measured. In less than 50 per cent of tests studied would it have been possible actually to measure differences between superphosphate and the other phosphorus sources when equivalent quantities of phosphorus were applied. And in the tests in which it was possible to measure differences among sources, in less than 10 per cent would it have been possible to measure a difference of from one-half to twice as effective as concentrated superphosphate. The latter range probably includes all new sources which are likely to be used commericially. The possibility of measuring differences among sources decreased with increase in soil phosphorus levels and with increase in the rate of application of phosphorus at which the comparison was made. Although experimental error was high in some of the tests, the main reason for the difficulty in measuring differences among sources was the low response to applied phosphorus in most of the tests. Low yield responses were not always associated with nonresponsive soils but apparently were often influenced by growth factors which placed a low ceiling on yields. The result was a narrow spread between the control plot yield and the potential maximum yields, providing a small range in which potential source or rate differences could be expressed. Failure to include each fertilizer source at sufficient rates to define a response curve restricted the methods of comparing source yields in some experiments. The results of this survey indicated that unless the availability

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coefficients of two fertilizers are extremely different, the yields in field experiments are not likely to be statistically difEerent. However, this does not mean that the fertilizers are equivalent either in terms of yields or in terms of profits. Many yield differences which are not statistically significant are nevertheless important in terms of profit to the farmer. The need to test phosphorus (and other nutrient) sources on rather poor soils and at rather low rates in order to obtain response data falling on the steep portion of the response curve raises questions as to the interpretation of the data obtained (Terman, 1960). Most of the fertilizers will, in fact, be used on soils richer in phosphorus than the soils used for the experiments and will be applied at rates near the rate needed for maximum yield. Thus, the actual need is for information on the availabilities of phosphates under soil and crop conditions where the fertilizers have little effect on yields and differences among sources cannot be measured accurately ( Mulder, 1953). Consequently, under such conditions the residual, rather than the immediate, effects of fertilizers become of primary importance. Methods and designs for experiments to measure residual effects of fertilizers will probably receive increasing attention in the future.

IV.

Methods Used in Fertilizer Evaluation Tests

A. FIELDPLOTEVALUATION OF FERTILIZERS The ultimate criterion in evaluating the effectiveness of a fertilizer is the measurement of its performance under field conditions. Literally thousands of field experiments have been conducted in the past to compare fertilizer sources under a wide variety of soil and crop conditions. Many of these tests were very simply designed and largely served for demonstrational purposes, while many others were designed for more comprehensive studies. Some question may exist as to the efficiency of the great amount of time spent in field evaluation work; nevertheless, there is no denying the fact that a large quantity of valuable information has emerged from these field studies. The field researcher faces an assortment of problems arising from the lack of uniformity in the natural environment in which he operates. Soil variation within the experimental area and weather variations are two major problems with which he must deal. Weather adversities such as wind, hail, drought, and floods may reduce yields and add to experimental error within individual experiments. Seasonal weather variation frequently make it difficult to compare results of tests conducted in different years. Insects, diseases, nonuniform plant stands,

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and other factors difficult to control, may limit yield responses or cause erratic results. Hence, the big problem in field plot tests is keeping the experimental error within limits which will permit measurement of treatment effects. This is a problem in all field research, but it assumes particular importance in fertilizer source comparisons because the differences in effectiveness among sources are frequently very small. However, such differences are important to farmers, especially when they occur among fertilizers of equal cost. The value of the increased yield may be larger than the cost of the applied fertilizer. For this reason much of the following discussion will deal with the problem of obtaining sufficient precision in field plot tests satisfactorily to evaluate fertilizer sources.

1. Source and Extent of Yield Variability In view of the above-mentioned problem it seems appropriate to examine the nature of the yield variability found in previous field evaluation tests. Unfortunately, few studies have been made on the variability found in large groups of experiments. Most of these have dealt with crop variety trials and were primarily concerned with the effect of experimental design on standard errors. The only comprehensive studies found in the literature dealing with fertilizer sources trials were reported by Terman ( 1957, 1960, 1961). His reports involved several hundred phosphorus source experiments sponsored by the Tennessee Valley Authority in cooperation with various State agricultural experiment stations located, for t h e most part, in the southeastern United States. The yield results from these experiments were examined in relation to the nature of the response and experimental error. Ternian (1957) reported that in one group of 433 experiments located in the southeastern United States only 71 per cent showed a significant response to phosphorus fertilization at the 10 per cent level of probability. The percentage of tests in which yield response was obtained increased in the following order of crops: corn, cotton, legumegrass hay, and small grain. For all crops, 20 per cent of the responsive tests showed differences among phosphorus sources that were statistically significant at the 5 per cent level of probability. The coefficient of variation was used as a simple measure of experimental precision in this group of tests. Sevently-eight per cent of the experiments had coefficients of variation within the range of 5 to 50 per cent, with all crops showing a similar distribution. It was estimated that coefficient of variation of 5 per cent or less was necessary to measure a 10 per cent difference in yield with the 4 replications employed in most of the experiments. Only 2 per cent of the experiments

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had a coefficient of variation of 5 per cent or less. Coefficients of variation were negatively correlated with yield levels for all crops. This summary, together with the later papers (Terman, 1960, 1961), indicate the magnitude of variability which may be encountered, even in well-conducted field experiments. They point out some of the factors contributing to yield variability and experimental error. Many other factors which were undoubtedly of importance were not considered. Only phosphorus fertilizer sources were included in these studies, but it seems reasonable to assume that under the same experimental conditions similar trends might have been observed in tests with other fertilizer nutrients. It is not to be implied that it is impossible satisfactorily to evaluate fertilizers in field test plots. Satisfactory precision was obtained and significant source differences could be measured in many of the individual experiments within the above groups. It was also possible to pool the data from several tests having fairly consistent trends and to measure rather small differences among sources compared under rather similar crop, soil, and climatic conditions. Some investigators have tended to overlook the fact that in order to have a valid evaluation of two or more fertilizers it is essential that a response be obtained to the nutrient or nutrients under study. This response may be in the more practical terms of yield of crop or in terms of uptake of the nutrient by the crop. Failure to include the necessary treatments to determine whether there is a yield response to applied fertilizer is common in fertilizer demonstrations on farms. Comparison of fertilizers in unreplicated plots or strips without adequate control plots can and frequently does lead to erroneous conclusions.

2. Field Plot Techniques The necessity for attaining a high degree of precision to be able to successfully evaluate fertilizer sources in field plots has previously been emphasized. Certain natural factors which influence experimental error can be controlled only in part by the researcher. However, the experimental techniques employed in the actual establishment and management of a field experiment are largely within the realm of control. If error from this source is to be minimized, the experimenter must assume the responsibility of using the best techniques known and carefully attending to details. Some of the practices which are common to all field evaluation studies and which will be considered in this discussion include selection of experimental site, choice of number of replications, preparation and application of fertilizer, control of limiting yield factors, sampling and harvesting techniques, and general observations.

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a. Selection of experimental site. The nature of the soil and topography in an area may limit the researcher in selection of desirable sites, but compromising for less than the best auailuble definitely reduces the chance for success. Two of the most important factors to consider in site selection are the potential responsiveness of the soil to the fertilizer element being tested and the degree of soil uniformity. The importance of obtaining a satisfactory yield response to evaluate fertilizers in terms of crop yields has been emphasized previously in this review and the importance of soil uniformity need not be elaborated. A general knowledge of the characteristics of a soil in an area greatly facilitates site selection. Information regarding crop growth and yields in previous years is also very helpful. Such information may be obtained by observation or from the farm operator. However, the final decision is normally dependent on a close examination of the prospective site. Chemical soil tests are generally considered to be the most reliable index of potential responsiveness. Su5cient soil samples should be collected to provide a reliable measurement. It is often necessary to test the subsoil as well as the surface soil. An adequate supply of an element in an underlying horizon may prevent deep-rooted crops from responding to the element in question even though the surface soil is deficient. Information regarding past fertility treatment and crop management is also often indicative of the size of response to expect. A recent application of manure may prevent or reduce the response to additional treatments. Nonleguminous crops which immediately follow a legume in the crop rotation may fail to respond to nitrogen. A past record of soildepleting crops with infrequent use of fertilizer may suggest a favorable chance for a yield response. Selection for soil uniformity not only demands a knowledge of the fertility but also information regarding depth, texture, drainage, and other soil characteristics. Uniformity of slope and other surface features is highly desirable but is not always indicative of uniform productivity. In some areas slight undulations which are hardly visible may reflect wide variations in soil characteristics, such as soil pH and associated phosphorus availability. The slope may vary several percent in other areas with little change occurring in the nature of the soil. Some familiarity with the soils in an area is very helpful in judging the significance of surface features. Careful examination of both surface and subsoil is usually necessary for a complete evaluation. Depth, texture, erosion, drainage, and other

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traits may be judged by examination. Chemical soil tests are necessary to detect variability in fertility and soil reaction. Often the farm operator can designate sites which should be avoided because of known differential past treatment. Examples of such sites might include those on which: nonuniform applications of manure or fertilizer were applied, livestock were fed in the past, plant refuse was piled and burned or allowed to decompose, snow drifted and resulted in an irregular moisture pattern, or numerous other variable treatments may have occurred. There are many other less important factors which should be considered in selecting an experimental site. The results will be much more applicable if the test is located on a soil which is representative of a large region. A site which is conveniently located and readily accessible will reduce the travel and labor required by the experimenter. If the test is located on a private farm, it is highly desirable that the farmer understands the significance of the work and is willing to cooperate fully. Uniform cropping of an area prior to establishing differential fertilizer treatments is usually helpful as a basis for determining the plot layout or for rejecting the site entirely. b. Number of replications. In designing and establishing an experiment the researcher is always faced with the problem of determining the number of replications needed to provide the desired precision. The required number is influenced by the experimental design, the number of treatments, and by the many factors which influence expermental error. An estimate of the number required for a specified level of precision can be calculated by methods presented in most textbooks on statistics. These methods require an expression of the mean yield difference which the experimenter desires to detect and an estimate of the standard error per plot which is expected in the experiment. The reliability of the answer is dependent on the accuracy with which the expected standard error is estimated. Often this can be done fairly well using the results of previous experiments as a guide. Terman (1957) used the results from 433 source of phosphorus experiments conducted in southeastern United States to estimate the number of replications which would have been necessary to measure certain yield differences. He estimated that 8 replications would have been necessary to measure a 10 per cent yield difference with a coefficient of variation of 10 per cent. A coefficient of variation of 5 per cent or less would have been required for measurment of a 10 per cent yield difference with the 4 replications employed in most of these experiments.

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Webb and Pesek (1958, 1959) observed little increase in precision from increasing the number of replications above six in phosphorus source tests on corn in Iowa. The coefficients of variation were approximately the same for a series of tests involving hill-placed fertilizer on small plots replicated 7 to 12 times and a series involving broadcast applications on larger plots replicated 5 or 6 times. Of course, fertilizer placement and test plot size were variables in the experiments. It was their feeling that 5 or 6 replications should be used in fertilizer source tests in that locality. In general, experimental error tends to decrease with an increase in the number of replications used. However, a point is reached where the gain in precision must be weighed against the problems associated with the increased size of the experiment. Larger experiments entail greater difficulty of locating uniform sites and add to the labor and resources required. In the end, this point must be determined by the individual experimenter after consideration of all factors involved. Experience and information from previous experiments with the crops and soils involved are very useful guides. With the objective of covering a range of environmental conditions in the field, Sandison (1959) stated for crop trials that a precision greater than 10 per cent within trials was uneconomic. He found 2 to 3 replicates per trial sufficient, with adequate site and season replication. These results appear to be pertinent in the evaluation of fertilizers to be used over wide areas. c. Preparation and application of fertilizer. In fertilizer evaluation tests every effort should be made to keep the properties of the fertilizer as nearly uniform as possible except for the character being studied. Otherwise confounding effects will reduce the value of the results. Fertilizers to be evaluated should be thoroughly characterized chemically and physically before being tested in field plots. Chemical composition is one characteristic to be controlled as nearly as possible. For example, in comparing the phosphorus in an ammonium phosphate with that in superphosphate it becomes necessary to add sufficient nitrogen to the latter to compensate for that in the ammonium phosphate. A chemical form of nitrogen similar to that contained in the ammonium phosphate should be used for this purpose. An ammonium salt such as the nitrate or sulfate would be preferable to materials such as sodium nitrate or urea. This example would involve the comparison of a chemically blended fertilizer with a physical mixture, which is not desirable but must be tolerated in many cases. This physical variation is generally considered to be less objectionable in the case of banded or concentrated placements than in broadcast applications.

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Fertilizer particle or granule size should be kept constant when evaluating the chemical properties of fertilizers. Fertilizer effectiveness may be influenced to a greater extent by variation in particle size than by difference in chemical properties. Regardless of their relative influence, it becomes difficult to separate the effects of two variable properties. Of course, two such properties can be varied simultaneously in a controlled manner to permit measurement of interacting effects. It is equally important to maintain constant placement of fertilizers in source trials. This may be difficuIt when different physical forms of fertilizer are involved, such as the comparison of a liquid and a solid in row applications. For example, in a comparison of anhydrous ammonia with solid nitrogen fertilizer, it is important that the applicators be run through all plots to eliminate the possibility of confounding the effects of tillage with fertilization. d. Control of limiting factors. The control of growth factors, other than the one under study, which limit yields in test plots is important for at least two reasons. First, as we have already mentioned, low yields and small responses reduce the chance of successfully detecting differences in fertilizer effectiveness. Secondly, source differences observed under conditions of low crop demand for applied nutrients, as suggested previously, are no guarantee of the same relative differences under conditions of higher demands at high yield levels. Since modern conditions demand high yield levels, information obtained by comparing fertilizers at low yield levels may be of limited practical value. The ideal situation would be one in which all yield factors were optimum except the one being studied. If nitrogen sources were being studied, the rate and effectiveness of the nitrogen sources would be the sole factors determining yields within the genetic capacity of the plants. It obviously is impossible to control some of the natural factors but others can be controlled in part by the experimenter. Some of the important factors falling into the latter category include plant nutrients, weeds, diseases, insects, plant stands, and moisture levels. Plant nutrients. Every effort should be made to ensure an adequacy of nutrients, including minor elements, except the one being evaluated. These can be applied at the time and by the methods recommended for the crop and region. Liming to the proper pH should be accomplished several months prior to establishment of a fertilizer test in regions of acid soils. Weeds. The competitive effect of weeds for light, moisture, and nutrients is well known. Weeds can completely eliminate any yield response or in the case of irregular infestations cause erratic yields.

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L. TERMAN, D. R. BOULDIN, AND J. R. WEBB

The use of chemicals for weed control on experimental plots should be confined to those which have been thoroughly tested and characterized. Insects a d diseases. Failure to control insects and diseases can result in total failure of a field test or cause erratic, distorted results. Yield responses are commonly reduced or prevented by the low ceilings placed on yields. An interesting example of insect damage is that caused to the corn crop in the United States by the European corn borer (Pyruusta nubilah). The moths seem to show a strong preference for the larger plants in a field as a place to deposit their eggs. Consequently the greatest damage occurs to the larger, fertilized plants in a test plot, preventing or reducing yields. Such a situation tends to bias the results in favor of the less effective sources. Less commonly, the unfertilized plants on the control plots will be more severely damaged by insects or diseases than the fertilized plants and the yield response is exaggerated. Plant stands. Nonuniform or low plant populations reduce the value of many field experiments. Plants insufficient in number to take full advantage of the potential productivity reduce yield responses, and uneven stands contribute to experimental error. Uneven stands are usually greater problems with row crops such as corn, cotton, and sugar beets than with broadcast crops because of the smaller number of plants per unit area. They are usually the least problem with small grains because the plants tend to tiller and utilize any extra space. Because there is no entirely satisfactory method to correct for uneven stands, it is a far better practice to prevent them if possible. With some crops poor stands may be avoided by overplanting and thinning the young plants to a constant density. Replanting and transplanting may be employed to fill in areas where plants are missing. Because such plants commonly fail to reach average size, they are often used solely to provide a competitive situation and are not harvested for yield. If the experimenter is confident that the stand variability is not due to the imposed treatments, it is possible to adjust yields by statistical techniques. The analysis of covariance is used for this purpose; it is described in most statistical textbooks. Adjustment of treatment mean yields to a uniform plant density within the experiment, by this method, often results in increased precision in measurement of fertilizer source differences. This procedure cannot be used for broadcast crops where it is impractical to count the number of plants in the harvest area. It should not be used if the stand has been influenced by treatment. In such a case the resulting stand is part of the treatment effect and should be considered as such. Leonard and Clark (1939) and Kempthorne (1957) discuss the

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subject of variable stands and the effect upon yields in experimental plots. Moisture levels. Lack of soil moisture frequently limits yields in field tests. In many localities it is possible to use supplemental irrigation to great advantage; however, care needs be taken to ensure uniform application of water to avoid an additional source of error. The problem of excessive amounts of water can be avoided only by selecting sites having uniformly well-drained soils. In areas where artificial drainage of some type is practiced, the plots should be carefully located with respect to tiles, ditches, and other drainage structures in order to avoid an uneven drainage situation. e. Sampling and harvesting techniques. The wide variety of conditions and crops involved in field evaluation tests prevent a detailed discussion of harvesting techniques in this review. Methods for harvesting specific crops have become fairly well standardized. Many of the errors encountered in harvesting are associated with the human element. Mistakes in weighing, recording data, labeling samples, and others of this nature cause much confusion and contribute to experimental error. Mistakes of this type can be kept to the minimum by careful organization and adequate supervision of the operation. Extreme caution should be taken to avoid faulty techniques which result in measurements that are consistently biased. The estimate of experimental error does not take account of such biases. Collecting samples of plants or plant parts for chemical assay provides similar problems. The samples need to be large enough to be representative of the plot from which they were taken. Differences in the stage of plant development that are due to the applied treatments often pose the question whether all plots should be sampled at one date or at a specified stage of development. The former is more convenient but consideration should be given to the latter possibility if large differences exist. f. General observations. Distance and lack of time may limit the frequency at which the experimenter can observe field plots during the growing season. Valuable information is often lost as a result of this situation. The final yield may be the main criterion of evaluation, but treatment effects which may not be reflected in final yields often appear during the season. Frequently these can be evaluated and provide worthwhile information. Differences in early season growth, plant color, plant maturity, susceptibility to weather hazards and diseases are examples of such effects. Even if these differences cannot be measured, knowledge of their occurrence may be useful in interpreting yield results and may provide suggestions for future research. One may likewise detect the

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appearance of diseases and other irregularities which can be corrected or used in interpreting the data.

3. Design of Experiments and Treatment of Data This important aspect of experimentation has been discussed in the above review of concepts useful in fertilizer evaluation and will be considered only briefly as related to field plots. An examination of the literature reveals that many types of experimental designs have been used in fertilizer evaluation tests. It appears that the randomized block design has been used most widely. This is understandable, since it is a relatively simple design which allows for satisfactory control of error. The Latin Square design has also been widely used. This design is somewhat less flexible than the randomized block design but is more efficient. Many tests have involved the use of more complex designs. In many of the latter, fertilizer sources were studied in conjunction with other factors such as fertilizer placement or time of application. G. W. Cooke reported (personal comunication, 1961 ) that lattice square designs were unsatisfactory for testing a large number of phosphates. Lattice designs were no better than simple randomized blocks involving areas of the same size. Central composite and lattice designs have been used recently in experiments to determine response surfaces arising from application of several rates of two or more nutrients. Incomplete designs would seem desirable in such experiments because of the very large numbers of treatments involved in complete factorials. More adequate discussions of types of designs and e.qerimenta1 techniques are given by Love ( 1943), Cochran and Cox ( 1957), and in numerous other texts. Munson and Doll (1959) have discussed experimental designs useful for studying the economics of fertilizer use. In past tests it appears that choice of treatments have limited the value of the results far more than has the experimental design. Insufficient rates of the fertilizers being tested has been the most common weakness. The restrictions which insufficient rates impose on the use of the experimental results was considered in the above discussion of concepts and was reviewed by Terman (1960) and by Munson and Doll ( 1959). Many of the earlier tests involved only a single rate of each source, which greatly limited mathematical treatment of the data. The inclusion of each source at a sufficient number of rates to define a response curve permits maximum treatment of the data. Such a procedure may require a large experiment, a fact which has undoubtedly limited its use in the past. The results from such experiments can be studied by conventional

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statistical analysis and tests of significance. They can also be studied by regression analysis and by what might be termed the bioassay methods applied by Black et aZ. ( 1956) and White et al. (1956).

4. The Future of Field Tests Despite the limitations which have been mentioned above, field tests definitely have a place in a fertilizer evaluation program. However, it does appear that in the future new sources may be screened more closely before they are tested in the field and that field tests will be conducted on a more selective basis. Such a procedure should prove to be equally as effective as the methods employed in the past and should be more efficient and economical. Any new material being considered for possible use as a fertilizer should be completely characterized in the laboratory. By using this information and the current knowledge of fertilizers one should be able to predict with a reasonable degree of accuracy the relative manner in which the new material will perform under various cropping and soil conditions. The next logical step would appear to be thorough agronomic evaluation in the greenhouse under controlled conditions. If the material appeared to have possibilities, the final step would involve field testing on a selective basis, the greatest emphasis being placed on the crops and soils which were likely to provide the most rigorous tests. In summary, meaningful results can be obtained from field tests only with careful attention to the following practices: ( a ) careful selection of sites for uniformity and response to the nutrient under test, ( b ) use of adequate replication, ( c ) use of the best, known techniques, and ( d ) use of several rates of each fertilizer to describe a response curve for each. Collis-George and Davy (1960) have questioned the utility of presentday field experimentation and advocated a restricted number of complety instrumented experiments to give the necessary information on the quantitative importance of environment and its interaction with fertilizer response. Bawden ( 1959) has also asserted that repeating experiments in many places or in many years not only is costly, but is no substitute for intensive research. What is needed is a detailed study of a fertilizer so that the conditions under which it will be beneficial can be precisely defined.

5. Use of Frames and Rims One research technique which might be placed in a category between greenhouse and field experimentation has involved the outdoor growth of plants in specially constructed frames of various types. One common method of construction has consisted of lining the sides of a pit with a

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G. L. TERMAN, D. R. BOULDIN, AND J. R. WEBB

frame constructed of wood, metal, or other materials and filling it with the soil to be studied. Such structures are embedded to depths ranging from a few inches to three or more feet and usually project slighly above the soil surface. They usually are 2 feet or more in diameter. Other various types of construction have been used. This technique has been employed in an effort to maintain a certain amount of control over various experimental factors. It has the advantage over greenhouse methods of allowing plants to be grown under natural weather conditions, thus providing results more comparable to those obtained in field test. Assuming uniform mixtures of the soil prior to placement in the frames, soil variation among the differently treated frames is minimized. The fact that fertilizers can be compared in soils from many areas under a single natural environment is a big advantage. This is of much importance in fertilizer evaluation tests because the relationship between fertilizer source and soil properties can be studied under uniform weather conditions. Depth and position of fertilizer placement can be conveniently attained at the time of placing the soil in the frames. Supplemental irrigation may be more easily provided than with field plots. The frames can be used for a succession of crops, thus providing an opportunity to study residual effects of fertilizer treatments. This technique has several disadvantages which have limited its use. The operation of constructing and filling the frames is costly. All the labor involved in producing and harvesting a crop must be performed by hand. These two factors frequently limit the size and number of frames available for use in an experiment. The small size of an individual frame restricts the number of plants which may be grown, thereby increasing the chance for error due to plant variability. Close-growing crops such as small grains and forage crops are usually chosen to reduce this possibility. The growing plants are exposed to the same natural hazards which endanger field-grown crops. This method appears to have been used much less extensively for fertilizer source evaluation than for placement and other types of fertility studies. Bennett et al. (1954) recently reported on the use of 3 X 3-foot frames for comparing the efficiency of 18 phosphorus fertilizers. Their work involved a number of crops grown over a 5-year period. B. EVALVATION OF FERTILIZERS IN GREENHOUSE POTS Much more of the variability inherent in research on evaluation of fertilizers can be controlled in greenhouse pot tests than in field plot experiments. For example, the coefficients of variability (C.V.) for yields of crops grown in pots in greenhouses at Wilson Dam, Alabama, to

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307

evaluate phosphorus sources and rates (Tennan, 1959) usually range from 5 to 10 per cent and occasionally are less than 5 per cent. Corresponding C. V. in field experiments to compare phosphorus source and rates (Tennan, 1957) range considerably higher. Because of the limited soil volume in greenhouse pots, crops are usually harvested at some stage prior to full maturity-small grains and grasses as heads begin to appear, legumes at early bloom state, and corn when 2 to 3 feet tall. Thus, the harvests are usually made at a time comparable to the early growth stage commonly observed in field experiments. In the field these early growth differences tend to become less as the crop matures. For example, under long-season conditions it is commonly observed in field experiments that early growth differences are not well correlated with final yields of forage, grain, or fiber. Another factor increasing the sensitivity of fertilizer comparisons in greenhouse tests is that soils are usually selected that are markedly deficient in the nutrient under test. On deficient soils plants use more of the nutrient applied in the fertilizer and less from the soil than they do on soils having a higher content of the nutrient. This allows more precise comparison among the nutrient-supplying capacities of the fertilizers under test. In soils higher in content of nutrient or at rates of application high on the response curve, the comparison is much less precise. Thus, in properly conducted greenhouse pot experiments, differences in response among a series of fertilizers are at the maximum. As a result, pot tests are particularly useful for screening large numbers of experimental products. If differences among fertilizer treatments cannot be measured in pot experiments, it would appear useless to make similar comparisons under the more variable conditions often encountered in the field. However, if greenhouse pot experiments indicate rather marked differences in response among the fertilizers, then consideration of field experiments is justified. Effects of water solubility, granule size, placement, nature of associated salts, and other factors can be explored to advantage in the greenhouse. Greenhouse and field results for certain types of experiments, such as effect of salt concentration in relation to moisture, may not be comparable. In general, however, properly conducted greenhouse and laboratory studies often give useful evidence as to the factors that merit investigation under field conditions. Pot experiments also are valuable in relating observed laboratory measurements to actual plant response and thereby are important in establishing basic principles on soilfertilizer-plant relationships. Because of low precision under field conditions, relationships observed in the laboratory and greenhouse might be missed entirely in field experiments.

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G . L. TERMAN, D. R. BOULDIN, AN D J. R. m B B

1. General Methods Used Xumerous investigators have used pot tests to evaluate fertilizers, but few have reported studies in which various methods were compared. MacIntire and Winterberg (1946) reported their methods used for comparing fertilizers in %gallon undrained glazed earthenware pots at Knoxville, Tennessee. Adequate aeration was provided by a glass tube extending down through the hole of a 3-inch clay pot inverted in the bottom of the large pot which contained 8kg. of soil. The pots were weighed at least weekly and soil moisture was brought to 50 per cent of field capacity as needed. They usually grew sudangrass in summer, annual ryegrass in fall and winter, and red clover in winter and spring, as well as alfalfa, sweet clover, lespedeza, and soybeans as crops for evaluating various fertilizers. Tennan (1959) found that variability in yields was essentially the same for crops grown in 2-gallon glazed pots and in No. 10 tin cans containing 3 kg. of soil. Anniger et al. ( 1958) grew alfalfa in containers from 1 quart to 24 gallons in size (3.6 to 201 pounds of soil) and concluded that this crop could be grown as accurately in 1-to 3-gallon pots as in large cans. Plants grown in small pots obtained slightly more phosphorus from the applied fertilizer than those grown in large cans. In a second study, Armiger and Fried (1958) grew German millet in 3-gallon and smaller sizes of pots. Small pots produced the highest yields of dry matter and of phosphorus per unit weight of soil. They concluded that small pots economized use of soil and space and gave yields comparable to those obtained in larger pots. Crowther and Cooke (1951) found 3.5 inch-diameter glass pots containing 50 g. of soil mixed with 200 g. of coarse, angular sand satisfactory for evaluating phosphate fertilizers. M7atering was accomplished by subirrigation from shallow dishes in which the pots stood. Mitscherlich (1909, 1930) developed a method for determining the fertilizer needs of soils in which 25 oat plants were grown in enameled metal pots 20 cm. in diameter and 20 cm. deep. Drainage through a hole in the bottom was collected in metal pans below each can and returned to the soil. This method has been used extensively in Germany for determining crop response to applied fertilizers. This method, as well as the method of growing Romaine lettuce used by Jenny et al. (1950), that of growing sunfiowers as the test crop used by Stephenson and Schuster (1941), and other methods, could no doubt be adapted for fertilizer evaluation studies. Vandecaveye ( 1948) has discussed more fully various biological methods, largely for determining nutrients in soil. Recently, No. 10 tin food cans containing about 3kg. of soil have

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become widely used for fertilizer studies. These cans placed on movable tables have been used satisfactorily in numerous greenhouse experiments by TVA at Wilson Dam, Alabama, and by many other investigators. At first the cans were coated with asphalt paint, but is was soon found that lining the cans with polyethylene bags was less expensive. Either method avoids the possible errors involved in reaction of phosphates with the rust in uncoated cans or perhaps the absorption of salts by imperfectly glazed earthenware jars used as the containers. The following methods have been used successfully in a large number of fertilizer evaluation experiments by TVA: Three kilograms of soil (dry weight basis) per can is commonly used. If an experiment requires the incorporation of lime or other amendment, mixing with the soil is usually done in a large twin-shell rotating soil blender in about 12-kg. lots. Phosphates or other fertilizer under study are weighed separately for each can and mixed with the soil by hand, using a uniform mixing procedure. As shown in Table VII, yield variability was lower as a result of individual rather than of bulk blending. TABLE VII Effect on Variability of Mixing Phosphate with Soil for Individual Pots (1 Replicate) and for 3 Replicates Togethep Yield of dry matter Method of mixing phosphate Individual pots 3 Replicates together a

Mean (g./pot) 6.8 6.5

Per cent P

C.V.

(%) 6.4 9.2

Mean 0.089 0.086

Uptake of P

C.V.

C.V.

(%)

Mean (mg./pot) 6.9 5.9 9.3 5.6

(%) 7.1 14.0

From Terman (1959).

An excess number of seeds are planted and the plants are thinned to a uniform number per can, usually 5 plants for corn and 30 plants for crops such as millet, oats, and wheat. Supplemental fertilizers are commonly added in solution to the soil after planting in uniform amounts for all cans. In case the fertilizers contain fertilizer nutrients in addition to the one under study (i.e., NP fertilizer in a phosphorus source study), supplemental amounts of nitrogen, etc., are added to give the same total in all cans. Additional amounts of nitrogen and other nutrients are added as required by crop growth. The cans plus soil are weighed twice weekly and deionized water added to bring the soil to two-thirds of the field capacity. When water needs are more frequent, as during warm weather and rapid growth, the amount of water required is usually estimated. No drainage is provided, and a tube to the bottom of the can for aeration has not been found to be necessary. However, data in Table VIII indicate a possible

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G . L. TERMAN, D. R. BOULDIN, AND J. R. WEBB

need for aeration throughout the soil, as indicated by lower yields with phosphorus placed near the bottom of the cans. TABLE VIII Effect on Response by Oats of Placement of Two Phosphorus Sources Banded Separately and Together with Lime at 2 Levels in the Pot in Acid Hartsells Fine Sandy Loam Previously Limed to pH 7.4 Placement of lime ( L ) and phosphate ( P ) a

P P P P P

in band A; L in band 3 in band B; L in band A and L in band A and L in band B and L mixed through soil

Monocalcium phosphate

Diammonium phosphate

100 82 56 36 79

104 89 89 68 78

5 Band A-a circular band 4 cm. from the walls of No. 10 tin cans and 4 cm. deep; Band B-a similar circular band 2.5 cm. from the bottom of the can; L ground limestone, 500 pounds per 2 M pounds of soil; P-60 and 120 mg. of P per 3 kg. of soil.

Thermostatic contTols are used to maintain the desired temperatures during most of the year. No refrigeration is provided, however, and slat shades are used to reduce temperatures during summer. During hot weather the tables and cans are moved daily to an outside wireenclosed growth area. Usually only the above-ground portions of the plants are harvested for determination of yield of dry matter and subsequent analyses for total content of phosphorus or other nutrient under study. In experiments designed to study residual effects of phosphorus sources, the surface 2 to 3cm. of soil is worked and a new seeding is made without further addition of phosphorus. Powdered and granulated materials usually are compared in greenhouse tests conducted by TVA at Wilson Dam. In case ordinary granulation techniques are not feasible or cause changes in the chemical composition, the powder is pressed into pellets. These are crushed and screened into the desired sizes.

2. Design of Experiments The type of design and number of replications for greenhouse pot tests to evaluate fertilizers depend primarily on the extent of variability of growth conditions within the greenhouse. At the TVA greenhouses at Wilson Dam, Alabama, which are 32 by 66 feet and oriented in a northsouth direction, randomized block experiments with 3 replications are commonly used. Pots of each replicate are grouped together on movable tables, and treatments are randomized within each replicate. The variation among replicates is usually so low, however, that in analyses of

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311

variance the among-replicate component is not removed, and the experiments are considered as completely randomized experiments. Under these rather uniform conditions, however, it was found (Terman, 1959) that 3 to 4 replicates were insufficient for a valid estimate of experimental error for a particular fertilizer treatment. Error mean squares based on 20 or more replicates were usually found necessary in order to establish relationships with mean yield. Other investigators have found much more variation in growth conditions and have recommended suitable experimental procedures. LeClerg (1935) cited an extreme example of the effect of a temperature gradient on stands of sugar beet seedlings. Cox and Cochran (1946) concluded that only large treatment differences could be measured with 3 or 4 replications under typical greenhouse conditions. 3. Agreement of Greenhouse and Field Results Collis-George and Davy ( 1960 ) have questioned the transference of greenhouse results to field conditions, but suggest that this might be possible, if the environments of the greenhouse and field situation were known quantitatively. Cook and Millar (1946) reported techniques which help to make greenhouse results comparable to those in field experiments. Higher nutrient levels in the greenhouse, adequate size of pots and other factors were found to be important. Good correlation resulted, particularly with soil from the same site being used for both the greenhouse and field experiments. Hausenbuiller and Weaver (1960) found with low to moderate rates of application that greenhouse evaluation with ladino clover and field evaluation with alfalfa were :qually satisfactory for determining the phosphorus-supplying capacity 3f several phosphorus sources. Stage of growth is important in regard to effects of water-soluble phosphorus on yields. Terman et al. (1956), DeMent and Seatz (1956), md others have found marked effects of level of water solubility on fields in greenhouse pots and on early growth in field experiments. However, the early growth differences did not persist in final yields of :orn or wheat grain in most experiments. Thus, different conclusions may 3e reached as a result of evaluations based on early growth response, i s compared to those based on final yields of grain or forage.

C. SEEDLING AND MICXOBIOLOGICAL METHODS OF EVALUATION 1. Short-Term Nutrient Uptake Methods The seedling method developed by Neubauer and Schneider (1923) or the determination of soil content of available nutrients has also Ieen used by Thornton ( 1931, 1935), McGeorge (1939), and others to

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G . L. TERMAX, D. R. BOULDIN, AND J. R. WEBB

study the availability of nutrients in fertilizer materials. In this method as originally designed, 100 rye seedlings are grown on 100 g. of soil diluted with washed sand, After 14 to 18 days of growth, the whole plants are harvested, washed, ashed, and analyzed for nutrient content. This method is relatively rapid but has the disadvantage that the results must be evaluated largely in terms of luxury uptake of nutrient by the seedlings, because the content of nutrient in the seed may be adequate for growth during this period. Jones (1949) modified the Neubauer method to study the effectivenes of phosphate fertilizers on Congo soils. He found that early availability of superphosphate and soda-phosphate was much greater than that of Uganda phosphate rock. Stanford and DeMent ( 1957) developed an improved short-term fertilizer evaluation method to help bridge the gap between laboratory chemical extraction methods for phosphates and conventional greenhouse pot methods. Plants deficient in the nutrient under study are grown in sand contained in small cardboard cartons with bottoms removed. The dense mat of roots formed at the bottom of the sand after 2 to 3 weeks is placed in contact with soil, or soil plus fertilizer, contained in a second carton. Uptake of phosphorus or other nutrient can be measured in 24 hours, but greater precision is obtained after longer growth periods. An uptake time of 1 week is commonly used. Uptake by this method is usually well correlated with uptake obtained in conventional pot tests over a period of several weeks (Terman et al., 1958). Later, DeMent et aI. (1959a, b ) adapted the short-term method for evaluation of potassium and nitrogen fertilizers. High correlations were found between amounts of nitrogen and potassium in oat tops and in the whole plants. This short-term method has also been adapted for other types of studies, such as the determination of available sulfur and available potassium in soils. 2. Microbiological Methods Several microbiological assay methods have been used for determinations of available nutrient content of soils, but also in some cases for availability of nutrients in fertilizers (see Vandecaveye ( 1948) for a more detailed discussion of these methods). Use of Aspergillus niger was studied extensively by Niklas and Poschenrieder (1932) and associates for determining available phosphorus, potassium, and magnesium in soils. Their method was to inoculate 2.5 g. of soil and 30 ml. of nutrient solution in 75-ml. flasks with A. niger spores, incubate at 35°C.for 4 to 6 days, wash and dry the mycelial pad and determine its weight as an indication of amount of available nutrient in the soil. Mehlich et al. (1933) modified this method for determining available potassium in soils.

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Mehlich et al. (1934) developed a plaque method for determining available phosphorus in soils based on extent of growth of a colony of Cunninghamella sp. They found that availability, as determined by the diameter of growth on the soil surface after 4-1/2 days at 28 to 29"C., was in good agreement with results obtained with the Mitscherlich pot method and chemical tests. Cunninghumella was found to make a quick response to available phosphorus and to be very sensitive to lack of phosphorus. Sackett and Stewart (1931) and others studied the plaque method developed earlier by Winogradsky ( 1927), using the growth of colonies of Axotobacter to study mineral deficiencies in Colorado soils.

D. EFFECT OF KIND OF CROPAND CLIMATE ON EVALUATION OF FERTILIZERS Nitrogen and potassium fertilizers. Most crop plants seem to utilize ammonium and nitrate forms of nitrogen equally well, so that, from this standpoint, the kind of nonlegume crop is not usually of great importance in the evaluation of nitrogen fertilizers. These include most nonlegume grain crops and most hay and pasture grasses. Crops such as tobacco, potatoes, and tomatoes, however, apparently require nitrate nitrogen and do not grow properly unless nitrate is present as a result of direct fertilizer application or nitrification of ammonium compounds. Legumes obtain much of their required nitrogen from the air and thus tend to respond less to applied nitrogen fertilizers than do nonlegumes. Soluble potassium sources such as chloride and sulfate applied at normal rates appear to be equally suitable for the growth of crop plants. Chloride in large amounts has detrimental effects on the quality of certain crops such as tobacco and potatoes, and this factor should be considered in any evaluation of the effect of potassium fertilizers on quality of such crops. Phosphorus fertilizers. Truog ( 1916), DeTurk ( 1942), Fried ( 19!3), and others have shown that buckwheat and legumes, such as alfalfa, red clover, and sweet clover, are much stronger feeders on phosphate rock than are cereals, which require a more soluble source of phosphorus for good growth. Thus, the kind of crop is important in the evaluation of phosphorus sources differing widely in solubility. It has been commonly observed that rapid-growing potato and other vegetable crops respond much more to phosphorus applied on soils having medium to high levels of soluble phosphorus than do crops such as oats and clover. For example, on the Caribou soils in Maine, Terman et al. (1952) found significant yield response by potatoes to 80 to 100 pounds of Pz05 applied to highphophorus soils, but oats grown on these soils did not respond to applied

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G . L. TERMAN, D. R. BOULDIN, AND J. R. WEBB

phosphorus. Similarly, on Sultan soils in western Washington, Mortensen (unpublished data, Washington State University) found marked yield responses by cucumbers, pole beans, and potatoes to applied phosphorus, but no response by oats on these soils. Level of water-soluble phosphorus in applied fertilizers was found to be important for vegetables. In a practical sense, the relative effectiveness of several fertilizers may vary so slightly under different environmental conditions that for all practical purposes general statements may be made about their relative effectiveness. As pointed out previously [Eq. ( 7 ) ] , numbers proportional to the availability coefficients of fertilizer (availability coefficient indexes ) are derived from fertilizer evaluation experiments. Hence, so long as the values of k y derived from two experiments with the same fertilizers are directly proportional to each other, there is reason to reject the hypothesis that the values of y are different. This means that, in general, the ratio of ky for two fertilizers will be the same in both sets of experiments. This behavior is illustrated by the experimental data in Table IX, obtained in a cooperative experiment between the Tennessee TABLE IX Average Values of the Availability Coefficient Index (ky) for Two Fertilizers Based on Yields of Wheat Forage and Corn Grain ~

Wheat forage

kv Ratio of ky for AOSP to ky for OSP a OSP phate.

= Ordinary

OSPa

AOSW

0.241

0.096

0.40

Corn grain OSP~ 0.470

AOSPa 0.212

0.45

superphosphate; AOSP = ammoniated ordinary superphos-

Valley Authority and the Mississippi Agricultural Experiment Station over the 3-year period 1957-1960 on Houston black clay. The results show that values of the availability coefficient index ( k y ) for wheat forage and corn grain are different, but that the ratios of ky are not very different for the two crops. Hence, so far as decisions regarding which of the two fertilizers to use are concerned, the results with both crops were in agreement. Terman et al. (1961) compared a series of fertilizers varying in water solubility of phosphorus for corn grown in the greenhouse at Wilson Dam, Alabama, at all 4 seasons with other crops grown at appropriate seasons-millet in spring and summer, wheat in fall, and oats in winter. Yield of crops at all seasons increased in a similar manner with increase in phosphorus water solubility. Relative effectiveness of the fertilizers for the various crops are given in Table X. Good agreement among the fertilizers was obtained, regardless of the season or crop.

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315

The results reported by Webb and Pesek (1958) indicate that generally the efficiency of fertilizer hill-placed for corn was much the same regardless of season or soil type. However, it is interesting to note TABLE X Relative Effectivenessa of 4 Fertilizers as Sources of Phosphorus for Various Crops Grown in Greenhouse Pots of Hartsells Fine Sandy Loam ( p H 5.6) at 4 Seasons of the Year Ammoniated Ammoniated Ammoniated ordinary concentrated ordinary supersupersuperDiammonium phosphate, phosphate, phosphate, phosphate, Crop and season 14%WSP 30%WSP 87% WSP 100%WSP Corn-Summer 22 26 100 105 24 Fall 18 100 109 27 100 Winter 26 136 Spring 100 36 35 113 100 Millet-Summer 18 27 120 24 100 118 Wheat-Fall 15 Oats-Winter 16 25 100 133 Millet-Spring 16 18 100 127 0 Based on availability coefficient indexes (Bouldin and Sample, 1959a) calculated from yields of dry matter with 30 and 60 mg. of applied phosphorus mixed throughout 3 kg. of soil per pot. WSP-water-soluble phosphorus.

that entirely different results were obtained with many of the same fertilizers applied broadcast and plowed under for corn (Webb and l'esek, 1959). E. USEOF GROWTHCHAMBERS Many of the factors affecting crop response to fertilizers in the field can be controlled in a greenhouse, but factors such as light and humidity cannot be controlled closely under usual greenhouse conditions. As a result, for more precise studies the use of growth chambers has increased rapidly in recent years. Provision is usually made for control of light, temperature and, in some cases, humidity. As a result, rather exact growth conditions can be reproduced over time, as contrasted to greenhouse, and especially to field conditions. Reproducibility of growth conditions and results over time is essential in many types of studies. At TVA's facilities at Wilson Dam, Alabama, experiments using the short-term uptake method (Stanford and DeMent, 1957) are routinely conducted in growth chambers. In an experiment to determine best growth temperatures for this method, oat plants were grown for 17 days under the following day-night temperatures: 9"-7"C., 2Oo-16"C. and 31"-27"C. Plants from each of these temperature regimes were then

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G . L. TERMAN, D. R. BOULDIN, AND J. R. WEBB

transferred to soil cultures and grown for 7 days at each of the same temperatures. The greatest uptake of phophorus was found at the intermediate temperature with plants grown at the higest temperature. Response to applied nitrogen and phosphorus was determined for oats grown during December and January in No. 10 cans in growth chambers maintained at a (1) 16-hour, 21°C. day and 8-hour, 16°C. night; and ( 2 ) 12-hour, 21°C. day and 12-hour, 16°C. night and on benches in the greenhouse. As shown in Table XI, growth response was greatest with the 16-hour day, intermediate with the 12-hour day, and TABLE XI Relative Effectiveness of Ammoniated Concentrated Superphosphate for Oats Grown in Greenhouse Pots at Wilson Dam, Alabama, as Affected by Placement and Growth Environment Relative effectiveness

Growth condition Growth chambers-light at 1800 foot-candles: 16 hour, 70°F. day; 8 hour, 60°F. night 12 hour, 70°F. day; 12 hour, 60°F. night In 70°F. greenhouse, with light prevailing, from Dec. 16 to Feb. 1

Mixed through the soil

Placed in spots, 30 mg. of P per spot

Banded in layer 4 cm. below surface

Limiting yield with mixed placement ( g./pot)

42

99

100

23.0

45

91

100

14.0

25

100

100

8.5

poorest in the greenhouse. Oats in all pots were germinated in the greenhouse before transfer to the subsequent growth conditions. Results published by Grunes et al. ( 1958), Robinson et al. (1959), and Ketcheson (1957) are indicative of the types of fertilizer studies which can be carried on only under conditions of controlled environment. V. Conclusions

In this chapter \?re have discussed ( a ) the relationship between the chemical properties of fertilizers and reactions which occur in the soil, ( bf the significance of the soil-fertilizer reaction products to crop uptake of fertilizer nutrients, and ( c ) some of the principles and practices involved in evaluating fertilizers. There is a need for continuing research on evaluation of fertilizers because of the increasing number and complexity of fertilizers being used commercially. Because of the difficulties of measuring differences

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317

in crop response among fertilizers in field experiments, increasing use should be made of laboratory and greenhouse pot methods in screening new fertilizers and fertilizer compounds. However, experiments conducted under field conditions are still necessary in some cases. Emphasis should be placed on the best possible techniques for carrying out field experiments, and response to applied fertilizers should be related to measured characteristics of the fertilizer, weather, and soil. Fields for worthwhile new research are those of explaining differences in limiting yields among fertilizers and methods for measuring differences in residual effects of fertilizers over a period of years. ACKNOWLEDGMENT This chapter was prepared under the auspices of the Subcommittee on Fertilizer Evaluation of the Fertilizer Committee of the Soil Science Society of America. Appreciation is expressed to Drs. C. E. Bardsley and J. L. Ragland, members of the subcommittee, and to Dr. G. W. Cooke, Rothamsted Experimental Station, Harpenden, England, for reviewing the manuscript. REFERENCES Armiger, W. H., and Fried, M. 1958. Agron. I . 50, 462-465. Armiger, W. H., Dean, L. A., Mason, D. D., and Koch, E. J. 1958. Agron. 1. 60, 244-247. Association of Official Agricultural Chemists. 1955. “Official Methods of Analysis,” 8th ed. Washington, D. C. Bawden, F. C. 1959. Rothamsted Agr. Expt. Sta. Ann. Rept. pp. 28-29. Bennett, 0.L., Longnecker, T. C., and Gray, C. 1954. Soil Sci. Sac. Am. Proc. 18, 408-412. Bingham, F. T. 1959. Soil Sci. 88, 7-10. Black, C. A., and Scott, C. 0. 1956. Soil Sci. SOC. Am. Proc. 20, 176-179. Black, C. A., Webb, J. R., and Kempthome, 0. 1956. Soil Sci. SOC. Am. Proc. 20, 186-189. Bouldin, D. R., and Sample, E. C. 1958. Soil Sci. SOC. Am. Proc. 22, 124-129. Bouldin, D. R., and Sample, E. C. 1959a. Soil Sci. SOC. Am. Proc. 23, 276-281. Bouldin, D. R., and Sample, E. C. 195913. Soil Sci. SOC. Am. Proc. 23, 338-342. Bouldin, D. R., and Sample, E. C. 1960. Soil Sci. Soc. Am. Proc. 24, 464-468. Bouldin, D. R., DeMent, J. D., and Sample, E. C. 1960. I . Agr. Food Chern. 8, 470-474. Clark, L. J., and Hill,W. L. 1958. I . Assoc. Ofic. Agr. Chemists 41, 631-637. Cochran, W. G., and Cox, G. M. 1957. “Experimental Designs,” 2nd ed. Wiley, New York. Collis-George, N., and Davy, B. G. 1960. Soils and Fertilizers 23, 307-310. Cook, R. L., and Millar, C. E. 1946. Soi2 Sci. SOC. Am. Proc. 11, 298-304. Cooke, G. W. 1956. J . Agr. Sci. 48, 74-103. Cooke, G. W., and Widdowson, F. V. 1959. 1. Agr. Sci. 53, 46-63. Cox, G. M., and Cochran, W. G. 1946. Soil Sci. 62, 87-98. Crowther, E. M., and Cooke, G. W. 1951. Ministry Supply ( G t . Brit.) Monograph 11, 108. DeMent, J. D., and Seatz, L. F. 1956. J . Agr. Food Chem. 4, 432-435.

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J. R. WEBB

DeMent, J. D., Stanford, G., and Bradford, B. N. 1959a. Soil Sci. SOC. Am. Proc. 23, 47-50. Dehlent, J. D., Stanford, G., and Hunt, C. M. 1959b. Soil Sci. SOC. Am. Proc. 23, 371-374. DeTurk, E. E. 1942. Illinois Agr. Expt. Sta. Bull. 484. Duncan, W. G., and Ohlrogge, A. J. 1958. Agron. J. 50,605-608. Eid, M. T., Black, C. A., Kempthorne, O., and Zoellner, J. A. 1954. Iowa Agr. Expt. Sfu. Research Bull. 406. Fried, M. 1953. Soil Sci. SOC. Am. Proc. 17, 357-359. Grunes, D. L. 1959. Adcances in Agron. 11, 369-396. Grunes, D. L., Wets, F. G., Jr., and Shih, S. H. 1958. Soil Sci. SOC. Am. Proc. 22, 43-48. Hagin, J. 1957. Plant and Soil 9, 114-130. Hagin, J. 1960. Plant and Soil 12, 285-296. Hausenbuiller, R. L., and Weaver, W. H. 1960. Soil Sci. 90, 298-301. Hunter, A. S., and Yungen, J. A. 1955. Soil Sci. SOC. Am. Proc. 19, 214-218. Jacob, K. D. 1959. Adrjances in Agron. 11, 233-332. Jacob, K. D., and Hill, W. L. 1953. In “Soil and Fertilizer Phosphorus in Crop Nutrition” (W. H. Pierre and A. G. Norman, eds.), Vol. 4, Academic Press, New York. Jenny, H., Vlamis, J., and Martin, 1%’. E. 1950. Hilgurdia 20, 1-8. Jones, G. H. G. 1949. Bull. agr. Congo Belge 6, 2051-2064. Kempthorne, 0. 1957. Aduonces in Agron. 9, 177-204. Ketcheson, J. W. 1957. Can. J. Soil Sci. 37, 41-47. Kolaian, J. H., and Ohlrogge, A. J. 1959. Agron. J. 61, 106-108. Lawton, K., and Vomocil, J. A. 1954. Soil Sci. SOC. Am. Proc. 18, 26-32. Le Clerg, E. L. 1935. Phytopthobgy 26, 1019-1025. Leonard, W. H., and Clark, A. G. 1939. “Field Plot Technique.” Burgess, Minneapolis, Minnesota. Lindsay, W. L., and Stephenson, H. F. 1959a. Soil Sci. SOC.Am. Proc. 23, 1222. Lindsay, W. L., and Stephenson, H. F. 1959b. Soil Sci. SOC. Am. Proc. 23, 440445. Lindsay, W. L., Frazier, A. W., and Stephenson, H. F. 1962. Soil Sci. SOC. Am. Proc. (in press). Lorenz, 0. A., and Johnson, C. M. 1953. Soil Sci. 76, 119-129. Love, H. H. 1943. “Experimental Methods in Agricultural Research.” Agr. Expt. Sta. Univ. Puerto Rico, Rio Piedras, Puerto Rico. MacIntire, W. H., and Winterberg, S H. 1946. Soil Sci. 62, 33-41. McGeorge, W. T. 1939. Arizona Agr. Expt. Stu. Tech. Bull. 82, 295-331. Mehlich, A., Truog, E., and Fred, E. B. 1933. Soil Sci. 35, 259-279. Mehlich, A., Fred, E. B., and Truog, E. 1934. Soil Sci. 38, 445-461. Miller, M. H., and Ohlrogge, A. J. 1958. Agron. J. 50, 95-97. Mitscherlich, E. A. 1909. Landwirtsch. Jahrb. 38, 537-558. h.litscherlich, E. A. 1930. “Die Bestimmung des Diingerbedurfnisses des Bodens,” 3rd ed. Paul Parey, Berlin. Moreno, E. C., Lindsay, W. L., and Osborn, G. 1960. Soil Sci. 90, 58-68. Mulder, E. G. 1953. Fertiliser SOC. (Engl.) Proc. X o . 26. Mulder, E. G. 1956. Plunt and Soil 7 , 341-376. Munson, R. D., and Doll, J. P. 1959. Adcunces in Agron. 11, 133-169.

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Neubauer, H., and Schneider, W. 1923. Z. Pflanzenerniihr. u. Dung. A2, 329-362. Niklas, H., and Poschenrieder, H. 1932. Erniihr. Pflanze 28, 86-88. Olson, R. A., and Dreier, A. F. 1958. Soil Sci. SOC. Am. Proc. 20, 509-514. Organization for European Economic Cooperation. 1952. “Fertilizers: Methods of Analysis Used in OEEC Countries.” Pans. Pesek, J. T., and Webb, J. R. 1957. In “Fertilizer Innovations and Resource Use” (E. L. Baum, ed.), pp. 15-28. Iowa State Coll. Press, Ames, Iowa. Prummel, J. 1957. Plant and Soil 8, 231-253. Robinson, R. R., Sprague, V. G., and Gross, C. F. 1959. Soil Sci. SOC. Am. Proc. 23, 225-228. Sackett, W. G., and Stewart, L. C. 1931. Colo. Agr. Expt. Sta. Bull. 376, 1-36. Sandison, A. 1959. Nature 184, 834. Sauchelli, V. 1960. I n “The Chemistry and Technology of Fertilizers,” pp. 1-9. Reinhold, New York. Stanford, G., and Bouldin, D. R. 1960. PTOC. 7th Intern. Soil Sci. Congr. Madison, Wisconsin, 1960 2, 388-396. Stanford, G., and DeMent, J. D. 1957. Soil Sci. SOC.Am. Proc. 21, 612-817. Starostka, R. W., and Hill, W. L. 1955. Soil Sci. SOC. Am. Proc. 19, 193-198. Steenbjerg, F., and Jakobsen, S. T. 1959. Plant and Soil 10, 284-295. Stephenson, R. E., and Schuster, C. E. 1941. Soil Sci. 62, 137-153. Terman, G. L. 1957. Agron. J. 49, 271-276. Terman, G. L. 1959. Agron. 1. 61, 67-71. Terman, G. L. 1960. Soil Sci. SOC.Am. Proc. 24, 356-360. Terman, G. L. 1961. Soil Sci. SOC.Am. Proc. 25, 49-52. Terman, G. L., Hawkins, A., Cunningham, C. E., and Carpenter, P. N. 1952. Maine Agr. Expt. Sta. Bull. 606, 1-24. Terman, G. L., Anthony, J. L., Mortensen, W. P., and Lutz, J. A., Jr. 1956. Soil Sci. SOC. Am. Proc. 20, 551-558. Terman, G. L., Bouldin, D. R., and Lehr, J. R. 1958. Soil Sci. SOC. Am. Proc. 22, 25-32. Terman, G. L., DeMent, J. D., and Engelstad, 0. P. 1981. Agron. J. 63, 221-224. Thomton, S. F. 1931. J . Assoc. O g c . Agr. Chemists 14, 292-295. Thomton, S. F. 1935. Indiana Agr. Expt. Sta. Bull. 399, 1-38. Truog, E. 1916. Wisconsin Agr. Expt. Sta. Research Bull. 41. Vandecaveye, S. C. 1948. In “Diagnostic Techniques for Soils and Crops,” (H. B. Kitchen, ed.), pp. 199-230. Am. Potash Inst., Washington, D. C. Webb, J. R., and Pesek, J. T. 1958. Soil Sci. SOC. Am. Proc. 22, 533-538. Webb, J. R., and Pesek, J. T. 1959. Sod Sci. SOC. Am. Proc. 23, 381-384. White, R. F., Kempthome, O., Black, C. A., and Webb, J. R. 1956. Soil Sci. SOC. Am. Proc. 20, 179-186. Winogradsky, S. 1927. Ann. imt. Pasteur 42, 36-62.

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ISOTOPES METHODS AND USES IN SOIL PHYSICS RESEARCHI Don Kirkham and Raymond J. Kunze Department of Agronomy, Iowa State University, Amer, Iowa

I. Introduction ................................................. 11. Soil Water ................................................... A. Measurement by the Neutron Method of Water Retained in Soil B. Measurement of Moving Water and Ions in Soil by Use of Various Isotopes ................................................. 111. Soil Density and Soil Structure ................................ A. Soil Density Measurements by Gamma Radiation .............. B. Isotopes in Soil Structure Experiments ...................... IV. Soil Aeration ................................................. V. Soil Temperature ............................................ VI. Soil Particle Movement ...................................... VII. Transformation of Soil Materials from One Form to Another ........ VIII. Soil Profile Formation and Dating .............................. IX. Disposal of Radioactive Waste .................................. X. Proposed Future Work ........................................ References ..................................................

Page 321 322 322 331 342 342 346 347 348 348 350 352 353 354 355

1. Introduction

This paper is a review of isotopes work in soil physics. Since the physical characteristics of soils important in plant growth are those concerned with the four items: (1) soil water, (2) soil air, ( 3 ) soil impedance to root growth, agd (4)soil temperature (Shaw, 1952), this paper will contain sections (11, 111, IV, and V ) on isotopes methods for investigating these four items .and/or items related to them, such as soil density, soil structure, and soil tillage. This paper will also contain several short sections (VI, VII, VIII, and IX), such as one on transformation of soil material from one form to another. These subjects are included because the subject matter is largely that of soil physics, even though it cannot be classified under soil water, etc. The isotopes methods will include use of neutron radiation, gamma radiation, beta radiation; 1 Journal Paper No. J-4233 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Project No. 998.

321

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DON KlRKHAM A h 9 RAYMOND J. W N Z E

also of nonradioactive isotopes. The isotopes which will be discussed are: H2, H3, C14, N16, 0 l 8 , P32,S35, C136,K42, Ca45,Cow, Rbs6, SrgO,Il3I, C S I ~ ~ , and radium-beryllium. Recent reviews on the use of radioisotopes, including some uses in soil physics, have been prepared by Fried et al. (1958) and by Raney and Thorne (1957). A recent general paper on radioisotopes and agronomy is that of Norman (1959). Most of the work reviewed on uses of isotopes in soil physics has been concerned with moisture determination by neutron scattering and with density determination by gamma-ray transmission or scattering. Very little other work on nuclear methods in soil physics has, at the time of this writing ( 196l), been reported. What work is known to the authors, including recent work in neutron and gamma ray scattering, will be reported here. This paper will include, primarily at the end, suggestions for future work. 11. Soil Water

A. MEASUREMENT BY THE KEIJTRONMETHODOF WATERRETAINED IN SOIL The most important information generally needed about soil water is the amount of it contained in the soil rooting depth. This amount, in humid regions where irrigation is not commonly practiced, will vary with the following seven factors: (1) rainfall, ( 2 ) evaporation, ( 3 ) transpiration, ( 4 ) runoff, ( 5 ) deep seepage, ( 6 ) condensation, and ( 7 ) rising ground water level. In the above list, for arid regions, irrigation must be considered with or in place of rainfall. With these many factors and their interaction with various soils, and without prior moisture tests of the soils, it is impossible to predict the soil moisture content; it must be measured. The most useful method we know for measuring soil moisture is the neutron-scattering method. We at Iowa State University have also tried nuclear magnetic resonance, a method which, with its present costly equipment ( $25,000 from the Varian Associates ) , does not seem practical. This technique was also tried by others ( Andreev and Martens, 1960). The neutron method appears much more practical and is becoming fairly well known to soil physicists.

1. How the Neutron Meter Works Early work on the neutron meter for use in agriculture was done by Gardner and Kirkham (1952). Even earlier Belcher et al. (1950) had worked on the problem and had developed equipment for soil tests in airport runway design. The Gardner-Kirkham equipment was not portable. Stone et aE. (1955) and van Bavel et al. (1956) developed portable meters. Commercial meters are now available. One commercial

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model including surface and depth gauges is described by Kuranz (1960). Other papers dealing with the calibration and the use of commercial models are those of Burn ( 1961), Neville and van Zelst ( 1961), and Gnaedinger (1961). The device works as follows: A source of fast neutrons, (about 1,000,000electron volts) usually from a mixture of radium and beryllium, is lowered into a pipe (called an access pipe) driven into the soil. With the neutron source at the desired depth the fast neutrons penetrate into the soil, bouncing about from soil atom to soil atom, some of them returning to the access pipe where a detector of slowed neutrons has also been located with the neutron source. Essentially, the only slowing of the neutrons is that resulting from collisions with hydrogen atoms. The other soil atoms, all much heavier than hydrogen, do not slow the neutrons appreciably. So the detector measures essentially the density of hydrogen atoms in the soil; and since the hydrogen atoms in soil are almost all associated with soil water, the number of slowed neutrons detected per second is a measure of the soil water per unit volume of the soil. 2. W h y Neutrons Are Slowed Down by Hydrogen Atoms To show the basic principle of the neutron moisture meter, which is that hydrogen atoms slow up neutrons rapidly whereas other soil atoms do not, a simple experiment may be performed. As a preliminary to such an experiment, Table I is presented which gives the masses and relative abundances of different soil atoms. The table shows that atoms comprising TABLE I Masses and Percentages of Elements in Soil@ Element

H

Mass

1

0 (not in water form) Si A1 Fe Ca Mg Na

K Other

16 28 27 56 40 24 23 39

Per cent in soil by weight (Depends on water content of soil) 46.9 32.3 7.6 4.0 1.1 0.7 1.0 1.7 4.7

100.0b

a

b

Calculated from data of Thompson (1957, p. 7 ) . Hydrogen of soil water is not considered as a part of the soil.

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DON KIRKHAhL A S 9 FIAYMOND J. KUNZE

the dry soil substance are all heavy (massy) compared to hydrogen. With the table in mind one can now perform the experiment. Take two glass marbles, an orange one and a blue one, say, each one weighing 5 g. These are standard-sized marbles used by boys. Take also a third ball about the same size made of steel and weighing about 45 g., 9 times as much as the glass balls. Now suppose that the blue glass ball represents a neutron, one such as would be shot from the radium-beryllium neutron source. Suppose also that the orange glass ball represents a hydrogen atom in soil. Let the steel ball represent a heavy atom in the soil. Now, in a plastic V-groove trough strike the ‘leavy atom” with the “neutron.” The “neutron” will bounce back from the “heavy atom” (steel ball) with very little loss of velocity, that is, energy. Now strike the “hydrogen atom” (orange glass ball) with the “neutron” (blue glass ball). Observe that the neutron stops; the hydrogen atom takes all the neutron’s velocity. In soil, of course, the blows are not head-on as in the V-groove trough, but the principle is the same. The hydrogen atoms slow up the neutrons. The more hydrogen, and hence water, the more slowing down.

3. Adoantages and Disadvantages of the Neutron Meter

The neutron meter has some outstanding advantages. Some of these are: 1. The method is rapid, about one-half minute being required for a determination at a single depth in the soil. The detector is easily moved up and down in the access pipe to get the soil moisture at any desired depth. Movement of the equipment between access pipes may be facilitated by a gocart ( Ameniya and Namken, 1960). 2. Fewer samples are required than for most other methods. A neutron determination is as good as six (Shaw et al., 1959) or seven (Stone et al., 1960) gravity determinations. 3. The neutron meter equilibrates instantaneously with the soiI moisture. Some other type instruments require 2 to 3 weeks to equilibrate. 4. One access pipe may be used the year round to determine moisture content in the soil. A small test plot need not be dug up for getting moisture samples. The method is nondestructive. 5. The meter gives the soil moisture on a soil bulk volume basis. Moisture on a volume basis is what is generally desired. Soil moisture detenninations by oven drying are on a weight basis. To get them on a volume basis the bulk density of the soil sample must be determined, Bulk density determinations may introduce large errors (Stone et al., 1960). 6 . The meter gives the average moisture of about a 6-inch diameter

ISOTOPES IN SOIL PHYSICS RESEARCH

325

sphere. Ordinarily this is an advantage, as variations over very small distances are not desired. In some cases a smaller distance is desired. Mortier et al. (1959,1960) has shown how a paraffin sphere surrounding the radium-beryllium source may be used to slow up the neutrons and reduce the sphere of influence, 7 . The meter is insensitive to changes in salt concentration in the soil. Different rates of fertilizer application will not upset the moisture reading. 8. The type of material of which the access pipe is made is not critical. Neutrons pass through steel tubes and aluminum tubes easily. 9. One calibration ordinarily serves for a number of soils (Stolzy and Cahoon, 1957). If soils which contain large amounts of organic matter are avoided (Gardner and Kirkham, 1952) and if certain clay soils which bind up a great deal of water in the clay crystal lattice (Mortier and De Boodt, 1956) are excluded, a high degree of accuracy in determining soil moisture can be obtained. Stewart and Taylor (1957) discuss the advantages of measuring soil moisture with a neutron meter as opposed to resistance and tensiometer-type measurements. They point out that the neutron meter is reliable in all types of inorganic soils of medium texture regardless of salt content and soil type. At the present time there are two marked disadvantages of the neutron meter. The initial cost is high: the scalar costs $1500, and the depth probe costs $1000. The other disadvantage is that skilled personnel are needed to service the unit. Presently available commercial units seem to get quickly out of repair. For an article giving 91 references on the neutron meter and the gamma-ray density meter (the latter to be discussed below) see a paper by Kuranz (1960).

4. Uses of the Neutron Meter in Soil Moisture Studies a. Field capacity measurements. Figure 1 shows curves of moisture content versus time and field capacities, F.C., at a number of depths in two soils. When the curves flatten, gravitational water is gone, and the field capacities, as shown by arrows in the figure, become evident. The neutron meter is especially useful in these field capacity determinations since one can follow the moisture content with time at one spot; variations in moisture content due to sampling at different sites do not confound results. b. Determination of water stored termporarily above the jield capacity. The neutron meter responds essentially instantaneously to the instantaneous moisture content in the soil. This characteristic has been used by Nielsen et al. (1959) to measure the water stored temporarily above the

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DON KIRKHAM A N D RAYMOND J. WNZE

field capacity. In Fig. 2 profiles of temporarily stored water are seen for a Monona silt loam, a loess. The thickness of the profiles represents moisture in excess of the field capacity at the time printed above the profiles. Zero time is the time when the profile is near water saturation to about the $foot depth and when water application to the profile has

MARSHALL SILT LOAM

SILT

IDA LOAM

Horiz. arrows indicote scole o f ordinate

36 34 32 34

Vert. arrows give

. 4EC = 25.6

32 30

-0

28

-2

7 26 28 26 m 27

L c

25

f 23

;28

26 24 26 I 24

38 42

22

20 18 16 ‘14 12 10

40 38 36

0

12

24 36 48 Time (Hr.)

60

72

I 0

+

8

16

I

I

24 32 Time (Hr.)

I

I

40

48

56

FIG. 1. Moisture vs. time curves for ( A ) Marshall silt loam and ( B ) Ida silt loam obtained with a neutron meter. Field capacities are given at the right-hand ends of the curves. (From Burrows and Kirkham, 1958.)

been discontinued. In Fig. 3 the storage profiles are for Webster clay loam, a Wiesenboden soil formed on glacial till. The neutron meter shows for the Webster, unlike the Monona, large amounts of water stored temporarily (above the field capacity) in the surface 12 inches. From these last two figures one sees that a considerable amount of soil moisture may be stored above the field capacity even after 5 or more

ISOTOPFS IN SOIL PHYSICS RESEARCH

327

days’ time. In a later paper Nielsen et al. (1961) showed, using neutron moisture meter data, that soil moisture profiles in the field can be theoretically calculated. Two other recent papers on more conventional use of the neutron meter may be cited here. Stolzy et al. (1959) calibrated the meter against the soil suction and various moisture contents so that the meter TIME IHOURSI 75 10 IS

20

mBo

;?42

40

54

so HCB%XNTAL S & s r

=

I O S R C E N T WATER

FIG.2. Moisture profiles, obtained by the neutron meter, of water stored temporarily above the field moisture capacity for a Monona silt loam, a loess. (From Nielsen et al. 1959.)

FIG.3. Same as Fig. 2, except that soil is a Webster silty clay loam, a Wiesenboden glacial till soil, with an obvious tight layer at 6 to 12 inches depth. (From Nielsen et al., 1959.)

could be used for determining soil suction. Weeks and Stolzy (1958) used the meter to measure soil moisture in columns of soil 3 feet in diameter. Satisfactory agreement between the calculated changes of total water content and the measured amounts of added water were obtained for two soils, a sandy loam and a silt loam. c. Water movement through soil profiles. Measurement of moving water and stored water are closely related. Water which is moving may be considered as stored instantaneously.

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DON KIRKHAM A?;D

RAYMOND J. KU’NZE

In the paper by Nielsen et al. (1961) the movement of water in the soil profile was determined by the neutron meter and the shape of the moving moisture profile related to the theoretical profiles determined on the basis of diffusion theory. In Monona-Ida loess soils the agreement of the measured profiles with the theory was fairly good. Kixon and Lawless (1960a, b ) measured soil moisture with a neutron meter down to 20-foot depth under brush cover. They found that the moisture content of various soil layers under their test plots varied with time t according to the relation W = At - b.

5. Uses of the Neutron Meter in Studying Soil-Water-Plant Relationships a. Water auailability for crops. Shaw (1959) covered some plots with plastic film; other comparison plots he left uncovered. He pierced neutron access pipes through the plastic-covered plots and installed access pipes also in the uncovered plots. In both plastic-covered and uncovered plots, corn (maize) was planted, holes for the corn seed being punched in the plastic covers. Irrigation water could be added under the plastic-covered plots and also onto the uncovered plots. There was no seepage loss. Since no evaporation in the plastic-covered plots could occur and since there was no seepage loss, a loss of moisture in the plastic-covered plots was due to transpiration alone. In the uncovered plots the moisture loss was due to both evaporation and transpiration. By subtracting values of the moisture content found in the uncovered plots from the moisture content found in the covered plots, evaporated moisture could be and was obtained. Evaporation and transpiration could be followed accurately and conveniently from day to day. At high sunlight intensities, such as 5% g.-~al./cm.~/day, the evaporation loss from the soil was a larger percentage of evapotranspiration than at lower sunlight intensities. Shaw found that on the average for the season there was about 50 per cent of the water evaporated from the soil; this 50 per cent the plants did not get-it was wasted. (Actual average vaIues were 46 per cent evaporation, 54 per cent transpiration. ) There was an interesting side effect in the experiment. Large amounts of rain water were found to be intercepted by the corn plants. This water, 2.21 inches between July 15 and September 16, was evaporated from the leaves and probably served little useful purpose. The writers of this review suggest that tritiated or deuteriated water might be applied to corn leaves to see how much of “intercepted water,” as noted by Shaw, is used by the plant. In a later paper Fritschen and Shaw (1961) pointed out that it is

ISOTOPES IN SOIL PHYSICS RESEARCH

329

not accurate to substract transpiration on plastic-covered plots from evapotranspiration on uncovered plots to obtain evaporation. A principal reason is that the plastic cover changes the microclimate of the soil. They gave some correction factors, which, when applied to their moisture data of 1961, indicated that only 11 per cent of moisture loss was due to evaporation, the other 89 per cent, to transpiration. Even without the correction factors the 1961 work indicated 27 per cent loss by evaporation (not about 50 as in 1959) and 73 per cent by transpiration. But the 1959 and 1961 data are not strictly comparable. The 1959 data were for evaporation for the whole growing season; the 1961 data for the portion of the growing season after the corn crop had reached its maximum height. It appears that further work on measurement of evaporation and transpiration on field plots is needed. Denmead (1961)used the technique of the neutron meter and plastic covers successfully in smaller “lysimeter” plots to study soil-plant-climate relations, work which we will now discuss. There has long been a controversy on whether or not water is equally available to plants over the whole range of available water; that is, from the field capacity to the wilting point. Veihmeyer and Henrickson (1955) have supported the equal availability concept; Slatyer (1956) and Richards and Wadleigh (1952) have disagreed. In his work Denmead used the neutron meter to follow the moisture in 136 “lysimeters” in which corn was grown. The lysimeters were 20-gallon garbage cans. They were buried in the field to the surface of the soil and were surrounded by growing corn. Evaporation was prevented by covering the lysimeters with plastic covers through which the corn grew. It would have been impracticable to attempt to weight this many lysimeters. Denmead (1961) found that each of the two viewpoints on plant available water were tenable. On overcast days in soils of high capillary conductivity the corn plants would transpire at about the same rate at moisture contents ranging all the way from the field capacity to the wilting point; whereas, in soils of very low capillary conductivity and on hot days, the soils would not supply enough water to the plant at any moisture content between the field capacity and the “wilting point.” Wilting would occur even at the field capacity. The neutron meter already has been adapted in a variety of agronomic moisture availability problems. Stolzy and Cahoon (1957) used the neutron meter in measuring soil moisture under citrus orchards. Marston (1958) used the meter in mountain soils for studying evapotranspiration. Reproducibility of moisture values was a problem because of rocks. Pits had to be dug for placing the access pipes. Merriam (1960) used the neutron method in sampling “wild land soils.” He found that use of

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DON KIRKHAM AND RAYMOND J. WNZE

herbicides resulted in a saving of 1 inch of water in 10-foot depth for west-facing slopes and resulted in saving of about 3 inches of water in 10-foot depth on east-facing slopes. This was a sandy soil which contained about 15 to 16 inches of water in 10 feet of profile. b. Plant spacing and water utilization. Neutron meters have been used by Yao and Shaw (unpublished data as of July, 1961) to test efficiency of water use by corn at various rates of planting. Since Shaw’s work of 1959 (cited above) indicated that about 50 per cent of soil moisture might be lost by evaporation, Yao and Shaw wondered if increased shade at the soil surface would decrease this loss. The increased shade could be obtained by planting the corn rows closer together or by planting the rows in such directions as to obtain more shadow on the ground. Table I1 shows the results for 21-inch spacing and 42-inch spacing. The TABLE I1 Water Use by Corn (Maize) for Different Rates of Planting and Different Directions of ROWS= Distance apart of corn plants Number of corn plants per acre

21 inches 14,000b

28,0006

Direction of corn row EWandNS EWandNS 132 153 Yield, bushels corn per acre Inches of water used 13.2 15.0 Bushels of corn produced per inch of water 10.0 10.2

42 inches 14,000b

28,OOoC

E-W N-S E-W N-S 120 124 148 143 16.1 15.3 17.4 18.8

7.4

8.1

8.3

8.5

0 Yao and Shaw, Iowa State University, Ames, Iowa, unpublished data; moisture data collected with neutron meter. 0 One corn plant per hill. c Two corn plants per hill.

table indicates that 1inch of water produces 10 bushels of corn per acre at 21-inch spacing of rows; and about 7 to 8 bushels per acres at 42-inch spacing of rows. Whether there were 14,000 or 28,000 plants per acre at a given spacing seemed to make little difference in the yield per inch of water. Also, the direction of the rows seemed to make little difference in the yield per inch of water. Bahrani and Taylor ( 1961) investigated the influence of soil moisture potential and evaporative demand on the actual evapotranspiration in an alfalfa field. Soil water was measured to a depth of 9 feet using a neutron meter. The actual evapotranspiration and its ratio to “potential evapotranspiration” ( calculated by Penman’s formula ) showed a curvilinear relation with the average moisture potential.

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331

B. MEASUIIEMENT OF MOVINGWATER AND IONS IN SOILBY USEOF VAFUOUS ISOTOPES In Section 11, A above, the papers reviewed were largely concerned with the static condition of water in soil. In this Section (B) papers on moving water and ions are reviewed. The movements are considered first for unsaturated flow and then for saturated flow. 1. Unsaturated Flow a. Miscible displacement. When one solution moves into another solution, both solutions being soluble in each other, and when movement of the solutions occurs in a porous medium such as soil, the original solution is moved by a process called miscible displacement. This term is used because when the added solution is washed through the soil it mixes with the solution which is already there while displacing it. To study how applied soil water displaces water already in the soil, Nielsen and Biggar (1961) originally displaced the soil solution with water containing a small concentration of chloride ion. They wondered how the tracing ion might influence the miscible displacement process; therefore they used, in some later experiments ( Biggar and Nielsen, 1!36l), tritium and chloride ion. The results showed (Fig. 4) that the chloride ions came ahead of the tritium ions. The movement of the chloride ions was faster in the soil column than was the tritiated water, even though the latter had a higher diffusion coefficient. In the unsaturated state the tritiated water could also diffuse in a vapor phase, where movement is faster than in the liquid phase. Thus under some unsaturated conditions, one might expect that the tritiated water would be detected first. b. Digusion flow of water. Moisture movement in soil using deuterium as a tracer was studied by Kunze and Kirkham ( 1961). They directed their attention to the determination of the self-diffusion coefficient of soil water because of the need for this coefficient in order to separate mass flow and diffusion flow-types of flow they had observed in an earlier experiment. In this earlier experiment they used a soil core, initially at constant moisture content and having its middle portion tagged with deuterium. Letting the core dry from one end, they found that a large part of the tagged moisture moved in a direction the reverse of that of the drying gradient. Since mass flow of water would, by definition of mass flow, have to move in the direction of the drying gradient, they reasoned that some of the water motion was due to molecular self-diffusion. They therefore turned their attention to the measurement of the self-diffusion coefficient of soil water. It is emphasized here that the self-diffusion coefficient of water as determined by

332

D O S KIRKHAhf A33l M Y M O N D J. KUNZE

Kunze and Kirkham does not refer to the type of diffusion of water spoken of in connection with capillary soil-moisture flow. This capillary flow diffusion coefficient is defined as the product of the capillary conductivity of moist soil and the rate of change of the soil water tension with the soil moisture content. The ( molecular) self-diffusion coefficient is a measure of the rate at which molecules of a material diffuse into other molecules of the same material. 1.0

COLUMBIA SILT LOAM

0.8

0-0

0.6

A-

CHLORIDE TRITIUM

@

f/ ,p d

8 = 0.482

0

y 0.4 V

0.2

0

2

0.0

CL

+ z

1.0

$

0.8

W V

w 0.6

L

2 =

0.4

f W

0.2 0.0 0

400

800

1200

VOLUME OF EFFLUENT (ml.)

FIG. 4. Miscible displacement curves of soil solution being displaced by water containing chloride ions and tritium, the displacing water moving into the soil solution at a velocity V = 0.20 cm./hr. The moisture content 0 for the replicated runs shown is about 0.48 cm."cm.3. C,, is the initial concentration of the ion or of the tritium in the displacing solution; C is the concentration of the chloride ion or tritium in the outflow solution (effluent) as measured in volumetric outflow incremants. (From Biggar and Nielsen, 1961.)

Figure 5 shows a curve used by Kunze and Kirkham in evaluating the self-diffusion coefficient. In Fig. 5 on the y axis, a function F is given which is essentially the concentration of deuterium atoms in a soil core, the left half of which contained originally deuterium-tagged soil water ( F = 1 ), and the right half, untagged water ( F = 0). The test soil core in these experiments was kept completely enclosed to prevent evaporation. The heavy line is a theoretical curve based on the hypothesis that self-diffusion in soil water is a linear process. The circle

i1 1

ISOTOPES IN SOIL PHYSICS RESEARCH

~

333

”\ COLO CLAY LOAM SOIL = HOURS t

57

DIFFUSION CURVE

0.8

-

EXPERIMENTALLY DETERMINED POINTS

Z-0.6X

u

-

0.4-

i

0.2

FIG.5. Experimental points and a theoretical curve, of a diffusion function F versus distance x along a Colo clay loam soil core, as determined by segmenting the core 57 hours after self-diffusion had begun. Deuterium is the soil-water tagging material. (From Kunze and Kirkham, 1961.) ~EI-MICRON DIAMETER GLASS BEADS t = 97 HOURS

0.8

THEORETICAL DIFFUSION CURVE

POINTS

U

a(2 4 . 6 * 6 . I o ‘ i 2 . -

DISTANCE

x (crn.1

+FIG.6. Same as Fig. 5 except that the “soil’’ is an artificial one made of 28-p glass beads and the diffusion time is 97 hours. (From Kunze and Kirkham, 1961.) -0.1-

334

DON KIRK€L%M AND RAYMOND J. ICUNZE

points are experimental data. They fit the theory satisfactorily. The segments referred to at the bottom of the curve refer to locations of slices made in the core to collect deuterium samples for analysis. When an artificial silt “soil” prepared from 28-p glass beads was used (Fig, 6 ) , the theoretical and experimental curves showed an even better fit than in the preceding figure. Kunze and Kirkham found that the self-diffusion coefficient of soil water was 0.61 times cm.2/sec. for the clay loam soil of Fig. 5; and cm.2/sec. for the artificial glass beads silt soil of Fig. 6. 1.36 times Other workers have found that the value of the self-diffusion coefficient cm.2/sec., which is of water (in the absence of soil) is 2.34 times higher than either of the preceding values, about 4 times as high as for loam and about twice as high as for the glass beads. The volumetric moisture contents of the Colo clay loam soil and the glass beads were 37.5 and 27.5 per cent, respectively. The variation of the self-diffusion coefficient with the soil moisture content (volumetric basis) for Colo soil and beads is shown in Fig. 7. The coefficient for Colo soil is almost constant with moisture content from about 8 to 18 per cent moisture; and there is only a little change in the coefficient in the much larger range, 5 to 35 per cent. One may thus conclude that the same correction term for self-diffusion in moisture tracer studies may be used over a wide range of moisture percentages. The values of the self-diffusion coefficients found by Kunze and Kirkham provide diffusion data, for comparison with mass movement data, for determining whether self-diffusion processes or mass flow processes are negligible in a water movement study. c. Ion movement in soil water. Jordan et al. (1958) used cobalt-60 cations in the form of CoC12 and sulfur-35 atoms in the form of H2S04, in attempts to obtain quantitative measures of water movement around socalled slick-spot soils. Results of their experiment indicate that radioactive sulfur is a suitable tracer for studying soil moisture movement in slick-spots and associated soils. They found the sulfur superior to the cobalt-60. Their preliminary experiments show that it was reasonable to assume that the tracer moved with the soil moisture. From their data it is seen that the ions moved during the experiment, but no proof is given that they moved at the same rate as the water. Here one remembers the experiment of Biggar and Nielsen described in Section 11, B, 1, a, The cobalt or sulfur ions might have moved by diffusion ahead of the true moisture front, which would indicate a faster movement than the water itself actually made. On the other hand, preferential absorption of the ions would have indicated a slower movement. Owens (1960) used the stable isotope NI5 to follow ions of ammonium

335

ISOTOPES IN SOIL PHYSICS RESEARCH

sulfate applied to soil in lysimeters. He applied the ammonium sulfate at the rate of 120 pounds of nitrogen per acre during each winter of two years that the experiment was conducted. Three moisture treatments, 12, 18, and 24 inches of water, were established on the soil during the 5 months prior to crop seeding. The test crop was corn (maize). He

-

I

I

I

1

I

I

SELF- DIFFUSION OF WATER IN 28-MICRON BEADS AND

-

-

COLO CLAY LOAM SOIL AS A FUNCTION OF MOISTURE CONTENT

-

-

-

0 0 \ u)

N

E

0

w

a"

--:

-

COLO SOIL

-

10-6

-

I

I

5

10

I 15

I 20

I 25

I 30

35

336

DON KIRKHAM A N I RAYMOND

J. KUNZE

to high moisture rates. The amount of fertilizer nitrogen remaining in the soils at the end of the experiment was the same (38 per cent) for all moisture treatments. Davis et al. (1953) used Ca45in lysimeters to find that 49.21 inches of rain removed about 10 per cent of a slag-derived Ca from Hartsell's fine sandy loam and about 6 per cent of the same type of Ca from a Claiborne silt loam. @en et al. (1959) reported that rubidium is not a very suitable tracer to replace potassium in soil leaching studies. The reader is here reminded that rubidium has been used in several soil studies in place of radioactive potassium because potassium has too short a half-life for most experiments. @ien et al. used K42 and Rbss as tracers. The half-life of K42 is 12.5 hours, the half-life of rubidium, 18.6 days. If rubidium was a suitable replacement for potassium, the isotopes should have behaved alike. The investigators found in one experiment that as much as 30 per cent of the added quantity of the rubidium was fixed in clay soils, but only 15 per cent of the potassium. It is suggested by the reviewers that concentration ratios of K:Rb other than K:Rb = 1, which was used by @en c t al. in their experiments, may give other results, especially at other concentration levels. Self-diffusion of rubidium chloride in moist glass beads was investigated by Klute and Letey (1958). They were interested in seeing how this self-diffusion coefficient would vary with moisture content. Their results indicate a very rapid decrease of diffusivity as the moisture content was decreased. The values for self-diffusion were probably on the high side compared with soil since the glass beads they used were larger than silt size. One size of glass beads was 200 p, another 75 p. Silt is of 20-1.1 size. Klute and Letey described their results in relation to diffusion path lengths of the tracer and the moisture content of the medium. In a later investigation Letey and Klute ( 1%0), using C136, studied self-diffusion of the chloride ion in soil and clay paste. The self-diffusion coefficients measured decreased with increasing salt ( chloride concentration). The apparent mobilities for chloride, which were calculated from self-diffusion results, were lower than those calculated from transference-conductivity experiments; both calculated mobilities, nevertheless, gave a similar relationship when plotted against the chloride concentration. In all cases the calculated apparent mobilities of chlorides were higher than those of potassium. Letey and Klute explained their results on the basis of an ionic distribution about charged surfaces of clay particles and on the basis of the viscosity of the water. Heslep and Black (1954) studied, in soil, one-dimensional diffusion

ISOTOPES IN SOIL PHYSICS RESEARCH

337

of phosphatic fertilizer containing P32. Under no conditions of the experiments did the phosphorus diffuse more than 3 or 4 cm. from the source. The extent of phosphorus diffusion was considerably less in calcareous soils than in acid soils. The extent of phosphorus diffusion increased with water content of the soil but was relatively unaffected by the degree of soil compaction encountered in the field. Other experimental observations were generally in accord with diffusion theory. There are several papers on ion movement in soils using autoradiographic methods. Bouldin and Black (1954) measured, by X-ray film and Geiger tubes, the diffusion of radioactive phosphorus in laboratory-prepared water-saturated soils columns. The X-ray film revealed many localized areas of P32concentrations 0.5 to 2 mm. in diameter. Large-scale irregularities were observed in the concentration-distance graph for some soils. It was postulated that precipitates in the form of CaHP04 were responsible for these irregularities. However, the majority of the soils tested did not show definite major irregularities in phosphorus distribution. Johnston ( 1954) described a radioautographic technique for studying the movement of phosphorus in soil. His technique consisted of freezing the test sample after phosphorus had moved in, sectioning the sample into thin sections 10 to 20 p thick with a high speed band saw, and exposing the sections to radiation film. A technique such as this might be helpful in making a very exacting study of irregularities of phosphorus concentration in diffusion problems as discussed, for example, by Bouldin and Black (1954). Datta and Srivastava (1958) used an autoradiographic technique in following P32 movement in soil columns. Their work indicated that organic molecules produced from farmyard manure chelated calcium or iron and thus led to increased movement of phosphorus. The chelation also apparently improved the soil structure. d. Vapor transfer studies by use of isotopes. Veinik (1958) studied evaporation inside a soil column by use of radioisotopes. A gammaradiating isotope in the form of a soluble salt was placed in the soil water. As the soil moved upward in the core, with evaporation taking place inside the core, the salt containing the radiotracer would deposit out along the walls of the soil pores where the evaporation was taking place. Thus, the greater the concentration of radioactivity along a portion of the soil column, the greater must have been the evaporation there. An iodine isotope, used as potassium iodide, was said to be suitable. Radioactive salts used in such evaporation experiments must not be absorbable

338

DO?: KLRKHAM AND RAYMOND J. KUNZE

to any signscant degree in the soil sample material and should not react with it. Also the radioactive salt selected should be one the solubility of which varies very little with temperature, as rather high temperatures (above 150OC.)may be used. From the variation of the measured activity with depth and from theoretical equations, the internal evaporation as it is related to temperature gradient, moisture content, capillary size, and the geometry of the body could be calculated. Veinik concluded, from an analysis of the experimental data, that the character and strength of the bond between the moisture and the soluble salt was an important factor influencing the process of internal evaporation. Zaslavsky ( 1960) , independently of Veinik, conceived this technique of crystallization of radioactive salts in the inside of the soil column for determining where evaporation takes place in soil.

2. Saturated Flow Under this heading three principal topics will be considered: movement and storage of ground water in an aquifer; water and ion movement in natural waters; and water and ion movement in plants. a. Movement and storage of ground water in an aquifer. Horton and Ross (1960) suggested, and made a study of, the use of the tritium present in liquid waste from the chemical separation of spent uranium fuel elements, as an aid in tracing the movement of ground water about ground disposal areas of radioactive waste. Their study indicates that tritium can be used to trace the movement of ground water from seepage basins that receive the waste. They remark: “The ideal tracer has the following characteristics: It is readily determinable, quantitatively, in low concentrations; it is either absent entirely or present in only very small amounts in normal ground water; it is cheap and readily available; it is not absorbed by the porous medium; it does not form a precipitate with materials in the test environment; and it is not so influenced by the test environment that precision of detection is impaired.” Their measurements indicated to them that tritium, although it is slightly retained by soil, can serve as a satisfactory ground water tracer, inasmuch as it satisfied the conditions given above. At the Symposium on Tritium in Tracer Applications (1958) the general field of tritium tracing was reviewed. A renewed interest in tritium as a tracer was attributed to three developments: (1) cost has been reduced from $100 to $2 per curie; ( 2 ) detection is more efficient ( 2 5 micromicrocuries can now be detected); and ( 3 ) labeling is easier and more versatile. Ground water investigations of tritium stemming from that in the atmosphere produced by hydrogen bomb explosions were initiated by

ISOTOPES IN SOIL PHYSICS RESEARCH

339

Von Buttlar and Wendt (1958). The “apparent age” of ground water samples was determined by measuring the tritium concentration of the ground water. The term “apparent age” is defined as the interval between the time of the H-bomb explosion and the time of detection of a tritium peak in the ground water. Young water would be that water which came into the aquifer shortly after the H-bomb explosion. By knowing the tritium concentration, inference could be made about the rate of flow of aquifers and the size of the recharge reservoirs. Begemann and Libby (1958) studied circulatory rates for waters of the Northern Hemisphere with tritium which had been produced during the H-bomb explosions in 1954. Their study revealed that about one-third of the rain in the upper Mississippi Valley is ocean water and about twothirds is re-evaporated local surface water. They concluded that the technique of studying the tritium content of well water is quite likely to be valuable in studying underground water supplies: one should be able to predict the susceptibility of the ground water supplies to drought, to depletion by pumping, and to replenishment by rain and snow. Giletti et al. (1958) studied the tritium in natural waters before and after the H-bomb test. The amount of tritium spread over the earth has doubled since 1954 when thermonuclear tests began. The change in tritium concentration has permitted an extensive study of the age and also the movement of water on the ocean floor and in hot water springs, lakes, and streams, etc. In work with tracers other than tritium, Barker and Scott (1958) showed that radioelements, specifically uranium and radium, may be used to support inferences as to ( a ) the direction of movement of ground water; ( b ) the lithology and geologic history of all the aquifer materials; and ( c ) the presence of materials having ion-exchange capacity. Of interest in the area of development of instrumentation in ground water studies is a recording apparatus for ground water level measurements ( Andreae, 1957-1958) that registers the water level on a recording tape by a radium beam rather than by a pen. Pen friction and ink problems are thus eliminated. b. Water and ion movement in natural waters. According to Bryant and Geyer (1958), the travel time of radioactive wastes or tracers when discharged into natural waters may be determined by any of, or all of, four processes. These are as follows: ( 1 ) precipitation and settling-out; ( 2 ) adsorption on suspended and fixed surfaces; ( 3) entrance into the biological cycle; and (4) movement through the action of the flowing, mixing water. A single tracer molecule may thus shift back and forth between these alternatives. The fourth alternative is controlled by factors such as gravity, wind, tide, density, impact, shear, and inertia. Bryant and

310

DON KIRKHAM A S D R\YMOXD J. KUNZE

Geyer point out that tracer studies offer a means whereby mixing and travel time of the tagged molecules may be studied. Studies of phenomena in hydrology that have not previously been amenable to investigation are now proceeding at a rapid rate, according to Parker (1958). He carried out experiments with P1in lakes and reservoirs to determine eddy diffusion coefficients. In other experiments Parker used P32 to determine stream velocity and dilution effects. The diffusion coefficients resulting from these measurements, when compared with theoretical coefficients of diffusion for straight pipes, were found to be larger owing to increased dispersion caused by bends and obstructions in the stream. According to Kaufman and Orlob ( 1956), the ideal ground water tracer should correctly depict the movement of water through a porous medium without modifying the transmission characteristics of the system. In their study of evaluation of radioisotopes and other materials as ground water tracers, Kaufman and Orlob found chlorides to be the most suitable where density effects could be avoided and dispersion of clay was not likely. Tritium also was evaluated and was found to move a little more slowly than chlorides. Kaufman and Orlob attributed this slower movement to the exchange of the tritium with soil-bound water: a resulting reduction in velocity of the tracer front would have to occur. In this regard, the results of Biggar and Nielsen ( 196l), already reported, are noted to be in agreement with those of Kaufman and Orlob. Kaufman and Orlob concluded that radioisotopes employed in cationic form will probably be unsatisfactory in any situation involving porous media having measurable exchange capacities. Similarly, anionic radioisotopes when used in a carrier-free form (that is, without other anions) will be retained largely by adsorption. The pattern of sewage water dispersion in the Mohawk River (New York) was traced by Simpson et al. (1958), using P32. The initial path of sewage water was strongly influenced by differences in density of sewage and river water due to temperature. For about the first 800 feet the concentration of the sewage water decreased logarithmically with distance from the outfall. Straub et al. (1958) conducted a time of flow study on the Ottawa River (Ohio) using RbS6, sodium chloride, and fluorescein dye as tracer material. Their results indicated that under the existing test conditions all tracers gave similar results and the maximum deviation between tracers was less than 10 per cent; however, recovery of Rbx6was higher than the recovery of the other tracers. The deuterium concentration in Arctic Sea ice sampled at various depths was investigated by Friedman et at. (1961). The low concentration of deuterium found was interpreted as being due to the formation

ISOTOPES IN SOIL PHYSICS RESEARCH

341

in the summer, and freezing in the early winter, of a surface layer of water of low deuterium concentration. This surface layer resulted from the melted snow, which is, in the Arctic Sea, naturally low in deuterium Concentration. The sea ice is thought to ride on this deuterium-poor layer and is the first material to be added to the ice floe when accretion by freezing begins in the early winter. c. Water and ion movement in plants. Some of the pioneering studies of translocation in plants by methods utilizing tritium were those of Biddulph and Cory (1957). They found the downward movement of tritiated water velocity in phloem of the bean plant to be 86 cm./hr. On the other hand, Gage and Aronoff (1960), under slightly different experimental conditions, could detect no appreciable movement of tritiated water although movement of tagged solutes was observed. Gage and Aronoff thus were led to the conclusion that there appeared to be insufficient movement of water in phloem tubes to satisfy the mass flow theory. Tracer techniques, which have been developed for plant translocation and related studies, are described by Aronoff ( 1960). Kunze (1960) studied the amount of enrichment of deuteriated water in plants that would result during transpiration because of slight differences in vapor pressure of the deuteriated and normal water. He developed theoretical equations that predict for ideal situations the correct amount of enrichment. These conditions are as follows: (1) the plant must be isolated from atmospheric water vapor; ( 2 ) the temperature must remain constant; ( 3 ) the liquid as it vaporizes from the plant must be removed; and ( 4 ) the deuterium content of the plant water in question must be homogeneous. Although enrichment as predicted by the physical laws undoubtedly occurs in plants, apparently other processes (possibly the rapid rate of exchange between leaf water and atmospheric water vapor) not only mask any enrichment process, but also dilute considerably the tagged water in the leaf. Kunze, working with oat plants in deuteriated nutrient solution, found that leaf water under no circumstances approached the deuteriated concentration of the nutrient solution. Nakayama (1960) found that the tritium concentration in leaves of a bean plant, grown in tritiated nutrient solution, was only 65 per cent of the tritium concentration in the nutrient solution. This concentration did not appear to increase after 72 hours. It appears that before tritium or deuterium can serve adequately as tracers in plants, basic studies are needed to learn definitely what factors are responsible for the observed reduction of the tagged water in the plant leaves. Mederski (1961) found, by counting beta-rays from C14 passed through undetached 18-inch-high soybean leaves growing in 2-gallon soil-filled greenhouse pots, that within 30 seconds after water was

342

DON KIRKHAM AND RAYMOND J. KUNZE

added to the soil near the wilting point the count rate sharply decreased, dropping from 4200 counts per minute (wilting turgidity) to 3200 counts per minute (100% turgidity) in 7 minutes. 111. Soil Density and Soil Structure

A. Son. DENSITY MEASUREMENTS BY GAMMARADIATION

Soil density is often taken as a measure of soil aeration and also as a measure of impedance of soil to root growth. Soil can either be too dense or too loose for optimum field conditions, the latter usually being a problem only at seedplanting time. A soil of high density is the result of compaction by farm equipment, rainfall impact, and/or other dispersion of aggregates by chemical or physical means. Soil density may be measured by methods consisting of a transmission or back-scattering of gamma rays. 1. Transmission Measurements Vomocil (1954a) did some of the initial work with the transmission technique of measuring soil bulk density. The resolution of the apparatus was about 2 inches (the density of a 2-inch layer of soil could be determined), and accurate measurements could be made within 3 inches of the soil surface. Bulk density in gram per cubic centimeter could be measured to the nearest 0.01 unit. Vornocil (1954b) pointed out that almost all the gamma-ray dissipation, when energies are in the range of 1 million electron volts, is due to the Compton effect. Since gamma-ray absorption by the Compton effect is proportional to the ratio of the atomic weight over the atomic number, the absorption is essentially constant for all elements except hydrogen and elements with an atomic number higher than 30. This range of atomic numbers 2-30 includes everything found in appreciable amounts in soil. Hence Vomocil concluded that absorption of gamma rays is independent of the chemical composition for the stated gamma-ray energy value. Wet density when plotted against the log of transmitted intensity did not yield the straight line predicted by the Beer-Lambert law l / l o= e--oxd where I,, is the initial intensity of the energy beam, 1 the intensity of the beam transmitted through a thickness x, 0 the mass absorption coefficient, and d the density. The nonlinearity of the count-density curve found by Vomocil implied that is was not constant. Vomocil attributed this curvature to scattering and secondary emission effects. He postulated that changes in ratio of scattering and secondary emmission to direct-beam

ISOTOPES IN SOIL PHYSICS RESEARCH

343

intensities could conceivably result in an increase in o with an increase in density. Vomocil found the edge effect of the gamma beam to be negligible; thus the size of a calibration chamber was not critical. Bernhard et al. (1956) presented a statistical technique for analyzing data obtained by the gamma-ray transmission method. The main purpose in their study was to determine parameters of the equation which govern the “best fit” lines, that is, correlate soil density with transmitted radiation intensity, and distance between the radiation source and the detector. They observed measurement deviations from supposedly correct density values of _t 2.3 per cent. The use of only primary radiation for measuring soil density by the transmission technique was emphasized by van Bavel et al. (1957). The inclusion of secondary radiation, they found, resulted in unwanted complications. They described a technique whereby a separation of primary and secondary radiation could be achieved by scintillation counting and electronic discrimination. Van Bavel (1959) described a gamma transmission technique for field measurement of soil density. This technique necessarily required that the source and the detector be placed some distance apart. In field measurements van Bavel obtained bulk density values the precision of which was of the order of 0.01 g. per cubic centimeter and for which the resolution was one-half inch. Moisture content had been taken into account to obtain dry-bulk density values. Volarovych and Churaev (1960) used gamma-ray transmission to measure soil density in peat soils. They also used other isotopes methods to study peat. ( S35 was used in a water movement study.) Their 200-page book came to our hands too recently for an adequate review. The book contains 314 mainly Russian references. About half of the references are on isotopes work and these isotopes references are largely from nonagronomic sources. 2. Back-Scattering Measurements The back-scattering principle involves the absorption and backscattering of gamma rays by the outer orbit electrons of all atoms present in the soil, atoms of water molecules included. Here again as with the transmission technique, in order to obtain dry bulk density, one must know the volumetric moisture content and subtract it, expressed as grams per cubic centimeter, from the wet-bulk density. Kuranz (1960) described the use and calibration of a commercial depth density gauge and a commercial surface density gauge. He pointed out that the sensitive volume of measurement is up to about one cubic foot of soil for the depth gauge. The radius of penetration for the density gauges is 3 to 8 inches, decreasing with an increase in wet density. Depth density measurements

344 DON KIRKI-IAM AND RAYMOND J. KUNZE

FIG.8. Radiation equipment for soil density determination: left, density unit; middle, counter; right, moisture unit. The central unit is 12 inches wide. (From Phillips et al., 1960.)

ISOTOPES IN SOIL PHYSICS RESEARCH

345

may be made from 1 to 200 feet. Calibrations methods and evaluations, among many others, of the same commercial soil density gauges described by Kuranz (1960) are given by Mintzer ( 1961),Carey et al. (1961), and Carlton (1961). Phillips et al. (1960) used a commercial gamma-ray density (backscattering) meter for determining soil density at different levels of artificially applied compaction. The equipment ( Fig. 8 ) included two surface probes, one to measure the moisture density and one to measure

FIG.9. Yield of corn (maize) versus bulk density on a dry soil basis, where the density is determined by radiation equipment, and also, for comparison, by cores. ( A ) no fertilizer added to soil; ( B ) fertilizer added. (From Phillips et al., 1960.)

the moisture-plus-solids density. Subtraction of the former density from the latter gave the soil-solids density. In Fig. 9 one sees how corn (maize) growth depends on the density. The density measurements plus root studies indicated that root impedance developed by the compaction and reflected by the bulk density was the factor which caused yield variation. Trouse and Humbert ( 1961 ) , working with radioactive rubidium, found that soils compacted to different densities showed decreasing rooting efficiency for Hawaiian sugar cane as bulk density increased.

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DON MRKHAM AND RAYhlOND J. KUNZE

Critical bulk densities for the rooting of sugar cane were empirically established for the principal cane sugar soils of Hawaii.

B. ISOTOPESLU SOIL STRUCTURE EXPERIMENTS A few papers on use of isotopes in soil structure investigations have been published. They deal with aggregate stability of the macrostructure and bonding and kinetic effects of the macrostructure. 1 . Aggregate Stability Toth and Alderfer (1960a) described a procedure for tagging waterstable soil aggregates with Corn. Their data indicated that with their techniques it is possible to tag different-sized aggregates uniformly throughout the aggregate. Tagged water-stable aggregates have been kept in distilled water for over a year without releasing Corn. In later work Toth and Alderfer (1960b) used their tagging technique for studying the formation and breakdown of water-stable soil aggregates. The most important findings from an incubation and greenhouse study \yere: “( 1) the physical composition of water-stable soil aggregates is constantly changing; ( 2 ) during incubation the contribution of smallsized tagged aggregates to the formation of larger ones decreased as the size of the tagged aggregates was reduced; ( 3 ) aggregates of all size ranges examined contained fragments of the original tagged aggregates; and ( 4 ) under bluegrass sod, the contribution of the tagged aggregates of small-sized ranges to the formation of larger one was greater than that noted under incubation.”

2. Bonding and Kinetic Effects Katz (1960), in work that is pertinent to the use of isotopes in soil structure studies, investigated the change in the physical and chemical properties of deuteriated compounds as opposed to normal compounds when subjected to certain tests. He pointed out that not only are equilibrium properties of deuteriated compounds frequently different from those of the corresponding protiated (normal hydrogen) compounds, but the rate at which deuteriated compounds undergo chemical reaction also may be different. These reaction rate differences are a function of the more stable bonds-to-deuterium as compared to bonds-tohydrogen. Since the masses, hydrogen and deuterium, are different, the vibrational frequencies of bonds-to-deuterium will be at slightly lower frequencies, and these bonds will thus be slightly more stable than corresponding bonds-to-hydrogen. Since type and strength of bonds is apparently a factor that cannot be neglected in soil structure studies, these techniques of exchanging deuterium for hydrogen and observing

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the resulting change of the physical properties in the soil material appear to justify some investigation. The deuterium-hydrogen exchange phenomenon in soils has been investigated. Faucher and Thomas (1955) found that the exchange of deuterium for hydrogen in montmorillonite is very rapid for 75 per cent of the total water associated with the clay minerals; and that very little further exchange occurred even after contact periods of 120 hours. Romo (1956) observed by infrared measurements the actual presence of deuterium in the clay lattice. He concluded that the rate of exchange appears to be characterized by two steps: one in which the exchange takes place predominantly on the surface hydroxyls, and the other one in which a process of diffusion takes place to influence exchange of the intralattice hydroxyls. No work seems to have been done on how the physical properties of deuteriated soils or clays may be changed as opposed to those soils or clays saturated with normal water. This could open up a completely new field of soil physics research, if it seemed warranted for an understanding of binding forces in soil. In place of deuterium, tritium could be used. The latter, being radioactive, would probably simplify the analysis procedure. IV. Soil Aeration

So far as the authors know, work with oxygen isotopes for studying soil aeration has been confined to the use of the nonradioactive isotope Ols. The radioactive isotopes of oxygen have half-lives too short for uses that may be envisioned. This is seen from the following tabulation: Isotope

Half-life

014

76.5 seconds 127 seconds 27 seconds

015 019 ~

~~

Jensen (1961) used Ol8 in studying oxygen diffusion through soil cores and plant roots growing in the soil cores. The diffusion of oxygen increased with increasing numbers of roots. His evidence indicated that this increase took place almost entirely in the soil, rather than inside the root. Although Jensen did obtain these and some other soil-plant measurements, his work dealt largely with the development of tagging and sampling techniques suitable for 0ls mass spectrometry. Danielson and Russell (1957) measured Rbss absorption by corn seedlings as affected by moisture and aeration. Their work indicates that the absorption of rubidium ions was not significantly influenced by oxygen concentration above 10 per cent when the flow rate through these

348

DON KIRKHAM AND RAYMOND J. W N Z E

samples was about 1liter per hour. The critical oxygen level for rubidium absorption decreased with decreasing soil moisture content. It is difficult to ascertain whether the critical oxygen level at various moisture contents was controlled by respirational activity or by the rate of diffusion of oxygen through moisture films. Their data indicate that both factors are important. Self-diffusion of radioactive carbon dioxide as a means of relating diffusion to porosity in porous media was studied by Rust et al. (1957). Diff usion-porosity relations, which were in good agreement with those obtained by other workers, were determined for air-dry nonsoil materials. Absorption of carbon dioxide precluded an evaluation of the diffusionporosity relationship of moist soils. Another experiment using C1*labeled CO, in a related area of interest was that of Rhykerd et uZ. (1959), who measured the uptake of COP by alfalfa, red clover, and birds-foot trefoil seedlings during a 15-minute exposure to natural light immediately after these plants had been exposed to various light treatments for 30 days. Alfalfa and red clover fixed significantly greater quantities of CO, under all light treatments than did birds-foot trefoil. The effect of various light treatments, but all of the same energy for any particular species, was found to be highly significant with the amount of CO, fixed. V. Soil Temperature

The use of isotopes methods in connection with the study of soil temperature is relatively unexplored. The authors, using deuteriated water as a tracer and a split root technique, presently are determining rates of water absorption by plant roots when roots from one plant are under two different temperature environments ( unpublished data). With this technique half the roots are kept in one environment of untagged moist soil and half in another where the moist soil is tagged. To detect from which soil environment the water is being absorbed it is necessary to analyze the transpired water for its deuterium content. In preliminary experiments, with oats growing in nutrient culture and with temperature regimes of 60" and 90"F.,it was found that the ratio of absorption from the two differently heated soils was initially proportional to the reciprocal ratio of the viscosity. As growth continued, more roots developed on the cooler side and the' proportionality did not hold. VI. Soil Particle Movement

Smith and Eakins (1958) have discussed various methods of marking sand grains or pebbles that are to be used in investigations of littoral drift. Approximately ten different radioisotopes suitable for tracing particle

349

ISOTOPES IN SOIL PHYSICS RESEARCH

movements are listed in their paper, choice of which will depend upon length of the experiment. These are shown in Table 111. TABLE I11 Suggested List of RadioisotoDes for Use in Studvine Sand Movement+ Duration of experiment Short

Element Sodium Na24 Lanthanum La140 Gold A11198

Medium

Barium-lanthanum Bal"o-La140 Rubidium Rb86 Chromium Cr51

-

Long

Antimony Sbl24 Iridium 1~192

Scandium Sc46 Tantalum Tal82 Zinc Zne5 Silver AglIO Cobalt COG0 a

Half-life

Specific activity in flux of 1011 n/cm.z/sec. for 1 week

Gamma radiation (in Mev.)

15.0 hr.

38 mc/g

1.37,2.76

40.2 hr.

88 mc/g

1.60 and others

2.69 days

620 mc/g

0.41

12.8 days

Fission product

1.60 and others

18.6 days

2.0 mc/g

1.08 (8.5%)

27.8 days

2.1 mc/g

0.32 (8%)

60 days

0.95 mc/g

0.6 to 2.1

74.4 days

175 mc/g

Up to 0.61

84 days

38 mc/g

0.89, 1.12

111 days

7 mc/g

u p to 1.2

245 days

100 Il.c/g

1.1 (45%)

270 days

320 pc/g

Up to 1.5

5.25 yr.

2.2 mc/g

1.17, 1.35

From Smith and Eakins (1958, Table I ) .

Inman and Chamberlain ( 1959), using irradiated quartz grains, have traced the movement of beach sand under the influence of wave action. The actual tracer isotope in the irradiated quartz grains was P32. The investigators pointed out that, to be satisfactory, the natural tracer of sand movements should be: "( 1) not a health hazard, ( 2 ) of the same size, density, and shape as one of the major components under study,

350

DON KIRKHAM Ah?) RAYMOND J. KUNZE

( 3 ) easily and rapidly distinguishable from the sand mass, ( 4 ) inexpensive and available in relatively large amounts, and (5) able to retain its distinguishing properties over time comparable to the time of the fluid processes.” The above tracer (P32 in irradiated quartz) was found to fulfill these requirements. Their experiment indicated a rapid rate of dispersion of sand from the point of release, this dispersion being most rapid in the offshore and onshore directions. It should be obvious that these same techniques used in tracing beach sands could also be applied to wind and water erosion of soil and to the resulting formations of sediment ( Anonymous, 1961) . The techniques appear to be equally suitable for studying results of various tillage practices, such as the uniformity and mixing of soil. Main (1959) gives some theoretical considerations for uniformity of mixing. The incorporation of small irradiated sand grains in aggregate stability studies should prove to be interesting. Results might be compared with those of Toth and Alderfer (196Ob). VII. Transformation of Soil Materials from One Form to Another

Carbon in the form of soil organic matter may break down into gaseous carbon dioxide. Oxygen may be taken from the soil air by organisms and bound up into a chemical form. Nitrogen may be in a plant-available form or a plant-unavailable form. Water may be in the liquid phase or the vapor phase. Thus there are many ways in which soil materials may change from one form to another. The rate of change from one form to another is important, but this rate of change cannot often be measured directly. With methods of mathematical physics, however, measured quantities can be converted into the desired rates. Details of the problem of determining the rate at which nitrogen goes from mineral form to organic form and the rate at which the organic form goes into the mineral form-both processes go on in the soil simultaneously-has been presented in detail by Kirkham and Bartholomew (1954, 1955) and by Kirkham (1956). The latter paper is a composite of the two former papers. In the work of Kirkham and Bartholomew the mineralization rate and the so-called immobilization rate of the soil nitrogen were desired from certain experimental data. By mineralization rate is meant the rate at which plant-unavailable nitrogen becomes plant available; by immobilization rate is meant the rate at which plant-available nitrogen becomes plant-unavailable. Mineralization rate is sometimes referred to as mobilization rate; available nitrogen, as mobile or mineral; unavailable nitrogen, as immobile. Soil organic matter ordinarily contains most of the unavailable soil nitrogen.

ISOTOPES IN SOIL PHYSICS RESEARCH

351

Measured quantities from which m, mineralization rate, and i, immobilization rate, are to be determined are ordinarily the concentrations of total and of tagged mineral nitrogen at the times when the concentrations are measured. In the experimental work reported, tagging was done with the heavy nitrogen W5. The mathematics was simplified

K

5 a

X

0

20 40 t (doyr)

60

0

20 30 x (milligrams)

10

FIG.10. Theoretical (solid curves) and experimental data (circled points) of the variation of x with time; and y with x; where x is the milligrams of available nitrogen per 3 grams of soil mulch material and y is the milligrams of tagged available nitrogen per 3 grams of the same mulch material (decomposing oat straw) (see Kirkham, 1956)

.

2.0

EK

'2 3 0.5

-J z

-

/ m

0.

I

I

I

I

I

FIG.11. Variation of mobilization 'and immobilization rates, m and i, of nitrogen in soil mulch material (decaying oat straw), with time. Multiply the values of m and i by 2/3 to obtain pounds of nitrogen mobilized or immobilized per day per ton of the oat straw; values of m and i are derived from the values of x and y of Fig. 10 (see Kirkham, 1956).

when there was a large amount of unavailable nitrogen in the soil as compared with available nitrogen. This situation was first analyzed by Kirkham and Bartholomew (1954). The unrestricted case, where the amounts of unavailable nitrogen and available nitrogen as well as other materials involved were finite, was analyzed in the 1955 paper. The complete details, which involve setting up differential equations and

352

DON KIRKHAM AND RAYMOND J. KUNZE

solving them, need not be given here. The results depended on what sort of transformation law was assumed or known to hold. The first case Kirkham and Bartholomew considered was that where m and i were constants. The second case was that where m was taken to depend on a mass action law. It turned out that the experimental data corresponded to the mass action assumption except for very short durations of transformation. Figure 10 shows the experimental curves that were analyzed by Kirkham and Bartholomew. The experimental data were those of Jansson as quoted in the Kirkham-Bartholomew paper. One sees that the theoretical curves could be fitted to the data; therefore the values of rn and i could be obtained. For these data the values of m and i varied as shown in Fig. 11. The figure shows that after a sufficient time has passed, the mineralization rate and immobilization rate are equal; that is, they are equal when the soil is at equilibrium. VIII. Soil Profile Formation and Dating

A good review of some of the early research work with isotopes in the general field of geology is given by Ingerson (1953). The use of isotopes in this field stems from the fact that a slight fractionation of isotopes sometimes occurs in nature. The quantity of fractionation observed is frequently used to deduce the age, rate of reaction, etc., of the material in question. By fractionation is meant a process whereby the concentration of the isotope in question is changed from the concentration normally found in nature. Scholtes and Kirkham ( 1957) have reported use of radiocarbon ( el4) for dating the formation of a soil profile. A fractionation process is involved. They explained the theory of the radiocarbon dating methodology in detail. The results they discussed indicate that the glacial material deposited in the Plcistocene Period is older than geologists had previously considered it to be. CI4 is a weak beta emitter (0.156 Mev.) and there is an appreciable absorption and scattering of radioactivity within the sample itself. Thus it is very important that adequate corrections be applied, especially if low count determinations are made as in radiocarbon dating. Hendler (1959) described a new technique for correcting radioactivity measurements for loss of radiation due to self-absorption. Hendler found that the absorption of radiation follows a hyperbolic law much more closely than an exponential one. Furthermore, the absorption coefficient for the sample itself has been found to be a function of weight, rather than a constant as had been assumed for the derivation of the law of exponential absorption.

ISOTOPES IN SOIL PHYSICS RESEARCH

353

IX. Disposal of Radioactive Waste

The disposal of radioactive waste may not be a basic part of isotopes methods in soil physics research; but, because the nature of the problem is closely allied to many interests of soil physicists, and because the problem will be an ever-increasing one, some of the work being done will be mentioned here. Interest in this area of waste disposal is necessitated by the possibilities of an atomic war and also by the increasing use of radioisotopes in power reactors, research, and other areas. Graham (1958) measured the uptake of SrWand by vegetation grown in a waste disposal area. He found the plant uptake of SrgOwas 0.057 to 0.178 pc, per 100 g. of dry plant material and was closely correlated to the concentration of Srgo in the soil saturation extracts. The total Cs137uptake varied from 0.133 to 0.334 pc. per 100 g. of dry plant material. The higher uptake of Cs13?was associated with the moderately low supply of available potassium in the soil sediment. Hess and Thurston (1958) reported on a two-year study made by the National Research Council of the disposal problem of radioactive waste. Their report indicates that the most practical immediate solution for reactor waste disposal is the utilization of cavities in salt beds or domes. The conditions of maximum absorption of radioactive waste by soil were studied by Prout (1958). Experimental data show that the adsorption of Sr9O by the soil is dependent on the strontium concentration in the waste solution and the pH of the soil. Strontium adsorption is negligible at pH of 2, increases sharply from 2 to 4,reaches a maximum at 7, and again declines above 8. A similar pattern holds for the adsorption of These pH and ion concentration considerations result in a distribution coefficient which Prout used in an equation to show the rate of SrgOor Cs13? movement in porous adsorption beds. It is of interest to note that this work is related closely to the work of Biggar and Nielsen (1961) cited earlier. After the 1956 nuclear bomb tests, high intensities of radioactive fallout were found to be associated with rainstorms. Setter and Straub (1958) studied the distribution of radioactive fallout in both rain and snow. They found that the monthly residual fallout (corrected for decay) reached a maximum of about 1 microcurie per square meter of surface area at its peak during the fallout period. Because little activity was detected in runoff after a rainstorm, it was presumed that the radioactivity remained on vegetation and was adsorbed by the soil. Menzel

354

DON KIRKHAM A h 3 RAYMOND J. KUNZE

(1960) reported that only a small portion of Srgo that fell on cultivated land was removed by runoff. Nevertheless, a considerable concentration of SrP0,found in the soil particles carried away by the runoff, may cause sediments to be appreciably higher in Srw than soil under normal field conditions. Other studies, many primarily soil physics problems, as to how to reduce the effective concentration of fallout nuclei and their absorption by plant and animal tisues are under way. X. Proposed Future Work

The use of isotopes in soil physics research and practice is still in its infancy. Fruitful investigations already begun should be continued; new work should be initiated. We have suggested a few researches in the foregoing sections; we present further work areas here. Neutron moisture meters, especially when improved to be more trouble-free and less expensive, should be used widely; also gamma-ray density meters. Kirkham (1961) has mentioned as a new research area the use of radiation for assaying the soil mineralogically by a back-scattering technique. The back-scattering technique offers tremendous possibilities in soil survey. Kirkham also lists the following seven items: ( 1 ) Simplify, and make trouble-free, neutron-scattering soil moisture measuring equipment. (2) Use isotopes in soil mixing studies in conection with ( a ) effectiveness of tillage operations; ( b ) insect activity; and ( c ) soil swelling and churning activity as in the soils now called vertisols. ( 3 ) Use 0 l 8 to and C14 trace sources of plant and soil aeration. (4) Use H2, 0l8, simultaneously to trace water movement. (5) Use labeled carbon dioxide to study whether decaying soil organic matter is a fertilizing source of carbon dioxide for increased plant growth. ( 6 ) Use deuterium- or tritiumtagged water, under different physical conditions, in “split root” studies, to see how the different physical conditions influence water uptake and ion uptake where the ions of interest in the two sides of the split system may also be tagged. Here we mention that the isotope P33 might be used. P33 is now hard to get, as it is prepared in cyclotrons. Its half-life is double that of P32. ( 7 ) Test a method proposed by Zaslavsky ( 1960) for using tagged krypton to measure the vapor pressure of soil water in its difficult-to-measure range. In reiteration of, or in addition to the above list are these items: ( 8 ) Use the miscible displacement technique to study what fraction of applied rain or irrigation water moves through the large pores in soil and what ions are carried by this water, when the soil is initially at

ISOTOPES IN SOIL PHYSICS RESEARCH

355

various soil moisture levels and the ions at various concentration levels. ( 9 ) Use the same miscible fluid technique (see Biggar and Nielsen, 1961) to see how applied water moves fertilizer, fumigants, insecticides, etc., in soil. (10) Use deuterium or tritium to see how OH groups interchange in clay minerals and hence learn of bonding forces of soil structure. Other techniques, not presented or discussed in our review will be discovered as the research work with isotopes progresses. Much insight as to the role of isotopes in research may be obtained from textbooks by Rochlin and Schultz ( 1959), Kamen ( 1957), and others. ACKNOWLEDGMENTS The illustrations in this contribution stem largely from work done by both the authors or by their associates. This is partly because the senior author was in Egypt at manuscript deadline time and did not have other suitable illustrations at hand. Part of this material was presented by the senior author at a nuclear methods conference (unpublished) of the International Atomic Energy Agency, Vienna, Austria, May 14-19, 1961. The junior author is a principal investigator in isotopes research under a National Science Foundation Grant awarded to the authors. Both authors are indebted to Mrs. Kirkham, who, under difficult conditions in Egypt, typed and helped edit the final copy of the manuscript.

REFERENCES Amemiya, M., and Namken, L. N. 1960. Soil Sci. SOC. Am. Proc. 2 4 ( 6 ) , 528. Andreae, H. 1957-1958. Wiss. Z. Humboldt-Univ. Berlin Math. Naturw. Reihe 7 ( 4 ) , 449-453. Andreev, B. V., and Martens, B. K. 1960. Pochvovedenie 10, 112-115. Anonymous. 1961. Agr. Research ( U S . Dept. Agr.) 10( l ) , 8-9. Aronoff, S. 1960. “Techniques of Radiobiochemistry.” Iowa State Univ. Press. Ames, Iowa. Bahrani, B., and Taylor, S. A. 1961. Agron. J . 63( 4 ) , 233-237. Barker, F. B., and Scott, R. C. 1958. Trans. Am. Geophys. Union 39(3), 459-466. Begemann, F., and Libby, W. F. 1958. Radioisotopes Sci. Research Proc. Intern. Conf. Paris 1957 2, 634-656. Belcher, D. J., Cuykendall, T. R., and Sack, H. S. 1950. Civil Aeronaut. Admin. Tech. Develop. Rept. 127. Bemhard, R. K., Chasek, M., and Griggs, P. 1956. Am. SOC. Testing Materials PTOC.66, 1288-1300. Biddulph, O., and Cory, R. 1957. Plant Physiol. 32, 608-619. Biggar, J., and Nielsen, D. 1961. Soil Sci. SOC. Am. P ~ o c .26, 1-5. Bouldin, D. R., and Black, C. A. 1954. Soil Sci. SOC. Am. Proc. 18(3), 255-259. Bryant, G. T., and Geyer, J. C. 1958. Trans. Am. Geophys. Union 39(3), 440-445. Bum, K. N. 1961. Am. SOC. Testing Materials. Spec. Tech. Publ. 295, 14-26. Burrows, W. C., and Kirkham, D. 1958. Soil. Sci. SOC. Am. Proc. 22(2), 103-105. Carey, W. N., Jr., Shook, J. F., and Reynolds, J. F. 1961. Am. SOC. Testing Materials Spec. Tech. Publ. 293. Carlton, P. F. 1961. Am. SOC. Testing Material Spec. Tech. Publ. 293, 27-35. Danielson, R. E., and Russell, M.B. 1957. Soil Sci. SOC. Am. Proc. 21, 3-6.

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Datta, N. P., and Srivastava, S. C. 1958. Proc. U. N. Intern. Conf. Peaceful Uses Atomic Energy 2nd Conf. 27, 190-192. Davis, D. E., MacIntire, W. H., Comar, C . L., Shaw, W. M., Winterberg, S. H., and Harris, H. C. 1953. Soil Sci. 76, 153-163. Denmead, 0. T. 1961. Ph. D. thesis. Iowa State University, Ames, Iowa. (Microfilm available.) Faucher, J. A., and Thomas, H. C. 1955. 1. Phys. Chem. 59, 189-191. Fried, h.I., Allison, F. E., and van Bavel, C . H. M. 1958. In “Radiation Biology and Medicine” (W. D. Claus, ed.), pp. 607-632. Addison-Wesley, Reading, Massachusetts. Friedman, I., Schoen, B., and Harris, J. 1961. Trans. Am. Geophys. Union 66, 1861-1 864. Fritschen, L. J., and Shaw, R. H. 1961. Agron. 1. 53, 71-74. Gage, R. S., and Aronoff, S. 1960. Plant Physiol. 36, 53-64. Gardner, W.,and Kirkham, D. 1952. Soil Sci. 73, 391-401. Giletti, B. J., Razan, F., and Kulp, J. L. 1958. Trans. Am. Geophys. Union 39, 807-818. Gnaedinger, J. P. 1961. Am. SOC. Testing Materials Spec. Tech. Publ. 293, 36-44. Graham, E. R. 1958. Soil Sci. 86, 91-97. Hendler, R. W. 1959. Science 130, 772-777. Heslep, J. M., and Black, C . A. 1954. Soil Sci. 78(5), 389-401. Hess, H. H., and Thurston, W. R. 1958. Trans. Am. Geophys. Union 39(3), 467-468. Horton, J. H., and Ross, D. I. 1960. Soil Sci. 90, 267-271. Ingerson, E. 1953. Bull. Geol. SOC. Am. 64, 301-374. Inman, D. L., and Chamberlain, T. K. 1959. 1. Geophys. Research 64( l ) , 41-57. Jensen, C. R. 1961. Ph. D. thesis, Iowa State University, Ames, Iowa. (Microfilm available. ) Johnston, W. B. 1954. Soil Sci. 78, 247-255. Jordan, J. V.,Lewis, G. C., and Fosberg, hl. A. 1958. Trans. Am. Geophys. Union 39 ( 3 ) , 446-450. Kamen, M. D. 1957. “Isotopic Tracers in Biology.” Academic Press, New York. Katz, J. J. 1960. Am. Scientist 48(4), 544-580. Kaufman, W. J., and Orlob, G. T. 1936. Trans. Am. Geophys. Union 37(3), 297-306. Kirkham, D. 1956. U . S. Atomic Energy Comm. Rept. TID-753.2, 349-359. Kirkham, D. 1961. Soil Sci. SOC. Am. Proc. 26(6),423-427. Kirkham, D., and Bartholomew, W. V. 1954. Soil Sci. SOC. Am. Proc. 18(1), 33-34. Kirkham, D., and Bartholomew, W. V. 1955. Soil Sci. SOC. Am. Proc. 19(2), 189-192. Klute, A,, and Letey, J. 1958. Soil Sci. SOC. Am. Proc. 22, 213-315. Kunze, R. J. 1960. Ph. D. thesis, Iowa State University, Ames, Iowa. (Microfilm available. ) Kunze, R. J., and Kirkham, D. 1961. Soil Sci. SOC. Am. Proc. 25( l ) , 9-12. Kuranz, J. L. 1960. Am. SOC. Testing Materials Spec. Tech. Publ. 268, 40-54. Letey, J,, and Klute, A. 1960. So8 Sci. 90, 259-265. Main, R. hi. 1959. Am. SOC.Testing Materials Spec. Tech. Publ. 268, 74-80. hlarston, R. B. 1958. Proc. SOC. Am. Foresters Meeting, 1958, Salt Lake City, Utah pp. 39-42.

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Mederski, H. J. 1961. Soil Sci. 92, 143-146. Menzel, R. G. 1960. Science 131, 499-500. Merriam, R. A. 1960. Iowa State Coll. J. Sci. 34(4), 641-648. Mintzer, S. 1961. Am. SOC. Testing Materials Spec. Tech. Publ. 293, 44-54. Mortier, P., and De Boodt, M. 1956. Neth. J. Agr. Sci. 4, 111-113. Mortier, P., De Boodt, M., and De Leenheer, L. 1959. 2. Pflanzenerniihr. Dung. Bodenk. 87( 3 ) , 244-250. Mortier, P., De Boodt, M., Dansercoer, W., and De Leenheer, L. 1960. Trans. 7th Intern. Congr. Soil Sci., Madison, Wisconsin, 1960, Comm. 1, 321-329. Nakayama, F. S. 1960. U . S. Dept. Agr. ARS41-37. Neville, 0. K., and van Zelst, T. W. 1961. Am. SOC. Testing MateriaZs Spec. Tech. Publ. 293, 3-13. Nielsen, D. R., and Biggar, J. W. 1961. Soil Sci. SOC. Am. Proc. 25( l), 1-5. Nielsen, D. R., Kirkham, D., and van Wijk, W. R. 1959. Soil Sci. SOC. Am. Proc. 23( 6 ) , 408-412. Nielsen, D. R., Kirkham, D., and van Wijk. W. R. 1961. Soil Sci. SOC. Am. Proc. 25( 2 ) , 165-168. Nixon, P. R., and Lawless, G. P. 1960a. Trans. Am. SOC. Agr. Engrs. 3( l ) , 5-8. Nixon, P. R., and Lawless, G. P. 1960b. J. Geophys. Research 65(2), 655-663. Norman, A. G. 1959. Agron. J. 61, 702-705. @en, A., Semb, G., and Steenberg, K. 1959. Soil Sci. 88, 284-287. Owens, L. D. 1960. Soil Sci. SOC. Am. Proc. 24(5), 372-376. Parker, F. L. 1958. Trans. Am. Geophys. Union 39(3), 434-439. Phillips, R. E., Jensen, C. R., and Kirkham, D. 1960. Soil Sci. 89, 2-7. Prout, W. E. 1958. Soil Sci. 86, 13-17. Raney, W. A., and Thorne, M. D. 1957. In “Atomic Energy and Agriculture” (C. L. Comar, ed.), pp. 81-95. Am. Assoc. Advance. Sci., Washington, D. C. Rhykerd, C. L., Langston, R., and Peterson, J. B. 1959. Agron. J. 51, 7-9. Richards, L. A., and Wadleigh, C. H. 1952. In “Soil Physical Conditions and Plant Growth” (B. T. Shaw, ed.), pp. 73-251. Academic Press, New York. Rochlin, R. S., and Schultz, W. W. 1959. “Radioisotopes for Industry.” Reinhold, New Pork. Romo, L. A. 1956. 1. Phys. Chem. 60, 987-988. Rust, R. H., Klute, A., and Gieseking, J. E. 1957. Soil Sci. 84, 453-463. Scholtes, W. H., and Kirkham, D. 1957. Pedologie 7, 316-323. See also Ruhe, R. V., Rubin, M., and Scholtes, W. H. 1957. Am. J . Sci. 255, 671-689. Setter, R., and Straub, P. 1958. Trans. Am. Geophys. Union 39(3), 451-458. Shaw, B. T. (ed.) 1952. ‘‘Soil Physical Conditions and Plant Growth.” Academic Press, New York. Shaw, R. H. 1959. Agron. J. 51, 172-173. Shaw, R. H., Nielsen, D. R., and Runkles, J. R. 1959. Iowa State Agr. Coll. and Home Econ. Expt. Sta. Bull. 465, 411-420. Simpson, E. S., Beetem, W. A., and Ruggles, F. H. 1958. Trans. Am. Geophys. Union 39( 3 ) , 427-433. Slatyer, R. 0. 1956. Neth. J . Agr. Sci. 4, 73-76. Smith, D. B., and Eakins, J. D. 1958. Radioisotopes Sci. Research Proc. Intern. Conf. Paris 1957 2, 619-633. Stewart, G. L., and Taylor, S. 1957. Soil Sci. 83, 151-158. Stolzy, L. H., and Cahoon, G. A. 1957. Soil Sci. SOC. Am. Proc. 21(6), 571-575.

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Stolzy, L. H., Weeks, L. V., Szuszkiewicz, T. E., and Cahoon, G. A. 1959. Soil Sci. 88, 313-316. Stone, J. F., Kirkham, D., and Read, A. A. 1955. Soil Sci. SOC. Am. Proc. 19(4), 419-423. Stone, J. F., Shaw, R. H., and Kirkham, D. 1960. Soil Sci. SOC. Am. Proc. 24(6), 435-438. Straub, C. P., Ludzack, F. J., Hagee, G. R., and Goldin, A. S. 1958. Trans. Am. Geophys. Union 39(3), 420-426. Symposium on Tritium in Tracer Application. 1958. Nucleonics 16( 3 ) , 62-67. Thompson, L. M. 1957. “Soils and Soil Fertility.” McGraw-Hill, New York. Toth, S. J., and Alderfer, R. B. 1960a. SoiZ Sci. 89, 36-37. Toth, S. J., and Alderfer, R. B. 1960b. Soil Sci. 90, 232-238. Trouse, A. C., Jr., and Humbert, R. P. 1961. Soil Sci. 91, 208-217. van Bavel, C. H. M. 1959. Sod Sci. 87, 50-58. van Bavel, C. H. M., Underwood, N., and Swanson, R. W. 1956. Soil Sci. 82, 29-41. van Bavel, C. H. M., Underwood, N., and Ragar, S. R. 1957. Soil Sci. SOC. Am. Proc. 21( 6 ) , 588-591. Veihmeyer, F. J., and Hendrickson, A. H. 1955. Tram. Am. Geophys. Union 36, 425448. Veinik, A. I. 1958. Radioisotopes Sci. Research Proc. Intern. Conf. Paris 1957 1, 460-474. Volarovych, M. P., and Churaev, N. V. 1960. “Isledovanye Torfa Pry Pomoshchy Radioaktyvnyh Isotopov” (Studies of Peat Soils by Radioactive Isotopes). 200 pp. Akademia Nauk, S.S.S.R., Moscow. ( In Russian. ) Vomocil, J. A. 1954a. Soil Sci. 77, 341-342. Vomocil, J. A. 1954b. Agr. Eng. 35, 651-654. V O Buttlar, ~ H., and Wendt, I. 1958. Trans. Am. Geophys. Union. 39(4), 660668. Weeks, L. V., and Stolzy, I. H. 1958. Soil Sci. SOC.Am. Proc. 22(3), 201-203. Zaslavsky, D. 1960. Ph. D. thesis, Iowa State University, Ames, Iowa. (Microfilm available.)

THE MANAGEMENT

.

OF SOYBEANS

.

Jackson L Cartter and Edgar E Hartwig United States Regional Soybean laboratory. Urbona. Illinois. and Stoneville. Mississippi

I. Introduction ................................................ A . World Production ........................................ B. United States Production Trends ............................ C . Utilization .............................................. I1. Soil and Climatic Adaptation .................................. A. Areas of Production in the United States .................... B. Soil Requirements ........................................ C. Climatic Adaptation ...................................... I11. Time of Planting and Varietal Adaptation ........................ A . Effect on Plant Characters ................................ B. Effect on Composition of the Seed .......................... IV . Planting Methods and Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Seedbed Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Row Width and Planting Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................ C . Double Cropping .................... D . Special Methods of Planting ................................ E . Types of Equipment ...................................... V . Rotation Practices and Erosion Control .......................... A . Effect on Soybean Yields .................................. B. Effect on the Following Crop .............................. C . Effect on Weed Population ................................ D . Soil Residues from Herbicides .............................. E . Erosion Control .......................................... VI. Weed Control ................................................ A . Effect of Planting Time on Plant Growth and Weed Competition B. Methods of Cultivation .................................... C . Chemical Weed Control .................................. VII . Seed Quality and Seed Treatment .............................. A . Factors Affecting Seed Quality and Germination .............. B . Seed Treatment .......................................... VIII. Nutrient Requirements ........................................ A. Nitrogen Requirements and Nodulation ...................... B. Liming and pH Levels .................................... C. Phosphorus .............................................. D . Potassium ............................................... E . Trace Elements .......................................... F . Fertilizer Practices and Recommendations .................... 359

Page 360 360 361 364 365 365 365 365 372 372 377 378 378 379 381 382 383 383 383 384 385 385 385 386 387 387 387 389 389 390 390 390 393 394 396 398 401

360

JACKSON L. CARlTFX A h 9 EDGAR E. HARTWIG

........

IS. Water Requirements and Utilization

.4. Water Needs in Relation to Pla B. Irrigation and Soil Management . .......... X. Growth-Regulating Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI. Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. When to Harvest . . . . . . . ........ .............. B. Harvesting Methods . . . . . ........................... XII. Seed Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . XIII. Discussion . . . . . . . . . . . . . . . . . . . . References ...................................

401 402 403 404

404 405

406 407 408

I. introduction

A. WORLDPRODUCTION

The soybean, Glycim mar ( L . ) Merrill, has become a major crop in the United States, the acreage harvested for beans increasing from only 190,000 acres in 19%)to over 26 million acres in 1961. It now ranks fourth

x

CHINA MAINLAND

I UNITED STATES

ESTIMATED W O R L D T O T A L

958,275,000

BUSHELS

FIG. 1. World soybean production, 1960. (Source of data: Foreign Agricultural Service, U . S. Department of Agriculture.)

among the cash crops in this country and first among oilseed crops of the Western Hemisphere. The United States now produces about 57 per cent of the total world crop of soybeans (Fig. 1 ) . Many developments have contributed to the rapid increase in soybean

THE MANAGEMENT OF SOYBEANS

361

production, among them a steady expansion in the market for soybean oil and meal in this country and a strong export demand for the crop. Research on production problems and the development of superior varieties by breeding have had a major role in increasing the efficiency of production. Agricultural engineering research leading to the development of the combine harvester has been of considerable importance in the complete mechanization of soybean production. Soybean management is the application of the sciences of plant breeding, plant pathology, plant physiology, soil management, engineering, and economics to the growing of soybeans. The most profitable soybean culture comes from the use of all these sciences. An old variety such as MAMMOTH YELLOW, under some conditions, could produce a yield of 40 bushels. However, MAMMOTH YELLOW is susceptible to several diseases which frequently limit seed yields and is subject to lodging and shattering. The newer, improved LEE variety is resistant to several diseases to which MAMMOTH YELLOW is susceptible, has greater lodging resistance and improved seed holding. These factors reduce production hazards and permit greater stabilization and efficiency of production. Literature on soybean management is extensive. One of the first, and little recognized, publications was that of Mooers (1908). He presented information on varietal interaction to date of planting and row width and gave directions for planting, cultivating, and harvesting the crop, as well as data on chemical composition of the seed. The first book on the subject was that of Piper and Morse (1923); twenty-six years later appeared a general review of soybean breeding and management by Weiss (1949) followed by a review of production by Morse ( 1950) and of structure and genetics by Williams (1950). Recently, reviews were made on physiology ( Howell, 1%0), nutrition ( Ohlrogge, 1960), and genetics and breeding (Johnson and Bernard, 1962). This review brings together the recent research on culture and management.

B. UNITEDSTATESPRODUCTION TRENDS The first commercial crushing of soybeans from domestically grown seed was in North Carolina in the fall of 1915 (Dies, 1943). By 1920, production was 3,000,000bushels and the leading states were North Carolina, Virginia, Alabama, Missouri, and Kentucky-North Carolina producing 55 per cent of the total. By 1931, the center of production had shifted to the North Central States, where it is at present (Fig. 2 ) . Soybean production has been spreading over additional areas of the United States as better varieties and improved production methods have been developed. Soybean acreage in the United States increased 124 per cent during the period 1949-1960, while cotton acreage dropped 44 per

FIG.2. Soybeans harvested for beans, acreage 1959. (From Bureau of the Census, U. S. Department of Commerce.)

TABLE I United States Soybean Acreage and Production Dataa by 5-Year Intervals, 1925-1960 Acres planted

Year 1925 1930 1935 1940 1945 1950 1955 1960 0

b 0

Grown alone (1000 acres )

Interplantedb ( 1000 acres)

Equivalent solid0 (1000 acres)

Acres harvested For beans (1000 acres )

For hay (1000 acres)

Grazed or plowed under (1000 acres)

476 1,785 1,539 415 1,175 195 3,072 786 3,473 1,074 337 2,082 6,966 1,028 7,503 2,915 544 4,044 2,589 11,782 4,807 10,487 4,819 2,156 1,505 13,807 13,056 10,740 1,940 1,127 1,184 15,640 15,048 13,807 963 870 19,658 603 19,959 18,620 711 628 24,275 375 24,463 23,516 521 426 Data from Economic Research Service, U. S. Department of Agriculture. Grown with other crops. Acreage grown alone, plus one-half the interplanted acres.

Yield per acre harvested

Production

For beans (bushels)

For hay (tons)

Beans (1000 bushels)

Hay (1000 tons)

11.7 13.0 16.8 16.2 18.0 21.7 20.1 23.8

1.01

4,875 13,929 48,901 78,045 193,167 299,249 373,522 558,778

1,185 1,938 5,422 6,450 2,451 1,260 910 751

.94 1.34 1.34 1.26 1.31 1.28 1.44

8m

F

zE 3 8

3 E

364

JACKSON L. CAR'ITER A h 3 EDGAR E. HARTWIG

cent, wheat acreage 31 per cent, oats 28 per cent, all hay 5 per cent, and corn acreage 4 per cent ( Kromer, 1961 ) . Up to 1941, over half of the soybean acreage was for hay, grazing, or green manure. The trend toward production for processing has been strong since then because of the demand for soybean oil and meal, so that at present virtually all soybeans are grown for processing (Table I ) . Long-run prospects indicate that as the demand for soybean meal increases, production lvill continue to expand.

C. UTILIZATION 1. Processing to Obtain Oil a d Meal Processing in this country was at &st by hydraulic press, later by expeller or screw press, and now almost entirely by hydrocarbon solvent extraction. A moist heat treatment is given to the solvent meal to destroy certain protein digestion inhibitors and improve the absorption of some of the amino acids, making the meal an escellent protein supplement in feeds. Soybean meal now supplies nearly 56 per cent of the protein concentrates in this country. For the 1959-1960 processing year, approximately 8,400,000 tons of meal were used in feed, 650,000 tons for export, and 30,000 tons for food and industrial purposes. The yield of meal is about 46.5 pounds per bushel of soybeans processed. Soybean oil, cIassed as a semidrying oil, is used for food purposes and also in several industrial products. Soybeans now supply about 35 per cent of the total fats and oils produced in the United States. The yield of oil per bushel has increased from 9.7 pounds in 1950 to 11 pounds in 1960. This increase has been due in part to the change in processing from the expeller method, which left 4 or 5 per cent of the oil in the meal, to the solvent method, which leaves only about 1 per cent in the meal, and also to the development of improved soybean varieties with higher oil content. The principal use of soybean oil is in food (margarine, 31 per cent; shortening, 34 per cent; other food uses, 24 per cent), constituting 89 per cent of the oil use. The remainder is used in paints and varnishes, other drying oil products, and miscellaneous nonfood products. Extensive research is being done to improve flavor stability of soybean oil for food use and to increase the list of products that can be made from the oil (Cowan and Witham, 1959). 2. Hay and Green Manure

The use of soybeans for hay and for grazing and plowing under increased in importance from the start of production in this country to about 1940, when nearly 7 million acres were grown for these purposes

THE MANAGEMENT OF SOYBEANS

365

(Table I ) . After that, the use of soybeans for hay and green manure decreased rapidly and the acreage has dropped to less than a million. II. Soil and Climatic Adaptation

A. AREASOF PRODUCTION IN THE UNITED STATES The principal area of soybean production is in the North Central region (Fig. 2 ) , where, in 1961, over 77 per cent of the United States crop was produced. Here, as in the eastern portion of the great central plain of Manchuria, a large area has been found to be particularly well adapted to soybean production ( Weiss, 1949). The two other major areas of soybean production in the United States are the Mississippi delta and the Middle Atlantic coast. Production has not extended very far west in the central Great Plains, except where the crop can be irrigated. In this area, low rainfall and the high temperatures frequently cause moisture shortages ( Swanson, 1951) . With supplemental irrigation, especially during the pod-filling period, soybean culture can be extended into this area successfully. The high plains of Texas and west-central Nebraska represent such areas. Good yields have been obtained under irrigation in southern California and in some areas of eastern Oregon and Washington, but here soybeans are competing with other crops having higher per acre value.

B. SOILREQUIREMENTS The soybean will succeed on nearly all soil types except extremely deep sands. Soybeans are better adapted for production on clay than either corn or cotton. The crop is also well suited for production on muck. For best results, soils should be limed to pH 6.0 to 6.5 and soils of low fertility should be supplied with those mineral elements in which they are deficient. C. CLIMATIC ADAPTATION

1. Effect of Temperature on Plant Growth The effects of temperature on soybean yields have not been studied extensively. Runge and Ode11 (1960) found that yields were slightly lower when temperatures were above average during July and August. They found above-average maximum temperatures in June and September resulted in small increases in yield. Prompt emergence in the field is important for weed control. Plants grown in the greenhouse under controlled conditions at 60°F. took 7 to 10 days to emerge, compared to 3 to 5 days for those in 70°, 80°, or 90" chambers. Early-planted soybeans often require 10 to 14 days to emerge, but later plantings, when the soil

366

JACKSON L. CARTIER AND EDGAR E. HARTWIG

is warm, will emerge in 5 to 7 days (Smith et d.,1961; Nagata, 1960; Hartwig, 1954 ). In addition to affecting rate of germination of soybeans, temperature also affects rate of growth and the time required for the plants to shade the ground between the rows, an important consideration in weed control as well as yield. Smith et al. (1961) found that soybeans planted on May 5 had shaded only 59 per cent of the ground in 2 months but those planted on June 5 had shaded 86 per cent. Hartwig (1954) found the rate of growth increased markedly as temperature at planting PLANTING

JUNE

D A m

10

M A Y 10

A P R I L 10

FIG. 3. Diagrammatic comparison of relative height and width at 6 weeks after emergence for the average of four varieties of soybeans planted at Stoneville, Mississippi, April 10, May 10, and June 10, 1944-1951.

time increased. Figure 3 shows the relative growth 6 weeks after emergence on the dates indicated. Temperature affects blooming date, as pointed out by Garner and Allard ( 1930). They stated that sustained summer temperatures below 75' to 77'F. will ordinarily delay blooming, a decrease of 1" causing a delay of 2 or 3 days. Variation from year to year in date of flowering of a given soybean variety planted on a particular date is due chiefly to differences in temperature, whereas differences between varieties are due chiefly to their response to length of day. There is a minimum temperature for most growth processes, which for all practical purposes appears to be about 50°F. Parker and Borthwick (1943) found that floral induction was greatly inhibited at 50°F.or

367

THE MANAGEMENT OF SOYBEANS

lower, and Brown (1960) found no growth of prebloom soybeans at 50°F. Using 50°F. as the base, Brown and Chapman (1961) calculated heat units required to mature soybean varieties adapted to the upper Great Lakes region. Good agreement was obtained between required and available units for varieties grown in the northern part of the region, but available units exceeded required units for varieties grown in the southern part. -MAXIMUM

----. MINIMUM

TEMPERATURES MAY I THROUGH OCT. 31, TEMPERATURES MAY I THROUGH OCT. 31,

1948-1957 1948-1957

FIG.4. Mean maximum and minimum temperatures by 5-day periods at Urbana, Illinois, and StoneviUe, Mississippi, during the growing season, 1948-1957. ( Source of data: Climatological Data, Weather Bureau, U. S. Department of Commerce.)

High temperatures (over 100°F.) early in the season may have adverse effects. For example, brief periods of high temperature greatly reduce the rate of node formation and the rate of growth of internodes (Howell, 1956). There is some indirect evidence that varieties differ in their temperature requirements and that some are adapted to higher temperature conditions than others. Varieties such as LEE grow well and produce good quality seed at Stoneville, Mississippi, where seed of CLARK is usually inferior. Green (1961) has shown that seed quality is adversely affected by high temperatures during seed development. At Urbana,

3f33

JACKSON L. CARTTER AND EDGAR E. HARTWIG

Illinois, where am^ produces excellent seed, it matures when maximum temperatures are approximately 70" and minimum temperatures around 50" (Fig. 4 ) . At Stoneville, it matures when maximum temperatures are 90" and minimum around 67". Although other factors are involved in this, it appears that temperature may be a predominant one. The variety LEE makes excellent growth and produces high yields of good quality seed at Brawley, California, where daily maximum temperatures are above 100°F. for June, July, August, and September.

2. Effect of Temperature on Composition of Seed Temperature is one of the basic elements of the environment that influences storage of oil in the seed. Howell and Cartter (1953, 1958) showed that temperature during certain portions of the pod-filling period was correlated with oil percentage in the mature seeds of soybeans produced in the North Central and Gulf coast areas. The highest correlation coefficients were obtained for the periods 20 to 30 and 30 to 40 days before maturity, results indicating that temperatures during these periods exert a greater effect on oil level than those at other times. This influence was subsequently studied under controlled conditions where soybean seed produced in the greenhouse contained 22.3, 20.8, and 19.5 per cent oil when grown at temperatures of 85", 77", and 70"F., respectively, during the pod-filling stage, To further determine the period of maximum sensitivity of oil formation to temperature, the effect of a brief period of elevated temperature was measured. One week of elevated day temperature during the fourth to seventh week before maturity produced seed with an oil content of about 22 per cent, compared to 19.6 per cent when temperatures were elevated the second week before maturity. Thus, the period of greatest temperature influence occurred prior to the period of most rapid oil synthesis in the seed, a sequence indicating that the influence of temperature was on the establishment of the metabolic system for the conversion of sugars to oil rather than on a specific reaction rate. hlost of the studies by Howell and his co-workers have been with daytime temperatures between 70" and 85°F. When temperatures under these controlled conditions were raised to W",there was a seed yield and oil content reduction indicating that the optimum temperature had been exceeded. 3. Effect of Light on Plant Growth Light is the source of energy for photosynthesis, as well as the control of many plant growth processes. Light saturation of photosynthesis in

THE MANAGEMENT OF SOYBEANS

369

individual soybean leaves is at about 2200 foot-candles (Bohning and Burnside, 1956), which is about one-fifth of the intensity of sunlight at midday in the central part of the United States. By the time the plants have reached an appreciable size, most of the leaves are receiving a light intensity far below this value and by the time the foliage covers the row, light intensity has been reduced to probably 2 or 3 per cent of this value on the lower leaves. During the flowering period, soybean plants produce three to four times the number of flowers that finally develop into pods, the number of pods that are finally set on the plant depending upon the vigor of the plant during the time of blooming. If the plants are shaded during this period, the proportion of pods that abort will be much higher, possibly owing to lowering of the sugar level in the leaves or other imbalance in the plant system. This effect of shading is of much significance in considering the importance of the natural variations in light intensity associated with periods of cloudiness, especially at critical periods in plant development.

4. Effect of Photoperiod on Flowering and Maturity In addition to furnishing energy, light also serves an important function in regulating blooming and maturity. Our present extensive knowledge of the effects of day length on flowering goes back more than fifty years: Mooers (1908) concluded that the agreement in length of season required by the same variety to reach maturity in the different years when planted at a given date is striking, and that there is not only a steady shortening of the season of growth as the date of planting is made late, but also that this shortening is much more marked in some varieties than in others. Garner and Allard (1920) recognized the significance of day length in the flowering behavior of soybeans and other plants and termed it photoperiodism. Later studies by Parker and Borthwick (1950), using different periods of artificial light and darkness, determined that the length of the period of darkness was the controlling factor. Since a soybean variety flowers in the field only when the days are shortened below a critical value for the variety, soybeans are called short-day plants. This photoperiodic response is an important factor in soybean production. Research on photoperiodism has been reviewed several times in recent years (Hamner, 1938, 1944; Murneek and Whyte, 1948; Parker and Borthwick, 1950; Leopold, 1951; Lang, 1952; Liverman, 1955; Doorenbos and Wellensiek, 1959; and Howell, 1960). One well-known example of photoperiodic effect in soybeans is the delay in date of blooming and maturing of a soybean variety as it is

370

JACKSOX L. CAR-

AND EDGAR E. HARTWIG

moved north. Figure 5 shows how the maximum day length on June 21 vanes with latitude, the difference amounting to some 80 minutes between the latitude of Urbana, Illinois, and that of Winnipeg, Manitoba. Assuming that a full-season variety at Urbana (CLARK, for example) flowers around the first of July when the day length is about 15 hours, this same shortness of day (long dark period) would not be reached in the vicinity of Winnipeg until around August 10. A corresponding delay IT

16

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0

<

12

a n U.

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a

3 0

= 10 9

8

7

.

.

growing season. (Source of data: The American Ephemeris &d Nautical AlGanac for the Year 1936. U. S. Government Printing Office, Washington, D. C., 1934.)

in maturing would mean that the variety would not ripen before frost

at the higher latitude. Actually, CLARK will be frosted most seasons when grown at Madison, Wisconsin, only 3" north of Urbana. The rather precise plant response to latitude is illustrated in Table 11, which shows the average maturity date for the soybean variety LINCOLN at several locations (Cartter, 1958). This delay in maturity illustrates why soybean varieties are said to be adapted to rather narrow belts of latitude. It becomes evident that the terms early-, medium-, or late-

37 1

THE MANAGEMENT OF SOYBEANS

maturing, when describing a soybean variety, are meaningless except when related to a specific location and uniform planting date. Johnson et aZ. (1960), using equipment in which the photoperiod could be closely controlled, have studied rate of development throughout the life cycle of the plant. They reported that under a d c i a 1 control where similar day-length experiments differed by as much as a month in planting date, the periods from emergence to flowering were essentially identical, though the plants were subjected to quite different and fluctuating environmental conditions. Only when the temperature drops below an optimum value does it begin to play a major part compared to photoperiod in determining rate of development following floral initiation. TABLE I1 Effect of Location on Maturity Date of Location

Latitude

Madison, Wisconsin DeKalb, Illinois Dwight, Illinois Urbana, Illinois Eldorado, Illinois Sikeston, Missouri Stoneville, Mississippi

42" 34' 41" 50'

LINCOLN

Soybeans

41" 8' 40" 8' 37" 52' 36" 23' 33" 25'

Date mature Oct. 2 Oct. 1 Sept. 27 Sept. 17 Sept. 8 Aug. 30 Aug. 12

In addition to control of blooming and maturing, the photoperiod reaction also controls other functions of the plant. Soybeans produced longer internodes when the plants entered the dark period with the photosensitive pigment system predominantly in the red-absorbing formthat is, under incandescent supplemental light high in infrared than under fluorescent light high in the red region of the spectrum. With either a 12-hour or 16-hour photoperiod, the stem length of AGATE, a very early strain, was doubled under the rich infrared light and BILOXI, a late strain, increased one-fourth when compared to plants exposed to fluorescent light (Downs, 1959).

5. Efect of Soil Moisture on Growth The period of germination is critical for soybeans; then excess moisture or prolonged drought may be injurious. After the plant is established, it withstands short periods of drought and is not seriously retarded in growth nor reduced in yield by a wet season, provided weed growth is controlled. Runge and Ode11 (1960), in corn, corn, corn, soybean rotation at Agronomy South Farm, Urbana, Illinois, 1909 through 1957, found that

372

JACKSON L. C A R T E R Ah?) EDGAR E. HARTWIG

above-normal precipitation during July (period of major vegetative growth) and from mid-August to midSeptember ( grain-filling period) increased soybean yields, but abundant rainfall during other periods decreased yields. In early spring there is normally plenty of moisture and above-average precipitation is detrimental. In the first half of August, soybeans are in the early pod stage. An inch of rain above the average during the last week of August or the first week in September increased soybean yields nearly 2 bushels per acre. Moisture deficit for 2 to 4 weeks immediately after flower bud differentiation reduced vegetative growth and caused heavy flower and pod dropping, according to Fukui and Ito (1951). They also observed that a sudden increase to high moisture after a severe drought caused another severe pod drop. A water table in the root zone is injurious, especially if it is near the surface early in the season and during the late fall (Fukui et al., 1954). Also, short periods of excessive moisture supplied to the plants after the period of bud differentiation resulted in very poor yields (Fukui and Ito, 1952). It has been shown that the crop is more susceptible to drought injury during the pod-filling period than during earlier stages of growth. An early and a late variety may be affected very differently at a given location during a period of moisture tension, in accordance with the stage of pod development at the time (Howell, 1956). This varietal interaction to a short period of drought in late summer is frequently observed in the evaluation of soybean strains and must be taken into consideration in a testing program. 111. Time of Planting and Varietal Adaptation

A. EFFECTON PLANT C H A R A ~ ~ Probably no single cultural factor is more important to soybean production than planting date. The effect of planting date was observed by Mooers (1908), in Tennessee, who planted several soybean varieties at several dates. When planting was delayed from May 15 to July 15, the delay in maturity of MAMMOTH YELLOW, a late variety, was only 19 days for the 60-day delay in planting, but for ITO SAX, an early variety, the delay was 52 days. Twelve years later, the significance of day length in the flowering behavior of soybeans was described by Garner and Allard (1920), who explained these phenomena as changes in day length accompanying changes in planting date. Time of planting was shown by Johnson et aZ. (1960) to affect development of the soybean plant at various stages through day-length response.

THE MANAGEMENT OF SOYBEANS

373

I. Maturity Date-of-planting studies have been conducted in nearly every area where soybeans are extensively grown. In the Midwest, the average maturity date was retarded approximately 1day for each 3 days’ delay in planting, although the varieties behaved differently ( Weiss et al., 1950; Osler and Cartter, 1954). In Illinois, using four dates of planting from May 1through June 12, the maturity date of the genetically later strains was affected less by delay in planting than was that of the earlier strains. BLACKHAWK, the earliest variety, was delayed 16 days for a 43-day delay in planting, but for WABASH, the latest variety, maturity was delayed only 8 days. Workers in the southern part of the United States have reported similar results for date-of-planting studies. In Virginia, LEE, a late variety, was delayed only 8 days at Warsaw and 10 days at Petersburg for a 61-day delay in planting (May 5 to July 5), while CLARK, an early variety, was delayed 21 and 29 days, respectively (Smith et al., 1961) . In Mississippi, WABASH, a very early strain for that area, matured August 20 when planted April 10, and September 22 when planted June 20, a delay of 33 days for a 72-day delay in planting. For the same planting dates, ROANOKE, a late variety, showed only a 5-day difference in maturity (Hartwig, 1954). Since in Mississippi the maturity date of many strains is essentially unaffected by planting dates from April 10 to May 10, dividing the acreage between two adapted varieties of different maturity is a surer method of spreading harvest dates than planting one variety over a range of planting dates. Leffel (1961), in Maryland, using several varieties of group IVYV, and VI maturity (Morse et al., 1949) at five dates, May 22 to July 18, concluded that the duration of the periods from planting to first flower, of flowering period, and from termination of flowering to maturity decreased with each delay in planting date. The reduction in the intervals flowering to termination of flowering, and termination of flowering to maturity, tended to be similar for each of the maturity groups studied (Fig. 6 ) . The decrease in the interval from planting to flowering, as a consequence of delayed planting, was greatest for the late varieties. These conclusions are in substantial agreement with Mooers ( 1908), who wrote that his field notes “show in extreme dates of planting of Mammoth Yellow less than a week’s difference in the period between flowering and maturity so that the variation in length of season takes place almost entirely previous to flowering.” Somewhat similar results were shown by Abel (1961) working under irrigated conditions in the Imperial Valley of southern California.

374

JACKSON L.

Ah7) EDGAR E. HARTWIG

In the northern part of the United States, the relation of planting time to maturity is somewhat different from that in the central and southern areas because of the greater importance of the temperature effects on flowering ( Gamer and Allard, 1930).Torrie and Briggs ( 1955) observed a l-day delay in maturity for 2 days' delay in planting for both early and late varieties in Wisconsin. Brown and Owen (1961) attribute the decrease in time from planting to flowering at Harrow, Ontario, to (1) time required for emergence, ( 2 ) temperature, and ( 3 ) photoperiod. Maturity

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NOVEMBER

Doyr from p l a n t i n g t o first flower Days f r o m f i r s t flower to terminotion of flowering D a y 3 from termination of flowering to maturity

FIG.6. Duration from planting to first flower, first flower to termination of flowering, and termination of flowering to maturity for maturity groups IV, V, and VI soybean varieties at various dates of planting at Trappe, Maryland, 1957-1958. (Redrawn from Leffel, 1961.)

They found 2 weeks were required for emergence for their May 15 date of planting and only 6 days for the June 26 date. The number of days from emergence to flowering was decreased 19 days as planting was delayed 41 days. The period from flowering to maturity was essentially constant for all planting dates. Luther Fitch (personal communication) at Ontario, Oregon, under conditions of low daily minimum temperature in the fall, observed an extreme delay in maturity due to delay in planting-28 days for an early variety and 34 days for a late one-for a %-day delay in planting. Fitch

THE MANAGEMENT OF SOYBEANS

375

concluded that, under his conditions, early planting was very important because of cool temperatures in the fall. 2. Plant Height Height and vigor of soybeans is of considerable importance owing to the possible effects upon yield, weed control, lodging, and harvesting. Very tall or extremely short varieties are not as easily harvested as those of medium height and are not generally recommended for production. A variety may vary considerably in height because of date of planting, spacing of plants in the row, spacing of rows, moisture supply, temperature and general growing conditions, soil fertility, and other factors of the environment. Weiss et al. ( 1950), Osler and Cartter ( 1954), Hartwig ( 1954), Caviness and Smith ( 1959), Abel ( 1961),Smith et al. ( 1961), and Leffel ( 1961) are in general agreement that midseason plantings produce taller plants than do earlier or later plantings. In Mississippi, early varieties tend to gain maximum height when planted around May 1, whereas, later strains, such as OGDEN and ROANOKE, attain maximum height when planted June 1 or later (Hartwig, 1954). Because of this height relationship to planting date, medium late or late varieties are better suited for late planting. When height is reduced by early or late planting, there is a tendency for pods to be formed closer to the ground. Gray (1959), in Louisiana, found most varieties to be severely stunted when planted in March or April. In these studies, plant height was closely correlated with seed yield. IMPROVED PELICAN (group VIII maturity) had the widest range in suitable planting dates. 3. Lodging

Lodging is very important to producers, especially if lodging is so severe that combine losses are high or the field has to be cut in one direction only to pick up the lodged plants. Soybean varieties differ greatly in their lodging susceptibility. For the planting dates extending from May 1to June 12, in Illinois Osler and Cartter (1954) observed that lodging increased as planting was delayed. Weiss et al. (1950), using five varieties at three locations in the Midwest over a three-year period, did find a small increase in lodging susceptibility with delayed planting, but, more important, found lodging increased with genetic lateness of the variety. Smith et al. (1961) observed that during seasons with relatively low rainfall and low yields, late plantings lodged less than early plantings. The greatest amount of lodging for the LEE and DORMAN varieties in the delta area of Arkansas occurred when the soybeans were planted late (Caviness and Smith, 1959). Similar results have been reported by Leffel ( 1Wl).

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JACKSON L. CARTIER AND EDGAR E. HARTWIG

4. Seed Quality Quality of the seed affects the market value of the crop sold for processing and the viability of the seed kept for planting. Studies conducted in the southern part of the North Central region, southeastern Missouri, Virginia, Mississippi, Arkansas, southern California, and Maryland show that the best quality seed has consistently been obtained with planting dates and varieties which resulted in maturity dates that are late for the area but before frost (Feaster, 1948; Hartwig, 1954; Cartter, 1958; Caviness and Smith, 1959; Abel, 1961; Smith et al., 1961; and Leffel, 1961). High temperature and humidity during seed development frequently cause poor seed quality and promote growth of microorganisms on the seed. Continued warm rains at maturity are especially detrimental to good seed quality and under severe conditions may cause the beans to sprout in the pods. Also, very high temperatures under dry conditions may arrest seed maturation, green and shriveled seed resulting. Summarizing the many recent date-of-planting studies with soybeans, it appears that seed quality is a function of the weather conditions during maturing and harvesting: cool dry conditions favor good quality seed; warm, wet weather with frequent rains produces low quality seed, frequently with a weathered appearance; and very hot, dry weather with drought tension or a frost causes small-size, greenish-colored seed.

5. Size of Seed One of the characteristics used to describe soybean varieties is seed size; it is generally expressed as weight in grams per 100 seed. Seed size ranges from 1 g. per hundred seeds for the small-seeded, wild soybean to 55 g. for the large-seeded, vegetable types, the more common commercial varieties ranging from 13 to 18 g. per hundred (3500 to 2500 seeds per pound ) . According to Cartter and Hopper ( 1942), seasonal conditions play an important role in modifying the size of soybean seeds, one of the important factors being the stage of development of the plant in relation to weather conditions. During flowering and early pod set, the soybean plant regulates, by physiological abortion, the number of pods that it can develop. Thus, if unfavorable growing conditions occur during blooming, less seed are set. Subsequently, favorable conditions, providing abundant food supply, tend to produce larger seed than would be produced if reverse conditions occurred. Osler and Cartter (1954) found that seed weight was not appreciably affected by delay in planting, although there was a difference in varieties in this respect. Weiss et al.

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(1950) found that varieties differed in seed size but varietal differences were not consistent for locations or years. The interaction of varieties with locations was largely attributable to moisture conditions at the various stages of development. According to Smith et al. (1961), seed size is normally of little concern to soybean growers except as it might affect seed quality. They found that size of seed varied considerably owing to location, year, and date of planting, but the relative seed size of the different varieties remained constant regardless of other factors.

6. Seed Yield Recommendations for date of planting soybeans in northern areas are usually based on the best date to plant full-season varieties-those that utilize the full growing season and ripen before the average frost date. Mooers (1908) recognized that extremely early planting was not desirable and that under southern conditions late May or June plantings would often produce the best yields. Later studies with improved varieties have confirmed Mooers’ results (Hartwig, 1954; Gray, 1959; Caviness and Smith, 1959; Abel, 1961; Rouse, 1961; Smith et al., 1961; Lelfel, 1961). In Illinois and other northern areas, plantings from May 1 to May 20 yield best (Burlison et al., 1940; Weiss et aZ., 1950; Osler and Cartter, 1954). In northern areas, yields of early varieties are generally affected less than the yield of late varieties by late planting. In southern areas, late varieties have a broader range in planting dates for producing high yields. B. EFFECT ON COMPOSITION OF THE SEED Much interest has been centered on the effect of environment on the synthesis of oil and protein. Viljoen ( 1937) and Weiss et aZ. (1950) have reported a decrease in oil content and a slight increase in protein content with delay in date of planting. This decrease in oil is assumed to be associated with slightly later maturity and lower temperatures during seed development. Iodine number of soybean oil is influenced inversely by the temperature at the time the oil is being synthesized in the seed (Washburn, 1916; Osler and Cartter, 1954; Howell and Collins, 1957). Osler and Cartter found that, for their earliest variety, BLACKHAWK, oil content and iodine number did not drop with the later dates of planting. It appeared that BLACKHAWK matured sufficiently early so that even at the late date of planting the air temperatures during the period of sensitivity for determining oil level were substantially the same for all dates. Leffel (1961) found an increase in oil content for some delay in planting of his early varieties and then a decrease with succeedingly

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JACKSON L. CARTfER AND EDGAR E. HARTWIG

later dates. For his later varieties, he reported a practically linear decrease in oil content from 21 to 17 per cent associated with delay in planting. He also reported a slight increase in protein content with delay in planting date. No appreciable effect of planting date on oil content was observed for two southern varieties of intermediate maturity (Hartwig, 1954), but oil content of an early variety, WABASH, increased as planting was delayed from April 10 to May 20 and then continued at the same level for later planting dates. On the other hand, ROANOKE, a late strain, decreased in oil content as planting date was delayed. IV. Planting Methods a n d Equipment

A. SEEDBEDPREPARATION I . Concentional Planting methods for other row crops have usually been applied to soybeans as they were introduced into an area. Fall or early spring plowing is practiced in most areas, followed by light tillage immediately preceding planting to destroy weeds. Better stands are usually obtained when the seedbed is moderately firm under the seed than when it is loose. In the south, the weed population can be reduced prior to planting by shallow harrowing at 14- to 21-day intervals from late March to planting time. Clay soils frequently present problems in obtaining stands not encountered in coarser textured soils. Hazards in obtaining stands have been minimized by using a spring-tooth harrow rather than a disk harrow for seedbed preparation and by planting with double-disk openers. Seed is placed in moist soil, below the depth of harrowing (Hartwig and Wooten, 1957). Studies in Ohio on clays with poor internal drainage showed that excellent stands were obtained when the surface 2 inches of dry soil were removed from the seed row and the seed was pressed into the soil by means of furrow openers and press wheels (Haynes et al., 1959). 2. Minimum Tillage In many areas, production costs have been reduced without a corresponding reduction in seed yields by reducing the amount of tillage preceding planting. The “plow-plant” or “tractor-plant” systems develop a loose rough seedbed between the rows and a fine, firm seedbed over the row. The loose areas soak up more water, have better aeration for root development, but retard germination of weed seeds. The firmed area in the row is ideal for soil-seed contact and water movement over the

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seed (Cook et al., 1958). Illinois studies showed little difference in seed yields when “conventional,” “plow-plant,” “plow-harow-plant,” and “tractor-track” systems were compared (Bowers et al., 1959). On a Sharkey clay in Mississippi, fall plowing with a moldboard plow increased soybean seed yields 5 per cent over yields in areas having no fall treatment but disked in early spring to destroy winter weeds. After the spring disking of plowed and not-plowed areas, both treatments were worked with a spring-tooth harrow at 14- to 18-day intervals prior to planting ( E. E. Hartwig, unpublished). 3. Deep Tillage

Soils having a silty-textured subsoil have a tendency to form compacted layers which restrict movement of water and also restrict root development. Shattering this layer results in improved yields, especially in dry years. In the fall of 1953, a Dundee silt loam in Mississippi was chiseled 16 inches deep. Soybeans planted on May 10, 1954, showed severe drought symptoms by late June on untreated areas, while soybeans on the treated areas were growing vigorously. In this very dry year, seed yields on the deep tilled area were 35 bushels per acre as contrasted to 8 bushels on the untreated area (E. E. Hartwig, unpublished). On soils with hardpans, deep tillage has increased the water intake, increased root development, reduced drought damage, and improved the stand of cotton. Deep tillage gave no benefits on Sharkey clay (Grissom et al., 1955). Hobbs et al. (196l), in Kansas, found that deep tillage at 6 to 24 inches, with chisel-type implements on soils with definite restricting layers, rarely increased yields sufficiently to pay the cost of the operation.

B. Row WIDTHAND PLANTING RATE 1. Row Width The row width which will result in maximum yield is dependent upon length of growing season, growth type of varieties, and fertility level of the soil. In general, with shorter growing seasons, row widths narrower than the conventional 36- to 40-inch will result in highest yields. However, when seed yields of 35 to 40 bushels are produced in 36- to 40-inch rows, increased yields from narrower rows are less frequent. If post-emergence cultivation is necessary for weed control, then it appears that row widths cannot be reduced below 28 to 30 inches. If preemergence chemicals satisfactorily control weed growth, the increased yield from narrow rows must cover the cost of extra planting seed and also increased quantities of chemicals. Rows too narrow to permit

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JACKSON L. CARlTER Ahm EDGAR E. HARTWIG

cultivation would require a broadcast application of chemicals whereas only a 12-inch band is treated in 36- to 40-inch rows. Consequently, the cost of chemicals would be at least tripled. Yields in Minnesota and Illinois in 18- to %-inch rows have been 15per cent greater than in 36- to 40-inch rows (Lehman and Lambert, 1960; Pendleton et al., 1960). In Manitoba, increases with narrow row spacing were so small that it seemed important to adopt the row width in which best weed control could be achieved with the equipment available (B. R. Stefansson, personal communication). In southeastern Kansas, 21-inch rows have produced higher yields than 42-inch rows. A 75-pound seeding rate has been superior in 21-inch rows while 45 pounds per acre has been the optimum rate in 42-inch rows ( V. H. Peterson, personal communication). In Virginia, North Carolina, Mississippi, and west Florida, narrow row widths did not increase seed yield (Hartwig, 1957; R. L. Smith, 1959). Plantings in Arkansas at lower yield levels have, under some conditions, shown a yield response from rows narrower than 36 to 40 inches (P. E. Smith, 1959). Results from the eastern shore area of Delaware and Maryland have been variable. 2. Planting Rate Results for planting rates within the row are, in general, similar for all production areas. Planting rates of 6 to 12 viable seeds per foot usually give most satisfactory results. Recommendations in most States will be nearer the 12 seeds per foot rate. This heavier rate is recommended largely from the standpoint of early season weed competition and also for its influence on height of lower seed pod development. Since varieties differ in seed size, growers should base planting rates on seeds per foot rather than pounds per acre. Results from planting OGDEN at rates of 30 to 150 pounds per acre in west Florida for the years 1950 to 1955 showed little difference in yields at rates of 30 to 120 pounds, but yields were reduced at rates of 135 and 150 pounds ( R . L. Smith, 1959). Plant spacings of 2 to 3 inches in the row produced slightly higher yields in Indiana than l-inch or 4-inch spacings. Plants spaced 1 inch in the row showed a greater amount of lodging and matured slightly later than did thinner plantings (Probst, 1945). Spacings of 6 and 12 plants per foot produced higher yields in North Carolina than did the thinner rates. There was greater lodging at the 12-plant spacing than with 6 plants per foot. Yield differences were small and not significant in Mississippi from plantings at 6, 9, 12, 18, and 21 seeds per foot. The 6-seed-per-foot rate required a longer time to give complete ground shading in the row. The 9 and 12 seeds-per-

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foot rate gave excellent early growth with a not too severe amount of lodging (Hartwig, 1957). Along with rates within the row, moderate skips in one row of an otherwise complete stand have little influence upon the total yield. Caviness (1961) found that a 2-foot gap in a 16-foot row resulted in a very small and statistically nonsigngcant yield reduction. A 4-foot gap resulted in a barely significant reduction, and with a 6-foot gap 95 per cent of the check yield was produced. Similar results have been secured in Ohio (L. C. Saboe, personal communication) and in Illinois (R. L. Bernard and J. L. Cartter, unpublished).

C. DOUBLE CROPPING 1. After Fall-Sown Grain Crops The growing seasons for fall-sown grain crops and soybeans are such that in many areas both can be grown during the same year with nearly full production of each. One of the requirements is that each be harvested as soon as it is mature and the other planted immediately. Weather conditions may upset this schedule. Studies at Stoneville, Mississippi, indicate that, for mid-June plantings, yields after oats are approximately 10 per cent lower than for similar plantings where no spring crop was grown. At Stoneville, October 15 is the best planting date for oats, and they mature between May 25 and June 15. One of the problems in double cropping is timely planting of soybeans after small grain harvest without excessive loss of soil moisture. Plantings on clay in the delta area of Mississippi have been most successful when grain straw is burned and soybeans planted immediately without any seedbed preparation, using planters equipped with a double disk opener. Equally satisfactory stands have been obtained by planting in shredded straw but the residue gives problems in cultivation. Burning straw also kills seedling weeds. Several workers (Brim et al., 1955; McAlister, 1958; Fullilove and Reid, 1959) in the coastal plain area have obtained good results in planting soybeans after small grain by using a lister type planter with no previous seedbed preparation. In cultivation, the soil thrown out from the lister furrow is worked back into the row. This method works best in relatively sandy soils with moderately high rainfall. In some areas use of oats for silage has proved desirable. This practice would permit earlier planting of soybeans and would reduce the straw problem. 2. After Peas In northern areas where peas are grown for canning or freezing, the crop is harvested in late June or early July, and soybeans have been

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JACKSON L. CARTER Ah’D EDGAR E. HARTWIG

planted after peas. Many of the problems are similar to those for planting after small grain, except that all top growth of the peas is removed in harvesting. Results obtained in Minnesota and Wisconsin indicate that soybean yields of 10 to 15 bushels can be produced after a crop of peas is harvested (J. W. Lambert, J. H. Torrie, personal communication). D. SPECIALMETHODSOF PLANTING Many soybean growers have been of the opinion that larger yields could be produced from a given land area if two crops differing in requirements were planted in alternate rows. A survey of farm practices with soybeans (Smith and Hope, 1920) showed that farmers believed that by planting soybeans and corn in alternate rows, each would produce 75 per cent of a full crop, thus giving a 50 per cent greater yield than if the two crops were grown separately. This theory has been tested in North Carolina, Ohio, Illinois, and Iowa with alternate rows, paired rows, and other interplanting patterns. At times interplanting has given small increases in total production, but it appears that increased management problems overbalance the production gains. In many of the older production areas, soybeans have been interplanted in the row with corn. In some cases the corn has been harvested and the residue plus soybeans grazed with livestock. In other cases the soybeans were grown only to add nitrogen for the following crop. As fertilization rates for corn in the southeast have increased and plant populations have also been increased, soybeans have failed to survive in the interplantings. Data from North Carolina show no difference in corn yield between corn planted alone and with soybeans interplanted. There was no seed production on the soybeans. The possibilities for planting two soybean varieties of different maturities or growth types in alternate or paired alternate rows has been explored. In Mississippi, interplanting the varieties JACKSON and LEE, which differ by 12 to 15 inches in height and 12 days in maturity, produced yields equal to the mean for the two varieties grown alone (E. E. Hartwig, unpublished ) . J. W. Pendleton (personal communication ) found no yield increases in Illinois from interplanting an early variety in the middle of the rows of a late maturing variety at the time of the last cultivation. Limited studies have been initiated to determine whether northsouth row direction might permit more light to be received by the soybean plant, resulting in higher yields. Results at Winnipeg, Manitoba, and at Urbana, Illinois, have shown no effect of row direction on yield (B. R. Stefansson, J. W. Pendleton, personal communication).

THE MANAGEMENT OF SOYBEANS

E. TYPESOF

383

EQUIPMENT

Soybeans may be planted with planters designed for row crops or with a grain drill with all feed cups covered except those needed for row planting. A row planter with good press wheels provides more uniform depth and better covering of the seed. In some heavy clays, the surface layer dries rapidly in the process of seedbed preparation. In such soils, a double disk opener has been found very beneficial in getting the seed placed below the loose dry surface into moist soil under conditions favorable for prompt germination (Fig. 7). If narrow rows are used, provision must be made for cultivating equipment to accommodate the narrower row spacing.

Double disc opener

Sword opener

FIG.7. Cross section of seed furrows. Effect of djfferent types of furrow openers on seed placement. The double disk permits placement of seed in moist soil with a minimum of soil disturbance. (From Mississippi State College Agricultural Experiment Station Information Sheet 576, March, 1958.)

The rotary hoe is very effective in early season weed control in soybeans (Lovely et al.,1958). For later cultivation, conventional row crop equipment is used. V. Rotation Practices and Erosion Control

A. EFFECTON SOYBEANYIELDS The soybean can be used advantageously in many crop rotations, and no standard rotation can be given that will apply to every farm. Depending upon the use of the crop, soybeans have been classed as soil improving or soil depleting. Sears (1939) found that where the crop was plowed under, it returned about 90 pounds of nitrogen per acre, and where it was harvested for beans and the straw returned to the soil, there was a gain of about 16 pounds of nitrogen per acre. When the

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JACKSON L. C A R T E R AND EDGAR E. HARTWIG

soybeans were combined and the straw burned, there was a loss of about 3 pounds of nitrogen per acre. Also, under comparable conditions, Sears estimated that corn, oats, and wheat would remove 40, 26, and 36 pounds of nitrogen, respectively, but his estimates were made for a rather low (40 bushel) yield of corn. Typical rotations for the midwest farming areas usually suggest soybeans following corn in the cropping sequence, as corn can utilize to advantage the nitrogen furnished by turning under a deep rooted, small-seeded legume sod, whereas, wellnodulated soybeans do not benefit from the high level of nitrogen. At Stoneville, Mississippi, with good weed control, soybean yields in a continuous cropping system have been similar to yields produced in a 2- or 3-year rotation with cotton. The advantage of soybeans in the rotation cannot be explained altogether on the basis of the returns from the crop, according to Pond (1950). Some of the other advantages given were ( 1 ) the labor requirements are low; ( 2 ) soybeans do not compete too seriously for labor at peak periods; (3) soybeans can be planted later than other crops with reasonable assurance that they will m a t u r e t h i s is especially important in a wet spring; ( 4 ) as a cultivated crop they aid in weed control; ( 5 ) they stand drought better than some other crops; ( 6 ) soybeans do better than many other crops on spring plowing; and ( 7 ) soybeans improve the physical condition of the soil. A crop rotation in which soybeans do not appear oftener than once in three or four years aids in controlling certain diseases such as brown stem rot. Where the soybean cyst nematode is a serious problem, nearnormal soybean yields may be obtained in a 2-year rotation if a nonsusceptible crop is grown during the year out of soybeans (J. M. Epps, personal communication).

B. EFFECTON

THE

FOLLOWING CROP

Soybeans improve soil tilth by shading and protecting the soil from rain. The roots and the bacterial action they foster tend to loosen the soil mass and make it more easily penetrated by nioisture and by roots of the succeeding crop. Soybeans leave heavy compact soils in much better physical condition than do corn and small grains (Calland, 1949). Soybeans as a crop seem to be less depressing on soil productivity than corn when judged by crop yield (Strickling, 1950). Results from studies conducted in Minnesota show that corn yields following soybeans are greater than those following oats. This yield increase was attributed largely to greater residual nitrogen in the soil after a crop of soybeans (Schmid et al., 1959).

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C. EFFECTON WE^, POPULATION Continuous cropping tends to build up the population of weeds of certain species. Rotating crops with different growth patterns and management practices aids in controlling weeds. Thus, the most effective approach to weed control is to consider it in relation to the entire rotation and utilize the advantage of good cultural practices on each crop. With the development of selective herbicides, chemical weed control is becoming more effective on many crops. Maximum pressure on the weed population can be maintained by use of various herbicides on tolerant crops in the rotation (Shaw, 1961).

D. SOIL RESIDUESFROM HERBICIDES In controlling weeds in one crop, due regard must be given to the problem of residues or the long-term carry-over in the soil. In recent years, many new herbicides have been developed for weed control in cotton and corn. Some of these have long-persisting soil residues so that they will give weed control throughout the growing season. Unfortunately, in some years, at some rates, and under some environmental conditions, this residue will carry through the winter and injure the soybean crop the following year. In most cases, the rates of application used are such that the rate cannot be lowered and effective weed control achieved. The residue in the soil, however, will be greater in some years than in others, primarily owing to differences in the rate of breakdown in the soil. Materials used at the present time which may give residual injury to soybeans are diuron on cotton, and simazine or atrazine and Randox-T on corn.

E. EROSION CONTROL Soybeans have a mellowing effect on the soil, leaving the surface loose and porous and in a favorable condition for seeding other crops. According to Browning et al. (1943), three factors are responsible for the looseness of the soil following a crop of soybeans: One factor is the protection of the soil surface by the plants themselves, commonly referred to as the “canopy effect”; if rainfall is limited up to the time when foliage is large enough to protect the surface, the soil under the soybeans will remain loose throughout the season. A second factor is the desiccating action of the plant roots on the soil during July and August when rainfall is often deficient and transpiration high. The root system of the soybean is not as extensive as that of corn, and the unusually heavy drain on the moisture supply of a limited soil mass reduces the moisture content to a low level. Such desiccation has a loosening effect

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JACKSON L. CARTIER AND EDGAR E. HARTWIG

on the soil. Since this zone is near the surface, the soil may be rewetted by showers and then dried several times in a season. The phenomenon has an effect on the soil similar to freezing and thawing. The third factor is aggregation resulting from decomposition of the roots, tops, and nodules. The incorporation of soybean roots and nodules increases ,the number of larger-sized aggregates. Studies in Iowa, Missouri, and Illinois show that land in soybeans is no more subject to erosion than land in corn if the beans occupy the same place in the rotation. Soybeans cause less erosion than corn when they follow meadow in the rotation. The soil losses from second year corn were larger because the soil tilth from meadow in the rotation largely disappeared after the first year. On a Marshall silt loam, the soil losses for soybeans were less than for corn under comTABLE 111 Soil Erosion from Corn and Soybeans with Different Tillage Practices (Marshall Silt Loam, Soil Conservation Experimental Farm, Clarinda, Iowa, 1944-1947 )

Soil loss (tons/acre) Tillage method

Up-and-down hill

Contoured

19.5 7.8 8.9 3.5

3.3 2.7 4.9 2.9

Corn listed in 40-inch rows Soybeans listed in 40-inch rows Soybeans surface-planted in 40-inch rows Sovbeans drilled in 7-inch rows

parable conditions (Table 111). Browning (1949) concluded that the frequent criticism of soybeans as causing more erosion than corn is not justified. VI. Weed Control

Weeds constitute a major hazard in successful soybean production. Yield reductions of 15 bushels per acre have been measured from competition of a moderate infestation of Johnsongrass in Arkansas ( Caviness and Taylor, 1960). Observations in Mississippi (E. E. Hartwig, unpublished) show yield reductions of 50 per cent from competition from pigweeds and 40 per cent from morning glory competition. Three-year average yields in Illinois show a 10 per cent reduction in soybean yields from competition of six giant foxtail plants per foot of row and a 28 per cent yield reduction from competition of 50 giant foxtail plants per foot of row. At least 50 per cent of the tillage requirements for producing a crop of soybeans has been attributed to controlling weeds (Shaw, 1961). In addition to yield reduction from competition, seed of plants such as crotolaria are toxic to livestock and poultry. Consequently, presence of any crotolaria seed requires the cleaning of soybean seed before it can be used as food or feed.

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A. EFFECT OF PLANTINGTIMEON PLANTGROWTH AND WEEDCOMPETITION Studies conducted at Stoneville, Mississippi, show that soybeans planted May 1 to June 20 after the soil has warmed will emerge in 5 to 7 days. Soybeans planted April 10 required 12 to 14 days for emergence (Hartwig, 1954). Similar results are reported for planting at a corresponding latitude in Japan (Nagata, 1960). In addition to more rapid emergence for the May and June plantings, the Stoneville studies show that these plantings grow more rapidly after emergence (Fig. 3). The combination of rapid emergence and rapid early growth results in earlier shading of the ground. Weeds will have greater difficulty becoming established under these conditions. The later planting permits several shallow cultivations prior to planting and destruction of many weed seeds in the upper soil layer. Under Mississippi conditions, soil conditions will be favorable for germination of weed seeds for a 6- to 8-week period if soybean planting is delayed until mid-May. Although preplanting shallow tillage has aided in controlling weeds in Mississippi, work conducted under the shorter growing season at St. Paul, Minnesota, showed that tillage of plowed ground prior to seedbed preparation for soybeans was of no benefit in the control of several annual weed species (Robinson and Dunham, 195s). Illinois studies show that foxtail seed begins to germinate in midApril and continues to germinate for about one month. Delaying planting until after May 15 permits many foxtail seedlings to be destroyed prior to planting and aids appreciably in reducing the foxtail problem (Slife, 1953). B. METHODS OF CULTIVATION Iowa studies show that rotary hoeing when weeds were germinating but not emerged and repeated once or twice at 5-day intervals reduced weed infestations 70 to 80 per cent and soybean stands approximately 10 per cent. Delaying use of the rotary hoe until weeds had emerged reduced the effectiveness of rotary hoeing (Lovely et al., 1958). Probst and Luetkemeier (1959) considered two rotary hoeings (16 and 22 days after planting) plus two cultivations (27 and 39 days after planting) to be satisfactory for controlling weeds in soybeans under most Indiana conditions. C . CHEMICAL WEEDCONTROL I. Pre-emergence Herbicides Numerous pre-emergence chemicals have been evaluated on soybeans but few have given consistently good control of weeds without injury to soybeans, CIPC [isopropyl N-( 3-chlorophenyl ) carbamate], DNBP ( 0-

388

JACKSON L. C A R T E R Ahm EDGAR E. HARTWIG

sec-butyl-4,6-dinitrophenol), PCP ( pentachlorophenol ) , CDAA ( 2chloro-N,N-diallylacetamide), and NPA (N-l-naphthylphthalamic acid) have been used to some extent in different areas. CIPC has given promising results in some tests, but results have not been uniformly good because of inability of this material to control broadleaf weeds at rates safe for use in soybeans. DNBP has appeared promising on some soil types but generally does not give acceptable weed control on the heavier soils. Volatility may cause injury to soybean seedlings. PCP and NPA give inconsistent weed control in soybeans. CDAA has not proved to be superior to PCP or NPA (C. G. McWhorter, personal communication ) . Amiben ( 3-amino-2,5-dichlorobenzoicacid ) appears to be one of the more promising pre-emergence chemicals for soybeans. Ohio results indicate that applications of 5 to 10 pounds per acre of DNBP gave satisfactory results over a four-year period. Damage to beans was observed, but this damage did not result in reduced yields of soybeans (Willard, 1952). Illinois data show Randox to be satisfactory for controlling annual grasses. Randox is relatively soluble and for this reason usually gives better results in seasons with limited rainfall (Knake et al., 1961). These workers considered amiben to be one of the most promising chemicals. A summary of weed control demonstrations conducted in Illinois in 1961 shows that 54 per cent of the growers using amiben reported good weed control whereas only 13 per cent of those using NPA reported good weed control. 2. Post-emergence Herbicides Several workers have reported good results from the use of 2,4-D to remove broadleaf weeds from soybeans, whereas other workers consider the material too hazardous to be used. In Ohio, studies indicated that in the cornbelt there are no important weeds which are more sensitive to 2,4-D than soybeans (Willard, 1952). In these studies, rates of 1/48 pound per acre to pound per acre were applied at three stages of growth from two true leaves to first flowers. Slife (1953) indicates that 2,4-D can be applied to soybeans 4 to 8 inches tall to remove broadleaf weeds. A rate of 1/16 pound of acid per acre is suggested to kill cockleburs, giant ragweed, and pigweed. The treatment is suggested only for areas where weeds are an extremely serious problem. The herbicide 4( 2,4-DB) gives promise of being less toxic to soybeans than 2,4-D but similar to 2,4-D for killing certain .broadleaf weeds. Preliminary results from Mississippi ( McWhorter et al., 1961) in-

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THE MANAGEMENT OF SOYBEANS

dicate that a mixture of pound of diuron plus 1% pounds of a surfactant may be used to remove young weeds from soybeans. The surfactant greatly increased the activity of diuron on weeds so that a concentration of herbicide, too weak to damage soybeans, could be used. VII. Seed Quality and Seed Treatment

A. FACTORS AFFECTINGSEH)

QUALITY AND

GERMINATION

Seed quality in the soybean is influenced by the variety and the environment during seed development, as well as by the conditions under which the seed is harvested and stored. Varieties must be able to withstand a period of rain and unfavorable weather which may frequently occur at harvest time. One of the objectives in varietal improvement is selection for good quality seed and resistance to weathering damage at maturity. Most of ,the recommended varieties have good seed quality when grown within their area of adaptation. Unfavorable weather during the ripening period, frost occurring while the beans are still green, or exposure to damp periods after the beans are fully mature may cause damage and poor seed quality (Morse et al., 1950; Howell et al., 1959). Severe drought can affect seed development, resulting in green seed and oil having high refining loss (Cartter and Hopper, 1942; Howell, 1956). Rough handling in threshing or cleaning, especially when moisture content of the seed is low, causes both externally visible and internal damage, though the latter may not be discovered until the seeds have been germinated (Moore, 1957; Humphrey, 1958; Colbry et al., 1961). Hard seeds, or those that fail to absorb moisture for several hours when soaked in water or placed in a germinator, are found occasionally in seed samples. Normally they present no serious problem. Soybean diseases, especially pod and stem blight, downy mildew, frogeye, and purple stain may affect the seed quality or injure germination. Comparative growth of normal and abnormal seedlings in germination tests indicate that the normal seedlings produce the most vigorous plants. Decay of cotyledons is a serious injury and only seedlings with healthy or very slightly decayed cotyledons should be included in the percentage of germination (Anderson, 1960). In Iowa, soybean germination tests in the seed laboratory correlated closely with those in the field. Greenhouse germinations in sand were lower, probably owing to a large population of seed-rotting fungi in the greenhouse sand (Sherf, 1953). Seeds of low vigor will be affected more by adverse field conditions than will seeds of high vigor (D. F.

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JACKSON L. CARITER AND EDGAR E. HARTWIG

Grabe, personal communication ) . Soybeans germinate most rapidly at 86" F., and if in sand, the most favorable moisture level is 15 per cent water based on dry weight of sand (Delouche, 1953). Methods of measuring seed viability are important to the seed trade (Sprague, 1958).Price per bushel of pure live seed, calculated by dividing the price per bushel by the purity times germination, is a simple way to determine planting value of the seed (Everson, 1957).

B. SEEDTREATMENT Seed treatment with a fungicide is not recommended as a general practice when seed with high germination is planted. Stands may be increased by seed treatment when seed having a germination of 85 per cent or less is planted. Although seed treatment seldom results in increased seed yields (Howard W. Johnson et al., 1954; Chamberlain and Koehler, 1959), the improved stands resulting from seed treatment aid in giving soybeans a competitive advantage with weeds. Studies by Howard W. Johnson et al. (1954) show that seed may be treated at any time between harvest and planting with equal effectiveness. The most satisfactory time for treating seed would be as it is cleaned. The materials Arasan, Captan, and Spergon have proved to be most satisfactory for treatment of soybean seed. Before any lot of seed is treated, it may be a good practice to check the germination with and without the fungicide to determine the beneficial effect of seed treatment on each seed lot. VIII. Nutrient Requirements

Nodulated soybeans do not respond to nitrogen fertilizer as do non-legume crops and because of this, gained a reputation of not responding to direct fertilization, a reputation that is not justified. In order better to understand the nutrient response of soybeans in comparison with corn, the total energy in the protein-oil seed of soybeans has been compared with the largely carbohydrate seed of corn (Howell, 1961). These studies show that in terms of total energy per acre, a 45-bushel soybean yield is equivalent to a 100-bushel yield of corn.

A. NITROGEN REQUIREMENTS AND NODULATION When properly nodulated, soybean roots may derive a considerable portion of the nitrogen needs of the plant from the nodules through the fixation of atmospheric nitrogen. Weiss (1949) presented a detailed review of the literature in this field, indicating the importance of nodulation to yield and composition of the crop.

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1. Effectiveness of Nodulution as a Source of Nitrogen A mutation type was reported by Williams and Lynch (1954) which does not develop nodules when inoculated with the soybean nodulating bacterium Rhizobium japonicum. This mutation, due to a single recessive gene, has been incorporated into several nodulating and nonnodulating near-isogenic lines of different maturities and is providing an excellent tool for the study of nitrogen fertility problems. Perhaps the best measures of effectiveness of nodules in supplying the soybean plant with nitrogen are the results of studies using one of these pairs of near-isogenic lines which differ in ability to nodulate. At Ames, Iowa, when 20 tons of ground corncobs per acre were added to the soil prior to planting to reduce the available nitrogen, nodulated soybeans produced 41 bushels per acre without nitrogen and 43 bushels when 600 pounds per acre of nitrogen was added. The nonnodulated strain produced 16 bushels per acre without nitrogen and 41 bushels with 600 pounds per acre of nitrogen. The 25-bushel yield increase for nodulated over nonnodulated soybeans where no nitrogen was applied was attributed to nodule activity. The 25 bushels of seed contained 96 pounds of nitrogen (C. R. Weber, personal communication). In a similar comparison of nodulated vs. nonnodulated soybeans on Sharkey clay at Stoneville, Mississippi, nodulated soybeans yielded at the rate of 42 bushels per acre with 41 per cent protein whereas nonnodulated soybeans yielded at the rate of 14 bushels per acre with only 28 per cent protein. The seed from nodulated soybeans contained 112 pounds more nitrogen per acre than seed from nonnodulated soybeans. In the same general area, other strains of soybeans produced seed yields as high as 60 bushels per acre which contained 192 pounds of nitrogen per acre, or an increase of about 160 pounds per acre over that for the nonnodulated strain. It appears that under conditions of low available soil nitrogen, nitrogen fixation from nodulation can furnish nitrogen for soybean yields of at least 60 bushels per acre with fixed nitrogen amounting to as much as approximately 160 pounds per acre. On soils high in available nitrogen, the amount of nitrogen supplied by the fixation process appears to be low. As far back as 1900, it was noticed that nodules of legumes inoculated with certain pure cultures were more active than those produced by bacteria already in the soil (Erdman, 1949). Some strains of bacteria are more efficient than others in fixing nitrogen, some are more aggressive in nodulating roots, and some are more competitive in the soil. Means et al. ( 1961), using a chlorosis-inducing strain of Rhizobium japonicum and mixing it in varying proportions with other strains, found that as little as 1.1 per cent of this strain in the mixture with another strain

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JACKSON L. CAR-

A N D EDGAR E. HARTWIG

caused 85 per cent of the nodules on certain soybean varieties. Almost without exception, a given nodule contains only one bacterial genotype. A chlorosis-inducing strain has been found in the Mississippi delta area that is aggressive on the variety, LEE, and causes typical chlorosis symptoms on plants around 6 weeks old (Clark, 1957). The symptoms are transient-that is, they are present for a short period and then new leaves growing out appear normal. Field observations indicate that the bacterial strain is relatively efficient in nitrogen fixation, though for a week or two symptoms may be alarming to a soybean grower. 2. Methods of Znoculation The generally accepted method for applying inoculum to the seed is to apply the bacteria adsorbed in a humus-peat carrier to slightly moistened seed or to make up a slurry of the inoculum in water and apply this. The quantity of seed inoculated should be limited to that which can be planted before the seed coats have completely dried. Results from several studies showed that 83 per cent of the inoculant was retained upon moistened seed whereas only 8 per cent of the inoculant was retained upon dry seed (Clark, 1956). Recent studies with precoating of soybean seed have shown no advantage over traditional methods of inoculation.

3. Survival of Bacteria in the Soil Studies in Illinois showed good survival of bacteria in soils that had not grown a crop of soybeans in thirteen years (Lynch and Sears, 1952). Similar results were obtained in North Carolina after twelve years of continuous cotton. Noninoculated plots were well nodulated and produced yields similar to yields of inoculated plots (E. E. Hartwig, unpublished).

4. Effect of Seed Treatment on Inoculation Under some conditions, seed treatment with the organic fungicides Arasan, Captan, or Spergon will result in improved stands of soybeans. Since rhizobia are already present in most soils in older soybean-growing areas, it is difficult to assess the effect of seed treatment upon survival of rhizobia. In 1950, planting were made on newly cleared land at the West Florida Experiment Station using ROANOKE seed which had been untreated or treated with Arasan or with Spergon. Both Arasan and Spergon reduced the effectiveness of inoculation. Seed yields from nontreated, inoculated seed were 22 bushels per acre as compared with 7 bushels for noninoculated seed and 13 bushels each for seed treated with Arasan or Spergon and inoculated (E. E. Hartwig, unpublished).

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5. Effect of Nitrogen Applications Applications of nitrogen tend to retard nodulation of seedinoculated soybeans planted in rhizobia-free soil. Applications of 100 pounds of nitrogen at planting time in the Imperial Valley of California, where rhizobia were not present in the soil, resulted in poorly nodulated soybeans which produced yields of 6 to 16 bushels per acre with 27 to 34 per cent protein on a dry matter basis. Omitting nitrogen application at planting time permitted good nodulation and resulted in yields of 35 to 40 bushels per acre with normal protein content of the seed (G. H. Abel, Jr., personal communication). An extensive field trial in Arkansas in which nitrogen was applied in a factorial experiment with phosphorus and potassium showed no significant response from nitrogen applied at different stages of plant development (Hardy, 1959). These results are typical of the many trials showing no appreciable benefit from nitrogen fertilizer on well-nodulated soybeans.

B. LIMINGAND PH LEVELS 1. p H and Plant Development There seems to be general agreement that a pH of 6.0 to 6.5 is desirable for soybean production (P. R. Smith, 1956). In North Carolina, under conditions where the pH of the soil was 4.2, nitrogen-fixing bacteria could not function actively and a yield response was obtained from added nitrogen. With additions of lime, soybean yields were related rather closely to the degree of calcium saturation regardless of the sourcecalcitic or dolomitic. On two soils, CaS04 had a detrimental effect due to the toxic effect of the sulfate ion and the salt injury (Welch and Nelson, 1950). In another series of soil treatments in North Carolina, a 2.8-bushel increase resulted from lime alone, and a 7.2-bushel increase from lime plus phosphate and potash (Collins et al., 1947). The pH of these soils before liming ranged from 5.1 to 6.0. A Richton silt loam in Arkansas with an initial pH of 4.9 produced significantly higher seed yields after applications of 4 and 8 tons of dolomitic limestone (Parks et al., 1959). Soils in some areas have shown a tendency to be short of magnesium, a minor element that is essential to green chlorophyll formation in leaves. Magnesium and calcium must be in proper balance for best growth. Surface applications of lime have been shown to move slowly downward in the soil under humid conditions; thus, there is relatively little advantage to deep placement by mechanical means. For a given soil

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JACKSON L. CAR'ITER AND EDGAR E. HARTWIG

and climate, the rate and h a 1 depth of effect are functions of the amount applied and the time elapsed (Brown et aZ., 1956). 2. Calcium and Magnesium Requirements A soil in Illinois, with a pH of 4.1 and an exchangeable magnesium level of 60 to 75 pounds per acre, gave increased yields of both corn and soybeans from applications of 75 to 150 pounds of magnesium per acre or 2 tons of dolomitic limestone (Key and Kurtz, 1960). The authors considered a level of 150 pounds per acre of exchangeable soil magnesium to be adequate for field crops on soils of moderate to low exchange capacity. A North Carolina soil low in magnesium showed a 4-bushel yield response from 60 pounds of magnesium (Nelson et al., 1945). A survey of farmer's fields in south Alabama showed that most fields which appeared to be low in vigor had a p H of 4.9 or lower and/or the available calcium below 200 pounds per acre. Soils with pH as low as 4.4 and calcium as low as 120 pounds per acre were observed. Many of these soils were also low in magnesium. A soil with a pH of 4.8 limed to give a pH of 6.2 gave a yield response of from 14 to 31 bushels per acre when adequate potassium was supplied (Rouse, 1961). C. PHOSPHORUS Approximately 16 pounds of phosphorus are present in 40 bushels of soybean seed. North Carolina results indicated that a yield response was obtained from applied phosphorus when their soil test showed less than 40 pounds available phosphorus in the soil (C. D. Welch, personal communication). Nelson ( 1946), on a coastal plain soil in North Carolina, obtained 6.4 bushels p e r acre of soybeans on a soil low in available phosphorus, but 33.8 bushels with added phosphorus. Response curves have been drawn by Bray (1961) which show that with 10 pounds of available phosphorus per acre soybean yields will be 75 per cent of the maximum expected, whereas with 30 pounds of available phoshoms soybean yields will be at 98 per cent of maximum. This response to phosphorus assumes other elements to be in adequate supply. The response curve for soybeans is almost identical with that for corn (Fig. 8). LINCOLN soybeans were grown on a Wooster silt loam in Ohio at soil phosphorus levels of 53, 30, and 11 pounds per acre. The percentage of total phosphorus derived from fertilizer was inversely related to the level of soil phosphorus. When radioactive superphosphate was applied, the plants near maturity had obtained about 25 per cent of their phos-

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395

phorus from the fertilizer on the high phosphorus soil and nearly 60 per cent on the low phosphorus soil. The high and medium levels of soil phosphorus gave increases in total dry matter of 38 per cent and 9 per cent over the low level, but all seed yields were at a 32-bushel level (Bureau et al., 1953). These results suggest that some other factors were limiting seed production, since North Carolina experiments on a low phosphorus soil showed grain : straw ratios to be relatively constant when additional phosphorus was added.

FIG.8. Relation of available phosphorus to percentage of maximum yield of soybeans and other crops. (From Bray, 1961.)

The availability of all the essential elements obtained by plants from the soil is affected in some way or another by the reaction of the soil. Phosphorus in particular becomes less available as the pH value drops below 6.5. This result may be due to the interaction of phosphate with hydrated iron oxides to form a basic iron phosphate (Truog, 1938; Klemme, 1949; Pearson, 1958). In a sand nutrient culture, the removal of magnesium did not retard phosphorus absorption, but did have a significant effect on the movement and final location of phosphorus in the plant, resulting in a higher percentage of phosphorus in the vegetative parts and a lower percentage

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JACKSON L. C A R T E R AND EDGAR E. HARTWIG

in the seeds. Thus, according to Webb et al. (1954), magnesium may function as a carrier of phosphorus in the plant. The ratio of phosphorus to potassium may be as important in some cases as the phosphorus level. Miller et al. (1961a, b), on a Dickinson fine sandy loam in Iowa with a pH of 6.6 and testing low in available phosphorus and very low in available potassium, showed that over 80 per cent of the variation in soybean yield which they obtained was accounted for by the variation in phosphorus and potassium content of some plant parts. The greatest yield increase was obtained from heavy applications of both phosphorus and potassium, but the greatest yield depression resulted from heavy additions of phosphorus and no addition of potassium. The variety they were using in the study (HAROSOY) is sensitive to high levels of phosphorus in relation to other elements in the nutrient solution. Howell (1954) found an increase in relative growth of soybeans as phosphorus level was increased from 2 parts per million (ppm) to 10 in the nutrient solutions, but a marked difference in varietal response to a wider range in phosphorus level. At levels as high as 112ppm, the variety CHIEF responded favorably in growth but LINCOLN showed definite symptoms of phosphorus toxicity. Howell and Bernard (1961) have classified commercial varieties with respect to phosphorus response. Weiss (1943) modified iron chlorosis by changing phosphorus level in the nutrient solution, but the iron inefficient character does not appear to be closely related to the phosphorus toxicity reported by Howell.

D. PoTAssNht Forty bushels of soybean seed will contain approximately 50 pounds of potassium. North Carolina data suggest that response to applications of potassium is likely when the available soil potassium is less than 75 pounds per acre ( C. D. Welch, personal communication). Response curves drawn by Bray (1961) suggest that when the soil test shows 50 pounds of potassium per acre the soybean yield will be approximately 50 per cent of the maximum whereas with a soil test of 200 pounds, the soybean yield will be at 97 per cent of maximum (Fig. 9). As with phosphorus, the expected response curve for soybeans closely approximates that for corn. On a Kalmia sandy loam in Alabama high in phosphorus but low in potassium, the five-year average yield increase from 50 pounds of potassium was 50 per cent (Rouse, 1961). Studies in North Carolina on a soil very low in potassium showed a fourfold yield increase from potassium. The addition of potassium caused greater retention of pods, increased the degree of pod filling, and improved seed quality. The

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397

application of 120 pounds per acre of potassium on this poor soil also increased oil content about 2 per cent and reduced protein content 5 per cent (Nelson et al., 1945). Indiana soybeans grown in rotation with corn and wheat gave equal response to row or broadcast applications of potassium. The response was proportional to the potassium applied and was closely correlated with rainfall during the growing season (Barber, 1959).

FIG.9. Relation of available potassium to percentage beans and other crops. (From Bray, 1981.)

of maximum

yield of soy-

The abilities of peanuts, soybeans, corn, and cotton to absorb potassium in small volumes of Ruston fine sandy loam were compared in the greenhouse by Reid and York (1955). Peanuts, soybeans, and corn absorbed essentially equal amounts of potassium, but cotton tended to absorb slightly more under low potassium conditions. Potassium deficiency symptoms appeared first and were most severe on corn followed by cotton, soybeans, and peanuts. All four crops responded in dry matter production to the application of potassium. In a second cropping, the dry matter production in unfertilized soil for peanuts, soybeans, cotton, and corn was 69, 85, 45, and 20 per cent, respectively, of the plants fertilized with potassium.

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JACKSON L. CARTIER AND EDGAR E. HARTWIG

Soybean seed is very sensitive to soluble-salt injury during germination, so potash should not be drilled directly with the seed. Placement studies show best results are obtained from band placement 2 inches to the side and 2 inches below the seed. When studying cation uptake, the soil temperature is important. Wallace (1957) found that the potassium content of soybeans increased with temperature to 90"F., but the potassium in barley increased from 54" F. to 72" F. and decreased from 72" to 90".

E. TRACE ELEMENTS In addition to calcium, magnesium, potassium, phosphorus, and nitrogen, several other elements are essential for satisfactory development of the soybean plant. The elements iron, manganese, cobalt, sulfur, boron, zinc, copper, and molybdenum are usually present in adequate quantities, but soils occur in which one or more of these materials might limit crop production. The visible sign of iron deficiency in the soybean is a yellowing or chlorosis of the leaves. Iron is required for chlorophyll synthesis and respiration. In most soils the iron supply is adequate and conditions are generally favorable for its absorption by soybean plants. Iron deficiency symptoms are generally observed only on soils with a high pH and a high calcium carbonate content. Weiss (1943) observed a differential response of soybean varieties growing on a calcareous soil in Iowa. Iron deficiency can be corrected by spray applications of ferrous sulfate. Manganese, like iron, aids in the formation of chlorophyll. Manganesedeficient soybeans have a light green to yellow mottling between leaf veins, Manganese deficiency symptoms are most likely to occur when soybeans are grown on soils limed to near the neutral level. According to Ohlrogge (1950),low levels of available manganese in soils are associated with a soil pH of above 6.3 on soils that have developed under a high water table. This condition encourages the reduction of the manganese to the soluble form which is consequently leached from the soil. Manganese sulfate has been found to be more effective than manganese oxide for correcting manganese deficiency in soybeans (Mederski et al., 1960). Low soil temperature combined with high soil moisture was conducive to the development of severe foliar symptoms of manganese deficiency. The relatively short time required for leaves to accumulate high concentrations of manganese probably accounts for the effectiveness of manganese sprays applied under a variety of field conditions. Manganese can be applied dry at a rate of 25 pounds or more of manganese sulfate per acre at planting time or as 10 pounds of manganese sulfate in 15

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gallons of water per acre as soon as deficiency symptoms appear. Repeated yearly applications of manganese are needed on fields where this trace nutrient is known to be deficient. Manganese applied to the soil changes to an unavailable form during the growing season (Mederski and Jones, 1961). Plant species and varieties differ in their capacity to take micronutrients from the soil. Studies at North Carolina showed that OGDEN soybeans took up only 114 ppm of manganese from a field where LEE soybeans in an adjacent row took up 197ppm. Molybdenum, unlike manganese, boron, iron, copper, and zinc, becomes increasingly available as the soil pH is raised (Reid et al., 1960). Molybdenum is needed by the plant and in the symbiotic fixation of nitrogen or in the reduction of nitrate (Anderson and Spencer, 1950; Evans, 1956). Soils in need of molybdenum are quite rare in the United States. Parker and Harris (1962), working in Georgia on a soil with a pH of 5.6, obtained yield responses from applications of 0.2 pound molybdenum per acre equivalent to applications of 2 tons of limestone. The application of molybdenum further increased protein content of the seed on a limed soil. Untreated plots yielded at the rate of 30 bushels per acre, but with the addition of molybdenum or limestone, yields of 50 bushels were produced. Caution should be observed by applying molybdenum only to soils that are deficient in this element, as plants may accumulate levels that are toxic to animals (Stout and Johnson, 1957). Cobalt is accumulated from the soil by plants, which in turn become the primary source of cobalt for animals. While cobalt is essential for animals and for synthesis of vitamin B12, plant needs are met by as little as 1part per billion in nutrient solutions. One part per million produced toxic symptoms and resulted in growth reduction (Toth and Romney, 1954). In many field trials in the Midwest, as little as 2.5 g. of cobalt applied on a bushel of soybean seed at planting time caused observable toxic symptoms on the unifoliate leaves (A. J. Ohlrogge, personal communication). If cobalt is applied to soil to enrich the resulting crop for animal feeding, extreme care should be exercised not to apply toxic smounts. Sulfur, essential to plant life, is a part of methionine and other amino acids in protein and also occurs in the vitamins thiamine and biotin. According to Baxter (1952), aside from functioning as a building material, sulfur is important in formation of chlorophyll and holds essential elements such as iron and manganese in solution. The average sulfur content of soybeans is about 0.16 per cent. Sulfur requirements of soybeans have been found to be lower than for cotton (Kamprath et al.,

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AKD EDGAR E. HARTWIG

1957). It usually enters the plant from the soil as the sulfate ion, and on most soils, especially those fertilized with superphosphate, the sulfur supply is adequate for the needs of the plant. Boron is essential to normal cell division and growth and the general metabolism of plants (Russell, 1957). In a few instances, yield increases have been obtained from boron application (S. R. Wilkinson, personal communication), but usually a soil that grows good alfalfa has adequate ‘loron for soybeans. Boron deficiency occurs at higher boron levels in the presence of high calcium levels (Reeve and Shive, 1944). Boron deficiency in soybeans is easily corrected by light applications of borax, but treatments should be made with caution as high levels of the element are toxic to the plant (Mederski and Jones, 1961). Boron toxicity decreases with increasing concentration of calcium ( Berger, 1949). Zinc is essential for soybeans, though needed in very small amounts (Seatz and Jurinak, 1957). Zinc deficiency produces a light brownishyellow color to the leaves. The symptoms are more severe in cold, wet weather, disappearing in warmer sunny weather. Where symptoms are severe, yield may be severely reduced (Weldon and Chesnin, 1959). Apparently, zinc is readily redistributed in the plant as the most severe symptoms occur on the older leaves (Viets et al., 1954). Heavy liming of a soil may lower the availability of zinc; also, heavy phosphorus fertilization may reduce zinc absorption (Bingham and Martin, 1956). Treatments of 50 pounds Es-Min-El or 5 pounds of zinc sulfate corrected zinc deficiency in soil from the Black Belt of Mississippi. In this case, a vegetative response of soybeans (LEE) to zinc fertilization was obtained, although severe chlorosis of the unfertilized plants did not occur (Nelson, 1956). Copper deficiency may cause severe stunting of growth, but moderate deficiency may merely reduce yields. The rate of photosynthesis is low in copper-deficient plants, and there is evidence that the element is involved in oxidation-reduction reactions and as an enzyme activator (Reuther, 1957). Response of soybeans to copper has been observed on peat and muck soils in the Everglades (Allison et al., 1927) and in Indiana ( S. R. Wilkinson, personal communication). Aluminum, commonly present in the soil in large amounts, may be essential to plants, though this is difficult to demonstrate, and it is not likely that it will ever be deficient (Bear, 1957). Aluminum is toxic to soybeans only in strongly acid soils, and proper liming to pH 6.0 will ordinarily correct any toxicity (Kamprath, 1958). Chloride content of the tops in a soybean variety (LEE), on plots receiving variable amounts of KCl, was found to be low and essentially constant, whereas, the chloride content of corn tissue varied with the

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amount of KC1 applied. In a greenhouse test when the nutrient solution contained 25 me. chloride per liter, the chloride concentration in the tops of three soybean varieties, JACKSON, OGDEN, and LEE, was 19.8, 16.3, and 3.9 me. per 100 g. of dry tissue, respectively. The data suggest that chloride accumulation by soybean tops is subject to genetic control ( McCollum, 1960).This low absorption of chloride may account, in part, for the excellent growth and yield of the LEE variety in the Imperial Valley of California.

F. FERTILIZER PRACTICES

AND

RECOMMENDATIONS

The mineral elements are raw materials in the development of soybean seed and an adequate amount of these mineral nutrients is necessary for the production of high yields. A soil test will serve as an indication of the supply of nutrients in the soil and should be obtained before any fertilizer is applied. In most areas, information is available as to the yield responses which can be expected with varying levels of available nutrients in the soil. Liming of soils with a low pH is a common practice. In addition to reducing acidity, dolomitic limestone supplies calcium and magnesium. In the Atlantic Coast States, where many soils are low in phosphorus and potash, 300 to 400 pounds of an 0-10-20 or 0-12-12 fertilizer are applied as a band application at planting time. Results in the Midwest suggest that supplying adequate nutrients for the rotation through fertilization of other crops is more effective than is direct fertilization of soybeans. IX. Water Requirements and Utilization

A. WATER NEEDS IN RELATIONTO

PLANT

GROWTH AND DEVELOPMENT

The capacity of soils to store water for plant use is largely a function of their porosity and depth. On a deep, permeable Brunizem soil, Peters and Johnson (1960) found that some water was extracted by soybeans from depths below 51 inches, but in covered, nonirrigated plots soybeans failed to make full use of water stored in the soil between 40-inch rows. This may partially explain the yield advantage of 20-inch rows over 40-inch rows in a dry season. The 20-inch rows also reduced the evaporation from the soil surface due to shading. Evaporation losses may amount to 50 per cent of the total water removal in a season of frequent rains. Root development in the soybean is more limited than that of corn and small grains (Ohlrogge, 1951) and is dependent to some extent on the physical properties of the soil. Under conditions of good soil tilth, the soybean may develop a tap root to a depth of 5 feet. Where the soil is shallow or an impervious layer is present in the subsoil, the tap root

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JACKSON L. CARTER AND EDGAR E. HARTWIG

does not extend so deep and branch roots are more developed. Most of the soybean root system is in the upper 2 feet of soil and in many cases the upper foot (Borst and Thatcher, 1931; Williams, 1950). B. IRRIGATION AKD SOIL MANAGEMENT The objective in irrigating is to supply sufficient water to keep the plants growing normally. This is usually accomplished by keeping the soil moisture within the root zones somewhere between the wilting point and field capacity. If too much water is applied during the period of early vegetative growth, lodging may occur, adversely affecting yields. The pod-filling period is the most critical from the standpoint of yield and frequently moisture from mid-August to mid-September is inadequate for maximum seed production (Odell, 1959). Irrigation is of advantage occasionally in the more humid areas of the North Central and Southern States. At Stoneville, Mississippi, yield increases from irrigation of sufficient magnitude to offer a profit over the cost of irrigation have been obtained in only two of the past eight seasons. In a dry season at McCredie, Missouri, on a claypan soil, Whitt (1954) obtained a yield of 31 bushels per acre following one irrigation of 4.7 inches on August 21, whereas, the nonirrigated plots yielded only 17 bushels. Irrigation increased seed size 50 per cent and raised oil content of the seed from 20.3 per cent to 22.5 per cent. At Urbana, Illinois, during the first week in August of 1959 when conditions were dry and unfavorable for growth, 1.5 inches of water was applied to soybeans, resulting in a yield of 45 bushels per acre compared to the unirrigated plot which yielded 34 bushels (D. B. Peters, personal communication). In a dry season at Stoneville, Mississippi, one irrigation during the period of fruit development gave as large yield increases as several irrigations during the season. At Conesville, Iowa, water applied during two dry seasons, 1953 and 1954, resulted in a 50 per cent yield increase for genetically early strains but only 19 per cent increase for late strains (Schwab et uZ., 1958). During the first season of this test, supplemental water was applied in June, July, and August which stimulated vegetative growth. Lack of moisture during the pod-filling period, especially on the later varieties, reduced seed size on the irrigated plots when compared to the nonirrigated. Oil content of the seed from the irrigated plots was also lower, showing the effect of the more unfavorable conditions during pod filling in the plots that had been irrigated early but had exhausted their moisture ahead of the pod-filling period. Thus, it is evident that proper timing of supplemental water additions is very important to soybean production.

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Irrigation is essential for satisfactory soybean production in westcentral Nebraska, and proper timing of water applications have been shown to be very important in securing high yields. During 1958 and 1959, on plots that were preirrigated to fill the profile to capacity, one irrigation at early bloom stage gave a 2-bushel increase over the nonirrigated check, but one irrigation at late bloom stage gave the highest yield, a 10-bushel per acre increase over the check (Somerhalder and Schleusener, 1960). In addition to west-central Nebraska, soybeans are grown on a commercial scale with supplementary irrigation on the high plains of Texas. In July and August, the average daily use of water by soybeans may run as high as 0.3 inch under some conditions (Whitt and van Bavel, 1955). The amount of water that soils can hold for the use of plants ranges from 0.5 inch per foot of depth in sand to 2.5 inches in clay. The need of the plant for water under equal fertility levels in a given locality is essentially the same regardless of the soil. Thus, soybeans grown on a sandy soil will need irrigation or rainfall more frequently than those produced on heavier soils. A good crop of soybeans requires about 20 to 30 inches of water. While most areas where soybeans are extensively grown have sufficient rainfall to supply that amount, there are times when a drought tension occurs, especially in late July, August, or early September-the critical period for maximum soybean yields. If a farmer is equipped to apply supplemental water to his cropping area, there may be times, especially during the late bloom or pod-developing stages, when an irrigation will substantially increase yield and seed quality. Irrigation, when moisture supply is low, may increase soybean yields from a subnormal level to a more nearly normal level, but irrigation has not resulted in abovenormal yields. X. Growth-Regulating Chemicals

Much interest has been indicated in growth-regulating chemicals, both from the standpoint of learning more about the behavior of the soybean plant and of controlling or modifying growth in the field. None have produced any beneficial effect on yield. Application of either 2 g. or 8 g. of potassium gibberellate per bushel of seed has reduced stands and yields of soybeans at eight locations from Winnipeg, Manitoba, to Coahoma, Mississippi. Although treated plants were taller in the seedling stage, at maturity the controls were more than 5 inches taller than treated plants at most locations. Reductions in stand were attributed to hypocotyl breakage and other injury during germination and emergence (Howell et d.,1960). One of the objectives, that

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of decreasing combine loss through increasing height of the first pods from the ground, was not achieved, owing to plant damage, severe lodging, and increased shattering. As chemicals to increase pod set during blooming, van Schaik and Probst (1959) tried six growth regulators, applied weekly during the entire blooming period, but in no case did any of the treatments increase pod set. XI. Harvesting

A. WHENTO HARVEST 1 . Moisture Content of Seed As plants reach maturity, the leaves yellow and drop and there is rapid loss of moisture from the seed. Seed can be threshed at 15 per cent moisture, but at t h s level it is too high in moisture for storage without drying. Threshing is most efficient when the moisture content of the seed is below 14 per cent. 2. Chemical DefoEiation The widespread use of defoliants and desiccants on cotton has probably contributed to the interest of their usage on soybeans both for hastening maturity of soybeans and for killing weeds. Defoliation studies conducted at Stoneville, hlississippi, showed that soybeans hand defoliated 3 weeks prior to normal maturity were reduced 30 per cent in seed yield, but were ready for combining only 3 days earlier than those maturing normally. These studies showed that any defoliation prior to yellowing of the leaves reduced yield. Studies at Urbana, Illinois (unpublished data ) , showed that any treatment applied sufficiently early to appreciably hasten the drying of the seed to the point where the crop could be combined ahead of normal maturity resulted in serious reductions in seed yield. When treatment was delayed until 50 per cent of the leaves had dropped, no serious yield reductions occurred but maturity was not hastened. Three-year studies in Arkansas (P. E. Smith, 1956) showed an advantage in using earlier-maturing varieties over attempting to hasten maturity of later-maturing varieties. Under conditions where weed-infested fields of soybeans matured before frost, desiccants such as pentachlorophenol mixed in diesel fuel have been used to kill weeds. Carlyle (1951) reports satisfactory results in killing weeds in soybean plantings in Illinois with rates of 2, 4, and 6 gallons of pentachlorophenol in oil. In the delta area of Mississippi, these materials have been effective in killing morning glories, but although effective in killing the leaves of large pigweeds and cockleburs,

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they have not hastened the drying of the large stems. Pentachlorophenol does not presently have label clearance for use on soybeans.

3. Losses from Respiration after Maturity Respiration of ripening soybean seed is closely correlated with moisture content of the seed. Temperature also affects the respiration rate but is of less importance than moisture. When soybean seed had 55 per cent moisture, the respiration at three temperatures, 70°, 84", and 90"F., consumed enough hexose to cause weight losses of 0.03, 0.04, and 0.05 per cent per hour ( Howell et al., 1959). Thus, when periods of wet, humid weather occur prior to harvest, weight losses can be considerable. There was little evidence of leaching of sugars from seeds in pods.

B. HARVESTING METHODS 1. Historical The earliest harvester designed specifically for soybeans was a twowheeled, horse-drawn machine which straddled the bean row (Piper and Morse, 1923). This special harvester was common in Virginia and North Carolina, but was never commonly used in the North Central States. Harvesting losses ranged from 20 per cent under favorable conditions to as high as 60 per cent under unfavorable conditions (Sjogren, 1939). In small-grain growing areas, the binder and thresher were adapted for soybean harvest. Harvest losses from using the binder or mower for cutting and then threshing ranged from 16 to 35 per cent of the total yield, with an average loss of 24 per cent ( Sjogren, 1939). The combine harvester was first used for soybeans in the mid-twenties. The combine harvester has been a major factor in the expansion of soybean production. This machine required less labor than earlier methods and was more efficient. Sjogren reports average harvest losses of 12.36 per cent in Virginia, 8.34 per cent in Indiana, and 8.99 per cent in Illinois. Observations of 62 combines operating in the coastal plain of South Carolina gave an average total harvest loss of 9.7 per cent. Harvest loss was associated with total yield. When only fields producing over 20 bushels per acre were considered, the average loss was 5.9 per cent. Several fields averaging 40 to 50 bushels per acre had a harvest loss of less than 4 per cent (Park and Webb, 1959). 2. Combine Haruesting a. Losses from machine adjustments. Harvest losses are principally either at the cutter bar or from failure to thresh or clean the seed properly. Losses at the cutter bar can result if ( 1 ) plants are cut too

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high and pods left on stalk; ( 2 ) plants are cut too high and beans shattered from cutting through pods; or ( 3 ) beans are beaten out by improper reel adjustment. Cutter bar losses may run as high as 20 per cent of the total yield in Iowa (Barger and Weber, 1949). Ridges caused by cultivation are a serious problem in cutter bar adjustment (Heitshu, 1956). South Carolina studies showed that an average of 2.2 per cent of the seed were left undisturbed below the cutter bar and 3.0 per cent were shattered to the ground by action of the cutter bar and reel (Park and Webb, 1959). Under extremely dry conditions, the reel is removed to avoid losses from its beating action on the plants. Cylinder losses are of two types-unthreshed pods or split beans. Unthreshed pods are usually the result of attempting to harvest too early in the morning after a heavy dew, too soon after a rain, or during damp weather. This loss is reported to be usually less than 1 per cent in Iowa (Barger and Weber, 1949), but was found to be as high as 16.5 per cent in South Carolina (Park and Webb, 1959). Split or cracked beans are the result of ( 1) too high a cylinder speed; ( 2 ) i n s a c i e n t clearance between cylinder and concave bars; and ( 3 ) too many bars in cylinder or concave. Excessive cylinder speed is damaging to seed viability (Moore, 1957). Separating losses result from beans being carried over the straw rack. This results from improper adjustment of the straw rack or from overloading the straw rack. Losses are frequently higher when the rack is overloaded with grass and weeds, especially so when these weeds are green. b. Losses from lodging. Soybean plants erect or nearly erect are considered best suited for efficient harvesting. Studies conducted in lowa comparing a variety which was nearly erect with a variety leaning considerably showed an increased harvest loss from lodging of 0.9 per cent in a 3-year average. There was a 2.3 per cent loss for each inch of cutter bar height above 3.5 inches up to 6.5 inches (C. R. Weber, personal communication). In a field survey, Park and Webb (1959) attributed a 1.1 per cent loss to lodging. They suggest that pick-up guards and tined reels were of considerable value in lodged fields. XU. Seed Storage

Soybeans are harvested over a short period of time and much of the crop is marketed directly from the field. Soybeans held on the farm should be stored in clean, dry bins, and at a moisture content not to exceed 13 per cent. At this moisture level, soybeans will keep for a year or more without deterioration and with substantially no insect damage.

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If the moisture content is around 14 to 15 per cent, soybeans will keep through the winter with little loss in quality, but serious deterioration begins when the weather warms up. If the moisture content is below 12 per cent, germination will stay good into the second year (Holman and Carter, 1952). During cold weather, because of air circulation moisture tends to accumulate in the surface layer of the beans near the center of the bin. This condition should be watched and the beans in the surface layer stirred or the bin ventilated to minimize losses from this cause. Forced ventilation also tends to equalize temperatures in the bin, reducing moisture movement. Adequate ventilation can be provided with relatively small volumes of air (Holman, 1960). Forced-air drying of soybeans allows one to combine earlier than could otherwise be done. Drying soybeans with heated air has the advantage that it can be done at any time, regardless of weather conditions. Maximum air temperature for drying soybeans should not exceed 110°F. for seed and 130" to 140°F. for market beans (Holman, 1951), and should be held lower than this during the initial stages of drying if moisture content is high (Brandenburg et al., 1961). XIII. Discussion

The rapid expansion in soybean acreage in the United States has pointed out new problems for which research has not fully provided answers. To produce maximum yields, soybeans must have an adequate supply of water and nutrients throughout the season and must be free from injury by diseases, insects, and nematodes. The possibility of raising the nitrogen supply by the development of a specific rhizobium strain to inoculate each soybean variety should be further explored. Studies of nitrogen metabolism, especially during seed development, and the effect of soil structure and humus content on root growth and nodulation may tell us how to provide an additional yield increase. Further studies appear to be needed on the balance between nutrients such as phosphorus and potassium and many other elements, especially at high yield levels where all factors of the environment must be at their optimum. Design of better planting equipment, including improved press wheels would insure more even soybean stands with a lower seeding rate. More specific weed control chemicals would reduce cost of production and loss from weed competition. As chemicals for weed control become more widely used on all crops, greater attention must be given to the effect of residual chemicals on the crop following in the rotation.

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Improved combines and better harvesting would materially reduce harvesting loss and raise the quality of the crop that is marketed. REFERENCES

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JACKSOX L. CAR’ITER A\?)

EDGAR E. HARTWIG

Stout, P. R., and Johnson, C. M. 1957. I n “Soil the Yearbook of Agriculture” (A. Steffenid, ed.), pp. 139-150.U. S. Govt. Printing Office, Washington, D. C. StrickIing, E. 1950 Soil Sci. Soc. Am. Proc. 16, 30-34. Swanson, A. F. 1951. Agion. J . 43, 397-400. Torrie, J. H., and Briggs, G. M. 1955. Agron. J. 47, 210-21-2. Toth, S. J., and Romney, E. M. 1954. Soil Sci. 78, 295-303. Truog, E. 1938. In “Soils and Men the Yearbook of Agriculture” ( G . Hambidge, ed.), pp. 563-580.U. S. Govt. Printing Office, Washington, D. C. van Schaik, P. H., and Probst, A. H. 1959. Agron. J . 61, 510-511. Viets, F. G., Jr., Boa\vn, L. C., and Crawford, C. I. 1954. Soil Sci. 78, 305-316, Viljoen, h . J. 1937. C‘nioti S. Africa Dept. Agr. Forestry Sci. Bull. 169. Wallace, A. 1957. Soil Sci. 83, 407-411. Washburn, W. F. 1916. X . Dakota Agr. Expt. Sta. Bull 118. Webb, J. R., Ohlrogge, A. J., and Barber, S. A. 1954. Soil Sci. SOC.Am. Proc. 18, 458-462. Weiss, M. G . 1943. Genetics 28,253-268. Weiss, M. C . 1949. Adcances in Agron. 1, 77-157. Weiss, M. G.,Weber, C. R., Williams, L. F., and Probst, A. H. 1950. U . S. Dept. Agr. Tech. Bull. 1017. Welch, C.D.,and Nelson, W. L. 1950. Agron. J. 42, 9-13. Weldon, M. D., and Chesnin, L. 1959. Soybean Dig. 19(8), 25. Whitt, D. M. 1954. Soybean Dig. l4(9), 10-11. Whitt, D. M., and van Bavel, C. H. M. 1953. In “Water the Yearbook of Agriculture” ( A . Stefferud, ed.), pp. 376-381. U. S. Govt. Printing Office, Washington, D. C. Willard, C.J. 1952. Soybean Dig. U ( 6 ) , 12-13. Williams, L. F. 1950. In “Soybeans and Soybean Products” ( K . S. Markley, ed.), VoI. I, pp. 111-134.Interscience, New York. Williams, L. F., and Lynch, D. L. 1954. Agron. 1. 46, 28-29.

AUTHOR INDEX Numbers in parentheses are reference numbers and indicate that an author’s work is referred to although his name is not cited in the text. Numbers in italic show the page on which the complete reference is listed.

A Abbott, A. T., 36, 56 Abel, G. H., Jr., 373, 375, 376, 377, 408 Adair, C. R., 74, 78, 83, 89, 95, 97, 99, 104,106, 107 Adams, E. L., 79,104 Adams, J. E., 126, 130,144 Alderfer, R. B., 346, 350, 358 Aldrich, D. G., Jr., 255, 261 Alessi, J., 242, 251, 254, 260, 261 Alexander, L. T., 6, 7, 8, 9, 10, 13, 14, 15, 16, 20, 21, 29, 31, 35, 36, 38, 39, 40, 55, 56, 57 Alexander, M. W., 366, 373, 375, 376, 377, 411 Allard, H. A., 366, 369, 372, 374, 409 Allen, D. I., 198, 218 Allison, F. E., 237, 261, 322, 356 Allison, R. V., 10, 57, 400, 408 Allston, R. F. W., 67, 79, 104 Amemiya, M., 324, 355 Anderson, A. J., 399, 408 Anderson, A. M., 389, 408 Anderson, J. C., 169, 220 Anderson, L. J., 115, 144 Anderson, M. S., 4, 57 Andreae, H., 339,355 Andreev, B. V., 322,355 Andres, J. M., 212, 218 Andrew, G., 53, 57 Andrews, J. B., 115, 120, 146 Anthony, J. L., 311, 319 Armiger, W. H., 308, 317 Aronoff, S., 341, 355, 356 Artsruui, A. B., 125, 147 Athow, K., 158, 170, 218 Athow, K. L., 154, 170,218,220 Atkins, J. G., 88, 89, 91, 104,107 Atlas, D., 115, 144 413

Aubert, G., 15,57 Aubertin, G. M., 231, 261 Augustine, M. T., 122,145 Ayers, A. D., 74,107

B Babb, A. F., 73, 107 Babington, B., 2, 26, 57 Bahrani, B., 231, 261, 330, 355 Bainer, R., 68, 83, 104 Baird, J., 37Q, 408 Baker, J. B., 87, 104 Baldwin, M., 4, 57 Ballard, L. A. T., 235, 261 Bancroft, J. B., 154, 218 Barber, S. A., 396,397,408,412 Barger, E. L., 68,104,406,408 Barker, F. B., 339, 355 Barnes, 0. K., 130,144 Bamett, A. P., 130, 144 Bartelli, L. J,, 134, 138, 147 Bartholomew, W. V., 350,351,356 Bartley, B. G., 181, 182,218 Batcher, 0. M., 102, 104 Bateman, H. P., 131, 147, 379, 408 Bates, H. G., 101, 108 Bauer, M., 3, 57 Baver, L. D., 46, 57, 110, 114, 129, 144, 146 Bawden, F. C., 305, 317 Baxter, A., 399, 408 Bay, C. E., 130,144 Bazan, F., 339,356 Beachell, H. M., 62, 82, 83, €25, 89, 95, 97,99,104,105,106,107 Beacher, R. L., 81,104 Beale, 0. W., 125, 146 Bear, F. E., 400, 408 Beasley, R. P., 136, 144, 145, 146

414

AUTHOR INDEX

Beater, B. E., 9, 57 Beetem, W.A., 340,357 Begemann, F., 339, 355 Belcher, D. J., 322, 355 Bell, F. G., 121, 122, 127, 129, 134, 135, 144,145,147 Benedict, H. M., 231, 261 Bening, W., 152, 218 Bennett, A. C., 136,147 Bennett, C. H., 68,83,107 Bennett, H. H., 10, 57, 113, 121, 144 Bennett, 0. L., 245,263,306,317 Bentley, W., 114,144 Benza, P. M., 26, 57 Berger, K. C., 400, 408 Berman, H., 10,59 Bernard, R. L., 160, 170, 198, 218, 219, 361, 380, 396, 409,410, 411 Bemhard, R. K., 343, 355 Bemstein, L., 75, 105 Berry, L., 22, 57 Bertoni, J., 129, 144 Bertrand, A. R., 113,126,144,146 Best, A. C., 114,144 Beotner, E. L., 112, 129, 144 Biddulph, O., 341, 355 Biggar, J., 331, 332, 340, 353, 355, 355 Biggar, J. W., 331, 357 Bingham, F. T., 287,317,400,408 Bisal, F., 111, 113, 121,144 Black, C. A., 256, 257, 261, 288, 289, 290,291, 293, 305,317,318,319, 336, 337, 355, 356 Black, J. N., 229, 261 Blackie, W., 15, 57 Blackman, G. E., 229,261 Blakeley, B. D., 138, 144 Blanchard, D. C., 115, 116, 144 Blaney, H. F., 224, 261 Blanford, W. T., 2, 18, 26, 57 Bloomfield, C., 43, 46, 57 Boawn, L. C., 257,263,400,412 Bohmont, D. W., 130,144 Bohning, R. H., 369,408 Bond, F., 67, 104 Bonifas, hl., 10, 57 Bonnault, D., 19, 24, 54, 57 Booher, L. J., 74,105 Borst, H. L., 112, 119, 121, 127, 129, 1%, 144,402,408

Borthwick, H. A., 153, 218, 219, 220, 366, 369, 371, 372, 410, 411 Bouldin, D. R., 276, 279, 280, 287, 291, 292, 312, 315, 317, 319, 337, 355 Bourbeau, G. A., 37,42,58 Bowen, H. D., 381,408 Bowers, W., 379,408 Bowling, C. C., 89, 91,104 Bradford, B. N., 312,318 Brandenburg, N. R., 407, 408 Bray, R. H., 394,395,396, 397,408 Briggs, G. M., 374,412 Briggs, L. J., 225, 232, 235, 261 Briggs, R. A., 384,411 Briggs, R. P., 29, 57 Brim, C. A., 160, 170, 176, 177, 192, 194, 195, 201,202,206,218,219, 381,403, 408, 410 Brown, B. A,, 394,408 Brown, C. A., 87,107 Brown, D. A., 243,261 Brown, D. M., 367,408 Browning, G. M., 126, 137, 144, 385, 386, 408 Bryan, B. B., 243,261 Bryan, 0. A., 400,408 Bryan, W. H., 5, 13,57 Bryant, G. T., 339, 355 Buchanan, F., 1,2, 57 Bucher, W. H., 36, 57 Buist, G., 26, 57 Bureau, M. F., 395, 408 Burkhart, L., 394, 397, 410 Burlison, W. L., 377, 408 Bum, K. N., 323,355 Bumside, C. A., 369, 408 Burrows, W. C., 326,355 Burton, G. W., 247,258, 261 Byers, H. G., 4, 40, 57, 58, 125, 146

C Cady, J. G., 6, 7, 8, 9, 10, 13, 14, 15, 16, 20, 21, 29, 31, 35, 36, 38, 39, 40, 55, 56, 57 Cahoon, G. A., 325, 327, 329, 357, 358 Caldwell, A. C., 384, 411 Caldwell, B. E., 160, 170, 218 Calland, J. W., 384,408 Campbell, J. M., 10, 14, 17, 18, 20, 21, 28,46,47,49,57

415

AUTHOR INDEX

Carey, W. N., Jr., 345, 355 Carlson, C. W., 240, 241, 242, 247, 251, 254, 260, 261, 262 Carlton, P. F., 345, 355 Carlton, R. C., 88,91, 104 Carlyle, R. E., 404, 408 Carpenter, P. N., 313, 319 Carr, R. B., 202, 219 Carreker, J. R., 130, 144 Carter, D. G., 407,409 Carter, H. J., 27, 57 Carter, L. C., 97,106 Cartter, J. L., 157, 158, 159, 1.60, 161, 162, 165, 169, 171, 197, 219, 220,368, 370, 373, 375,376, 377,389,408, 409, 410, 411 Casas, E., 154, 218 Cassady, C. F., Jr., 235, 237, 261 Caviness, C. E., 375, 376, 377, 381, 336, 408 Chamberlain, D. W., 390,408,410 Chamberlain, T. K., 349,356 Chambliss, C. E., 86,79, 80,97, 104 Chapman, H. D., 255,261 Chapman, L. J., 367,408 Chasek, M., 343, 355 Chatterjee, D., 92, 94, 104 Chesnin, L., 400, 412 Churaev, N. V., 343,358 Clark, A. G., 302,318 Clark, F., 73, 104 Clark, F. E., 198, 218, 392, 408 Clark, J., 26, 57 Clark, L. J., 287, 317 Cline, M. G., 40,57 CIore, W. J., 257, 261 Cochran, W. G., 304, 311,317 Cockerham, C. C., 176, 177, 192, 194, 195, 206,218 Colbry, V. L., 389,409 Cole, L. J., 165, 221 Cole, R., 28, 57 Collins, E. R., 393,409 Collins, F. I., 191, 196, 218, 219, 377, 389, 405, 410 Collis-George, N., 305, 311,317 Colwell, W. E., 394, 397, 410 Comar, C. L., 336,356 Comstock, R. E., 181, lI82, 183, 184, 187, 188, 190, 192,219

Converse, C. D., 244, 263 Cook, H. L., 113, 144 Cook, R. L., 311,317, 379,409 Cooke, G. W., 279,291,308,317 Cooper, C. W., 248,253,263 Copeland, E. B., 62,104 Cory, R., 341,355 Cospter, H. R., 255, 262 Costin, A. B., 14, 58 Covey, W., 230, 262 Cowan, J. C., 364,409 Cox, G. M., 304, 311,317 Coyle, J. J., 135, 136, 138, 144, 147 Crafts, A. S., 87, 106 Cralley, E. M., 89,104, 107 Crawford, C. I., 400, 412 Crawford, C. L.,257,263 Criddle, W. D., 224,261 Crook, T., 3, 57 Crowther, E. M., 308,317 Cunningham, C. E., 313, 319 Cuykendall, T. R., 322,355

D Dachtler, W. C., 68,83,105 Danielson, R. E., 347, 355 Dansercoer, W., 325, 357 Darby, R. E., 92,105 Datta, N. P., 337,356 Davis, D. E., 336,356 Davis, L. L., 71, 77, 79, 82, 85, 89, 91, 95, 97, 104, 105,106,107 Davis, W. C., 70, 71, 77, 80, 106 Davis, W. M., 53, 57 Davol, F. D., 20,23, 24,25, 55, 58 Davy, B. C.,305, 311,317 Dawson, E. H., 101,102,104,106 Dean, L. A., 308,317 De Boodt, M., 325,357 Deffes, J. J., 68, 83, 107 De Leenheer, L., 325, 357 Delouche, J. C., 390, 409 DeMent, J. D., 279, 292, 311, 312, 314, 315, 317,318,319 Denmead, 0. T., 225, 261, 329,356 DeTurk, E. E., 313,318 Dever, R. F., 10, 13, 14, 29, 38, 39, 55, 57 De Wit, C. T., 232,233,235,261 D'Hoore, J., 15, 17, 19, 20, 21, 27, 29,

416

AUTHOR Ih?)EX

31, 3 4 39, 42, 44, 45, 50, 53, 54, 35, 57 Dies, E. J., 361, 409 Dillman, A. C., 232, 261 Diseker, E. G., 127, 144 Doar, D., 66, 67, 105 Dockings, J. O., 70, 71, 77, 80, 106 Doll, J. P., 281, 290, 304, 318 Domingo, W.E., 157, 159, 160, 168, 218 Doorenbos, J., 369, 409 Dopson, R. N.,88,91,104 Dortignac, E. J., 113, 129,144 Douglas, W.A,, 89, 90, 105 Downs, R. J., 371, 409 Doyne, H. C., 4, 59 Dreibelbis, F. R., 260, 261 Dreier, A. F., 251,254,263,276, 319 du Bois, G. C., 3, 58 Duley, F. L., 112, 121, 127, 131, 145 Duncan, IV. G., 276,318 Dtinham, R. S., 387, 41 1 Dunleavy, J., 197, 198, 211, 218 Dunshee, C. F., 79, 105 du Precz, J. \I7.,4, 5, 19, 47, 53, 58

E Eakins, J. D., 348, 349, 357 Earley, E. B., 198, 218 Efferson, J. S., 6-7, 105 Ehrler, \I7., 7.5, 105 Eid, hf. T., 291, 318 Ekern, P. C., 111, 116, 119, 120, 121, 122, 145 Eley, G. W., 126, 138, 145 Ellison, If’. D., 111, 112, 119, 121, 145 Engelstad, 0. P., 314, 319 Engler, K., 74, 104 Erdman, L. \V., 198, 219, 391, 409, 410 Evans, C. E., 395, 408 Evans, H. J.. 399, 409,411 Evans, J. W., 3, 58 Evatt, N. S., 82, 105 Everson, L. E., 390, 409

F Fairbanks, G. E., 379, 409 Falconer, J. D., 6, 58 Faucher, J. A,, 347, 356 Faulkner, M. D., 72, 105 Faust, G . T., 29, 31, 36, 57

Feaster, C. V., 162, 170, 218, 376, 409 Fermor, L. L., 2, 3, 8, 19, 20, 58 Fernando, L. J. D., 20, 58 Feuer, H., 50, 58 Fielding, hl. J., 89, 104 Finfrock, D. C.. 71, 74, 76, 77, 79, 80, 82, 83, 92, 105, 106, 107, 108 Fitts, J. W., 399: 410 Foote, R. B., 26, 27, 58 Fosberg, hj. A., 334, 356 Fox, C. S., 2, 6, 7, 8,41, 58 Fox, D. R., 73, 107 Fox, R. L., 217,253, 255, 261, 262 Foy, C. D., 393,411 Frankart, R., 43, 58 Franzke, C. J., 213, 218, 220 Fraps, G. S., 75, 105 Frazier, A. W., 275, 277, 318 Fred, E. B., 312, 313, 318 Free, G. R., 119, 121, 131,145 Fribourg, H. A., 195, 218 Fried, hl., 308, 313, 317, 318, 322, 356 Friedman, I., 340, 356 Fripiat, J. J., 38, 42, 43, 58 Fritschen, L. J., 328, 356 Frondel, C., 10, 59 Fukuda, Y., 207,218 Fukui, J., 372, 409 Fukuyama, J., 159, 161, 16.3, 164, 167, 220 Fulgham, F. E., 379, 409 Fullilove, H. h4., 381, 409

G Gaastra, P., 229, 261 Gaebe, R. R., 112, 129,144 Gage, R. S., 341, 356 Gard, L. E., 127, 147 Gardner, C. O., 210, 211, 220 Gardner, H. H., 134, 147 322, 325, 356 Gardner, W., Garner, W. W., 366, 369, 372, 374, 409 Gastuche, M. C., 38, 42, 43, 58 Gates, C. E., 173. 176, 178, 218 Geeseman, G. E., 159, 170, 218 Geyer, J. C., 339, 355 Chose, R. L. hl., 94, 107 Gieseking, J. E., 348, 357 Gif€ord, R. O., 255, 262 Giletti, B. J., 339, 356

AUTHOR INDEX

Glass, J. A., 126, 137, 144 Glasser, A. H., 230, 231, 262 Glinka, K. D., 15, 28, 45, 58 Gnaedinger, J. P., 323, 356 Gobs-Sonnenschein, C., 212, 213, 218 Goldin, A. S., 340, 358 Gordon, M., Jr., 36, 58 Gottman, J., 155,58 Govindaswami, S., 94, 107 Grabe, D. F., 166, 167, 197,218 Graham, E. R., 353,356 Gray, C., 306, 317 Gray, J., 375, 377, 409 Gray, L. C., 62, 63,67, 69, 105 Green, B. L., 70, 105 Green, D. E., 367,409 Green, V. E., Jr., 69, 105 Greene, H., 18, 22, 48, 50, 53, 58 Grigarick, A. A., 91, 92, 10.5, 106 Griggs, P., 343, 355 Grissom, P., 378, 379, 409 Gross, C. F., 316, 319 Grunes, D. L., 249, 252, 263, 276, 316, 318 Guaid, A. T., 152, 218 Gunn, R., 116, 117,145

H Haas, H. J., 246, 253, 262, 263 Hackleman, J. C., 377, 408 Haddock, J. L., 246,262 Hadley, H. H., 380,411 Hagan, R. M., 76, 107,234,262 Hagee, G. R., 340, 358 Hagin, J., 286, 291, 318 Haise, H. R., 226, 227, 228, 231, 244, 246,248,262,263 Halick, J. V., 101, 105 Hall, V. L., 75, 105 Hallsworth, E. G., 14, 58 Halstead, M. H., 230, 262 Hamada, H., 152, 218 Hamner, K. C., 369, 409 Hanks, R. J., 245, 262 Hanlon, F. N., 10, 39, 45, 58 Hansen, C. M., 379,409 Hanson, W. D., 153, 154, 173, 174, 176, 177, 178, 179, 181, 182, 183, 184, 187, 189, 194, 195, 197,202, 203, 205, 206, 207, 218, 219, 221

417

Hanway, D. G., 210, 211, 220, 221, 251, 254, 263 Hanway, J. J., 396,410 Hardy, F., 10, 30, 58 Hardy, G. W., 393,409 Hargrove, B. D., 111, 122,145 Harrassowitz, H., 25, 29, 46, 58 Harris, H. B., 399, 411 Harris, H. C., 336, 356 Harris, J., 340, 356 Harris, W. W., 379, 409 Harrison, J. B., 2, 9, 10, 18, 20, 29, 30, 34, 35, 36, 58 Harrold, L. L., 111, 145, 260, 261 Hartwig, E. E., 161, 162, 163, 170, 191, 202, 219, 336,373, 375, 376,377, 378, 380, 381, 387, 389, 393, 403, 409, 410 Harvey, W. A., 85,105 Hausenbuiller, R. L., 311, 318 Hauser, V. I., 136, 145, 148 Hawkins, A., 313, 319 Hay, R. C., 131, 147 Haynes, J. L., 378, 409 Hays, 0. E., 112, 127, 130, 132, 135, 144, 145, 147 Heitshu, D. C., 406, 409 Helmintoller, K. F., 102, 104 Henderson, M. T., 94, 108 Henderson, R. C., 126, 130, 136, 144, 147 Hendler, R. W., 352, 356 Hendricks, S. B., 29, 31, 36, 57 Hendrickson, A. H., 234, 263, 329, 358 Hendrickson, B. H., 112, 113, 121, 145 Herring, R. B., 379, 409 Heslep, J. M., 336, 356 Hess, H. H., 353, 356 Hildebrand, A. A., 198, 219 Hilder, G. B., 101, 106 Hill, H. O., 135, 145 Hill, W. L., 267, 274, 276, 287, 317, 318, 319 Hinson, K., 161, 163, 183, 202, 203, 219 Hobbs, J. A., 379, 409 Hoff, D. J., 398, 410 Holland, T. H., 3, 18, 24, 27, 28, 58 Holman, L. E., 407, 409 Holmen, H., 240, 241, 247, 260, 262 Holmes, A., 17, 58 Holmes, G. K., 63, 105

418

AUTHOR INmEX

Holstun, J. T., Jr., 388, 410 Holt, R. F., 394, 408 Holtan, H. N., 112, 146 HoItz, H. F., 235, 263 Hope, C. E., 382,411 Hopper, T. H., 376, 389,408 Horiuchi, S., 202, 221 Homer, G. M., 129, 145 Homer, T. W., 173, 176, 178, 201, 218, 219, 221 Horton, J. H., 338, 356 Horton, R. E., 112, 129, 144 Hough, G. J., 40,58 Howell, R. W., 187, 189, 196, 197, 198, 218, 219, 361, 367, 368, 369, 372, 377, 389, 390, 396, 403, 405, 409,410 Huberty, M. R., 255, 262 '. W., 111, 115, 117, 118, 137, Hudson, h 145 Humbert, R. P., 10, 14, 15, 17, 21, 26, 36, 39, 58, 345, 358 Humphrey, L. hf., 209, 210, 219, 389, 410 Hunt, C. M., 312, 318 Hunter, A. S., 281,318 Hunter, J. H., 400, 408 Hurst, ?Ir. kf., 68, 83, 107 H! der, D. N.,247, 253, 263

I Ingebretsen, K., 80, 105 Ingebretsen, K. H., 91, 106 Ingerson, E., 352, 356 Ingram, J. W., 89, 90, 105 Inman, D. L., 349, 356 Isely, D., 91, 105 Ito, R., 372, 409 Izzard, C. F., 122, 145

J Jackson, J. E., 247, 261 Jackson, M. L., 37, 42, 58 Jacob, K. D., 266, 267, 274, 318 Jakobsen, S. T., 291, 319 Jarnibon, V. C., 130, 147 Jenkins, J. M., 78, 80, 89, 97, 104, 105, 106,107 Jenny, H., 308, 318 Jensen, C. R., 344, 345, 347, 3.56, 357 Jensen, L. R., 246, 262

Jensen, M. E., 238, 239, 240, 241, 247, 260,262 Jessop, R. W., 25, 26,58 Jeter, R., 80, 105 Joachim, A. W. R., 4, 9, 10, 15, 17, 20, 22, 58 Jodon, N. E., 78, 95, 96, 99, 104, 105, 106

Johnson, C. M., 284, 287, 318, 399, 412 Johnson, H. W., 153, 157, 181, 182, 183, 184, 187, 188, 190, 192, 194, 195, 198, 202, 204, 206, 218, 219, 361, 371, 372, 381, 390, 391, 408, 410 Johnson, I. J., 195, 218 Johnson, L. C., 401,411 Johnson, W. H., 409 Johnston, J. R., 385, 408 Johnston, T. H., 99, 105 Johnston, W. B., 337,356 Jones, B., 16, 58 Jones, D. M. A., 115, 116,145 Jones, G. H. G., 312,318 Jones, J. B., 399, 400, 41 0 Jones, J. W., 70, 71, 77, 79, SO, 82, 85, 89, 95,97, 99, 104, 105, 106, 107 Jordan J. V.,334,356 Jordon, G. S., 80,106 Jurinak, J. J., 400, 411

K Kadam, B. S., 95,106 Kalton, R. R., 174. 204, 205, 219, 221 Kamel, M. S., 229, 262 Kamen, M. O., 355,356 Kamprath, E. J., 399,400, 410,411 Kandiah, S., 4,9, 10, 15, 17, 20, 22, 58 Kapp, L. C., 81, 106 Karasawa, K., 158, 167, 208, 219 Kato, I., 153, 157, 219 Katz, J. J., 346, 356 Kaufman, P. B., 87, 106 Kaufman, W. J., 340,356 Kaufmann, M. J., 160, 170,218 Keeney, G. H., 67,104 Kelaart, E. F., 26, 58 Keller, W., 244, 262 Kelley, 0. J., 226, 234, 244, 246, 262, 263 Kellogg, C. E., 4, 20, 23, 24, 25, 55, 57, 58

AUTHOR INDEX

Kelly, L. L., 121, 145 Kelly, V. J., 101, 105 Kempthorne, O., 288, 290, 291,293, 305, 317,318,319 Keneaster, K. K., 101, 105 Keogh, J. L., 393,411 Kepner, R. A., 68,104 Kester, E. B., 83, 106 Ketcheson, J. W., 316,318 Key, J. L., 394,410 Kidder, E. H., 134,147 Kiesselbach, T. A,, 235, 262 Kihara, H., 94, 106 Kik, M. C., 62,84,106 King, A. V., 394,408 King, B. M., 97,106 King, W., 26, 58 Kinzer, G., 115, 117, 145 Kirkham, D., 126, 144, 322, 324, 328, 327, 328, 331, 333, 344, 345, 351,352, 354, 355, 356,357,358 Klemme, A. W., 395,410 Klute, A., 336, 348,356,357 Kmoch, H. G., 247,253,262 Knake, E. L., 388,410 Knapp, S. A., 66, 67, 79,106 Koch, E. J., 308,317 Koehler, B., 390, 408 Koehler, F. E., 247, 249, 252, 253, 262 Kolaian, J. H., 318 Kozachyn, J., 119,148 Krall, J. L., 130, 145 Kramer, H. H., 101, 105, 179, 181, 219 Krimgold, D. B., 111, 145 Kriz, W., 255,262 Krober, 0. A., 196, 197, 219 Xromer, G. W., 364,410 Kuehl, R. O., 156, 157,219 Kuiken, K. A., 197, 219 Kulp, J. L., 339, 356 Kunze, R. J., 331,333,341,356 Kuranz, J. L., 323, 325, 343, 345, Kuron, H., 120,145 Kurtz, L. T., 394,410

L Lacroix, A., 35, 59 Laflen, J. M., 136,145

419

Lake, P., 19, 26, 27, 59 Lambert, J. W., 187, 202, 219, 380, 403, 410 302, Lang, A., 369, 410 Lange, W. H., 91,92,105, 106 Langston, R., 348, 357 Larson, W. E., 129, 144 Latham, E. E., 125,146 Latimer, W. M., 52, 59 LauEer, C. H., 88,91, 104 Lawless, G. P., 328, 357 Laws, J. O., 114, 115, 115, 117, 119, 121, 145 Lawton, K., 275,318 Learner, R. W., 257,263 Le Clerg, E. L., 311, 318 Leffel, R. C.,153, 175, 176, 177, 187, 189, 197, 199, 205, 206, 219, 371, 372, 325, 373, 374, 375, 376, 377,410 350, Leggett, G. E., 249,262 Lehman, S. G., 152, 170,219,390,410 Lehman, W. F., 187, 202, 219, 380, 410 Lehr, J. R., 312,319 Lemon, E. R., 229,230,231,262 Leneuf, N., 29, 59 Leonard, W. H., 302,318 Leopold, A. C., 369,410 Letey, !., 336, 356 w4, Levine, G., 121,145 Lewis, D. C., 73,107 Lewis, G. C., 334, 356 Libby, W. F., 339,355 Lincoln, C., 84,107 182, Lindberg, G. D., 88, 91,104 Lindsay, W. L., 275, 276, 277, 287, 318 Little, J. M., 122, 145 Little, R. R., 101,106 Liu, H.-L., 169, 219 Liveman, J. L., 359, 410 Lloyd, C. H., 126, 138,145 Lloyd, L. C., 129,145 Longnecker, T. C., 306,317 Loo, w. s., 212, 220 356 Lorenz, 0. A., 284,287,318 Lorenz, R. J., 240, 241, 247, 253, 260, 262, 263 Love, H. H., 304, 318 Love, J. R., 225, 263 Love, L. D., 129, 144 Lovely, W. G., 383, 387, 410

420

AUTHOR ISDEX

Lowdermilk, it'. C., 112, 113, 121, 145 Luclzack, F. J., 340, 358 Luetkemeier, 0. \\'., 387, 411 h t z , J. A., Jr., 311, 319 Lutz, 3. F., 111, 122, 14.5 Lyman, C. M., 197, 219 LJnch, D. L., 159, 170, 221, 391, 392, 410,412

M %la,R. H., 186,219 hlcdister, J. T., 381, 410 hlcCal1, A. C., 122, 127, 129, 134, 135, 144,145, 147

blcCollum, R. E., 198, 219, 401, 410 XlcCoIIy, H. F., 379, 409 McCool, D. K., 136, 146 hlcCune, D. L., 113, 146 AlcDonaid, J. E., 115, 146 XtcGee, \V. J., 26, 59 MeGeorge, u'. T., 311, 318 MacIntire, 11'. H., 308, 318, 336, 356 Mclntyre, D. S., 111, 121, 146 hlackie, W.W., 75, 106 htaclaren, M.,14, 21, 24, 28,46, 59 hlcSeal, X., 83, 106 LlcWhortcr, C. C., 388, 410 hlagono, C., 116, 146 Xlahmucl, I., 159, 16.5, 179, 181, 182, 205, 219

hlaignien, R., 14, 13, 16, 18, 19, 20, 21, it, L J , 39, 30, 51, 53, 54, 35, 59 Slain, R. Xl., 350, 356 hlakkink, G . F., 225, 262 Xlallet, F. R., 3, 27, 59 hlannering, J. V., 130, 131, 146 hlaples, R., 393, 41 1 hlarshall, J. S., 115, 146 hlarston, R. B., 329, 356 Martens, R. K., 322, 355 Martin, J. P., 400, 408 hlason, R. J., 115, 120, 146 Massee, T. W., 130, 145 Marbut, C. F., 16, 20, 21, 24, 29, 40, 46, 47,48,54,59 Martin, F. J., 4, 59 Martin, W. E., 80, 105, 308,319 Mason, D. D., 201, 218, 233, 262, 308, 317 Maximov, N. A,, 225, 227, 262

hlazurak, A. P., 235, 261, 262 Means, U. M., 198, 219, 391, 410 hlederski, H. J., 341, 357, 395, 398, 399, 400, 408, 410 Mehlich, A., 312, 313, 318 Menzel, R. G . , 353, 357 hlerriam, R. A., 329, 357 hieyer, L. D., 113, 130, 131, 136, 144, 146

Xlickelson, R. If., 242, 5 1 , 254, 260, 261 htiddleton, H. E., 12.5, 146 Miears, R. J., 72, 105 hlihara, Y., 111, 115, 116, 117, 119, 120, 121, 146 hlikkelsen, D. S., 73, 77, 80, 106, 107 .Iliksche, J. P., 152, 219 hlillar, C. E., 311, 317 hfiller, E. C., 232, 235, 262 Miller, hI. D., 71, 74, 77, 79, 80, 82, 83, 105, 108

Miller, M. F., 113, 138, 146, 147 Miller, M. H., 276, 318 hliller, R. J., 396, 410 Milne, G., 54, 59 Mintzer, S., 345, 357 Misra, R. N.,94, 107 hlitscherlich, E. A., 308, 318 hlohr, E. C. J., 4, 14, 21, 24, 25, 28, 29, 30, 36, 37, 46,47, 48, 59 hloldenhauer, R. E., 231, 262 hloldenhauer, W. C., 134, 146 hloncrief, J. B., 70, 106 hlontgomery, E. C., 235, 262 Mooers, C. A,, 361, 369, 372, 373, 377, 410

%loore, R. P., 389, 406, 409, 410 hloorthy, 13. R., 181, 1843, 188, 190, 221 Moreno, E. C., 276, 318 hlorse, W. J., 150, 151, 152, 157, 158, 159, 160, 161, 162, 185, 1'69, 171, 220, 361, 373, 389, 405, 410,411 Slortensen, W. I'., 311, 319 Mortier, P., 325, 357 h4uckenhirm, R. J., 119, 145 Mulcahy, M. J., 17, 21, 22, 25, 44,53, 59 htulder, E. C . , 284, 295, 318 hfullins, T., 70, ,53, 105, 106, 107 hluma\v, C. R., 202, 204, 220 Munsell, R. I., 394, 408 Munson, R. D., 281, 290, 304, 318

AUTHOR INDEX

Mumeek, A. E., 369, 410 Musgrave, G. W., 128, 137,146 Musick, J. T., 238, 239, 262

N

421

Olson, R. A., 251, 254, 255, 261, 263, 276, 319 O’Neal, A. M., 125, 146 Orlob, G. T., 340, 356 Ormrod, D. P., 69, 107 Osbom, G., 276, 318 Osenbrug, A., 248, 249, 263 Osler, R. D., 373, 375, 376, 377, 411 Owen, C. W., 408 Owen, F. V., 157, 158, 159, 161, 162, 165, 166, 168, 189, 171, 172,220 Owens, L. D., 334, 357 Ozaki, K., 181, 182, 183, 186, 188, 191, 221

Nagai, I., 158, 159, 160, 161, 163, 164, 165, 167, 168, 169, 171, 172,220 Nagao, S., 96, 106 Nagata, T., 151, 152, 153, 181, 220, 396, 387, 410 Naito, Y., 153, 157, 219 Nakatomi, S., 161, 171, 220 Nakayama, F. S., 341, 357 Namken, L. N., 323, 355 Nandi, H. K., 92,106 P Neal, J. H., 112, 114, 123, 127, 146 Nearpass, D. C., 73, 104 Packer, P. E., 113, 146 Nelson, C. E., 257, 263 Painter, C. G., 257, 263 Nelson, L. B., 110, 146 Palache, C., 10, 59 Nelson, L. E., 400, 410 Palmer, W. McK., 115, 146 Nelson, M., 80, 81, 97, 105, 106 Papa, K. E., 211,220 Nelson, R. J., 80, 82, 106 Parish, C. L., 126, 137, 144 heison, W. L., 2154, 262, 393, 394, 397, Park, J. K., 403, 405, 406, 410, 411, 399, 409,410, 412 Parker, E. R., 255, 261 Neubauer, H., 311, 319 Parker, F. L., 340, 357 Neville, 0. K., 323, 357 Parker, M. B., 399, 411 Newbold, T. J., 2 , 5 , 17, 26, 27, 45, 59 Parker, M. W., 153, 218, 220, 366, 369, Newsom, L. D., 88, 91,104 411 Nichols, M. L., 112, 146 Parks, C. L., 393,411 Nicholson, R. P., 254, 263 Parr, J. F., 113, 126, 144, 146 Nielsen, D., 331, 332, 340, 353, 355, 355 Parsons, D. A., 111, 112, 114,145,146 Nielsen, D. R., 324, 325, 327, 328, 331, Passerini, G., 113, 146 357 Pearson, G. A., 74, 107 Niklas, H., 312, 319 Pearson, R. W., 241, 246, 263, 264, 395, Nitta, K., 191, 220 411 Nixon, P. R., 328, 357, 402, 411 Peaslee, D. E., 379, 409 Norman, A. G., 322,357 Peele, T. C., 125, 146 Norum, E. B., 252, 264 Peevy, W. J., 135, 145 Penck, W., 53, 59 Nye, P. H., 16, 22, 23, 46, 49, 59 Pendleton, J. W., 380, 411 0 Pendleton, R. L., 2, 3, 4, 5, 8, 9, 14, 15, Odell, R. T., 365, 371, 402, 410, 411 17, 20, 21, 22, 44, 47, 59 @lien,A., 336, 357 Penington, R. P., 37, 42, 58 Ohlrogge, A. J., 276, 318, 361, 396, 398, Penrun, H. L., 225, 229, 242, 260, 263 401, 411, 412 Persons, T. D., 88, 91, 104 Oinuma, T., 212,213,220 Pesek, J. T., 254, 263, 285, 286, 288, 290, Oldham, R. D., 4, 15, 16, 17, 18, 21, 27, 292, 300, 315, 319, 396, 410 51, 59 Peters, D. B., 229, 231, 261, 263, 401, Ollier, C. D., 16, 22, 59 411 Olsen, K. L., 88, 106 Peterson, A. E., 225, 263

422

AUTHOR INDEX

Peterson, J. B., 348, 357 Phillips, R. E., 344,345,35> Pierre, J. J., 138, 146 Pillsbury, A. F., 254,262,263 Piper, C. V., 361, 405, 411 Plank, V. G., 115, 144 Poehlman, J. M., 217, 220 Pomerene, W. H., 112,145 Pond, G . A., 384, 41 1 Porter, K. B., 162,212,220 Portman, R. F., 92, 107 Poschenrieder, H., 312, 319 Power, J. F., 130, 145, 249, 252, 263 Prescott, J. A., 2, 3, 5, 9, 14, 15, 17, 21, 22, 44, 59 Prince, A. H., 85, 107 Prine, G. M., 247,261 Probst, A. H., 153, 158, 159, 160, 162, 164, 165, 168, 169, 170, 171, 182, 183, 187, 191, 196,202, 204,205, 218,219, 220, 221, 373, 375, 376, 377, 380, 387, 404, 411, 412 Prout, W. E., 353,357 Prummel, J., 286, 319

Q Quackenbush, F. W., 196, 221 Quereau, F. C., 75, 107

R Radwanski, S. A., 22,59 Raeber, J. G., 205,220 Ragar, S. R., 343, 358 Ramanathan, K., 151, 220 Ramiah, K., 94, 95,106,107 Ramig, R. E., 247, 250, 253, 255, 262, 263 Raney, F. C., 74, 75, 76, 105, 107 Raney, W. A., 237, 261, 322, 357, 379, 409 Rao, D. R. R., 94,107 Rao, M. B. V. N., 94,107 Rawvlings, J. O., 210, 211, 220 Raychaudhuri, S. P., 23, 59 Read, A. A., 322, 358 Redit, W. H., 68, 83, 107 Reed, J. F., 82, 107 Reeve, E., 400, 411 Reichman, G. A., 249, 252, 263

Reid, J. T., 381, 409 Reid, P. H., 397, 399, 411 Reuther, W., 400, 411 Reynolds, E. B., 71, 76, 78, 79, 82, 107 Reynolds, J. F., 345, 355 Rhoades, H. F., 255,262 Rhykerd, C. L., 348,357 Richards, L. A., 225,226, 263, 329,357 Richards, S. J., 25.5,263 Ricker, P. L., 150, 220 Robertson, L. S., 379, 409 Robins, J. S., 226, 228, 232, 248, 262, 263 Robinson, B. D., 121, 144 Robinson, H. F., 181, 182, 183, 184, 187, 188, 190, 192, 219 Robinson, R. G., 387, 411 Robinson, R. R., 316, 319 Rochlin, R. S., 355, 357 Rodrigues, S., 10, 30, 58 Rogler, G. A., 253, 263 Roller, E. M., 237, 261 Rolston, L. H., 84, 90, 91, 107 Romney, E. M., 399, 412 Romo, L. A., 347, 357 Rosanow, I. M., 245,263 Roschevicz, R. J., 94, 107 Rose, C. W., 111, 113, 121,146 Rosenberg, L. E., 92, 107 Rosevear, R. D., 16, 55,59 Ross, D. I., 338, 356 Ross, J. G., 213,218, 220 Ross, J. P., 160, 170, 218 Rouse, H. K., 246, 264 Rouse, P., 84, 90, 91, 107 Rouse, R. D., 377, 394,396, 411 Rowe, P. B., 112,146 Ruggles, F. H., 340, 357 Ruhe, R. V., 16, 18, 20, 22, 53, 54, 60 Runge, E. C. A., 365,371,411 Runkles, J. R., 324, 357 Russell, D. A., 400, 411 Russell, E. J., 257, 263 Russell, E. W., 2-57, 263 Russell, M. B., 234, 263, 347, 355, 385, 408 Rust, R. H., 348,357 Ruxton, B. P., 22, 57 Ryker, T. C., 87,107

AUTHOR INDEX

S Sack, H. S., 322, 355 Sackett, W. G., 313, 319 Saito, M., 181, 182, 183, 186, 188, 191, 221 Saito, S., 159, 160, 165, 167, 172, 220 Sakaguchi, S., 153, 157,219 Sakai, K. I., 92,107 Salley, A. S., 63,107 Sampath, S., 94, 107 Sample, E. C., 276, 279, 287, 291, 292, 315, 317 Sanders, M. E., 213,218,220 Sandison, A., 300, 319 Sattenvhite, L. E., 230,231, 262 Satyanarayana, K. V. S., 26, 60 Sauchelli, V., 268, 319 Scarsbrook, C. E., 248, 263 Schade, H., 39, 60 Schleusener, P. E., 403, 411 Schmid, A. R., 384,411 Schmitthenner, A. F., 160, 170,218, 220 Schneider, W., 311, 319 Schoen, B., 340, 356 Schofield, R. K., 225,243,260,263 Scholtes, W. H., 126, 144, 352, 357 Schultz, W. W., 355,357 Schuster, C. E., 308, 319 Schwab, G. O., 402, 411 Schwardt, H. H., 91,105 Scofield, C. S., 237, 263 Scott, C. O., 288,289,317 Scott, R. C., 339, 355 Scott, V. H., 73, 107 Scott, W. O., 388,410 Scrivenor, J. B., 14, 60 Sears, 0. H., 383, 392, 410, 411 Seatz, L. F., 400,411 Sedgwick, V. E., 389,405,410 Seigler, W. E., 410 Semb, G., 336, 357 Sen, N. K., 212, 220 Senewiratne, S. T., 72, 107 Setter, R., 353, 357 Sexton, H. D., 112, 146 Shantz, H. L., 225,232,235,261 Sharasuvana, S., 4, 5, 8, 20, 59 Sharp, A. L., 112,146 Shastry, S. V. S., 94, 107

423

Shaw, B. T., 321,357 Shaw, R. H., 225, 261, 324, 328, 356, 357, 358, 402, 411 Shaw, W. C.,85, 86, 87, 107, 385, 386, 411 Shaw, W. M., 336,356 Sheets, T. J., 388, 410 Shed, A. F., 389, 411 Sherman, G. D., 46,60 Shih, S. H., 316,318 Shive, J. W., 400, 411 Shook, J. F., 345,355 Shrader, W. D., 129,144,402,411 Siemer, E. G., 246, 264 Simons, J. W., 407, 408 Simpson, E. S., 7, 14, 20, 21, 24, 28, 46, 49,60,340, 357 Sinah, M. N., 77, 106 Singh, M. P., 169,220 Sivarajasingham, S., 13, 40, 60 Sjogren, J. W., 405, 411 Slater, C. S., 125, 146 Slatyer, R. O., 329, 357 Sletten, W. H., 238, 239, 262 Slife, F. W., 387,388,410,411 Slusher, M. W., 70,83, 106, 107 Smerdon, E. T., 136, 146 Smika, D. E., 253, 263 Smith, A. G., 382,411 Smith, D. B., 348,349,357 Smith, D. D., 117, 122, 123, 125, 126, 127, 128, 129, 134, 135, 136, 137, 138, 140, 146, 147 Smith, L. L., 407,408 Smith, P. E., 160, 170, 218, 220, 375, 376,377, 380,404,408,411 Smith, P. R., 393, 411 Smith, R. J., Jr., $5, 86, 87, 88, 107 Smith, R. L., 126, 147, 380,411 Smith, R. M., 126, 130, 136, 144, 147 Smith, T. J., 366, 373, 375, 376, 377, 411 Smith, W. D., 68, 83, 85, 107 Sneva, F. A., 248,253,263 Somerhalder, B. R., 403, 411 Sonnenschein, C., 212, 220 Specht, A. W., 73, 104 Spencer, A. T., 115,144 Spencer, D., 399, 408 Spies, C., 254, 263 Spilhaus, A. F., 115,147

424

AUTHOR IhiDEX

Spink, N’. T., 88, 91, 104 Sprague, H. B., 390, 41 I Sprague, 1.. C., 316, 319 Srivastava, S. C., 337, 356 Stallings, J. H., 110, 147 Stanbern,, C. O., 238, 244, 263 Stanford, G., 254, 262, 280, 312, 313, 318, 319 383, 387, 410 Staniforth, D. W., Stansel, J. W., 101, 105 Starostka, R. W., 276, 319 Stauffer, R. S., 134, 147 Steele, J. G., 138, 144 Steenberg, K., 336, 357 Steenbjerg, F., 291, 319 Stefansson, B. R., 403, 410 Steinmetz, H. J., 120, 145 Stephens, C. G., 16, 17, 60 Stephenson, H. F., 275, 277, 287,318 Stephenson, R. E., 308, 319 Stewart, G. L., 325, 357 Stewart, L. C., 313, 319 Stewart, R. T., 157, 158, 160, 165, 167, 171, 174, 220, 221 Stirling, A,, 2, 60 Stoizy, I. H., 327, 358 Stolzy, I.. H., 325, 327, 329, 357, 358 Stone, J. F., 372, 324, 358 Stout, P. R., 399, 412 Strauh, C. P., 340, 358 Straub, P., 353, 357 Strickling, E., 255, 262, 384, 412 Stringfield, G. H., 378, 409 Stubbe, H., 210, 220 Sturgis, M. B., 70, 82, 107, 108 Swamy Rao, A. A., 131,147 Swanson, A. F., 365, 412 Swanson, N. P., 113, 147 Swanson, R. W., 322, 358 Swidler, R., 231, 261 Swofford, T. F., 389, 409 Szuszkiewicz, T. E., 327, 358

Tang, P. S., 212, 220 Tanner, C. B., 225, 229, 230, 231, 232, 234, 245, 262, 263 Tawlings, J. 0 , 220 Taylor, hi., 386, 408 Taylor, R. E., 130, 147 357 Taylor, S.,3Z, Taylor, S. A., 231, 261, 330, 355 Terao, H., 159, 161, 162, 166, 171,220 Terman, G. L.. 294, 295, 296, 297, 299, 304, 307, 308, 309, 311, 312, 313, 314,

T

Uchiyama, Y., 372, 409 Uhland, R. E., 1.23, 12.6, 128, 133, 140, 147 Underwood, N., 322, 343, 358

Takagi, F., 158, 159, 161, 165, 168, 171, 220 Takahashi, hl. E., 96, 106 Takahashi, N.,157, 159, 167, 220 Takahashi, Y., 159, 161, 163, 164, 167, 220

319

Thatcher, L. E., 402, 408 Theobald, W., 26, 6U Thom, C. C., 215, 263 Thomas, H. C., 347,356 Thomas, J. R., 248, 249, 263 Thomas, P. K., 26, 60 Thompson, E. K., 62, 63,67, 69, 105 Thompson, J. R., 403, 410 Thompson, L. M., 323,358 Thorne, M. D., 322, 357 Thornton, J. F., 130, 147 Thornton, S. F., 311, 319 Thornthwaite, C. W., 22.5, 263 Thorp, J., 4, 57 Thurston, W. R., 353, 356 Ting, C. L., 157, 158, 167, 172,208, 220 Tippit, 0. J,, 126, 147 Tisdale, W. H., 89, 107 Todd, E. H., 89, 107 Torrie, J. H., 204, 205, 220, 374, 412 Toth, S. J., 346, 350, 358, 399, 412 Tower, H. E., 134, 147 Tracey, J. I., Jr., 36, 58 Trouse, A. C., Jr., 345, 358 Trumble, H. C., 245, 263 Truog, E., 312, 313,318,319,395, 412 Tsai, H. Y., 101, 108 Tnllis, E. C., 89, 80, 105, 107 Tyler, S. A., 37, 42, 58

U

V van Baren, F. A, 4, 14, 21, 24, 25, 29, 30, 36, 37, 46, 48, 59

AUTHOR INDEX

van Bavel, C. H. M., 229, 232, 263, 322, 343, 356,358,403,412 Vandecaveye, S. C., 308, 312, 319 Van Doren, C. A., 127, 134, 138, 147, 377, 408 Van Royen, W., 82, 108 Van Schaik, P. H., 153, 1\60, 168, 182, 1883, 191, 221, 404,412 van Wijk, W. R., 325, 327,328,357 van Zelst, T. W., 323, 357 Veatch, C., 158, 162, 174, 221 Veihmeyer, F. J., 234, 263, 329, 358 Veinik, A. I., 337, 358 Vidyabhusan, R. V., 212, 220 Viets, F. G., Jr., 226, 227, 248, 257, 261, 262, 263, 316, 318,400, 412 Viljoen, N. J., 377, 412 Vincenheller, W. G., 66,108 Vine, H., 28, 55, 60 Viste, K. L., 85,86, 87, 88, 105, 107, 108 Vlamis, J., 80, 105, 308, 318 Voigt, R. L., 206, 221 Volarovych, M. P., 343, 358 Vomocil, J. A., 275,318,342, 358 Yon Buttlar, H., 339, 358 Voysey, H. W., 2, 27, 60 Voznesensky, A. S., 125, 147

W Wadleigh, C. H., 225, 226, 263, 329, 357 Waegemans, G., 10, 60 Walker, A. J. K., 245,263 Walker, R. K., 70, 71, 76, 77, 80, 106, 108 Wallace, A., 398, 412 Walther, J., 3, 21, 24, 60 Ware, L. M., 257, 263 Wargel, C. J., 403, 410 Warth, F. J., 3, 7, 8, 60 Warth, H., 3, 7, 8, 60 Washbum, W. F., 377, 412 Wasson, R. A., 76,80,108 Watson, D. J., 2228,264 Weatherspoon, J. H., 186, 188, 192, 221 Weathenvax, P., 95,108 Weaver, H. A., 241, 264 Weaver, W. H., 311,318 Webb, B. K., 403, 405,406, 410, 411 Webb, J. R., %85, 286, 288, 290, 291,

425

292, 293, 300, 305, 315, 317,319, 3W, 412 Weber, C. R., 153, 154, 162, 171, 173, 174, 176, 177, 178, 181, 182, 184, 186, 187, 188, 190,201,202,204, 205,206, 207,208, 218, 219,220,221, 373, 375, 376, 377, 383, 387,406,408, 410 Weeks, L. V., 327,358 Weihing, R. M., 70, 106 Weiss, M. G., 157, 158, 162, 170, 171, 174, 175, 177, 187, 190, 204,205, 212, 219, 220, 221, 361, 365, 373, 375, 376, 377, 390,396,398,412 Welch, C. D., 393, 412 Weldon, M. D., 400,412 Wellensiek, S. J., 369, 409 Wells, J. P., 81, 104 Wendt, I., 339, 358 Wentworth, C. K., 49, 60 Wentz, J. B., 157, 160, 167, 174, 186, 188, 192, 220, 221 Whitaker, F. D., 130, 147 White, H. B., 196, 221 White, R. F., 288, 290, 291, 293, 305, 319 Whitehouse, F. W., 20, 26, 60 Whitt, D. M., 122, 127, 134, 135, 137, 138, 147, 402, 403,412 Whittig, L. D., 10, 13, 14, 29, 38, 39, 55, 57 Whyte, R. O., 369, 410 Widdowson, F. V., 291, 317 Wiggans, R. G., 202, 221 Willard, C. J., 388, 412 Willhite, F. M., 246, 264 Williams, A. H., 70, 71, 77, 79, 85, 92, 99, 106, 107 Williams, J. H., 195, 211, 220, 221 Williams, L. F., 157, 159, 160, 161, 162, 164, 165, 167, 170, 171, 172, 187, 208, 210, 221, 361, 373, 375, 376, 377, 3 1 , 402, 410, 412 Williams, R. E., 85, 108 Williams, R. R., 62, 84,106 Williams, V. R., 101, 108 Williams, W. A., 82, 108 Williamson, E. B., 379, 409 Willis, A. L., 37, 42, 58 Wilm, H. G., 112, 147 Wilson, J. H., 398, 410

426

AUTHOR INDEX

Wingate, G., 26, 60 Winogradsky, S., 313, 319 Winterberg, S. H., 308, 318, 336, 356 Wischmeier, W. H., 111, 117, 123, 124, 125, 126, 127, 128, 129, 132, 134, 138, 140, 141, 142, 146,147 Witham, W. C., 364, 409 Wojta, A. J., 135, 147 Woodbum, R., 119, 121, 127, 144, 147, 148 Woodruff,C. M., 123,148 Woodward, L., 112, 129, 148 Woodworth, C. M., 157, 158, 159, 160, 161, 162, 163, 164, 165, l&, 167, 168, 169, 171, 172, 174,186, 188,221 U'oolnough, W. G., 3, 17,60 Wooten, 0. B., 378, 379,409 Worley, L. D., 136, 148 Wu, W.T., 101,108 Wunnecke, G. W., 130, 144

Wyche, R. H., 80,81,97, lOS, 106,108 Wynne, A. B., 26, 60

Y Yamada, T., 202,221 Yarimizu, H., 372, 409 Yeh, B., 94, 108 Yoder, R. E., 127, 144 York, E. T., Jr., 397, 411 Yoshino, Y., 181, 182, 183, 186, 188, 191, 221 Yungen, J. A., 281,318

Z Zacharias, M., 209,210,212,221 Zaslavsky, D., 338,354, 358 Zingg, A. W., 112, 122, 127, 128, 134, 135, 136,137,147,148 Zoellner, J. A., 291, 318 Zubriski, J. C., 252, 264

SUBJECT INDEX A Advection, 230, 231-233 Aeration, soil, 342, 347-348 Aeschynmlene virginica, 85 Aldrin, 90, 91 Alfalfa, 230, 231, 232, 237, 244, 250, 311,313,330,348 Algae, 88 Allophane, 29 Almond moth, 84 Alumina, 8, 9, 10, 27, 29, 31, 39, 45, 49, 50, 55, 50 Aluminum, 3,7,275,287,400 Amiben, 388 3-Amino-2,5-dichlorobenzoic acid, 388 Ammannia coccinea, 85 Ammonia, 82, 255 Ammonium, 275, 278 Ammonium chloride, 80 Ammonium nitrate, 81, 255, 274 Ammonium nitrogen, 73, 80 Ammonium phosphate-sulfate, 81 Ammonium sulfate, 79, 80, 81, 82, 281, 282,284,285,300,334,335 Angoumois grain moth, 84 Aphelenchoides oryzae, 88 Arasan, 392 Arrowhead, 85 Aspergillus niger, 312 Atrazine, 385 Atropine, 212 Availability coefficient index, 389 Azotobacter, 312

B Bacterial pustule leafspot, 170 Bahiagrass, 247 Barley, 235, 238, 258 Barnyardgrass, 85, 86, 87 Beans, 150, 388 Beets, 233 Belle Patna rice, 99 Bermudaerass, 247 Biotite, 34

Bird's-foot trefoil, 348 Blast, 88, 89 Blue gramagrass, 248 Bluegrass, 348 Blue rose rice, 99 Boehmite, 10, 13, 58 Boron, 74, 82, 398, 400 Bouteloua gracilis, 248 Brickstone, 2 Bromegrass, 230, 240 Brown leaf spot, 88, 89 Buckwheat, 313 Bulrush, 85 Burclover, 82 O-sec-Butyl-4,8-dinitrophenyl, 387-388

C Cadelle beetle, 84 Calcium, 29, 48, 74, 75, 198, 338, 393, 394, 400, 401 Calcium nitrate, 285 Calcium phosphate, 274, 275, 276, 282, 283, 290 Caloro rice, 69, 74, 75, 77, 97, 99 Calrose rice, 99 Caperonia castaneaefolia, 85 Capillary flow, 332 Captan, 392 Carbon dioxide, 348, 350 Catalase, 73 CercospoTa oyzae, 88 Cercospora sojina, 170 Cesium, 353 Chloride, 198, 313, 331, 338, 340, 400401 2-Chloro-N,N-diallylacetamide, 388 Chlorophyll, 171, 231, 398, 399 Citrus, 255 Clay, 81, 125, 126, 347 Cliachite, 10 Clover, 230, 235 Cobalt, 334, 398, 399 Cocklebur, 388, 404 Coff eeweed, 85 Colchicine, 212, 213 Colusa rice, 79, 97, 99

427

488

SUBJECT L\mEX

Confused flour beetle, 84 Evapotranbpiration, 224, 225, 226, 227, Consumptive use, 227 259,329,330 Copper, 82,398,400 fertilizer and yield, 223-246 Copper sulfate, 88 validity of data, 228-233 Corn, 123, 131,132,133,242,245,251, F 254,256, 257,259,260, 281,285, 300,309,314,315,328, 329,330, Ferric sulfate, bO, 335,345,347,364,365,371, 382, Fertilizer, chemical characteristics, 266-280 384,385,386,397,400 efficient water-use, 223-264 Cornstalks, 130 water infiltration, 254-256 Cotton, 69,123,133,230,243,246,257, Fertilizer evaluation, 264-319 365,392,397 concepts of, 580-295 Cottonseed meal, 79,81 methods used, 295 p-Coumaric acid, 77 Ferulic acid, 77 Crested wheatgrass, 248,252 Field beans, 71,86 Cucumbers, 245,314 Field capacity, 325,326,329 Cunninghamella sp., 313 Field peas, 82 Curly indigo, 85 Flat grain beetle, 84 Cyanamid, 82 Floral induction, 366 Cyperus sppp., 85 Fortuna rice, 97 Foxtail, 387 D Frogeye leafspot, 170,389 Dactylis glomerata, 242 Fungicides, 390,392 Def 01 iat ion, 404-405 FW-450, 154 Dehydroabietylamine acetate, 88 G Deuterium, 331,340,341,346,355 Diammonium phosphate, 81 Germination inhibitors, 77 Diaporthe phaseoIotum var. sojae, 198 Giant ragweed, 388 Diazinon, 92 Gibberellin, 403-404 Dichlone, 88 Gibbsite, 10, 13,32,33,35,36-37,39, 2,4-Dichlorophenoxyacetic acid, 87,388 43,45 3,4-Dichloropropionanilide,88 Glycine clandestina, 151 Glycine falcata, 151 Dieldrin, 90,91,92 p-[2-( 3,5-Dimethyl-2-oxocyclohexyl)-2- Clycine gracilis, 207 hydroxyethyl] glutarimide, 88 Glycine javanica, 151 Glycine latrobeona, 151 Diuron, 385,389 C h i n e max, 82,149,150,207,208, 360 Dolerite, 8 Glycine petitiana, 151 Do\vnv mild en^, 170,197,389 Glycine sericea, 151 Glycine tabacincz, 151 E Chycine tomentella, 151 Echinoclzola ~pp.,85,87 Glycine ussuriensis, 151,207,208,209 Eleocharis spp., 85 Goethite, 10,13, 14,33,39,41,42,43, Enhpoasca fabae, 167 56 Environment, 186,106,197 Gooseweed, 85 Erosion, Grape colaspis, 90 control, 134-136 Grapes, 257 rainfall, 109-148 Grasshopper, 91 Erosion-index, 124 Green manure, 82,364 European corn borer, 302 Groinid-water laterite, 23,24,25

429

SUBJECT INDEX

H Hairy vetch, 82 Heat budget, 229,233 Helminthosporium oryzae, 88 Hematite, 10,13,39,41,43 Herbicides, see also individual compound, Herbicides, 330,385,387-389 Heteranthera sp., 85 Heterodera &cines, 170 High-level laterite, 18,19,51 Hoja blanca, 88,91,98 Horsebeans, 82 Hydrogen, 323 p-Hydroxybenzaldehyde, 77 p-Hydroxybenzoic acid, 77

I Indian-meal moth, 84 Indoleacetic acid, 77 Intermediate wheatgrass, 255 Iodine, 337 Ion movement, 334-342 in plants, 341-342 in water, 334-337, 339-341 Iron, 7,8,10,27,29,31,38,40,43,45,

55,82,170, 198,275,395 398 movement of 45-53 Iron clay, 2 Irrigation, 67,73-77, 246,255,402-403 Isopropyl N- ( 3-chlorophenyl ) carbamate, 87,387

mineralogical characteristics, 10-14 parent material, 16 physical characteristics, 5-7 pisolitic, 6 profiles, 20-26 softening of, 55-56 topography, 16-20 vegetation, 15-16, 44 vesicular, 5-6 Leaching, 335 Leafhopper, 82 Legumes, see also individual, 132 Lespedeza, 70, 71 Lesser grain borer, 84 Levee, 72-73 Lime, 79,80,255,393-394 Limiting yields, 282-295 Linoleic acid, 196 Linolenic acid, 195,196 Littoral drift, 348 Low-level laterite, 18,19,51 Lodging, 199,375,406 Lysimeter, 229,230,231,238,243,329

M Magnesium, 29, 48, 74, 75, 198, 312,

393,394,395,396,401

Malathion, 90,92 Manganese, 3,9,10,73,82,287,398 Manure, 130,255 Mass flow, 341 Medicago hispida, 82 Isotopes, Methionine, 196, 197 soil physics, 321-358 2-Methyl-4-chlorophenoxyacetic acid, 87 Mexican weed, 85 K Midge, 92 Kainite, 79 Kaolin, 6,8,13,29,33,35,36,37,38, Millet, 309 Milo, 235-236 42,43,51,55,56 Mineralization, 351 Kernel smut, 88 Miscible displacement, 331 L Molybdenum, 398,399 Ladino clover, 71,311 Montmorillonite, 13,347 Laspeyresia glyciniuorella, 167 Morning glory, 404 Laterite, 1-60 Mudplantain, 85 cellular, 6 N chemical characteristics, 7-10 climate environment, 14-15, 43-44 N-1-Naphthylphthalamic acid, 388 definition, 1-4 Narrow brown leaf spot, 88,89 formation of, 26-53 Nematode, 89 geomorphic relationships, 53-55 Neorjossia barclayana, 88

430

SUBJECT

Seutron irradiation, 209, 211, 212 Neutron moisture meter, 332-330 Nira rice, 97 Nitrogen, 383 evduation, 274, 275, 276, 281, 301, 309, 312, 313, 316 rice culture, 73, 75, 79, 80, 81, 82, 86, 89 soybeans, 391-393 transformation, 350-352 water-use efficiency, 235 236, 238, 239, 240, 241, 242, 265, 246, 247, 249, 251, 253, 254, 255, 256, 257 Nitrogen fixation, 213, 391, 399 Nitrate, 235 Nitrate nitrogen, 73, 275, 313 Nodulation, 390-393

0 Oats, 70, 71, 86, 233, 245, 251, 256, 309, 314, 315, 316,348,364,381,384 Orchard grass, 242, 245 Oryza glaberrima, 92 Oryza satioa, 85, 92, 94 Oryza satica var. fatua, 94 Oryzea perennis, 94 Oryzea perennis var. balunga, 94 Oxygen, 347

P Pangolagrass, 247 Peanuts, 397 Peas, 233, 381-382 Pentachlorophenol, 388, 404-405 Peronospora manshccrica, 170 Photoperiod, soybean, 153, 369-371 Phosphate, 250, 256, 393 evaluation, 267, 274, 275-280, 281, 287, 292,294, 309, 312 rice culture, 79, 80, 81, 86, 87 Phosphoric acid, 82 Phosphorus, 198, 337 evaluation, 267, 275-280, 283. 285, 291, 294, 295, 297, 299, 300, 307, 311, 312, 313-315, 316 soybean, 393, 394-396, 400 water-use efficiencv, 236, 249, 252, 257 Photosynthesis, 69, 229, 368, 400 Phtrtovhthora mePmverma var. soiae. 170

TNDEX Phytophthora rot, 170, 198 Pigweed, 388, 404 Piricularia oryzae, 88 Planthopper, 91 Plinthite, 4 Plow-plant, 378, 379 Pod blight, 197, 389 Pod borer, 167 Pole bean, 314 Ponderosa pine, 129 Potash, 80, 82, 89, 236 Potassium, 29, 74, 198, 257, 336, 353 evaluation, 274, 275, 278, 312, 313 soybean, 393, 396-398 Potatoes, 69, 256, 313, 314 Potato leafhopper, 167 Purple stain, 389 Purple vetch, 82 Pyrausta nubilalis, 302

Q Quartz, 8, 10, 13, 41

R Raindrop characteristics, 114-118 Rainfall erosion, 109-148 mechanics of, 113-123 tools used, 111-113 Rainfall-erosion equation, 138-144 Rainfall-erosion index, 124, 125, 138 Red clover, 313, 348 Red flour beetle, 84 Hed rice, 85 Redstem, 85 Rexoro rice, 97, 99, 103 Rhizobium, 407 Rhizobium japonicum, 170, 391 Rice in United States, 61-108 Rice, botany of, 92-96 breeding, 96-104 culture, 68-84 diseases, 88-89 drying and storing, 83-84 harvesting, 83 history of, 62-68 insects, 89-92 milling, 84-85 uses, 61-62 weeds. 85-88

SUBJECT INDEX

Rice dwarf virus, 88 Rice leaf folder, 92 Rice leaf miner, 91 Rice moth, 84 Rice stalk borer, 90-91 Rice stinkbug, 90 Rice stripe virus, 88 Rice water beetle, 91 Rice water weevil, 91 Rice weevil, 84 Rill erosion, 110 Roots, moisture extraction, 252-254 Root knot nematode, 89 Root temperature, 75 Rotating vane sampler, 111 Rubidium, 336, 340, 345, 347 Runoff erosion, 121 Rye, 312 Ryegrass, 133, 245

S Safflower,71, 86 Sagittaria spp,, 85 Sand, 119, 120 Savannah, 15 Saw-toothed grain beetle, 84 Sdrpus, sp., 85 Sclerotiurn myzae, 88 Self-difFusion coe$cient, 332, 334, 336 Sesbania eraltata, 85 Sheet erosion, 110, 119, 122 Silica, 3, 7, 8, 10, 27, 28, 31, 34, 37, 45, 55 Simazine, 385 Smooth bromegrass, 247, 248, 255 Snap beans, 257 Sodium, 29, 74, 255 Sodium hypochlorite, 77 Sodium nitrate, 300 Sogato orizicola, 91 Soil, see also laterite soil, aeration, 342, 347-348 density, 342-346 erodibility, 125-126 soil loss, factors affecting, 123-136 prediction, 137-144 Soil moisture, 324, 325

431

Soil moisture tension, 230, 234 Soil physics, aeration, 347-348 density, 342-346 isotope methods in, 321-358 particle movement, 348-350 profile formation, 352 radioactive waste, 353-354 structure, 346-347 temperature, 348 transformation of materials, 350-352 water, 322-342 Soil profile, 352 Soil suction, 327 Soil structure, 346-347 Sorghum, 71,86, 213, 238,251, 254 Soybean, breeding, 199-218 climatic adaption, 305-372 erosion control, 385-386 genetics, 157-158 growth regulators, 403-404 harvesting, 404-406 management, 359-412 nutrient requirement, 390-401 origin, 151-152 photoperiod, 369-371 planting methods, 378-383 planting time, 372-378 production, 360-365 reproduction, 152-157 rotation practice, 383-385 seed quality, 389-390 seed storage, 406-407 taxonomy, 150-151 water requirements, 401-403 weed control, 386-389 Soybean cyst nematode, 170 Soybean meal, 364 Soybean oil, 187-191, 194, 195-196, 197, 210, 364, 377 Soybeans, 70, 82, 86,257 Spergon, 392 Sphenoclea zeyanica, 85 Spikerush, 85, 86 Splash erosion, 119, 121 Spring wheat, 249, 253 Stem blight, 197, 389 Stem rot, 88, 89 Straighthead, 88, 89

432

SUBJECT INDEX

Strawberry clover, 71 Straw mulch, 119 Strip cropping, 134-135 Strontium, 353 Sudangrass, 241 Sugar beet, 246, 256, 257 Sugar cane, 345 Sugarcane beetle, 91 Sulfur, 334, 398, 399-400 Summer fallow, 70 Superphosphate, 244, 394 evaluation, 267, 274, 279, 283, 286, 287, 290, 300 Sweet clover, 313 Sweet corn, 257

T Tadpole shrimp, 91-92 Temperature, effect on growth, 365-368 iodine number, 377 oil formation, 368 root, 75 soil, 348 soil loss, 123 Tmndipcdidue, 92 Terracing, 135-136 Thernial neutrons, 210 Timothy, 235 Titanium, 10 Tobacco, 313 Tobacco ringspot virus, 154 Tomatoes, 257, 313 Toro rice, 99, 103, 104 Toxaphene, 90 Tractor-plant, 378 Trefoil-grass, 71 Triops longicaudatus, 92 Tritium, 331, 338, 339, 340, 341, 355

U Umbrellasedge, 85 Urea, 81, 82, 255, 275, 300 V Vaniilic acid, 77 Vapor transfer, 337-338 Veratrine, 212 Vetch, 71, 82, 1.33 Viciu atropurpirreu, 82 Viciu faba, 82 Vicia uillosa, 82

W Water, intercepted, 328 measurement of soil, 322-342 movement, 338-348 soybean requirements, 401-403 Water erosion, 119 Watergrass, 86 Water requirement, 227 Water-use efficiency, 223-264 definition, 227-228 Wheat, 62, 71, 235, 246, 251, 252, 253, 309,314,364, 384, 397 White tip, 88, 89 Winter wheat, 130, 239, 249, 250, 251, 257

X Xanthomonas phaseoli var. sojensis, 170 X-rays, 154, 209. 210,211,212

Y Yellow-striped annywoim, 92

z Zinc,82, 198,257,287,398,400