Advances in Agronomy, Volume 53

Advisory Board Martin Alexander

Eugene J. Kamprath

Cornell University

North Carolina State University

Kenneth J. Frey

Larry P. Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the

American Society of Agronomy Monographs Committee P. S. Baenziger J. Bartels J. N. Bigham L. P. Bush

M. A. Tabatabai, Chairman R. N. Carrow W. T. Frankenberger, Jr. D. M. Kral S. E. Lingle

G. A. Peterson D. E. Roiston D. E. Stott J. W. Stucki

D V A N C E S I N

ono V O L U M5E3 Edited by

Donald L. Sparks Department of Plant and Soil Sciences University of Delaware Newark, Delaware

ACADEMIC PRESS San Diego New York Boston London Sydney Tokyo Toronto

This book is printed on acid-free paper. @ Copyright 0 1994 by ACADEMIC PRESS, INC. All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Academic Press, Inc. A Division of Harcourt Brace & Company 525 B Street, Suite 1900, San Diego, California 92101-4495 United Kingdom Edition published by Academic Press Limited 24-28 Oval Road, London NWI 7DX

International Standard Serial Number: 0065-2 I 13 International Standard Book Number: 0-1 2-000753-3

PRINTED IN THE UNITEDSTATES OF AMERlCA 94 95 9 6 9 7 98 9 9 B B 9 8 7 6

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Contents CONTRIBUTORS ........................................... PREFACE.................................................

vii ix

CROPROTATIONSFOR THE 2 1 s CENTURY ~ D . L . Karlen. G. E. Varvel. D . G. Bullock. and R . M . Cruse I. Origin of Crop Rotations ................................... I1. 2 0 t h Century Crop Rotations ................................ I11. Agronomic Impacts of Crop Rotation ......................... IV. Soil Quality Effects ........................................ V. Biological Diversity ......................................... VI. Economics of Crop Rotation ................................ VII . Policy Impacts on Crop Rotations ............................ VIII . Summary and Conclusions .................................. References ................................................

2 5 11 22 30 32 33 36 37

ROLEOF DISSOLUTION AND PRECIPITATION OF MINERALS INCONTROLLING SOLUBLE ALUMINUMIN ACIDICSOILS G. S . P. Ritchie I . Introduction .............................................. I1 A Framework for Understanding Mineral Dissolution and Precipitation in Soils ....................................... I11. Factors Affecting Dissolution and Precipitation of AluminumContaining Minerals ....................................... Iv. Modeling Soluble Aluminum ................................ V. Aluminum in Acidic Soils: Principles and Practicalities .......... References ................................................

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47 50

51 64 77 80

CONTENTS

vi

MANAGINGPLANTNUTRIENTS FOR OPTIMUM WATERUSEEFFICIENCY AND WATER CONSERVATION Jessica G. Davis I. Introduction .............................................. 11. Conserving Water Supply by Optimizing Water Use Efficiency . . . 111. Conserving Water Quality through Nutrient Management . . . . . . . Iv Needs for Further Research ................................. References ................................................

INTERPARTICLE FORCES: A BASIS FOR THE INTERPRETATION OF SOILPHYSICAL BEHAVIOR J. P. Quirk Introduction .............................................. Interparticle Forces ........................................ Soil Water Relations: Swelling and Shrinkage . . . . . . . . . . . . . . . . . .

85 86 92 108 109

Iv. Swelling of Sodium Clays ................................... v. Swelling of Calcium Clays .................................. VI. Surface Area and Pore Size .................................. VII . Water Stability of Soil Aggregates ............................

VIII. Sodic Soils and the Threshold Concentration Concept . . . . . . . . . . Ix. Concluding Remarks ....................................... References ................................................

122 124 143 146 152 161 166 169 176 177

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

185

I. 11. 111.

Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.

D. G. BULLOCK (l), Department ofAgronomy, University of Illinois, Urbana, Illinois 61801 R. M. CRUSE (I), Department of Agronomy, Iowa State University,Ames, Iowa 5001 I JESSICA G. DAVIS (85), Department of Crop and Soil Sciences, University of Georgia, Coastal Plain Experiment Station, Tifton, Georgia 3 1 793 D. L. KARLEN (I), National Soil Tilth Laboratory, United States Department of Agriculture, Agricultural Research Service, Ames, Iowa JOOl I J. P. QUIRK (1 2 l), Department of Soil Science and Plant Nutrition, School of Agriculture, The University of Western Australia, Nedhnd, WesternAustralia 6009, Australia G. S. P. RITCHIE (47), Department of Soil Science and Plant Nutrition, School of Agriculture, The University of WesternAustralia, Nedlands, Western Azlsh-alia 6009, Australia G. E. VARVEL (l), Soil/Water Conservation Research Unit, United States Department of Agriculture,Agricultural Research Service, University of Nebraska, Lincoln, Nebraska 68583

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Preface Volume 53 contains four excellent reviews that cover a broad spectrum of important advances and topics in the plant and soil sciences. Sustainable agriculture is one of the most discussed issues and venues for research in agronomy at the present time. The first chapter comprehensively reviews the history of crop rotations and future directions in this important area. Topics that are covered include twentieth century rotations, agronomic impacts of crop rotations, effects of rotations on soil quality, economics of crop rotations, and policy impacts. The second chapter provides a thorough discussion on how dissolution and precipitation affect soluble aluminum in acid soils. The author reviews factors that affect dissolution and precipitation, ways to model soluble aluminum including thermodynamic and kinetic approaches, and the effects of aluminum on aspects of acid soils. Water quality and conservation are of paramount importance in protecting and preserving our environment and are among the most active areas of research in agronomy. The role that nutrient management has on optimal water use efficiency and conservation is the topic of the third chapter. Discussions on conserving water supplies via optimization of water use efficiency and preservation of water quality through nutrient management are thoroughly covered. The fourth chapter is a definitive treatise on how interparticle forces affect soil physical behavior which, of course, has immense effects on plant growth and yield. Topics that are discussed include interparticle forces, soil water relations, swelling of clays, surface area and pore size, water stability of soil aggregates, and sodic soils. I thank the authors for their comprehensive and timely reviews.

DONALD L. SPARKS

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CROP R~XITONS FOR THE 21sr C

~

D. L. Karlen,' G. E. Varve1,z D. G. Bullock,3 and R. M. Cruse4 'National Soil Tilth Laboratory United States Department of Agriculture Agricultural Research Service Ames, Iowa 50011 *Soil/Water Conservation Research Unit United States Department of Agriculture Agricultural Research Service University of Nebraska Lincoln, Nebraska 68583 3Department of Agronomy University of Illinois Urbana, Illinois 61801 4Deparunent of Agronomy Iowa State University Ames, Iowa 5001 1

I. Origin of Crop Rotations 11. 20th Century Crop Rotations A. Pre-World War I1 B. Post-World War I1 Developments C. 2 1st Century Outlook 111. Agronomic Impacts of Crop Rotation A. Crop Yield B. Water Use Efficiency C. Nutrient Use Efficiency D. Disease and Pest Interactions E. Allelopathy W. Soil Quality Effects A. Soil Structure B. Aggregation C. Bulk Density D. Water Infiltration and Retention E. Soil Erodibility F. Organic Matter V. Biological Diversity A. Effects on Wildlife B. Alternative Land Uses VI. Economics of Crop Rotation VII. Policy Impacts on Crop Rotations VIII. Summary and Conclusions References 1 Advance in A p n q , Vdume 53

Copyright Q 1994 by Academic Press, Inc. All rights of reproduction in any form reserved

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D. L. KARLEN ETAL.

I. ORIGIN OF CROP ROTATIONS The practice of crop rotation or sequentially growing a sequence of plant species on the same land (Yates, 1954) has been in existence for thousands of years. As noted by Parker (1915), crop rotation developed primarily from the experiences of mankind relative to soil productivity. MacRae and Mehuys ( 1 985) stated that it was practiced during the Han dynasty of China more than 3000 years ago. Early agriculturists experienced low yields that resulted from continuous cropping, and throughout history, crop rotation was found to be necessary to maintain productivity. However, it was seldom, if ever, understood why. Early writers noted that crop rotation was in use in ancient Greece and Rome. Pliny mentioned the use of four rotational schemes, including two for rich soils and one each for second- and third-quality soils (White, 1970a,b). For rich soils, the two rotations mentioned included three-crop sequences of barley (Hordeurn vulgare L.), millet (Panicurn rniliaceurn L.),and turnip (Brussica r a p L.) or wheat (Triticurn aestivurn L.), millet or turnip, and emmer (Triticurn dicoccon Schrank), and 4 months of fallow followed by spring beans (Varafaba L.) or no fallow with winter beans (Hiemalis faba L.). For second-quality soils the suggested 2-year rotation was wheat and beans or another legume and for thirdquality soils it was emmer and beans or legume followed by a fallow period. Pliny also recommended that when fallow is not an option, the field should be put down to lupins (Lupinus albus L.), vetch (Vicia sativa L.), or beans. These crops could then be incorporated as green manure in preparation for growing emmer. Columella recommended similar crop rotation practices to those described by Pliny (White, 1970a,b). Systems cited by these ancient writers generally described the typical legume and cereal crop rotation in which some form of legume [pea (Pisurn sativurn L.), bean, vetch, or lupin] was alternated with a cereal crop. The actual use of crop rotations by the Romans has been debated at length by both medieval and classical historians, some of which insist the practice of rotation was rare. According to White (1970b), these historians argued that rotation systems were widely recommended by Roman agronomists, but in practice were not used extensively by farmers of the time. However, it is interesting to note accounts mentioning crop rotation by other writers not considered agronomists, which included Virgil’s detailed account of crop rotations as alternatives to traditional fallowing in his poems. Whether crop rotations were in widespread use is not known, but the practice was used and its benefit was known. According to Brehaut (1933), in his translation of Cato’s De agricultura, other writers, including Cat0 the Censor, indicated the use of rotations was prevalent. Cat0 noted the beneficial effects of lupins, beans, and vetch and indicated the

CROP ROTATIONS FOR THE 2 1st CENTURY

3

likely use of this crop rotation was in a legume-cereal system. In Italy during the first century B . c . , Varro also noted the importance of a green manure crop, especially legumes, in cropping systems prevalent at that time. Despite the beneficial effects of crop rotation, the practice fell out of favor with the demise of Roman power throughout Europe (White, 1970b). Less use of crop rotation and a return to the old crop and fallow system appeared to occur with the return to a more rural civilization as the more urban civilization prevalent in the Roman Empire disappeared. Throughout the Middle Ages, little mention is made of crop rotation and as noted earlier, the prevalent practice was probably the crop-fallow system. One exception mentioned for this period was the practice of alternating 2 years of wheat and 5 years of grass in a system called ley farming (crop rotation). This sequence was used by the Monks of Couper around 1400 in Britain (Franklin, 1953). Crop rotation was probably used to some extent during this period, but it appears that a crop-fallow system, with the use of manure, was the general system in use. Crop rotations, as we now know them, are often traced back to the Norfolk rotation. This was popular in England about 1730 (Martin et al., 1976). The Norfolk rotation, which was widely used at the time, consisted of turnip, barley, clover, and wheat in a 4-year sequence. The Norfolk and many other similar rotation systems were in use throughout the 18th century, but little was actually known about the specific benefits of rotating crops. The prevailing thought was that each of the crops in the rotation obtained their nutrients from different zones or parts of the soil. This perception was used to explain why a sequence of different crops yielded better than a single crop grown year after year. Between 1730 and 1840 the practice of crop rotation and the use of artificial manure (lime and other soil minerals) to supplement animal manure had become almost universal in England (Parker, 1915). One early English agricultural writer, Arthur Young, was not necessarily a proponent of this system. Young was a great apostle of mixed farming. He lauded the value of legumes, the use of crop rotation, and the feeding of livestock on the farm and the return of the manure to the land. Young insisted grass land and grazing were of primary importance and management of arable land of secondary importance to English agriculture. However, he did emphasize the importance of crop rotation and animal husbandry to agriculture at the time (Parker, 1915). As would be expected because of the heavy influence of English and Scottish settlers, most early agriculture in the United States was based on English customs. Several letters between Thomas Jefferson and George Washington (Bureau of Agricultural Economics, 1937) support this statement and indicate that crop rotation was also the prevalent practice in the United States. Jefferson wrote in a letter addressed to President Washington in 1794 that he was going to have to use a milder course of cropping because of the ravages brought about by overseers

4

D. L. KARLEN E T A .

during his absence. His rotation was first year, wheat; second, corn (Zea mays L.), potatoes (Solanum tuberosum L.), or pea; third, rye (Secale cereale L.) or wheat, according to the circumstances; fourth and fifth, clover or buckwheat (Fagopyrum esculentum Moench); and sixth, something he described as folding or buckwheat if it had not been used in the fourth or fifth years. Another letter, dated 1798, indicated he was using a triennial rotation of 1 year of wheat and 2 years of clover in his stronger fields or 1 year of wheat and 2 years of pea in the weaker fields followed by a crop of Indian corn and potatoes between every other rotation. Jefferson commented in both cases that he felt these types of cropping systems, with the addition of some manure, would help his fields recover their pristine fertility at Monticello. In later years, after retiring from the presidency, Jefferson returned to Monticello and noted in a letter to C. W. Peale in 1811 that his rotations were mainly corn, wheat, and clover; corn, wheat, clover, and clover; or wheat, corn, wheat, clover, and clover. It was apparent that he knew well the benefit of rotation with legumes by the prevalence of clover in each of these systems. In some of his letters and papers, George Washington described a good crop rotation plan that he found in use on Long Island in 1790. It consisted of corn with manure, oats (Avena sativa L.) or flax (Linum usitatissimurn L.), wheat with 4 to 6 pounds of clover and 1 quart of timothy (Phleum pratense L.), and meadow or pasture. From 1800 to 1810 this same rotation with some slight modifications came into quite general use in Pennsylvania. However, in Virginia, a rotation similar to that of Jefferson’s was used by many farmers (Parker, 1915). Jefferson, Washington, and many other progressive farmers of the time used rotations and manure extensively in an attempt to regain productivity levels similar to those when the virgin soils of the United States were first broken out. In other parts of the world, it was apparent crop rotations and other systems similar to the Norfolk rotation were in extensive use by farmers during the 19th century. Despite their extensive use of rotations, agriculturists of the time, such as Baron Justis von Liebig (1 859), believed that although crop rotation improved the physical and chemical condition of the soil, all plants would eventually exhaust the soil. Liebig felt that unless soils were heavily manured, all fields would eventually lose their fertility, regardless of crop rotation. Hall (1905) presents an excellent summary of the prevailing thoughts and experiments concerning crop growth and production during the 19th century. It was during this time period that researchers discovered legumes had the ability to assimilate and utilize nitrogen from the atmosphere, which enlightened researchers regarding the benefit of growing crops in rotation with legumes. As described by Hall (1905), this discovery provided an explanation as to the benefit of existing crop rotation studies and led to new investigations on crop rotations during the 19th century at Rothamsted, England (the world’s first agricultural

CROP ROTATIONS FOR THE 2lst CENTURY

5

research station). These studies further identified that some of the nitrogen fixed by the legumes in a cropping system becomes available for succeeding crops and clearly identified at least part of the beneficial effects of crop rotations.

11. 20th CENTURY CROP ROTATIONS A. PRE-WORLD WAR 11 The discovery in the latter part of the 19th century that legumes could fix nitrogen from the atmosphere was a major reason rotations remained popular into the early part of the 20th century. Nitrogen was the major limiting nutrient for most crops and it could only be supplied by the addition of manure or by incorporating a legume of some type in the cropping system. Use of crop rotation during this period, similar to patterns established throughout history, was greatly dependent on the amount of new or virgin land available for crop production. If cheap and plentiful amounts of fertile land were available, crop rotations were not extensively used. Only as land became more expensive and less plentiful were crop rotations utilized more extensively. Johnson (1927) presented examples of rotation experiments conducted in several different areas of the United States during the early part of the 20th century. In Georgia, the suggested rotation was corn, cowpea (Vigna unguiculata L.), oat, and cotton. Cowpea was sown during the last cultivation. The corn was harvested for grain and the cowpea was worked into the soil. Oat was sown in late fall and harvested in late May or early June. Cowpea was sown again as a green manure crop to be incorporated the next spring just before planting cotton (Gossypiurn hirsuturn L.). This crop sequence increased cotton yields as much as 100% after the first series of the rotation and even greater increases in productivity were maintained in successive rotations. Rotation experiments at the University of Missouri that began in 1888 included a 6-year corn, oat, wheat, clover, timothy, and timothy rotation and a 3-year corn, wheat, and clover rotation. According to Johnson (1927), after 30 years, yields of corn were increased 60.4%, oat 3%, and wheat 32% in the 6-year rotation and 30.8% for corn and 40.8% for wheat in the 3-year rotation over the yields of the corresponding continuously cropped areas. Manure applications averaged 6.8 tons annually in both rotation and continuous cropping systems. Results from an Ohio experiment were similar (Johnson, 1927). The main difference between the Ohio and Missouri experiments was the use of fertilizers instead of manure. Yields of corn, oat, and wheat in rotation were increased 29.9, 30.8, and 42.5%, respectively, above yields of those crops in continuous culture. In Delaware, corn grain yields increased 156.9% in a rotation of corn

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D. L. KARLEN ET AL.

(including a cover crop of rye and vetch), soybean [(Clycine m u (L.) Merr.)], wheat, clover, and timothy as compared to continuous corn when no fertilizer or manure was used. With nitrogen, phosphorus, and potassium applications corn grain yields still increased 24.4% in rotation as compared to the continuous corn system (Johnson, 1927). Cover crops were widely recommended for many cropping systems in the early 20th century (Johnson, 1927). Among these systems was one including two crops of kale (Brassica oleracea var. acephala DC.), three of cabbage (Brassica oleracea var. capitata L.), three of potatoes, three of sweet potatoes [(Zpornoea batatas (L.) Lam.)], and one of German millet [(Setariaifulica (L.) P. Beauv.)]. These 12 crops were grown in rotation during the 9-year period from 1912 to 1921 both with and without cover crops at the Virginia Truck Experiment Station. Similar to the experiments just described, both rotations received the same amount of fertilizer, nitrogen, phosphorus, and potassium. Rotation again increased yields, but yields were increased to an even greater extent with the use of cover crops. Yield increases ranged from 12.5% for kale to 62.5% for millet with cover crops. In this experiment, rotation was considered a major factor contributing to disease control in truck crops and probably contributed greatly to the yield increases. Other popular rotations for truck farmers in Virginia at this time included a 3-year rotation of potatoes, corn, and rye grown during both the first and second year, and sweet potatoes and rye grown during the third year. In Norfolk County, Virginia, early potatoes were grown as a spring crop. This was followed by a cover crop of native grass, which was cut for hay, or legumes such as soybean or cowpea, which were incorporated in the fall. Cabbage was then planted in November and harvested the following April or May just before planting a corn crop. Sometimes, soybean was planted directly in the row with the corn and rye was sown between the rows at last cultivation as intercrops. When the corn was harvested, the cycle, starting with early potatoes, was repeated. This Norfolk County, Virginia, rotation consisted of two main crops (potatoes and cabbage), two catch crops, the hay, and corn crop (Johnson, 1927). With small modifications, this was similar to most of the truck crop rotations used throughout the eastern United States. In the same symposium, Lyon (1927) described the effects of legumes and grasses in different crop rotations. Most of the systems he described were similar to a corn, oat, wheat, and hay rotation, where the hay was usually either a legume or a grass such as timothy. He concluded that with few exceptions, experiments conducted at eight experiment stations in the humid regions of the United States generally showed legumes to be superior to grasses for increasing yields of the following crops. In the drier parts of the country, however, grasses were generally superior to legumes because they usually did not deplete soil moisture as extensively as legumes. Crop rotation was not a widely accepted practice in the United States corn belt during the early 20th century. The soils were extremely fertile and after the virgin

CROP ROTATIONS FOR THE 2 1st CENTURY

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sod was plowed, they sustained corn yields at sufficiently high levels for many years. However, even on these extremely fertile soils, crop rotation greatly increased yields at several locations compared to growing monoculture corn (Wiancko, 1927). Despite the superiority of rotated corn yields, none of the other crops in the rotation produced net returns anywhere close to that of corn. Therefore, farmers wanted to grow continuous corn even though its production had greatly reduced the fertility of many soils. Wiancko (1927) concluded that corn was the principle crop of the corn belt and that fact had to be recognized and considered in crop rotations proposed for general use in the region. Crop rotations in the southern and southeastern United States usually revolved around the staple crops of cotton, tobacco (Nicotiana tabacum L.), rice (Oryza sariva L.), and peanut (Arachis hypogaea L.). Parker (1915) discussed several rotation schemes used for these crops during the late 19th and early 20th centuries. He presented rotations for both livestock and mixed grain and livestock farms. They usually had corn, oat, wheat, clover, and meadow in various combinations and sequences, with the main emphasis being on feed for livestock. Rotations for tobacco were usually tobacco, wheat, and clover; tobacco, wheat, and cowpea; or tobacco, wheat, red clover (Trifolium pratense L.), meadow, and corn. Cowpea was generally included where the legumes were used as a green manure to maintain the soil humus supply. Cotton rotations were similar to those of tobacco in that emphasis was placed on one crop, while other crops in the system were selected for maintenance of soil humus levels and/or their potential as livestock feed. Crops in the cotton-based rotations included corn, wheat, oat, peanut, cowpea, and crimson clover (Trifolium incartum L.) in 2- and 3-year sequences with cotton. Rice was most often grown continuously, but progressive farmers of the time were becoming aware of crop rotation benefits, and if possible they used a rice, rice, rice, fallow, corn, and pea or bean (as green manure) rotation. Western regions of the United States also utilized crop rotations extensively during the early part of the 20th century. Crop rotations varied widely because of large growing season precipitation differences across the region. In more humid parts of this region, crop rotations were similar to those discussed for the corn belt states to the east. Drier areas of the Great Plains used cropping systems developed for the region with respect to water conservation. Parker (1915) presented several of the rotations used during this period for what he termed grain farming and mixed grain and livestock operations (Table I). In these rotations, grain was Durum wheat (Triticum durum Desf.), winter wheat, rye, emmer, awnless barley, or 60-day oat. The specific selection depended on local conditions. Green manure/fallow referred to growing crops such as Dakota vetch, Canadian field pea, sweet clover, common millet (Panicurn miliaceum L.), or Hungarian millet [Setaria italica (L.) P. Beauv.], which were plowed under in early summer, and then allowing the land to rest for the remainder of the season. The term cultivated crop referred to such crops as Indian corn, Kafir corn (Sor-

Table I Qpiieal3- to 7-YearCrop Rotations Used for Grain Farming and Mixed Grain and Livestock Operations in the Western United States during the Early 20th Centuryn

Farming system Grain only

Option

Year 1

1

Grain

2

Grain Grain c-P c-crop c-crop Grain c-crop

3

c-crop

Grain

4

c-crop Grain Grain

g-m-f s-clovere s-clover

2 3 4

5 Grain and livestock

1

5 6 After Parker (1915). Green manure/fallow. Cultivated crop. Bromegrass (Brornus srerilus 1.). Melilotus oficinalis Lam.

Year 2

g-m-f Grain Grain Grain Bromed Grain

Year 3

Year 4

g-m-fb Grain g-m-f g-m-f g-m-f Brome Pea or vetch hay g-m-f

Grain Grain

Grain

c-crop Grain

csrop c-crop

Grain Grain

c-crop Millet or sudan grass Grain

Year 5 c-cropc Grain g-m-f g-m-f Grain Pea or vetch hay -

Year 6 -

g-m-f -

Grain

g-m-f

Year I

CROP ROTATIONS FOR THE 21st CENTURY

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ghum bicolor L. Moench), durra (Sorghum bicolor L. Moench), and proso millet (Panicurn miliaceum L.), which again depended on local conditions and the particular needs of the individual farmer. In western areas, where irrigation was available, crop rotations differed greatly from those used for dryland situations. The emphasis in these cropping systems was usually on some form of high value cash crop, similar to those in the more humid areas of the East and South. The high value cash crops were mainly potatoes and sugar beet (Beta vulgaris L.), usually in some form of rotation with a legume and grain crop. The importance of crop rotation was evident even with irrigation (Powers and Lewis, 1930). Substantial increases in soil nitrogen and total carbon were found where irrigation and crop rotation or use of manure occurred. They reported striking differences in crop yield, water use efficiency (yield per acre inch applied), net profit per acre, and water cost per unit of dry matter. Powers and Lewis (1 930) suggested that settlers on newly irrigated arid land should utilize a crop rotation to improve the nitrogen and organic matter content of their soils and that such practices would help ensure economical use of water and establishment of profitable crop production under irrigation. Examples of irrigated crop rotations used in different parts of the West included a 5-year rotation in Utah that consisted of sugar beet, oat and pea for hay, sugar beet, oat, and a fifth field in alfalfa (Medicago sativa L.); and a 5-year rotation used in Colorado that consisted of oat, pea, potatoes or sugar beet, barley or wheat, and a fifth field in alfalfa (Parker, 1915). These and many other similar cropping systems were used throughout irrigated areas of the western United States. The emphasis in most of the rotations was on some form of staple crop and an adaptable soil renovating crop, such as the alfalfa in the rotations just described. Alfalfa was well adapted to irrigation, provided an excellent source of forage, and as a legume, it contributed greatly to rebuilding the fertility of the soil in these irrigated systems. As noted earlier, selection of the specific cropping system during the latter part of the 19th and early part of the 20th centuries was based on the individual needs of the farmer, selection of crops adapted to a particular region, and the climatic limitations of the area. The other basic requirements in a crop rotation during this time period, regardless of the region, were the need for some sort of legume to provide nitrogen for successive crops and different forms of green manure crops, which were used to maintain fertility of the soils.

B. POST-WORLD WARI1 DEVELOPMENTS Cropping systems before World War I1 changed little, but following this world event, crop rotations that included legumes were de-emphasized. Increased avail-

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D. L. KARLEN ETAL.

ability of nitrogen from industrial sources during the 1950s and early 1960s hastened this change throughout the United States (Tyner and Purcell, 1985). Plentiful and inexpensive nitrogen fertilizer following World War I1 devalued legume rotations except for farmers with livestock systems that required the legume as a feed source (Olson and Sander, 1988). Post-World War I1 research reports document a consensus that synthetic fertilizers and pesticides could forever replace crop rotation without loss of yield (Aldrich, 1964; Benson, 1985; Shrader et al., 1966). Melsted (1954) concluded that to achieve maximum production with minimum soil deterioration, an adequate supply of fertilizer nitrogen was essential. Increased availability of nitrogen fertilizers, herbicides for weed control, and pesticides for insect and disease control reduced the use of extended rotations (Rifkin, 1983; Crookston, 1984; MacRae and Mehuys, 1985). These changes have resulted in extensive monoculture corn throughout most of the corn belt, with generally increasing yields. The introduction of improved crop varieties was a major factor (Power and Follett, 1987), but mechanization (i.e., replacement of draft animals, which required feed and land devoted to its production, with tractors and combines) also contributed to the general decline among farmers and researchers in perceived need for, and therefore use of, crop rotations. Mechanization and adoption of short, 2-year corn and soybean rotations or continuous monocropping enabled farmers to benefit from the economy of scale by specializing their operations, improving marketing practices, and having to invest in fewer pieces of equipment (Bullock, 1992; Colvin et al., 1990; Power and Follett, 1987). Furthermore, many of the government production control and income stabilization programs limited rotation options and forced farmers to abandon extended crop rotations (Francis and Clegg, 1990).

c. 2 1ST CENTURY OUTLOOK Intensive monoculture cropping has increased throughout the United States since World War 11, and crop rotations have diminished. However, in many areas, crop rotation has steadfastly remained the major cropping system. Current consensus is that crop rotation increases yield and profit and allows for sustained production (Mitchell et al., 1991). In many areas, including several different types of crops remains the most economical and feasible method for crop production because it is one of the most effective disease and pest control systems. More recently, increased energy costs resulted in renewed interest in crop rotations as a source of nitrogen (Tyner and Purcell, 1985). However, interest in rotations as a source of nitrogen is present only when energy and fertilizer costs are high. Both are uncertain at this time. Post-World War I1 abandonment of extended crop rotations, in favor of short rotations and monocropping systems, has generally been profitable. However,

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11

the change has had negative consequences, especially if on- and off-site environmental consequences are considered. Many effects are site specific, but they include decreased soil organic matter content, degraded soil structure, increased soil erosion, increased sedimentation of reservoirs, increased need for external inputs, and increased surface and groundwater contamination. Long-term effects of not using crop rotations are not clear, but it is reasonable to question if the substitution of capital, energy, and synthetic chemicals is sustainable (Bullock, 1992). These questions are raised as we look toward the 21st century, because, as stated by Hauptli et al. (1990), “Modern agriculture is a very recent development, when considered in the context of evolution or even human history.” Crop rotation has not been abandoned in the United States. Approximately 20% of the corn is grown in continuous monoculture, but most of the remaining 80% is grown in a 2-year rotation with soybean or in short (2- or 3-year) rotations with alfalfa, cotton, dry beans, or other crops (Power and Follett, 1987). The primary crop rotation change involves use of pasture and green manure crops. Few are included in current crop rotations. Many of the rotation factors, processes, and mechanisms responsible for increased yield remain unknown. Increased nitrogen supply is sometimes responsible (Russelle et al., 1987), but improvements in soil water availability (Benson, 1985; Roder et al., 1989), soil nutrient availability (Bolton et al., 1976; Higgs et al., 1976; Peterson and Varvel, 1989a,b,c), soil structure (Barber, 1972; Dick and van Doren, 1985; Griffith et al., 1988), soil microbial activity (Cook, 1984; Williams and Schmitthenner, 1962), and weed control (Bhowmik and Doll, 1982; Slife, 1976); decreased insect pressure (Benson, 1985), nematode populations (Dabney et al., 1988), and disease incidence (Dick and van Doren, 1985; Edwards et al., 1988); and presence of phytotoxic compounds and/or growthpromoting substances originating from crop residues (Barber, 1972; Benson, 1985; Bhowmik and Doll, 1982; Welch, 1976; Yakle and Cruse, 1983, 1984) have also been identified as contributing factors. Currently, no amount of chemical fertilizer or pesticide can fully compensate for crop rotation effects, and analysis of these individual factors generally does not explain the entire yield response associated with crop rotation. Determining how the factors associated with crop rotations interact and contribute to the currently undefined “rotation effect” will apparently continue to provide a major research challenge.

111. AGRONOMIC IMPACTS OF CROP ROTATION A. CROPYIELD Increased yield may be one of the most practical justifications for reintroducing crop rotations (Wikner, 1990; Karlen et al., 1991). Several studies showing

D. L. KARLEN ET AL.

12

that corn, grown in a 2-year rotation with soybean, yields 5 to 20% more than monoculture corn have been published (Strickling, 1950; Welch, 1976; Kurtz et al., 1984; Voss and Shrader, 1984; Peterson and Varvel, 1989c; Crookston et al., 1991). Data from a 15-year study in Iowa (Table 11) show the typical response. Crookston et al. (1991) reported that annually rotated corn yielded 10% more than continuous corn, and that first-year corn, following 5 years of soybean, yielded 15% more than continuous corn. Based on these results, they suggested Minnesota farmers consider using longer crop rotations. However, yield response to rotations greater than 2 years may (Crookston et al., 1991) or may not (Lund et al., 1993) occur. Increased emphasis on crop residue management to reduce soil erosion may also encourage crop rotations because they can largely eliminate corn yield decrease observed between no-tillage and conventional tillage production practices (Karlen et al., 1991). This response is particularly evident on poorly drained soils (Dick et al., 1991). Furthermore, because many cropping systems have a small profit margin, a 5% yield increase for corn may result in a 50% profit increase (Crookston, 1984).

Table I1 Crop Rotation Effect on Corn Grain Yield

in Northeast Iowa Year

Continuous

Rotation

Mg ha-l 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992

8.0 9.5 9.5 9.9 7.5 5.3 5.5 7.6 10.2 8.1 5 .O 6.6 10.6 8.3 9.1

9.4 10.2 9.7 10.4 7.7 6.8 7.3 8.8 10.8 9.0 6.3 8.0

8.0

9.0

11.2 9.4 10.0

LSD(O.05) = 0.3

cv = 4.3%

15-year average LSD(O.05) = 0.2

CROP ROTATIONS FOR THE 2 1st CENTURY

13

Crop yield increases due to rotation are not limited to corn. Grain sorghum in rotation with soybean (Brawand and Hossner, 1976; Clegg, 1982; Gakale and Clegg, 1987; Peterson and Varvel, 1989b; Roder et al., 1988; Langdale et al., 1990) or corn (Robinson, 1966) showed increased yield compared to continuous grain sorghum. Soybean yield also increased when grown in a rotation with corn, grain sorghum, or simply following a fallow period (Crookston, 1984; Dabney er al., 1988; Peterson and Varvel, 1989a).

B. WATERUSEEFFICIENCY The need to develop more water-efficient crop management practices may be one of the strongest incentives for adopting crop rotations. Crops should be managed in a rotation sequence so that complementary root systems fully exploit available water and nutrients (Karlen and Sharpley, 1994). Sadler and Turner ( 1994) suggested “opportunistic cropping” as a means for increasing agricultural sustainability through water conservation or by increasing productivity from applied water. Opportunistic cropping is not crop rotation in the typical sense, but this management practice requires farmers to remain sufficiently flexible to adapt their farming practices to utilize rainfall and/or irrigation water as efficiently as possible. Therefore, opportunities to rotate crops spatially and temporally may become increasingly important. Roder er al. (1989) evaluated yield and soil water relationships for a sorghum and soybean cropping system. They found that crop rotation increased soybean yield, but that nitrogen fertilization did not. The soybean yield advantage from rotation decreased as the amount of spring rainfall increased. Increasing temporal and spatial diversity by using different crop rotations may mimic natural ecosystems more closely than current farming practices. This change may lead to increased agricultural sustainability (Karlen et al., 1992). One example is in semiarid areas where saline seeps began to develop about 30 years after cultivation began, and especially after about 10 years of an alternateyear, crop-fallow rotation (Ferguson er d.,1972). Formation of saline seeps gradually became a problem as production agriculture disrupted annual crop growth associated with native plant communities in semiarid regions. Ferguson and Bateridge (1982) found that 50 years of crop-fallow farming significantly reduced soluble salt content of some soils. Although this was beneficial from an edaphic perspective, they found that up to 90 Mg ha-1 of salt was moved toward the water table where it resulted in groundwater salinization and became a source of salts for saline seeps. Undoubtedly, some water moves below the root zone of native vegetation, but the quantity is not large. Native vegetation is diverse with varying growth habits and rooting depths. Therefore, most precipitation infiltrating the sod is transpired

14

D. L. KARLEN ETAL.

(Ferguson et al., 1972; Halvorson and Black, 1974). With cultivation during periods of above-normal annual precipitation, and with improved soil water storage and conservation during fallow, increased use of summer fallow enhances percolation of water below the root zone and thus contributes to formation of saline seeps (Halvorson and Reule, 1976). By using flexible crop rotations involving small grains, grasses, deep-rooted crops, and a minimum amount of summer fallow, soil water loss by deep percolation could be prevented and development of saline seeps could be alleviated (Halverson and Black, 1974).

C. NUTRIENT USEEFFICIENCY 1. Nitrogen

Increased use of crop rotations may be mandated to improve nutrient use efficiencies and reduce losses of nitrogen to surface and groundwater resources. Crop rotation per se is important, but the sequence with which crops are grown may be more important (Carter et al., 1991; Carter and Berg, 1991). Karlen and Sharpley ( 1994) reviewed several studies showing how crop sequence could influence nitrogen movement through the soil profile and ultimately into groundwater resources. Several studies showed that soybean and alfalfa, which do not require supplemental nitrogen inputs, can effectively use or “scavenge” residual nitrogen remaining in the soil from previous crops (Johnson et al., 1975; Mathers et al., 1975; Muir et al., 1976; Olson er al., 1970; Stewart et al., 1968). Alfalfa roots may grow to depths greater than 5.5 m in some soils, and research has shown that nitrate can be utilized by the crop from any depth where soil solution is extracted by plant roots. Mathers et al. ( I 975) reported that alfalfa removed nitrate from the soil profile at a depth of 1.8 m during the first year of establishment and to a depth of 3.6 m during the second and third years. Olson et al. (1970) found that crop rotation reduced soil solution nitrate concentrations at a depth of 1.2 to 1.5 m by 34 to 82% compared to continuous corn. They found that the decrease in solution nitrate was directly proportional to the number of years in oats, meadow, or alfalfa production, and attributed this to combined recovery of nitrate by shallow-rooted oat crops followed by deep-rooted alfalfa crops. Soybean can also effectively scavenge residual soil N (Johnson et al., 1975; Havlin et al., 1990; Karlen e t a l . , 1991), but in Wisconsin, soybeans were not as effective as alfalfa because of their more shallow rooting depth (Jackson et al., 1987). This finding was supported by Olson et al. (1970), who also concluded that recovery of subsoil nitrates by deep-rooted legumes such as alfalfa will probably be more effective on medium and heavy textured soils than on sands. One of the persistent nutrient management questions associated with crop

CROP ROTATIONS FOR THE 2 1s t CENTURY

1s

rotation is whether the nitrogen contribution from legume fixation is responsible for much, if not all, of the beneficial rotation effect. Bullock (1992) reviewed several studies focusing on the fertilizer replacement value as the method for assessing nitrogen contributions from legumes grown in rotation with nonlegume crops such as corn or grain sorghum. He reported that this method overestimated the nitrogen contribution by legumes and underestimated the rotation effect. For example, soybean is given a fertilizer replacement value of 25 to 40 kg ha-1 in many midwestern states. The actual nitrogen contribution by the soybean crop is often much less or even negative. In the midwestern United States, soybean in a 2-year corn and soybean rotation may acquire only 40% of its nitrogen from dinitrogen fixation, while the remaining 60% is taken up from the soil (Heichel, 1987). When the grain is harvested and removed, there is an estimated net loss of 84 kg N ha-1 due to the large nitrogen content of soybean grain: The nitrogen contribution from alfalfa in rotation with maize in the upper midwestern United States is also less than suggested by fertilizer replacement value methodology. Fertilizer recommendations for corn following alfalfa in most midwestern states credit the alfalfa crop with a nitrogen contribution of 100 to 125 kg N ha-' (Bruulsema and Christie, 1987; Fox and Piekielek, 1988) based on fertilizer replacement methodology. However, the actual contribution measured with '5N methodology was only 24 kg N ha-1 (Harris and Hesterman, 1990). Based on these studies, Bullock (1992) concluded that rotation with legumes does not provide as much nitrogen as fertilizer replacement methodology estimates and that much of the yield benefit which has been credited to nitrogen contribution is actually due to other factors. Jensen and Haahr (1990) also concluded that with winter cereals, the rotation effect of pea was probably more important than the residual nitrogen effect. For winter oilseed rape (Brussicu nupus L.), the residual nitrogen effect from pea was equivalent to 30 to 60 kg N ha-1 if applied following oats. Removal of the above-ground pea residues, which contained less than 1% nitrogen, had no effect on the residual nitrogen value.

2. Phosphorus, Potassium, and Other Nutrients There is very little direct evidence that crop rotation affects phosphorus relationships (Bullock, 1992). Karlen and Sharpley (1994) concurred, but suggested that appropriate selection and use of a crop with a higher affinity for phosphorus may reduce soil phosphorus stratification and increase phosphorus-use efficiency, particularly if the nonharvested portion of the crop is returned to the soil. They suggested that selection of crops which can more efficiently utilize residual soil inorganic and organic phosphorus may be economically viable for farmers and enhance the sustainability of soil phosphorus fertility. Vivekanandan and Fixen (1991) reported that corn sampled at the six-leaf

16

D. L. KARLEN ET AL.

growth stage had a higher phosphorus concentration when following soybean than when following corn. Similarly, Copeland and Crookston (1992) observed that corn in a 2-year rotation with soybean accumulated significantly more phosphorus than did corn in continuous monoculture. This suggested that corn yield increases associated with crop rotation may have been due to improved general plant nutrition. However, in the same study, total phosphorus content of soybean was not affected by crop rotation except for the very early vegetative stages. Similarly, there was no consistent increase in leaf phosphorus concentration when sorghum was grown in rotation with cotton (Brawand and Hossner, 1976). They concluded that although there was a rotation effect, it could not be attributed to improved phosphorus nutrition. Copeland and Crookston (1992) reported that K and total micronutrient content increased for corn in a 2-year rotation with soybean as compared to continuous monoculture. They proposed that general improvement in plant nutrition may have been due to an improvement in corn root function and that causal agents such as mycorrhizae may have played a role. Following a similar argument, the same research group (Copeland et al., 1993) reported that increased water use by first-year rotated corn or increased water use efficiency of rotated soybean, as compared to continuous monoculture, demonstrated that rotation increased root surface and/or root activity which in turn improved water relations and increased grain yield. Inclusion of legume cover crops into a crop rotation in the southeastern United States also resulted in a beneficial redistribution of potassium to the soil surface from deeper in the soil profile (Hargrove, 1986). Extractable calcium and magnesium levels do not appear to be affected by crop rotation. Increased availability of micronutrients including iron, copper, and zinc because of microbiologically enhanced chelation may also be a beneficial effect of crop rotation and cover crops (King, 1990).

D. DISFASE AND PESTINTERACTIONS Crop rotation is a fundamental tool of integrated pest management. Francis et al. (1986a) coined the term “biological structuring” to describe the use of crop

rotations, management alternatives, biological phenomena, environmental conditions, and interactions of these factors to manage crop pests such as disease, weeds, and insects. Crop rotation affects pest pressure in various ways, but in general the literature supports Francis and Clegg (1990) who stated that “the greater the differences between crops in a rotation sequence, the better cultural control of pests can be expected.” While crop rotation does reduce pest pressure it should be noted that even when pest pressure is minimal, the rotational effect still exists. This suggests that pest control is a contributor to the benefit of crop rotation, but is not responsible for the rotation effect itself. However, it should be

CROP ROTATIONS FOR THE 2 1st CENTURY

17

recognized that we are unaware of all pests which detrimentally affect crops and thus it can be hypothesized that much of the rotational effect is due to alleviation of unknown pests. Crop rotation is an effective tool against certain pests, and efficacy may contribute to the rotation effect, but rotation does not control all pests (Bullock, 1992). Pests which are controlled by crop rotation have the following characteristics (Flint and Roberts, 1988). First, the pest inoculum source must be from the field itself. Crop rotation does not control highly mobile pests since they have the ability to invade from adjacent fields or other areas. Pests which can be controlled by rotation include soil and root-dwelling nematodes, soilborne pathogens (if they do not produce airborne spores), and vegetatively propagated weeds such as nutsedge (Cyperus). Second, the host range of the pest needs to be fairly narrow or at least must not include plants which are reasonably common in a given area. Third, the pest must be incapable of surviving long periods without a living host. In other words, the pest populations must decrease substantially within a year or two of removing a living host plant. 1. Weeds

Weeds can reduce crop yields provided their densities reach a biological threshold. Most research indicates that biological thresholds are greater than zero (Aldrich, 1987), but there are scattered arguments in the literature that biological thresholds are zero (Cousens, 1985). A combination of crop rotation, smother crops, and mechanical cultivation were used to control weeds prior to the introduction of the synthetic herbicide 2,4-D [(2,4-dichlorophenoxy)acetic acid]. Crop rotation alone was not sufficient to control weeds; all three methods had to be used as an integrated program with a primary goal of preventing weed reproduction (Regnier and Janke, 1990). Crop rotation helps control weeds because they thrive and increase in crops which have similar growth requirements to their own. For example, grasses thrive in continuous corn, while broadleaf weeds thrive in continuous soybean. In Nebraska, rotating corn and grain sorghum with a broadleaf crop is an effective method of controlling shattercane (Sorghum bicolor L.) because it allows for the use of herbicides which are phytoactive on cereals (Francis and Clegg, 1990). Similarly, Dale and Chandler (1979) reported that a corn and cotton rotation enabled growers to control johnsongrass (Sorghum halepense L.) much better than a continuous corn rotation because grass-specific herbicides could be used during the cotton phase of the rotation. Crop rotation introduces conditions and practices that are not favorable for a specific weed species and thus growth and reproduction of that species are hampered. For example, Forcella and Lindstrom (1988) found 25 weed seeds m-2 in a continuous corn field, but only 4 weed seeds m-2 where corn was

18

D. L.KARLEN ET AL.

grown in rotation with soybean. Not all crops are equal in their competitiveness with weeds. Van Heemst (1985) ranked 25 crops for their ability to compete with weeds based on a mean reduction in yield. Wheat was considered the most competitive and given a rank of first. Soybean ranked fourth and corn seventh. Regnier and Janke (1990) suggested that factors such as rate and extent of canopy development, plant spacing, and life cycle all contributed to a crop’s competitiveness. It has also been noted that cultivars within a species also compete differently with weeds. Bullock (1992) cited several references suggesting this difference may be attributable to production of allopathic compounds, especially if small grains such as rye, wheat, oats, or barley are included in the rotation. The importance of crop rotation was diminished with the advent of synthetic herbicides. However, there is ample evidence confirming that crop rotation irnproves weed control even with synthetic herbicides (Bullock, 1992). For example, after 7 to 8 years of standard chemical and mechanical weed control from 1500 to 3000 weed seed m-2 were found with continuous corn, while in a cornsoybean rotation, the soil had from 200 to 700 weed seed m-2 (Forcella and Lindstrom, 1988). Withholding herbicides for 1 year reduced continuous corn yield by 10 to 27% but did not reduce corn yield in the 2-year corn and soybean rotation. Interest in using crop rotation to control weeds is gaining popularity, especially among those persons focusing on sustainable agriculture. For example, compared to growing continuous corn, growing corn in a 2-year soybean and corn rotation or a 3-year soybean, wheat, and corn rotation reduced giant foxtail (Seturiafuberi Herrm.) seed at the 0- to 2 . 5 , 2.5- to lo-, and 10- to 20-cm depths (Schreiber, 1992). Similarly, Ball ( 1992) reported that cropping sequences were the most dominant factor influencing species composition in weed seed banks. Temporal diversity achieved through crop rotation and spatial diversity achieved through intercropping can markedly reduce weed population density and biomass production (Liebman and Dyck, 1993). Among 26 comparisons between monoculture and rotation cropping systems, they found that emerged weed densities with rotations were lower in 21 studies, higher in 1 study, and equivalent in 5 studies. They concluded that the success of rotation systems for weed suppression appears to be based on the use of crop sequences that create varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable or inhospitable environment that prevents the proliferation of a particular weed species. Liebman and Dyck (1993) concluded that the relative importance and most effective combinations of various weed control tactics have not been adequately evaluated and recommended, therefore three research thrusts should be addressed. These included (1) determining effects of crop rotation and intercropping on weed population dynamics including weed seed longevity, weed seedling emergence, weed

CROP ROTATIONS FOR THE 2 1st CENTURY

19

seed production and dormancy, agents of weed mortality, resource competition between cultivated crops and weeds, and allelopathic effects; (2) determining how to combine specific components of rotation and intercropping strategies that may be important for weed control; and (3) designing and testing new integrated approaches for weed control at the scale of complex farming systems.

2. Insects Insect pests which have specific or at least narrow host ranges and which are incapable of extended migration are particularly susceptible to crop rotation (Ware, 1980). An example of this is the control of northern corn rootworm (Diabrotica sp.) in the central United States. In a monoculture corn production system, rootworm reaches an economic threshold about 30% of the time, but in a 2-year corn and soybean rotation, the economic threshold is reached less than 1% of the time. However, increased use of a 2-year corn and soybean rotation throughout much of the northern corn belt has resulted in selection for corn rootworms with a 2-year rather than a 1-year diapause. Therefore, reports exist of economic damage for corn grown in a short, 2-year rotation (Ostlie, 1987). For some insect species, crop rotation is not an effective control practice. For example, Johnson et al. (1984) reported that black cutworms (Agrotis ipsilon) are more of a problem when corn is rotated with either soybean or wheat than when it is grown continuously. Apparently, black cutworm moths are less attracted to corn residue than to either soybean or wheat residues for oviposition (Busching and Turpin, 1976).

3. Diseases Crookston (1 984) suggested that decreased crop yields associated with monoculture cropping systems were caused by increases in some unknown soil pathogen. Although attractive and frequently used to justify crop rotation as a method for preventing fungal diseases (Curl, 1963), Crookston’s suggestion is not universally accepted (Roder et a f . , 1988). It is not necessarily clear to what extent disease prevention contributes to the rotation effect. Bullock (1992) found that monoculture wheat has problems with fungal diseases, in particular take-all (Gaeumannomyces graminis var. tritici), but the severity of fungal diseases in continuous wheat often decreases within 3 to 5 years. The reduction in severity is known as “take-all decline,” and as a natural control of the disease it is effective. The mechanisms responsible for take-all decline are not completely understood, but changes occur in the microflora and microfauna in soils where take-all fungus is established. Part of the take-all decline is due to a build up of competitive and predatory microorganisms which control the take-all fungus (Crookston, 1984). Curl (1963) suggested that in some cases the control mechanism may be a pest-

20

D. L. KARLEN ET AL.

predator type of relationship, while in others, the organisms are simply competing for limited resources. Crookston et al. (1991) postulated that a buildup of beneficial organisms which help to control detrimental organisms might explain why second-year yields for a continuous corn cropping system often show a greater decline compared to yields of rotated corn than that observed during the later years. Meese et al. (1991) reported that withholding corn for 1 year is sufficient to obtain the maximum rotation effect, but Crookston et al. (1991) reported that withholding corn for more than 1 year would increase the rotation effect slightly. Both reported that soybean requires more than 1 year of absence to negate deleterious soybean yield effects. The exact nature of the agent responsible for the deleterious effect of continuous soybean is not clear, but Whiting and Crookston (1993), working in the northern U.S. corn belt, have reported that plant diseases are not playing a major role. Thus, it is reasonable to conclude that the yield increase observed for soybean in a soybean and corn rotation is not necessarily due to a reduction in seventy or incidence of plant diseases. Time does not decrease the seventy of all diseases (Bullock, 1992). Studies by Stromberg (1986) showed that gray leaf spot in corn (Cercosport zeae-maydis) in the southeastern United States becomes a severe problem if corn is grown continuously using no-tillage production practices. However, a rotation in which corn is absent for at least 1 year prevents the disease from becoming an economic problem.

4. Nematodes The use of crop rotations to control Meloidogyne and Heteroderu glycines species of plant parasitic nematodes on tobacco and soybean crops in North Carolina was established during the 1950s and 1960s (Barker, 1991). This approach for control is currently increasing in importance once again because many chemical nematicides are no longer available (Flint and Roberts, 1988). Negative impacts of nematodes on crop production are decreased by crop rotation because changing plant species generally reduces population levels of most plant parasitic nematodes (Dabney e f ul., 1988; Ferris, 1967; Edwards et al., 1988). A reduction of nematode pressure may account for most of the rotation benefit for soybean in the southeastern United States since cyst nematodes (Heteroderu glycines Inchinohye) in soybean can generally be controlled by crop rotation (Dabney et al., 1988). Bailey et al. (1978) reported similar conclusions with regard to root knot nematodes. Sasser and Uzzell (1991) reported that soybean yields were improved most by increasing the number of years during which a nonhost crop was grown. In other nematode studies, a 1-year rotation with barley (Carter and Nieto, 1975), clean fallow (King and Hope, 1934), or planting a resistant processing tomato cultivar (Flint and Roberts, 1988) were effective in controlling the cotton root knot nematode (Meloidogyne incognita).

CROP ROTATIONS FOR THE 2 1st CENTURY

21

The economic impact of 3-year cotton and soybean rotations in soils with varying population densities of Hoplolairnus Columbus was estimated by Noe et al. (1991). They calculated maximum yield losses to be 20% for cotton and 42% for soybean. Maximum nematode population densities at harvest were estimated to be 182 per 100 cm-3 of soil for cotton and 149 per 100 ~ m for - soybean. ~ They projected net incomes to range from a loss of $17.74 ha-1 for a soybean, cotton, and soybean rotation to a profit of $46.80 ha-' for a cotton, soybean, and cotton sequence. A range of economic assumptions and management conditions are considered in this study. Crop rotation research was de-emphasized in the early 1960s as priorities shifted to the development of resistant cultivars and the evaluation of nematicides (Schmitt, 1991). Resistant soybean lines (Brim and Ross, 1965, 1966)performed well in cyst-infested fields and gave the maximum yield potential for the environment in which they were grown. As these and other nematode resistant cultivars became widely grown, often in monoculture, nematode races shifted and, consequently, the use of resistant cultivars as a management tool is now limited. Schmitt (1991) reported that following 2 or more years of a nonhost crop, nematode populations were at low or undetectable levels and that soybean yields were not affected. Those results suggested that a more prudent use of resistant cultivars grown in rotation with nonhost crops would increase their longevity in fields infested with cyst nematode races 1, 3, or 4.

E, ALLELOPATHY Allelopathy occurs when one plant species releases chemical compounds, either directly or indirectly through microbial decomposition of residues, that affect another plant species. Liebman and Dyck (1993) stated that including allelopathic plants in a crop rotation or as part of an intercropping system may provide a nonherbicide mechanism for weed control. They found few studies that focused on use of allelopathy in rotations, but management of allelopathic cover crops for weed control has been extensively investigated (Bullock, 1992). Results of those studies are directly applicable to crop rotations. Liebman and Dyck (1993) found that exudation of allelochemicals from living roots of barley and oats have been suggested, but most studies of allelopathy have been conducted with cover crops or dead crop residues associated with notillage production practices. Studies such as those by Yakle and Cruse (1983, 1984) are typical of those efforts to understand the effects of this complex process, especially for monoculture corn production. With respect to weed suppression, Putman er al. (1983) found that compared to unplanted control treatments, residues of several fall-planted cereal and grass cover crops significantly reduced growth and dry matter production by several weed species during the following summer. Rye, wheat, and barley, which survived the winter and were

22

D. L. KARLEN ET AL.

subsequently killed by nonselective herbicides, had greater a~l~lopathic effects and suppressed weeds much more than oats, grain sorghum, or sorghum-sudan grass (Sorghum urundinaceum [Desv.] Stapf var. sudunense [Stapf] Hitchc.), which were killed by winter conditions. Shading and cooling of the soil may have contributed to this control, but several other studies have shown suppressive effects that cannot be attributed to the physical presence of mulch. Identification of the mechanisms governing the differential effect of cover crop residues on weed and crop species provides a major challenge for persons studying and using crop rotations.

IV, SOIL QUALITY EFFECTS The need to reduce negative on- and off-site impacts of agricultural practices will probably provide one of the s~ongestincentives for reintr~ucingcrop rotations into farm management plans. Kay (1990) reached a similar conclusion in stating that a major goal for agricultural research will be to identify and promote cropping systems which sustain soil productivity and minimize deterioration of the environment. To assess the effects of soil and crop management practices such as crop rotation on both factors, several projects focus on the concept of soil quality as an assessment tool (Karlen el a / . , 1992; Doran and Parkin, 1994; Karlen and Doran, 1993; Karlen and Stott, 1994). Using different crop rotations may improve soil quality by more closely mimicking natural ecosystems than current farming systems (Karlen et al., 1992). This woutd occur because temporal and spatial diversity across the landscape would be increased. Furthermore, management strategies that maintain or add soil carbon have good potential for improving the quality of our soil resources. Critical factors being included in most soil quality assessments include measurements of soil structure, aggregation, bulk density, water infiltration, water retention, soil erosivity, and organic matter (Karlen and Stott, 1994).All of these factors are influenced by crop selection and rotation. Therefore, it is logical to examine the effects of crop rotations on the various soil quality indicators as we assess the need to re-emphasize rotations in 2 1st century farming systems.

A. SOILSTRUCTURE Kay (1990) stated that the characteristics of plant species being grown, the sequence of different species, and the frequency of harvest were ail aspects of cropping systems that affected soil structure by influencing the formation of biopores by plant roots and soil fauna. The network of biopores subsequently

CROP ROTATIONS FOR THE 2 1st CENTURY

23

determined the amount and distribution of organic materials throughout the soil. Kay (1990) also stated that the effectiveness of different crops for improving soil structure at different soil depths was related to the amount of water extracted and photosynthate deposited by the plant, as well as persistence of photosynthate carbon at different depths. Elkins (1985) demonstrated how different crops can affect soil quality by creating biopores in compacted soils at depths that were not economically tilled. He found that bahiagrass (Paspalum notatum Flugge ‘Pensacola’) roots could penetrate soil layers that impeded cotton roots and that including bahiagrass in a rotation increased the number of soil pores that were 1.0 mm or larger in diameter. The biopores enabled the cotton to obtain water and nutrients from a depth of at least 60 cm and were effective for 3 years after the grass had been incorporated by plowing. Plants are important factors influencing the amount of stress imposed on soil structure at or near the surface because rooting densities generally decrease exponentially with depth in well-structured soils (Hamblin, 1985; Dwyer et al., 1988). The total amount of photosynthate deposited below ground and the resistance of this material to complete mineralization also varies considerably between plant species (Kay, 1990). For example, Davenport and Thomas (1988) reported that the total amount of rhizodeposition by bromegrass (Bromus inermis L.) can be twice as high as the amount deposited by corn. Furthermore, carbon originating from bromegrass was more persistent in the soil than that from corn (Davenport et al., 1988). Russell (1973) stated that, in natural grasslands, over 2.5 Mg ha-’ dry matter can be added to the soil each year as roots, and total root systems may contain more than 12 Mg ha-1 of dry matter, compared to only 2 to 5 Mg ha-1 of aboveground material. Abandonment of multiyear rotations in favor of short rotations has generally resulted in a degradation of soil structure as measured by soil aggregate stability, bulk density, water infiltration rate, and soil erosion (Bullock, 1992). Much of the blame for this degradation is attributed to decreases in soil organic matter content, but Bruce et al. (1990) found the relationships to be complex and easily erased or modified by tillage. Langdale et al. (1990) reported that crop rotations did not affect soil physical properties on selected Ultisols, but those findings are not predominant in the literature. Demonstrating a direct linkage among crop rotation, soil structure, and crop yield is very difficult, even though it is generally accepted that improved soil structure is beneficial to crop production (Johnston et a l . , 1942; Page and Willard, 1947). Strickling (1950) and Morachan et al. (1972) found no correlations between physical improvement in silt loam soils and corn yield in the midwest United States and both concluded that physical conditions were not limiting to yield. Bullock ( 1 992) suggested that the differences in reports may be due to climate since significant correlations of soil structure with crop yield seem to be weather dependent (De Boodt et al., 1961).

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D. L. KARLEN ET AL.

B. AGGREGATION Crop rotations that include legumes and/or grasses are generally beneficial to aggregate stability and formation of favorable soil structure (Robinson et al., 1994; Kay, 1990). Using measurements of mean weight diameter, Van Bavel and Schaller (1950) and Wilson and Browning (1945) showed that soil aggregation with continuous corn was half of that found with a corn, oat, and meadow rotation. Bullock (1992) argued that it is convenient to suggest crop rotations beneficially affect soil aggregate formation and stabilization, but quantifying the relationships is not that simple. When rotations involve numerous years of a sod, pasture, or hay crops, improvements in soil structure do occur (Olmstead, 1947; Strickling, 1950; Adams and Dawson, 1964; Tisdall and Oades, 1982; Power, 1990), but short rotations often reduce soil aggregation. For example, a cornsoybean rotation results in greater yield for both corn and soybean, but because soybean returns less crop residue, the rotation often degrades soil aggregation faster than does well-fertilized continuous maize (Power, 1990). This finding was consistent with that of Olmstead (1947), who reported that short rotations and commercial fertilizers do not maintain soil aggregates. Raimbault and Vyn (1991) observed improved aggregate stability due to crop rotation in Ontario. They reported that compared to measurements under continuous corn, soil aggregate stability in most years was highest under continuous alfalfa or where a legume (either alfalfa or red clover) was grown in rotation with corn. They also found that first-year corn grown in rotation yielded 3.9% more than continuous corn grown using conventional tillage, and 7.9% more than continuous corn grown using minimum tillage practices. Hussain et al. (1988) also concluded that crop rotation increased soil aggregation over time based on geometric mean diameter (GMD) values. Their calculations for continuous corn, corn following soybean, soybean following corn, and an oat/red and yellow sweetclover mixture following corn showed average GMD values of 150, 2 I 1, 225, and 31 1 pm, respectively. Evaluations of farming systems that include crop rotation have also shown significant differences in soil aggregation. For example, Jordahl and Karlen (1993) compared alternative and conventional farming systems in central Iowa and found that combined effects of alternative practices (i.e., a 5-year crop rotation including oats and meadow, manure/municipal sludge application, and ridge-tillage) resulted in greater water stability of soil aggregates than the conventional practices (i.e., a 2-year corn-soybean rotation with reduced tillage). They attributed the increased aggregation to the longer crop rotation, which included the oat and meadow, but also to application of 45 Mg ha-1 of a mixture of animal manure and municipal sewage sludge during the first 3 years of each 5-year rotation. Similarly, Reganold (1988) found a more granular soil structure and a more friable consistence in soil managed without the use of commercial

CROP ROTATIONS FOR THE 2 1st CENTURY

25

fertilizers and only limited use of pesticides (organic) than in that managed with recommended rates of commercial fertilizers and pesticides (conventional) in the Palouse region of eastern Washington. Other studies related to soil structure and aggregation also show that soil amendments, including animal manures and municipal sewage sludge, can lead to increased water stability of aggregates, decreased susceptibility to crust formation, and an increased proportion of large pores (Kay, 1990).

C. BULKDENSITY Cropping systems which return the most residue to the soil usually result in the lowest soil bulk density. Therefore, continuous corn will frequently result in lower soil bulk densities than corn-soybean rotations, even though crop rotation generally results in greater grain yield (Bullock, 1992). In plots with a pigeon pea (Cajunus cujun L.) and corn rotation, Hulugalle and La1 (1986) found that bulk density was always lower than in well-fertilized continuous corn. Hageman and Shrader (1979) found that after 20 years, soil bulk density following continuous corn was slightly lower than after a 4-year corn, oats, meadow, and meadow rotation (1.13 vs 1.17 g cm-3, respectively). They also found that annual application of 134 kg N ha-1 increased soil organic matter concentrations (52.1 vs 53.3 g kg- 1) and decreased soil bulk density (1.10 vs 1.20 g ~ m - compared ~ ) to the 0 kg N ha-1 treatment. These differences were attributed to less machinery travel and greater organic matter production with the corn, oats, meadow, and meadow rotation. Hageman and Shrader (1979) concluded that as soil organic matter increases, soil bulk density decreases. Bullock (1992) suggested that several factors, including traffic patterns, tillage, or sampling technique, may complicate assessments of crop rotation effects on soil bulk density. He concluded that benefits obtained from short rotations, such as a 2-year corn and soybean sequence as compared to wellfertilized continuous corn, were probably not attributable to lower soil bulk density. However, reduced bulk densities may contribute to the rotation benefit obtained from sod, pasture, or hay crops. Hammel (1989) measured bulk density and soil impedance after 10 years of continuous management in a long-term tillage-crop rotation experiment on Palouse (fine-silty, mixed, mesic Ultic Haploxeroll) and Naff (fine-silty, mixed, mesic Ultic Argixeroll) silt loam soils. He concluded that crop rotation did not significantly influence either soil property. Logsdon et al. (1993) reported that bulk densities were sometimes lower and the volume of large pores was slightly higher in fields where a 5-year corn, soybean, corn, oats, and meadow rotation was being used compared to that for a 2-year corn and soybean rotation.

26

D. L. KARLEN ET AL.

D. WATERINFILTRATION AND RETENTION In the southeastern United States, soil organic matter content, water infiltration rate, and aggregate stability all increased as the proportion of sod in the rotation increased (Adams and Dawson, 1964). Wischmeier and Mannering (1965) also reported a positive correlation between water infiltration rate and soil organic matter content for several midwestern soils with organic matter concentrations from 1 to 14%. Allison (1973) attributed increased water infiltration to improved soil structure and higher soil organic matter content. Recent farming systems studies in Iowa support this conclusion, i.e., steady-state infiltration measurements were somewhat higher for longer rotations where soil organic matter concentrations were slightly higher than those for shorter rotations (Logsdon et al., 1993; Jordahl and Karlen, 1993). Bullock (1992) concluded that crop rotation did not benefit production by increasing water-holding capacity, even in situations such as long-term pastures which resulted in substantial increases in soil organic matter content. This conclusion is based on several studies. Among these are results from Jamison (1953) who stated that organic matter does have a large water-holding capacity and that most of the water held by organic matter is held at potentials far less than - 1.5 MPa (the water potential at which water is not sufficiently available for survival of most plants). A second reason is that increased soil aggregation results in decreased plant available water (Jamison, 1953; Hillel, 1980). Bullock (1992) stated that this occurred because a larger fraction of the water is held at potentials less than - I .5 MPa and because of an increase in macropore volume and a decrease in the micropore volume. He supported these arguments with field data from Johnston et al. (1942) who reported that during dry years, continuous corn may yield more than rotated corn. However, during years of adequate rainfall, rotated corn generally yielded more than continuous corn (Johnston et al., 1942; Sahs and Lesoing, 1985). Hudson (1994) used a critical review of literature on soil organic matter effects on plant available water capacity to argue against this position. He found that for sand, silt loam, and silty clay loam soils, the volume of water held at field capacity increased at a much faster rate than that held at the permanent wilting point. Hudson (1994) concluded that on a volumetric basis, soil organic matter is an important determinant of available water-holding capacity, thus indicating a reevaluation of crop rotational effects on plant available water might be warranted.

E. SOILERODIBILITY Soil erosion requires two processes: (1) detachment of soil particles, and (2) transportation of the soil material by erosive agents such as water or wind

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27

(Hussain et a l . , 1988). Soil detachment associated with water erosion can be initiated by raindrops or overland water flow during a rainfall event. Detachment by wind involves skipping or saltation of soil particles across the soil surface. Soil management practices such as crop residue placement, application of animal manure, or using crop rotation can have both direct and indirect effects on soil physical properties which subsequently affect the detachment process (Bullock, 1992). With regard to crop rotation, Hussain et al. ( 1 988) reported that the rate of splash detachment from continuous corn treatments was higher than from soil under crop rotations. They found splash detachment rates of 48 mg ~ m for - ~ continuous corn, 40 for corn following soybean, 39 for soybean following corn, and 31 for an oat/red and yellow sweetclover mixture following corn. Johnston et al. (1 942) evaluated continuous corn, a corn-oat-sweetclover rotation, and continuous bluegrass and reported that over a 9-year period, the continuous corn treatments lost 793 Mg ha-’ of soil, while the rotation and continuous bluegrass treatments lost 202 and 42 Mg ha-’, respectively. They noted that runoff was in the same general order as soil losses. Stewart et al. (1976) reported that soil losses from corn in rotation with meadow were 14 to 68% of the soil loss from continuous corn. Reganold (1988) found a 16-cm difference in topsoil depth between adjacent organic and conventional farms in the Palouse. This difference was attributed to significantly greater erosion on the conventional farm between 1948 and 1985. He attributed the difference in erosion rates to crop rotation since the organic farm included green manure crops within the rotation, while the conventional farm did not. Contrary to the benefit of rotations which include forages or other surface cover during the spring, 2-year corn and soybean rotations can result in greater soil erosion than continuous corn (Bullock, 1992). For example, over an 18-year period, soil loss from a 2-year corn and soybean rotation was 45% higher than that from continuous corn (van Doren et al., 1984). This often occurs because the amount of surface residue following soybean is very low (Stewart et a l . , 1976; Laflen and Moldenhauer, 1979; Papendick and Elliott, 1984). Alberts et al. (1985) reported that soybean production results in an annual soil loss 3.4 times greater than that seen with corn production but noted that differences in erosion were not simply a function of less biomass. They concluded that corn residue is better at preventing soil erosion than is soybean residue, even when they are present in similar amounts. Laflen and Moldenhauer (1979), in a 7-year study, found that average annual soil losses were about 40% greater when corn followed soybean than when corn followed corn. They concluded the difference was caused by a “soil effect” because major differences in soil loss occurred during the period 30 to 60 days after planting, a point at which canopy development and residue cover were almost identical.

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F. ORGANIC MATTER Soil organic matter is the soil quality indicator for which the most information relative to crop rotation exists, but it is also the indicator for which the most unanswered questions remain. Crop rotation affects soil organic matter in several ways. Factors affecting it include rotation length, losses caused by tillage operations, mineralization, and interaction with fertilization practices.

1. Rotation Length Crop rotations which involve long periods of sod, pasture, or hay crops generally increase soil organic matter content during periods of these crops. This increase is presumably a primary factor that beneficially affects subsequent crops and contributes to the rotation effect (Bullock, 1992). Hussain et al. (1988) reported increased soil organic matter content with a 2-year corn and soybean rotation, but such findings are the exception. Generally, this short rotation results in lower soil organic matter levels than continuous corn, even though it provides a rotation effect (Dick et al., 1986a,b). The primary cause for this response appears to be that soybean simply does not produce as much biomass as corn (Dick et al., 1986a,b). An exception to this general conclusion often occurs in the southeastern United States where nonirrigated corn growth is frequently reduced by drought stress, and full-season, nonirrigated, determinate soybean cultivars have been shown to produce more than 7 Mg ha-' of aerial biomass (Hunt and Matheny, 1993). Results from Havlin et al. (1990) demonstrated that including grain sorghum in a rotation, rather than growing continuous soybean, increased organic carbon and nitrogen in the soil. They concluded that increasing the quantity of residue returned to the soil through higher yields or through greater use of high residue crops in the rotation, combined with reduced tillage, could improve soil productivity. Juma er al. (1993) concluded that after 50 years of research on Gray Luvisolic soils at the Breton Plots in Alberta, Canada, soil organic matter content is about 20% higher where a 5-year rotation has been used than where a 2-year, wheat and fallow rotation was followed. Similarly, Unger (1968) found that when tillage treatments were kept constant, continuous cropping resulted in a significant increase in soil organic matter concentrations compared to a cropfallow system.

2. Tillage Losses Tillage, which inverts and mixes the soil, introduces large amounts of oxygen into the soil and stimulates aerobic microorganism consumption of organic matter as a food source (Doran and Smith, 1987). When virgin eastern Oregon soils

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29

were cultivated, some lost over 25% of their organic matter in the first 20 years, with 35 to 40% being lost in 60 years (Rasmussen er al., 1989). Tillage for weed control during the fallow period was the primary cause for the loss of soil organic matter. Ridley and Hedlin (1968) found that after 37 years, soils which had initial organic matter concentrations of nearly 10% had 7.2% organic matter if cropped every year, compared to 3.7% in those fallowed every other year. Soils fallowed after every two or three crops had intermediate soil organic matter concentrations. Use of no-till systems can reduce the rate of soil organic matter loss, but not completely stop it. Collins er al. (1992) reported that after 58 years, total soil and microbial biomass carbon and nitrogen were significantly greater in annualcropping treatments than for wheat-fallow rotations. They concluded that residue management (i.e., reduced tillage) significantly affected the level of microbial biomass carbon and that annual cropping significantly reduced declines in both soil organic matter and soil microbial biomass. Similarly, Havlin et al. (1990) found that compared to native grassland, a 12-year wheat and fallow rotation resulted in total soil organic matter concentrations that were 4, 14, and 16% lower with no-till, stubble mulch, and conventional tillage, respectively.

3. Mineralization Effects FrequentIy, crop rotation benefits derived from organic matter are attributed to the release of nitrogen through mineralization (Bullock, 1992). However, Doran and Smith (1987) concluded that relationships among soil organic matter content, management practices including crop rotations, and nitrogen availability were not always predictable, constant, or direct. It is generally accepted that soil organic matter affects many of the soil quality indicators influencing mineral availability. These effects include increased water infiltration (Wischmeier and Mannering, 1965; Adam et al., 1970; Allison, 1973; MacRae and Mehuys, 1985), improved aggregate formation and stability (Spurgeon and Grisson, 1965; Harris er al., 1966; Fahad er al., 1982; MacRae and Mehuys, 1985), lower bulk density (De Kimpe et al., 1982), higher water retention capacity (Jamison, 1953; Hudson, 1993), improved soil aeration, and reduced soil erosion (USDA, 1980; Bezdicek, 1984; Reganold, 1988). Commercial agriculture has altered both the quality and quantity of soil organic matter in many soils (Robinson er al., 1994). Often, these soils may have taken hundreds or even thousands of years to reach stable soil organic matter conditions (Rasmussen et al., 1989). Destruction of soil organic matter by short rotations does not continue unabated until the soil is devoid of organic matter, but rather the soil organic matter reaches an equilibrium level (Joeffe, 1955; Allison, 1973; MacRae and Mehuys, 1985). When alternative tillage or crop rotations are used, a new equilibrium point is established. For example, Larson et al. (1972)

30

D. L. KARLEN ET AL.

indicated that the addition of 5 Mg/ha of maize and alfalfa residue applied annually could maintain organic C at a level of 1.81%. However, this soil organic matter level is considerably lower than that found in its precultivation state. No-till and reduced tillage (Karlen et al., 1989, 1991) cropping systems have shown gradual increases in soil organic matter content when compared to more intensive tillage management practices. Similarly, Haas et al. (1957) reported that within the top 30 cm of medium-textured soils in the Great Plains, organic carbon concentrations ranged from 20 to 36% of that found in adjacent virgin grassland. They attributed the differences to cropping practices that included rigorous tillage and fallow periods. Different crop rotations seem to result in different soil organic matter equilibrium levels, but Miller and Larson (1990) predict that soil organic matter concentrations will never return to levels observed in their virgin state.

4. Fertilizer and Manure Interactions Juma et al. (1993) concluded that application of nitrogen, phosphorus, potassium, and sulfur fertilizer and animal manure to Gray Luvisolic soils increased soil organic matter by increasing crop yields. They also reported that application of manure increased soil organic matter even more than fertilizer. This presumably occurred because in addition to its nutrient value, the 9 Mg ha-1 of manure that was added each year represented an additional source of organic matter. The report by Juma et al. (1993) supports conclusions by Boyle et al. (1989) who suggested that returning carbon to the soil is “a necessary expense that insures a sustainable harvest.” Both support suggestions by Karlen et al. (1992) that crop rotation, cover crops, and conservation tillage as the practices most likely to improve soil quality as we enter the 21st century.

V. BIOLOGICAL DIVERSITY Crop rotations have more biological diversity than that which occurs with monocultures. This diversity can be divided into temporal and spatial components. Temporal diversity results from a sequence of crops being grown in a given field (i.e., crop change with time), and has been an important tool in breaking pest cycles, reducing soil erosion, and increasing yields. The second component, spatial diversity, results from greater numbers of crops being grown at a given time on the landscape. Managing spatial diversity for improved crop production is poorly understood and little used in modern agriculture. While

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examples of managing spatial diversity to increase crop yields and conserve resources exist, i.e., use of tree shelterbelts for grain yield increases and soil conservation (Burvill, 1950; Ferber, 1974; Kort, 1986), little research emphasis has been placed on this component of crop production. Strip intercropping is the practice of growing crops in a series of narrow, adjacent strips that uses spatial diversity to increase yields. A well-designed strip intercropping system uses complimentary growth habits of adjacent crops to reduce plant competition in the strip border positions (Cruse, 1990). Crops are rotated annually in this system, resulting in a production practice which effectively utilizes both temporal and spatial diversity to improve yields. Strip intercropping research has dominantly addressed corn and soybean, i.e., a two-crop strip system (Francis et al., 1986b). Corn yields benefit while soybean yields are normally reduced. Recent additions of a third crop strip containing small grain has resulted in strip border interactions different from, and more favorable than, those of the corn/soybean strip system (Cruse, 1990). Furthermore, improved soil conservation has been observed by cooperating farmers, but replicated research is yet to be done. Temporal life cycle differences between adjacent strips when three or more crops are included apparently reduces crop competition and increases yield. Growing only two crops with temporally similar life cycles (i.e., corn and soybean) appears to create competition for water, light, and nutrient resources in the border positions and can result in lower yields for one or both crops than if they were grown in larger blocks or fields. Crop spatial arrangements effectively add another dimension, or opportunity, to rotation management in developing environmentally sound and highly productive cropping systems.

A. EFFECTS ON WILDLIFE Quality and spatial diversity of landscape cover has repeatedly been shown to influence wildlife abundance on agricultural landscapes (Kendeigh, 1982; Vance, 1976). Studies conclusively illustrate, for example, the favorable impact of increased crop diversity in the midwest United States on the ring-neck pheasant (Phasianuscolchicus) population (Fanis et al., 1977; Taylor et al., 1978; Vance, 1976). This is particularly true when small grain and/or forages are rotated with row crop production. Most wildlife species that rely on agricultural habitat for survival sustain their populations much more effectively on landscapes with diverse rotations than on those with monocultures or continuous row crops (Moen, 1983). It is well recognized that crop rotations can be managed for increased crop production. It is less well recognized that rotations can also increase wildlife populations on the landscape.

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B. ALTERNATIVE LAND USES Agricultural crop products traditionally have been used for food and fiber. A new role of agricultural crops is that of energy production. Crops for bioenergy production are receiving increased emphasis for at least three reasons: (1) petroleum energy reserves are finite; (2) selected bioenergy products are relatively clean and less polluting than petroleum counterparts; and (3) farmers view the energy market as very large and with significant economic potential. Traditional energy plants such as trees used for firewood have not integrated well into conventional crop rotation schemes due to their semipermanency. However, crops such as corn and switchgrass (Panicurn virgatum L.) are agricultural crops manageable in conventional farm systems and may well serve the cropsfor-energy nitch (McClelland and Farrell, 1992). This type of plant material may integrate well with other agricultural crops in a rotation. Furthermore, perennial energy crops such as switchgrass may serve a secondary purpose-that of soil erosion control. A seemingly large potential exists for sod-forming biomass crops on conservation reserve program land when the contracts for these lands expire. These crops could be worked into a rotation with limited row crop production or could conceivably remain as “permanent” vegetative cover. Currently, crops grown for bioenergy have at least one significant technological drawback. When those such as corn are produced for energy products like ethanol, there is a negative energy balance because of the nitrogen fertilizer requirement (Pimentel, 1991; USDA, 1986). This occurs because approximately one-third of the energy required to produce corn in the United States is attributable to nitrogen fertilizer. Increasing the use of nitrogen-fixing legumes in crop rotations, and producing corn following them, could solve this dilemma. Studies have repeatedly shown that legumes in rotation can contribute sufficient nitrogen to meet the nitrogen demands of various succeeding crops (Francis and Clegg, 1990). Rotations coupled with technological advances in ethanol manufacture may be imperative in creating a positive energy balance in the crops-for-energy agricultural system.

VI. ECONOMICS OF CROP ROTATION Crop rotation will have an impact on the returns associated with alternative tillage systems, and responses will be different on different soils and in different regions. In northeastern Iowa, Chase and Duffy (1991) found that for continuous corn, returns to land, labor, and management were higher for moldboard plow and chisel plow than for no-tillage or ridge-tillage treatments. For a cornsoybean rotation, moldboard plow, chisel plow, and no-tillage performed equally

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well. In Quebec, Canada, Lavoie ef al. (1991) used linear computer models to evaluate net farm income as affected by crop rotation, farm size, tractor size, and weather conditions. They projected that continuous corn grain production would be most profitable for wet, normal, and dry weather conditions. Their empirical results tended to verify the profit maximization behavior of farmers since corn grain production in Quebec increased from 62,600 ha in 1976 to 260,000 ha in 1986. Their model predictions suggested that reduced tillage and crop rotation practices would reduce net farm income by 10 and 45%, respectively. Lavoie et al. (1991) concluded that the relative importance of cropping systems would likely change as more data on long-term and environmental benefits of crop rotations become available, especially if the information caused major changes in public policies regarding land use and crop production practices. Carter and Berg (1991) compared a traditional 7-year, furrow-irrigated rotation of (1) alfalfa, (2) alfalfa, (3) dry edible bean, (4) dry edible bean, (5) winter wheat, (6) silage corn, and (7) spring wheat/alfalfa with a rearrangement that placed silage corn and winter wheat in the third and fourth years instead of in the fifth and sixth years because of concern for inefficient use of residual soil nitrogen following the alfalfa crops. They found the alternate rotation reduced the number of tillage operations over the entire cropping sequence. This reduced soil erosion 47 to loo%, sustained crop yields, and increased net farm income by an average of more than $125 ha-' each year for a 5-year cropping sequence. Questions regarding optimum crop rotations for various regions are not new. With regard to the U.S. corn belt, Wiancko (1927) concluded that because of the favorable soil and climatic conditions, corn would be the principle crop for this region, and crop rotations proposed for general use in the region must recognize this characteristic. A 1991 survey, conducted in three Iowa counties where Iowa State University had conducted an integrated crop management project (Table III), revealed that a majority of the project cooperators, neighbors of those cooperators, and randomly sampled farmers from throughout those counties considered it important to include small grain or forage in a crop rotation. The economic challenge seems to focus on maintaining profitability, while producing crops with practices that prevent soil degradation and loss of nitrogen and other nutrients to surface and groundwater resources.

VII. POLICY IMPACTS ON CROP ROTATIONS American farm policies have traditionally dealt with food cost, commodity supply, and farmer income (Doering, 1992). Farm policy influences profitability of crop rotation through five processes: deficiency payments, acreage reduction, base-acreage levels, crop prices, and risk reduction (Young and Painter, 1990).

D. L. KARLEN ET AL.

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Table I11 Iowa Farmer AttitudesUwith Regard to the Statement “A Good Cropping System Should Include Rotations of Small Grains or Forage Along with Row Crops” Survey grouph ICM cooperators ICM neighbors Random sample

Agree (%)

Undecided (%)

Disagree (%)

56 74“ 78

13

32 17 14

10 8

* Information provided by Dr. Steve Padgitt, Iowa State University. Ames, Iowa, from a survey associated with participants and nonparticipants in Iowa State University’s Integrated Crop Management (ICM) research program. ICM cooperators included those persons in Caroll, Kossuth, and Sioux Counties who had participated in a Model Farms program with Iowa State University. Neighbors are persons living adjacent to a coopcrator. Random includes persons living in those counties. L‘ In these counties 14% of the cooperator group had small grains or forages in addition to row crops. Among neighbors and participants in the random sample, 50% had small grains or forages in addition to row crops.

Concerns about environmental impacts have been peripheral to date, but questions regarding agricultural policy impacts on practices such as crop rotations are being asked more frequently. Factors including production surpluses, rising commodity program costs, and environmental degradation are encouraging a reexamination of current programs (Moore, 1989). Participation in U.S. agricultural commodity programs has generally resulted in decisions to use more erosive crop rotations (Poe et al., 1991). This has tended to occur because under conditions of program participation, on-site and off-site erosion costs that can affect crop rotation decisions have been internalized. Therefore, these factors influence field-scale management decisions only when long time periods (>40 years) are considered. The implications of crop rotation on susceptibility of the Texas High Plains to wind erosion and groundwater depletion were evaluated by Lee et al. (1989). Their simulations indicated that farm program participation, coupled with base acreage restrictions, encouraged production of continuous cotton. They projected that average annual wind erosion under continuous cotton would be two to six times greater than with alternate cropping systems. However, compliance with base acreage restrictions prior to the 1985 Food Security Act limited adoption of multiyear or multicrop production systems. Changes in policy were viewed as supporting 2-year rotations such as cotton and wheat or 3-year rotations such as cotton, sorghum, and wheat, both of which provide substantial wind erosion control. Until recently, agricultural policy has reflected goals of increased farm and

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rural income, low-cost food, improved rural conditions, improved technical efficiency of farming operations, and natural resource conservation as part of the agricultural productivity base (Doering, 1992). At times, programs designed to enhance the various goals have conflicted. Furthermore, the market often fails to alert agricultural producers to the real costs associated with on-site and off-site environmental damage (Doering, 1992). Agricultural policies, however, provide only one part of a farmer’s decision framework. Other factors include the relative costs associated with alternatives such as tractors vs draft animals, fertilizers vs manure, and pesticides vs cultivation. Nonfarm policies that affect the economy, trade, industry structure, resources, and the environment can also have more impact on the way farmers manage their land than official agricultural policy (Doering, 1992). He also states that national decisions about health, safety, and environmental quality have had and will continue to have a great influence on the way farmers farm. From this perspective, Doering (1992) concludes that federal policies toward agriculture do not appear to provide an incentive or disincentive for less intensive and more environmentally benign agricultural practices and cropping systems. He suggests that new policy approaches should target specific management practices, cropping patterns, input use, or sensitive locations. This approach will require new policy mechanisms to deal specifically and equitably with environmental concerns and society’s changing values, while recognizing actual production decisions that farmers face daily. In a report compiled from 12 interviews with agricultural policy-making individuals in Washington, D.C., Moore (1989) found that crop rotations, in principle, were viewed as beneficial for American agriculture. However, unqualified support for a crop rotation policy was not expressed. The primary concerns were focused on how uncontrollable conditions, such as international market prices or drought, would impact successful implementation of a crop rotation policy. Two perspectives that emerged from the interviews were a desire to deter monocropping practices and concern for maintaining farm incomes (Moore, 1989). The first perspective focused on total resource efficiency for society as a whole and emphasized benefits to be gained by encouraging crop rotations. These benefits included improved soil and water quality, increased farm flexibility, reduced program costs, increased diversity, reduced dependence on nitrogen fertilizers, reduced chemical input costs, and reduced insect pests. The second perspective emphasized known benefits of crop rotation to individual producers, i.e., maintaining productivity at the microeconomic level. To be effective, crop production or land use subsidies would be needed to compensate farmers for using crop rotations (Moore, 1989). Information and education regarding site-specific crop rotation practices and impacts, profitability of rotated crops, market infrastructures for new crops, an integration of new livestock production practices, new equipment, and reduced exports are some needs iden-

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tified as being crucial for increased adoption of crop rotations as we enter the 2 1st century. Potential incentives to encourage crop rotation include monetary compensation, long-term program stability, provision of appropriate knowledge and skills, and development of market infrastructure for various new crops (Moore, 1989). Several disincentives for continuing monoculture include regulations, liabilities for on-site and off-site damages, and internalization of external costs. It was suggested to Moore ( 1 989) that crop rotations could be required under certain site-specific circumstances and that routine groundwater monitoring might be required. The projected impacts of crop rotation policies were that corn would be less widely distributed, especially in nontraditional corn-growing areas. Farm labor and management requirements would increase-perhaps increasing opportunities for rural employment. General environmental quality would improve, although changes could not be guaranteed. Improved rural aesthetics, increased requirements for educational and training programs, and some redistribution of income among companies as they develop uses and markets for alternative crops would be expected. There would also be increased demand for production consultants and a probable reduction in the volume of U.S. exports. However, international prices may rise and actually result in higher export earnings. Changing current agricultural policy to accommodate crop rotations would focus on the core of policy issues by raising questions regarding the ultimate goals for U.S. agriculture (Moore, 1989). When determined, costs and benefits of the alternatives, trade-offs, and impacts of all aspects must be resolved to establish a solid basis for policy consensus. When this is accomplished, the policy stage will be set for encouraging and facilitating adoption of crop rotations in farm management practices.

VIII. SUMMARY AND CONCLUSIONS Advantages and disadvantages of crop rotation have undoubtedly been debated for thousands of years, as documented by historians (White, 1970b) who have stated that rotation systems were widely recommended by Roman agronomists, but often not adopted by local farmers. One reason for farmer hesitancy to use crop rotation may be that agricultural scientists are still unable to explain the mysterious “rotation effect.” Macroeconomic and microeconomic considerations have and presumably will always influence land use decisions, such as adoption of crop rotation. For the U.S. corn belt, this was well documented by Wiancko (1927), but economic considerations must include a more complete accounting for both on-site and offsite impacts of our soil and crop management practices.

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Benefits of crop rotation for land and water resource protection and productivity have been identified, but processes and mechanisms responsible for those benefits need to be better understood. This is a critical area for basic and applied research. Public policies that influence land use decisions, such as crop rotation, need to be as flexible as possible to encourage adoption of practices that are economically viable, environmentally sustainable, and socially acceptable. Following this agenda will ensure that crop rotations have a major role in 21st century agriculture.

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Muir, J., Boyce, J. S., Seim, E. C., Mosher, P. N., Deibert, E. J., and Olson, R. A. 1976. Influence of crop management practices on nutrient movement below the root zone in Nebraska soils. J. Environ. Qual. 5 , 255-259. Noe, J. P., Sasser, J. N., and lmbriani, J. L. 1991. Maximizing the potential of cropping systems for nematode management. J. Nemarol. 23, 353-361. Olmstead, L. B. 1947. The effect of long-time cropping systems and tillage practices upon soil aggregation at Hays, Kansas. Soil Soc. Am. Proc. 11, 89-92. Olson, R. A., and Sander, D. A. 1988. Corn production. In “Corn and Corn Improvement” (C. F. Sprague and J. W. Dudley, eds.), Agronomy, No. 18, pp. 639-686. ASA-CSSA-SSSA. Madison, Wisconsin. Olson, R. J., Hensler, R. F., Attoe, 0. J., Witzel, S. A., and Peterson, L. A. 1970. Fertilizer nitrogen and crop rotation in relation to movement of nitrate nitrogen through soil profiles. Soil Sci. SOC.Am. Proc. 34, 448-452. Ostlie, K. R. 1987. Extended diapause-northern corn rootworm adapts to crop rotation. CropsSoils 39, 23-25. Page, J. B., and Willard, C. J. 1947. Cropping systems and soil properties. Soil Sci. SOC. Am. PFOC. 11, 81-88. Papendick, R. I., and Elliott, L. F. 1984. Tillage and cropping systems for erosion control and efficient nutrient utilization. In “Organic Farming: Current Technology and Its Role in a Sustainable Agriculture” (D. F. Bezdicek, ed.), Am. Soc. Agron. Spec. Publ. No. 46, pp. 69-81. ASA-CSSA-SSSA, Madison, Wisconsin. Parker, C. P. 1915. “Field Management and Crop Rotation.” Webb, S t . Paul, Minnesota. Peterson, T. A., and Varvel, C. E. 1989a. Crop yield as affected by rotation and nitrogen rate. I. Soybean. Agron. J. 81, 727-731. Peterson, T. A., and Varvel, G. E. 1989b. Crop yield as affected by rotation and nitrogen rate. 11. Grain sorghum. Agron. J. 81, 731-734. Peterson, T. A., and Varvel, G. E. 1989~.Crop yield as affected by rotation and nitrogen rate. 111. Corn. Agron. J. 81, 734-738. Pimentel, D. 1991. Ethanol fuels: Energy, security, economics, and the environment. J. Agric. Environ. Ethics 4, 1-13. Poe, G. L., Klemme, R. M., McComb, S. J., and Amb~sious,J. E. 1991. C o m m o d i ~programs and the internalizationof erosion costs: Do they affect crop rotation decisions?Rev. Agric. Econ. 13, 223-235. Power, J. F. 1990. Legumes and crop rotations. In “Sustainable Agriculture in Temperate Zones” (C. A. Francis, C. B. Flora, and L. D. King, eds.), pp. 178-204. Wiley, New York. Power, I. F., and Follett, R. F. 1987. Monoculture. Sci. Am. 256, 78-86. Powers, W. L., and Lewis, R. D. 1930. Nitrogen and organic matter as related to soil productivity, J. Am. SOC. Agron. 22, 825-832. Putman, A. R., DeFrank, J., and Barnes, J. P. 1983. Exploitation of allelopathy for weed control in annual and perennial cropping systems. J. Chem. Ecol. 9, 1001-1010. Raimbault, B. A., and Vyn, T. J. 1991. Crop rotation and tillage effects on corn growth and soil structural stability. Agron. J. 83, 979-985. Rasmussen, P. E., Collins, H. P., and Smiley, R. W. 1989. “Long-Term Management Effects on Soil Pr~uctivityand Crop Yield in Semi-And Regions of Eastern Oregon,” Stn Bull. No. 675. USDA-ARS and Oregon State Univ. Agric. Exp. Stn., Pendleton. Reganold, J. P. 1988. Comparison of soil properties as influenced by organic and conventional farming systems. Am. J. Alternative Agric. 3 , 144-145. Regnier, E. E., and Janke, R. R. 1990. Evolving strategies for managing weeds. In “Sustainable Agriculture Systems” (C. A. Edwards, ed,j, pp. 174-202. Soil Water Consent Soc., Ankeny, Iowa.

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Ridley, A. 0..and Hedlin, R. A. 1968. Soil organic matter and crop yields as influenced by frequency of summer fallowing. Can. J . Soil Sci. 48, 315-322. Rifkin, 5. 1983. “Algeny.” Viking, New York. Robinson, C. A., Cruse, R. M., and Kohler, K. A. 1994. Soil management. In “Sustainable Agricultural Systems” (I. L. Hatfield and D. L. Karlen, eds.), pp. 109-134. Lewis Publ., CRC Press, Boca Raton, Florida. Robinson, R. G. 1966. Sunflower-soybean and grain sorghum-soybean rotations versus monoculture. Agron. J. 58, 475-477. Roder, W., Mason, S . C., Clegg, M .D., and Kniep, K. R. 1988. Plant and microbial responses to sorghum-soybean cropping systems and fertility management. Soil Sci. SOC.Am. J . 52, 13371342. Roder, W., Mason, S . C., Clegg, M. D., and Kniep, K. R. 1989. Yield-soil-water relationships in sorghum-soybean cropping systems with different fertilizer regimes. Agron. J. 81, 470-475, Russell, E. W. 1973. “Soil Conditions and Plant Growth,” 10th Ed. Longman, New York. Russelle, M. P., Hesterman, 0. B., Sheaffer, C. C., and Heichel, G . H. 1987. Estimating nitrogen and rotation effects in legume-corn rotations. In “The Role of Legumes in Conservation Tillage Systems” (J. F. Power, ed.), pp. 41-42. Soil Conserv. SOC.Am., Ankeny, Iowa. Sadler, E. J., and Turner, N. C. 1994. Water relationships in a sustainable agricultural system. In “Sustainable Agricultural Systems” (J. L. Hatfield and D. L. Karlen, eds.), pp. 21-46. Lewis Publ., CRC Press, Boca Raton, Florida. Sahs, W. W., and Lesoing, G . 1985. Crop rotations and manure versus agricultural chemicals in dryland grain production. J. Soil Wafer Conserv. 40, 5 11-5 16. Sasser, J. N . , and Uzzell, G., Jr. 1991. Control of the soybean cyst nematode by crop rotation in combination with a nematicide. J. Nematol. 23, 344-347. Schmitt, D. P. 1991. Management of Heterodera glycines by cropping and cultural practices. J . Nematol. 23, 348-352. Schreiber, M. M. 1992. Influence of tillage, crop rotation, and weed management on giant foxtail (Serariafaberi) population dynamics and corn yield. Weed Sci. 40, 645-653. Shrader, W. D., Fuller, W., and Cady, F. B. 1966. Estimation of a common nitrogen response function for corn (Zea mays) in different crop rotations. Agron. J. 58, 397-401. Slife, F. W. 1976. Economics of herbicide use and cultivar tolerance to herbicides, Proc. Annu. Corn Sorghum Res. Conf., 3Ist, Chicago pp. 77-82. Spurgeon, W. I., and Grisson, P. H. 1965. Influence of cropping systems on soil properties and crop production. Miss. Agric. Exp. Sfn. Bull. No. 710. Stewart, B. A., Viets, F. G . , and Hutchinson. G. L. 1968. Agriculture’s effect on nitrate pollution of groundwater. J . Soil Water Conserv. 23, 13-15. Stewart, B. A., Woolhiser, D. A., Wischmeier, W. H., Caro, J. H., and Frere, M. H. 1976. “Control of Water Pollution From Cropland,” Vol. 2. U.S. Dep. Agric. and Environ. Rot. Agency, Washington, D .C . Strickling, E. 1950. The effect of soybeans on volume weight, and water stability of aggregates, soil organic matter content and crop yield. Soil Sci. SOC. Am. Proc. 15, 30-34. Stromberg, E. 1986. “Gray Leaf Spot Disease of Corn,” Publ. No. 450-072. Virginia Coop. Ext. Serv., Blacksburg. Taylor, M. W., Wolfe, C. W., and Baxter, W. L. 1978. Land-use change and ring-necked pheasants in Nebraska. Wildl. SOC. Bull. 6 , 226-230. Tisdall, J. M., and Oades, J. M. 1982. Organic matter and water- stable aggregates in soils. J . Soil Sci. 33, 141-163. Tyner, F. H., and Purcell, J. C. 1985. Forage production economics. In “Forages, the Science of Grassland Agriculture”(M. E. Heath, R. F. Barnes, and D. S. Metcalfe, eds.), 4th Ed., pp. 4350. Iowa State Univ. Press, Ames.

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Unger, P. W. 1968. Soil organic matter and nitrogen changes during 24 years of dryland wheat tillage and cropping practices. Soil Sci. SOC. Am. Proc. 32, 426-429. USDA 1980. “Report and Recommendations on Organic Farming.” U.S. Dep. Agric., Washington, D.C. USDA 1986, “Fuel Ethanol and Agriculture: An Economic Assessment,” Agric. Econ. Rep. No. 562., U.S. Dep. Agric., Off.Energy, Washington, D.C. van Bavel, C. H. M., and Schaller, F. W. 1950. Soil aggregation, organic matter, and yields in a long-time experiment as affected by crop management. Soil Sci. SOC. Am. Proc. 15, 399-408. Vance, D. R. 1976. Changes in land use and wildlife populations in southeastern Illinois. Wildl. SOC. Bull. 4, 1-15. van Doren, D. M., Jr., Moldenhauer, W. C., and Triplet, G. B., Jr. 1984. Influence of long term tillage and crop rotation on water erosion. Soil Sci. SOC.Am. J . 48, 636-640. van Heemst, H. D. J. 1985. The influence of weed competition on crop yield. Agric. Sysr. 18, 8193. Vivekanandan, M . , and Fixen, P. E. 1991. Cropping systems effects on mycrorrhizal colonization, early growth, and phosphorus uptake of corn. Soil Sci. SOC. Am. J. 55, 136-140. Voss, R. D., and Shrader, W. D. 1984. Rotation effects and legume sources of nitrogen for corn. In “Organic Farming: Current Technology and Its Role in a Sustainable Agriculture” (D. F. Bezdicek, ed.),Am. SOC. Agron. Spec. Publ. No. 46, pp. 61-81. ASA-CSSA-SSSA, Madison, Wisconsin. Ware, G. W. 1980. “Complete Guide to Pest Control, With and Without Chemicals.” Thomson, Fresno, California. Welch, L. F. 1976. The Morrow plots-hundred years of research. Ann. Agron. 27, 881-890. White, K. D. 1970a. Fallowing, crop rotation, and crop yields in Roman Times. Agric. His?. 44, 281-290. White, K. D. 1970b. “Roman Farming.” Cornell Univ. Press, Ithaca, New York. Whiting, K. R., and Crookston, R. K. 1993. Host-specific pathogens do not account for the cornsoybean rotation effect. Crop Sci. 33, 359-543. Wiancko, T. 1927. Crop rotation in relation to the agriculture of the corn belt. J. Am. Sor. Agron. 19, 545-555. Wikner, I. 1990. Crop management research and groundwater quality. Proc. Best Manage. Pract. Maintain Groundwater Qual. pp. 41-45. Iowa State Univ., Ames and Pioneer Hi-Bred Int., Inc., Johnston, Iowa. Williams, L. E., and Schmitthenner, A. F. 1962. Effect of crop rotation on soil fungus populations. Phytopathology 52, 241-247. Wilson, H. A . , and Browning, G. M 1945. Soil aggregation, yields, runoff, and erosion as affected by cropping systems. Soil Sci. Soc. A m . Proc. 10, 51-57. Wischmeier, W. H . , and Mannering, J. V. 1965. Effect of organic matter content of the soil on infiltration. J. Soil Water Consew. 20, 150-152. Yakle, G. A., and Cruse, R. M. 1983. Corn plant residue age and placement effects upon early corn growth. Can. J. Plant Sci. 63, 871-877. Yakle, G. A,, and Cruse, R. M. 1984. Effects of fresh and decomposing corn plant residue extracts on corn seedling development. Soil Sci. SOC. Am. J. 48, 1143-1146. Yates, F. 1954. The analysis of experiments containing different crops. Biomerrics 10, 324-346. Young, D. L . , and Painter, K. M. 1990. Farm program impacts on incentives for greenmanure rotations. Am. J. Alternative Agric. 5 , 99-105.

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ROLEOF DISSOLUTION AND PRECIPITATION OF MINERALS IN CONTROLLING SOLUBLE ALUMINUM IN ACIDICSOILS G. S. P. Ritchie Department of Soil Science and Plant Nutrition School of Agriculture The University of Western Australia Nedlands, Western Australia 6009, Australia

I. Introduction 11. A Framework for Understanding Mineral Dissolution and Precipitation in Soils 111. Factors Affecting Dissolution and Precipitation of Aluminum-Containing Minerals A. Solution Properties B. Solid Properties N. Modeling Soluble Aluminum A. Chemical Thermodynamic Approaches B. l n e t i c Approaches to Modeling V. Aluminum in Acidic Soils: Principles and Practicalities References

I. INTRODUCTION Acidic soils are a worldwide phenomenon that may be natural or anthropogenic in origin. Acidic precipitation and farm management practices that disrupt the carbon, nitrogen, and sulfur cycles have apparently resulted in contemporary acidification rates that are much higher than rates estimated to occur in their absence (Binkley et af., 1989; Robson, 1989). Agricultural production on acidic soils may be severely limited by a number of nutritional (e.g., nitrogen or molybdenum deficiencies) or toxicity (e.g., aluminum or manganese) problems 47 Advances in Aponomy, Volume 53

Copyright 0 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.

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G. S . P. RITCHIE

(Robson, 1989). Aluminum (Al) toxicity, however, is considered to be the most common cause of decreased plant growth in acidic soils. The quantity of toxic A1 in acidic soils has apparently defied prediction by chemical principles because the dynamic and diverse nature of soils distinguishes reality from ideality. The ultimate aim of soil scientists is to be able to predict Al speciation (solid and solution) in time and space and then deduce the quantity of A1 that is toxic to plants. There are several different forms of A1 in soils (Adams, 1984; Ritchie, 1989; Sposito, 1989a) which can all contribute to the toxic quantity of A1 in solution either directly or indirectly. AI-containing minerals are the ultimate source of A1 in most soils whereas organically bound, exchangeable, interlayer, and soluble, complexed A1 are sinks for Al3+ released during mineral dissolution. The sinks provide AP+ to the soil solution in the short term and hence, separately or collectively, may be seen as controlling the amount of AI3+ in solution. In the long term, even though A1 may be derived from mineral c o m ~ u n d sthe , quantity released cannot necessarily be predicted from equilibrium thermodynamics because morphological characteristics may result in the surface-free energy of the mechanism of structural breakdown being greater than the standard free energy of the reaction. When this occurs, kinetic considerations become more important than therm~ynamicsin controlling solution quantities of AP+ (Morse and Casey, 1988). Lewis and Randall (1923) pointed out that “thermodynamicsshows us whether a certain reaction may proceed and what maximum yield may be obtained, but gives no information as to the time required.” Hence our deductions about the processes controlling the dissolution and precipi~ationof A1 will always be at the mercy of the time scale of our observations. The processes and mechanisms of dissolution and precipitation have been under consideration by soil scientists and mineralogists for many years. In the context of A1 solubility, an understanding of dissolution mechanisms and kinetics helps us see the limitations of trying to apply equilibrium thermodynamics to predicting activities in soil solutions and to decide on the most appropriate course of action for our needs. The quantity of A1 in the soil solution is dynamic in time and space and the measurements we make represent one moment in the time and space of a pathway. Soluble A1 due to mineral dissolution and precipitation is the net result of the balance between thermodynamic and kinetic considerations, as affected by surface morphology, the uptake and release of nutrients and toxic ions by plants, and as affected by the composition and flow of water through the volume of soil being studied. When a mineral dissolves, whether it is a grain of feldspar in a granitic rock or kaolinite in a soil that is rewetting at the beginning of the wet season, the sequence of events that follows cannot be predicted by equilibrium

MINE^ D~SSOL~ION/PRECIPITATION

49

thermodynamics alone. A process or sequence of events begins which can be described in terms of a pathway. The pathway is controlled by thermodynamics, kinetics, and surface morphology, which answer the questions: (1) what is it and where can it go? (thermodynamics), (2) how quickly will it get there? (kinetics), and (3) what does it look like? (surface morphology). For soil scientists and others working in the field, there is a fourth question: how do I know when it’s there? Many mechanisms have been put forward to describe dissolution but few have addressed all three scientific components in~uencingthe process. Early work assumed the pathway was simply controlled by equilibrium thermodynamics (Garrels and Christ, 1965; Lindsay, 1979) but the inability of the theories to describe bulk solution concentrations led workers to postulate on nonequilibrium thermodynamics or on the physical structure of the dissolving surface and how they could lead to deviations from theoretical predictions based on the assumption of equilibrium (Helgeson, 1968; Hemingway, 1982; Hochella, 1990). In addition, the role of kinetics was also recognized to be so important (Morse and Casey, 1988) in some cases that it overshadows predictions from thermodynamic considerations. All the theories and mechanisms that have been suggested to explain dissolution have one aspect in common: they cannot be proved unequivocally. Hypotheses that explain behavior in terms of surface morphology require experimental evidence on the molecular scale (Sposito, 1986). Until now most of the evidence has come from bulk solution measurements or spectroscopic analyses that are limited in their ability to distinguish between the surface and the interior of a mineral. However, recent advances in spectroscopic and microscopic techniques are providing methods that can study the hydrated surface layers of a dissolving grain (Hochella, 1990; Brown, 1990; Mogk, 1990). This review considers the role of mineral dissolution and precipitation in c o n ~ l l i n gsolution quantities of A1 and our attempts to predict the outcome of these processes. Its purpose is to broaden our perspective and thereby increase our ability to predict (Al3+) accurateiy by providing soil scientists with possibilities for looking at the problem from a different perspective by drawing on examples from related disciplines such as geochemistry. The dissolution and precipitation of Al-containing minerals are by no means the only mechanisms controlling AP+ in soil solutions (Ritchie, 1994). It is an area, however, that requires more clarity so that its contribution to the overall scheme of events can be appreciated more appropriately. The new perspectives may then enable us to predict more accurately the variation in solution composition with time and space of acidic or acidifying soils, before and after amelioration. Within this framework, the chemical paradigms that have been mistaken for principles and the paradigms of mineral and solution phases that exist in our soils in apparent defiance of chemical principles will be discussed.

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G. S. P. RITCHIE

11. A FRAMEWORK FOR UNDERSTANDING MINERAL DISSOLUTION AND PRECIPITATION IN SOILS In a closed system, the amount and composition of a mineral that dissolves or precipitates may be described in terms of chemical thermodynamics and kinetics as affected by the surface morphologies of the dissolving and precipitating species (Fig. 1). It is not possible to understand fully the processes and pathways of precipitation and dissolution without considering the interactions among thermodynamics, kinetics, and surface morphology. Chemical thermodynamics describes the pathway and predicts mineral and solution speciation from the standard free energy change of a chemical reaction (AGO,) and the composition of the soil solution and the minerals present. Such considerations may assume that equilibrium can be achieved [i.e., the free energy ( G ) of the system reaches a minimum]; that non- or quasi-equilibrium exists [i.e., metastable products (e.g., smectites, Al-substituted goethite, and hematite) persist on a time scale considered long for soils]; or that an irreversible reaction occurs (i.e., a rock component dissolves completely). Even though the driving force for precipitation or dissolution may be great from a thermodynamic standpoint (i.e., a lot of free energy, AG,can be lost), the thermodynamic potential for a mineral to form or dissolve [( 1) in Fig. I ] may be overshadowed by kinetic considerations. The rate of precipitation or dissolution

,

Equilibrium

Transport

Irreversible

-

- Precipitation \

Equilibrium Solution

Surface Morphology

Topography Figure 1

Surface area

Structure

The three components influencing dissolution and precipitation.

MINERAL DISSOLUTION/PRECIPITATION

51

may be very small because the driving force (i.e., change in energy) is small. Thermodynamics indicates which reactions are possible whereas kinetics stipulate the time required for transformations and hence can frequently mediate the pathway of a reaction [(2) in Fig. 11. Kinetic considerations include transport of ions in solution, reaction rates in solution, and rates of nucleation, crystal growth, and dissolution. The energy changes described by chemical thermodynamics and kinetics during dissolution and precipitation may be modified by the surface morphology of the mineral (i.e., composition, structure, topography, thickness, and surface area). The surface morphology is the physical manifestation of the processes and rates of dissolution and precipitation. The soluble components predicted by thermodynamics can influence all the aspects of surface morphology [(3) in Fig. 11. For example, nucleation and crystal growth could generate new species on a surface. Conversely, the processes of dissolution could modify the surface by producing leached layers or crystal ripening (Morse and Casey, 1988) could produce crystals of smaller surface area. In turn, surface morphology can affect the release or incorporation of solution components which change the free energy of solution and hence mineral reaction pathways may be altered [(4) in Fig. 11. Kinetic factors can affect surface morphology [e.g., incongruent dissolution creates “leached layers” at a surface; ( 5 ) in Fig. I] just as much as surface morphology will dictate the speed of dissolution and precipitation [(6)in Fig. 11.

111. FACTORS AFFECTING DISSOLUTION AND PRECIPITATION OF ALUMINUM-CONTAINING MINERALS The surface and bulk properties of a mineral and the intensive and extensive properties of a solid-solution system can affect dissolution and precipitation by affecting each of the three components in the framework of Fig. 1 (Table I). Many of these factors are interrelated and hence the following discussion assumes all factors are constant other than the one being considered.

A. SOLUTIONPROPERTIES The state of saturation of a solution plays a fundamental role in determining the reaction pathway and rate, and the surface mechanism controlling precipitation and dissolution (Van Straten et al., 1984; Nagy and Lasaga, 1992). The dissolution of a solid may be represented by the following type of reaction:

52

G. S. P. RITCHIE Table I System, Solution, and Solid-Phase Properties That Influence the Dissolution and Precipitation of Al Mineralsa System

I . Temperature and pressure

Solution

2. .Saturation 3. pH 4. co, 5. Activity of water 6. Cations

7. Inorganic anions 8. Organic ligands 9. Ionic strength 10. pH buffeting 1 1 . Polydispersity a

Solid 12. Bulk composition

13. Surface composition 14. Activity of solid I S . Surface structure 16. Surface transmissivity 17. Surface thickness 18. Particle size 19. Particle surface area 20. Particle surface tension 21. Precipitation of other minerals

The numbers are used to refer to this table in Tables 11 and VI.

The extent to which the reaction proceeds to the right-hand side of Eq. (1) depends on the solubility product constant, Ksp: Ksp = (A13+)(OH)3

(2)

where round brackets denote activities. The right-hand side of Eq. (2) is referred to as the ion activity product (IAP) and can be used to estimate the saturation of a solution with respect to a particular mineral by estimating the relative saturation (RS):

RS

=

IAPIK,,

(3)

If RS < 1, the solution is undersaturated; if RS is > 1, the solution is supersaturated. The logarithm of RS is sometimes referred to as the saturation index (SI). The extent of saturation affects the reaction pathway of both dissolution and precipitation. Taking the dissolution of microcline in rainwater as an example, and assuming the very simple case that thermodynamic, partial equilibrium is possible (Tsuzuki, 1967), Fig. 2a shows that the reaction pathway depends on the initial A1 saturation of the solution as the microcline begins to dissolve. As the microcline reacts with water it releases A1 and Si into solution (A) at a rate that is sufficiently low for saturation to be < 1 with respect to any Al-OH or Al-Si-OH mineral. When the solution becomes saturated with respect to gibbsite (B), A1 will precipitate from solution while microcline continues to release A1 and Si, Eventually the Si activity will be high enough for kaolinite to precipitate (C) which will lower the A1 activity below that controlled by gibbsite. Hence, gibbsite will start to dissolve and, even though microcline and kaolinite are both present, A1 activity in solution will be controlled by gibbsite. During this stage,

MINERAL DISSOLUTION/PRECIPITATION

53

b

a 0

0

-a -:>: U W

-5

-5

ln

0

0

- -5

-5

m

= > ln

.-ln

'0 d

m

L

m

0 -10

0

-10

L

&

m

m

- = L

c 0

-10

-10 t

-

m

m

0

0

-5 loglH,SiO,

0

I

-5 log IHLSi0'1

0

Figure 2 The variation in A1 solubility at pH 5 (a) and pH 4 (b) during the weathering of microcline. The lines represent the ion activity product predicted from the K, of minerals at equilibrium: G , gibbsite; K, kaolinite; S, amorphous silica. (After Tsuzuki, 1967.)

both gibbsite and microcline will be sources of A1 for the kaolinite that precipitates. When all the gibbsite has dissolved, microcline continues to react to form kaolinite and the A1 activity decreases whereas Si activity continues to increase until it is equivalent to that associated with amorphous silica at equilibrium. At this point, kaolinite and amorphous silica are in equilibrium (D). If the microcline dissolved more quickly in the initial reaction with rainwater, then the line AB would not be so steep and there would be less likelihood that gibbsite would form before kaolinite precipitated. This is the first example of how three mineral phases can be present but A1 in solution is controlled by the least thermodynamically stable mineral. Even so, this is a very simplistic picture of what is happening and does not address the irreversibility of some of the reactions that occur (e.g., the precipitation of quartz). The state of saturation also affects reaction kinetics. The rates of dissolution and precipitation slow down as equilibrium is approached. Hence, as water flows through a soil, the rate of dissolution in each successive volume of soil decreases because the flowing water contains an increasing amount of A1 and is therefore nearer to equilibrium. This hypothesis is only relevant if other factors (such as pH, soluble organic ligands) that affect dissolution rates do not vary significantly between successive volumes of soil. With respect to mechanisms acting in situations far from equilibrium (i.e., the magnitude of the driving force is large), the rate of dissolution is controlled by the soluble quantity of the mineral components and the presence of other ions that may inhibit or catalyze the dissolution process (Nagy and Lasaga, 1992). In the case of precipitation, the rate-

54

G. S. P. RITCHIE

controlling step may be diffusion to the surface because surface reactions could have become very rapid at high supersaturations (Zhang and Nancollas, 1990). Hence, the nucleation rates for all possible intermediary phases become very rapid and essentially similar. As the driving force for the reaction decreases, the total change in free energy for the reaction, AG, (this includes the Gibbs free energy change, AGO,) may also influence the rate of reaction and alter the ratecontrolling step. In addition, even if the variation in the rate of reaction with AG, has the same shape (e.g., linear) for both precipitation and dissolution in solutions near equilibrium, one cannot necessarily conclude that the same mechanism is controlling both reactions. In the case of gibbsite at pH 3 and 80°C, Nagy and Lasaga (1992) found that the variation in dissolution rate with AG, could be explained most easily by postulating that dissolution occurs at dislocation screw defects on basal surfaces at saturations near equilibrium. In solution far from equilibrium, however, the dissolution rate was much greater and was consistent with the formation of etch pits. It was also possible that the functional dependence of rate on AG, was due to changes in solution or surface speciation of A1 with the extent of solution saturation. pH affects dissolution and precipitation because it takes part in the reaction, it acts as a catalyst, or it changes the reaction pathway or surface morphology. Lowering the pH (as in an acidifying soil) can change the reaction pathway by changing the extent of saturation (Tsuzuki, 1967). Figure 2 indicates that as the pH falls from 5.0 to 4.0 the reaction pathway of dissolution of microcline changes from: microcline + gibbsite .--, kaolinite + kaolinite

+ amorphous silica

to microcline + kaolinite + kaolinite

+ amorphous silica

Specific adsorption of H+ and OH- can alter the surface charge of a mineral and hence decrease the rate of nucleation by lowering the interfacial tension (Van Straten et al., 1984). Stumm and co-workers (Stumm and Wieland, 1990, and references therein) consider adsorption to consist of several stages of which the detachment of an activated surface complex is the rate-limiting step and hence controls the dissolution rate (Fig. 3). They found that the rate of dissolution of metal oxides was proportional to the surface concentration of H+ ions raised to the power equivalent to the charge of the metal cation (Fig. 4). Understanding the effect of pH on the dissolution rate of Al from layer silicates is not as straightforward because of the presence of pH-independent sites. In general, it appears that Al dissolution from kaolinite, anorthite, and montmorillonite is independent of H+ concentration in the pH region -4-9 whereas at pH <4, A1 dissolution rates can be explained by the metal oxide model (Amhrein and Suarez, 1988; Wieland and Stumm, 1992; Furrer et al., 1993).

MINERAL DISSOLUTION/PRECIPITATION

55

Figure 3 Schematic representation of proton-promoted dissolution of a metal oxide, M,O,. (After Stumm and Wieland, 1990, in "Aquatic Chemical Kinetics," copyright 0 1990, by permission of John Wiley and Sons, Inc.)

- 8.0

/

- 8.2

- 8.4 I

a

3.1

-o - 8.6 - 8.8 0

- 9.0 log CH8 I

I

I

- 5.8

-6.0

- 5.6

I

I

I

I

6.0

5.0

4.0

3.0

I

- 5.4

PH Figure 4 The relationship between the rate of proton-promoted dissolution (RH,mol m-2 sec-I) and pH or the concentration of protonated surface hydroxyls, CHs, in mol m-*. (After Stumm and Wieland, 1990, in "Aquatic Chemical Kinetics," copyright 0 1990, by permission of John Wiley and Sons, Inc.)

56

G. S. P. RITCHIE

In soils, the effect of pH on dissolution may be confounded by precipitation or adsorption of A1 on the mineral surface at pH 4-5 (Wieland and Stumm, 1992; Furrer et al., 1993). This mechanism appears to block dissolution sites and hence decreases the rate of dissolution of Al. Increasing the pH to very high values (pH > 12; as may occur temporarily in soil around a dissolving grain of lime) dehydrates AI(OH), linkages to A10,and changes the reaction pathway to favor the precipitation of fine-grained, poorly crystalline boehmite (referred to as pseudo-boehmite) rather than bayerite (Hemingway, 1982). As mixing of OH- with the soil increases with time, the localized ratio of OH and Al will decrease until dehydration is no longer favored. At this stage, the pseudo-boehmite will stop precipitating and an Al(OH), solid phase will form. The pseudo-boehmite will then dissolve in response to the removal of A1 from solution as Al(OH), precipitates. Exchange of H+ for A13+ in the surface layers of a dissolving mineral will change the surface morphology and temporarily affect the dissolution rate (Casey and Bunker, 1990). Ionic strength (I) affects dissolution and precipitation by changing the activity of soluble mineral components, the relative amounts of the species of each component and by changing the surface concentration of H/OH ions. Increasing ionic strength decreases the activity of Al3+ and hence more Al3+ is released by the mineral dissolving in an attempt to restore the original equilibrium. This is balanced partially by a simultaneous increase in the ratio of AP+ and monomeric hydroxy species. Such changes can affect the reaction rate and pathways and the surface morphology. Accordingly, Furrer el al. (1991) found that the dissolution rate of montmorillonite was approximately doubled when the ionic strength was raised from 0.1 to I M . The presence of cations and anions other than those forming the minerals under consideration can change the speciation of soluble mineral components and hence the reaction pathways. They can also affect reaction rates and surface morphology by being specifically adsorbed, incorporated as an impurity, coprecipitating, or by precipitating on a mineral surface. Inclusion (as defined in Sposito, 1989b) lowers the activity of the solid and produces a strain on the crystal structure, both of which decrease solubility (Sposito, 1984). Precipitation of a new phase on a mineral will change the surface area and tension and may block sites for nucleation or dissolution, or hinder crystal growth. The presence of anions can alter the reaction pathway and rate by inhibiting or promoting polymerization and precipitation, forming new compounds or solid solutions with the components of pre-existing minerals, and by retarding crystallization (Zawacki et al., 1986; Hemingway, 1982; Bertsch, 1989; Davis and Hem, 1989). For example, specific adsorption onto variable charge surfaces of anions that form bidentate mononuclear surface complexes (e.g., oxalate, salicylate, citrate) will enhance short term (< 50 hr) dissolution (Fig. 5 ) , whereas

I " O \

I

P

/ \

O\/

I/=\

O\

I I

P

I/=\ /" /=\

O\

57

58

G. S. P. RITCHIE

specific adsorption of ligands that form multinuclear surface complexes or block surface reactive groups retard short-term dissolution (Fig. 5 ) (Stumm and Wieland, 1990). The extent to which an organic ligand increases the short-term dissolution rate of a-Al,O, correlates with the ability of an anion (within a given structural class) to complex A13+ in solution (Furrer and Stumm, 1986). In contrast, the presence of organic ligands does not significantly enhance the longterm dissolution of corundum (Carroll-Webb and Walther, 1988). The results for layer silicates are also inconclusive. The long-term dissolution of anorthite increases in the presence of oxalate at pH 4.2-9 (Amhrein and Suarez, 1988) whereas organic ligands do not affect kaolinite dissolution (Carroll-Webb and Walther, 1988). The presence of ligands that form soluble complexes with A1 can prevent the formation or rapid polymerization of hydroxy-Al at pH <6.5 which can favor the formation of AI-0-Si or Al-ligand bonds. Therefore, kaolinite may be more prevalent than AI(OH), minerals in surface soils where organic matter contents are higher than in subsoils. Complexation also inhibits the formation of AI(0H); and favors A105 and the formation of boehmite. This could be why boehmite has been found in soils (Hsu, 1989). If the pH increases above neutrality (pH 7-12), OH- can compete more effectively with the ligand for A1 so polymerization becomes more prevalent and hence gibbsite may form. At even higher pH values (> 12), dehydration of AI(0H); to 0x0 linkages will occur and boehmite will become the favored precipitate again (Hemingway, 1982). If iron is present, thermodynamic considerations indicate that the simultaneous precipitation of goethite and gibbsite at Si activities less than that required for kaolinite precipitation can affect the reaction pathway by favoring the formation of Al-substituted goethite or hematite rather than pure A1 hydrous oxides (Tardy and Nahon, 1985). Field evidence suggests that this could be important in some acidic soils. Fitzpatrick and Schwertmann (1982) found that the crystallinity of kaolinite and the Al substitution of ferric hydrous oxides in lateritic profiles increased with depth and with decreasing pH. In contrast, equilibrium modeling indicates that A1 contents of goethite tend to decrease as aridity and the concentration of Si in the soil solution increase (Tardy, 1971) and that Al-substituted goethite is thermodynamically more metastable than gibbsite at low activities of A1 (Figs. 6a and 6c) (Tardy and Nahon, 1985). These predictions assume that ideal solid solutions can exist in soils and that they are in equilibrium with other A1 minerals, such as kaolinite and gibbsite. Solutions well-buffered with respect to pH increase the rate of precipitation of aluminum hydrous oxides (May et af., 1979) but do not affect the dissolution of feldspars (Wollast, 1967). For A1 hydrous oxides, the reaction rate decreases as the difference between initial and final pH values gets larger in poorly buffered solutions, even if the solution is initially supersaturated with respect to a solid phase.

MINERAL DISSOLUTION/PRECIPITATION

10.0

(luartz /

,Gibbsite

< 4 o1

-a I

8.0

-

-b

Quartz ' 1 6 i b b s i te

I I

I

6.0

I I

-

Kaol ini te

10.0

I I I

'

-C

4.0

I

8.0

(0.01 )

-

.

-5.5

I 1

I

Quartz

I

6.0

6.0

I

I

I

4.0

6,0

I I i

_I 0

m

10.0

59

Al-GOETHITE

1 AI-HEMATITE

1

1

-5.0

-4.5

-4.0

-3.5

L o g L 1i4s1041 Figure 6 Equilibrium solubility diagram for gibbsite, quartz, and kaolinite at activities of water (a,) of 1.0 (a) and 0.5 (b) and for goethite (c) and hematite (d) with substituted A1 varying from 0.001 to 0.1%. (After Tardy and Nahon, 1985, Am. J. Sci., reprinted by permission of American Journal of Science.)

Carbon dioxide may influence both the reaction pathway and rate. Increasing partial pressure of CO, (as may occur in the rhizosphere) decreases the dehydration of Al(0H); and favors the formation of gibbsite rather than boehmite (Hemingway, 1982). An increase in the level of dissolved C 0 2 may increase the pH buffering of the soil solution and affect the rate of reaction as discussed earlier. Raising the temperature (as may occur in dry, hot weather experienced in arid and mediterranean climates) increases the rate of reaction and influences the reaction pathway by increasing the likelihood of dehydration of Al(0H); to A10, and changing the relative values of AGO, of minerals that may form. For example, gibbsite converts to boehmite at T > 368 K (Hemingway, 1982). Polydispersity of a species in solution with respect to size or molecular weight can affect its dissolution (Parks, 1990). The smallest particles with the highest surface area tend to dissolve first but reprecipitate as more well-ordered, larger crystals. Hence a polydisperse system may take a lot longer to dissolve unless the rate of reprecipitation is much slower than the rate of dissolution. Lowering the activity of water (as a soil dries or as water enters a smaller pore

G. S. P. RITCHIE

60

size) affects the reaction pathway, equilibrium activities of mineral components, and the composition of solid phases (Tardy and Nahon, 1985). Assuming that equilibrium is achievable, decreasing the activity of water increases the activity of A P + in equilibrium with hydrous A1 oxides and decreases Si activity at which gibbsite and kaolinite are in equilibrium (Fig. 6). Lowering the activity of water favors the formation of diaspore and boehmite over gibbsite but this depends on the choice of the equilibrium constant (Fig. 7). Thermodynamic considerations indicate that the percentage of A1 that can substitute in goethite or hematite, in equilibrium with kaolinite and quartz, increases as water activity decreases (Tardy and Nahon, 1985). The influence of water activity on mineral solubility indicates that the formation of boehmite rather than gibbsite would be favored in dry soils, particularly with a large clay-sized fraction. Gibbsite would tend to precipitate in larger pores whereas boehmite would precipitate in smaller pores in which water activity would be lower. Alternatively, if Si was present, gibbsite precipitation would be more prevalent in large pores and channels whereas kaolinite would be more stable in the fine pores.

t

11 0

'

'

*

'

'

'

0.2 0.4 0.6 0.8 activity o f water

' 1.0

Figure 7 The relationship between log[A13+]/[H+]3 and the activity of water (a,,,) for corundum (log K , = 9.73), diaspore (log Ksp = 7.92 or 8.95), boehmite (log KIP = 8.13). and gibbsite (log K , = 8.04). Log K , values taken from Lindsay (1979) or Tardy and Nahon (1985).

MINERAL DISSOLUTION/PRECIPITATION

61

B. SOLIDPROPERTIES The aggregation and composition of the bulk mineral and its surface layers will affect the composition of the solution and hence the reaction pathway and rate and surface morphology. It is still not clear which minerals dissolve congruently or incongruently and whether dissolution and precipitation occur through surface-controlled reactions or the development of leached layers. In addition, it has yet to be established unequivocally whether dissolution and precipitation occur at specific sites or uniformly across the surface of a mineral. These uncertainties all affect the activity of the solid and the quantity of soluble components in equilibrium with it. Solubility of a mineral decreases when the solid activity is < I which may be due to inclusion or the mineral surface having concave interfaces rather than flat surfaces. Precipitation on to interlayers, lattice defects, convex interfaces, low crystallinity, and small grain size increase the activity of a solid above the ideal value of 1.0 (Tardy and Nahon, 1985; Sposito, 1981, 1989b; Schott, 1990). Solubility increases with surface area which can result from increasing disorder (amorphous versus crystalline) or more structural defects (Parks, 1990). Pits, fractures, ledges, comers, and edges are all structural defects that may contribute to dissolution to different extents depending on the relative rates and qualities dissolved (Schott el al., 1989) (Fig. 8). The relative contribution of each defect to the overall dissolution of a mineral depends on the degree of saturation. For example, as relative saturation increases from values far less than unity (i.e., highly undersaturated), the fewer the sites at which a pit may form and hence the smaller the contribution of this process to overall dissolution (Schott er al., 1989). As dissolution proceeds, however, a decrease in surface strain energy at structural defects may counterbalance the increase in surface area and hence the increase in dissolution rate due to a high density of defects may not be as great as expected (Schott, 1990). In supersaturated solutions, amorphous materials tend to precipitate more quickly because the rough surface provides more sites for nucleation than the smooth surfaces of crystalline phases. Crystalline materials have a higher activation energy barrier to be overcome for precipitation to occur and a higher surface tension (or free energy) which limits solubility and decreases the dissolution rate (Van Straten er al., 1984). Hence, it is possible to have highly supersaturated solutions of sparingly soluble minerals. A less structured, higher specific surface and spongy solid phase would be expected to nucleate and grow a precipitate more quickly than a well-structured, low surface area solid. The phrase “crystal ripening” was coined by Ostwald to describe the process by which small grains tend to dissolve to form fewer grains which are larger (Morse and Casey, 1988). This process tends to lead to a wider distribution in grain sizes as time progresses and thereby affects the rates of dissolution and

62

G. S. P. RITCHIE What Determines Measured Dissolution Rate With Parallel Processes? Fastest process is normally rate-determtning unless i t s contribution to total dissolved concentration is insignlflcant

Point Defects Dislocations Microfractures

Kinks Grain o r Twin Boundaries

Comers Edges. Ledges

Entire Face With All Defects

Figure 8 A schematic illustration of the parallel processes involved in crystal dissolution. The horizontal length of each arrow indicates the relative rate of each process (actual rates can differ by many orders of magnitude). The vertical thickness of each arrow represents the relative quantity of material dissolved and delivered to aqueous solution by that process. Thus, while point and linear defects react most rapidly. they deliver less dissolved material to solution than slower dissolution of faces and pits occurring at edges, ledges, and corners. (Reprinted from Geochim. Cosmochim. Acru. v. 53, Schott, J., Brantley, S., Crerar, D., Guy, C., Borcsik, M., and Williams, C., Dissolution kinetics of strained calcite, pp. 373-382, Copyright (l989), with kind permission from Pergamon Press, Ltd., Headington Hill Hall, Oxford OX3 OBW, UK.)

precipitation because of the dependence of solubility on grain size and because the rate of nucleation decreases with increasing surface tension. Particle size also affects solubility because thermodynamics predicts that the heat of dissolution varies with particle size in different ways for different minerals. For example, hematite is less soluble than goethite at equal or large grain sizes, but more soluble when it is smaller (Tardy and Nahon, 1985). Similarly, amorphous silica is less soluble than quartz until the grain size of quartz become <5 nm (Parks, 1990). However, if minerals have very small particle sizes, then these effects are minimal in comparison to those that alter precipitation and dissolution kinetics. Adsorption of a solution component or the presence of a foreign surface can

MINERAL DISSOLUTION/PRECIPITATION

63

influence the kinetics of precipitation by affecting nucleation or crystal growth (Zhang and Nancollas, 1990). A certain level of supersaturation (S,) has to be achieved before nucleation will occur, unless a foreign surface is present which can induce nucleation in the metastable region defined by 1 < supersaturation < S,. Adsorption can block precipitation sites on a nucleated surface and hence decrease crystal growth rates. However, if the adsorbate is similar in size to the lattice ion, it will promote growth. The rates of dissolution and precipitation are a function of molecular structure, microtopography, transmissivity, and thickness of the surface layer (Casey and Bunker, 1990; Lasaga, 1990). The reactivity of a mineral and the probability of a monomer sticking to a surface increase as the roughness of the surface increases. Minerals with extensive cross-linking tend to dissolve slowly and incongruently to produce leached surface layers. The cross-links help to preserve the original structure once the leached ions have been released by hydration, hydrolysis, or ion exchange. Aluminum in octahedral arrangement is released more readily than tetrahedrally coordinated A1 (Casey and Bunker, 1990). The extent to which leaching occurs depends on the mineral structure’s rigidity and transmissivity to water and solutes. Once the leached layer has formed, it is just as dynamic as the interface between a solid and solution. Silanol groups may repolymerize and solution components can adsorb at specific sites or detach from the surface and they can diffuse along or into the leached layer (Hochella, 1990). All these factors produce a surface that is unique and distinct from the bulk mineral and can have a significant effect on the reaction pathway and rates of dissolution and precipitation. Evidence for the presence of leached layers, precipitation on mineral surfaces, and incongruent dissolution was originally deduced from determination of components in solution or from spectroscopic techniques that are limited in their ability to distinguish between the composition of a surface and the bulk mineral. Since then spectroscopic and microscopic methods that can identify changes in structure at the molecular level initially indicated that leached layers or precipitated coatings did not occur; dissolution was controlled by surface reactions rather than diffusion; and that surface reactions did not occur uniformly over the surface but at weak points in the mineral structures (Mogk, 1990). Incongruent dissolution was considered to be due to the dissolution process not occurring uniformly over the surface and some of the data was interpreted as leached layers occurring nonuniformly, suggesting that the rate of dissolution depended on the number of reactive sites rather than the total surface area (Mogk, 1990). However, the approaches used were unable to measure the thickness of the reactive layer directly but had to estimate it from mass balance calculations or from the path length of the excited electrons in the spectroscopic technique used. Even more recent studies have included techniques that can directly measure elements in the surface of minerals in layers that are as thin as 1 nm (Mogk, 1990; Hochella, 1990; Brown, 1990). These methods have confirmed that incon-

64

G . S. P. RITCHIE

gruent dissolution occurs but have not found evidence for dissolution being controlled by surface reactions. Compositional changes as a function of depth up to 100 nm can also be detected but interpretation of the data is limited by the method of collection which assumes that every surface component has been identified and that they can be expressed as a function of the sum of all the components (i.e., normalized data). This means that an apparent decrease in the relative concentration of one component may be an artifact of a large increase in the relative concentration of another component. Aggregation of mineral particles decreases the dissolution rate of montmorillonite possibly by changing the rate-limiting step from surface complexation to diffusion of the dissolution products through the aggregate (Furrer er al., 1993).

IV.MODELING SOLUBLE ALUMINUM Attempts to model A1 dissolution and precipitation have been fragmentary even though the effect of the factors described in the previous section has been reasonably well known for some time. A major limitation has been the lack of appropriate data with which to test models. This section considers the development of models that predict solution composition in a general way rather than just models relating specifically to Al. Models developed by researchers in the field of acidic precipitation (Cosby er al., 1985; Furrer et al., 1990) are not necessarily based on dissolution and precipitation alone and are considered elsewhere (Eary er al., 1989; Ritchie, 1994). Models have been developed for both open and closed systems and are based on thermodynamics and/or kinetics as related to the three components in the framework of Fig. 1. Models of surface morphology that are purely descriptive in nature, rather than predictive, also exist and have been reviewed by Hochella (1990) and references therein. Each model has limitations and assumptions. Their accuracy depends on the derivation and choice of constant parameters and analytical errors in the experimental data used to develop them (May et al., 1986; Nordstrom and May, 1989; Hemingway and Sposito, 1989). It is not within the scope of this review to discuss these aspects of the models in great detail, but examples of the effects of different sources of inaccuracy on the predictive ability of models will be given.

A. CHEMICAL THERMODYNAMIC APPROACHES The application of chemical thermodynamics to predicting solution quantities of mineral components has been a popular approach for many years (Garrells and

MINERAL DISSOLUTION/PRECIPITATION

65

Christ, 1965; Helgeson, 1968; Lindsay, 1979; Hemingway, 1982). Initially, it was assumed that the rate of precipitation was greater than the rate of dissolution and so equilibrium conditions prevailed instantaneously. The roles of irreversible and partial equilibrium thermodynamics were then considered as well as the existence of metastable minerals.

1. Equilibrium Thermodynamics Models Models based on equilibrium thermodynamics have been the most commonly used approach to predicting A1 quantities in soil solutions (Lindsay, 1979). This approach develops mass balance equations for each component in solution along with the equation for conservation of neutrality. The equations are written in terms of the free form of each component (e.g., AP+) and the equilibrium constant that defines the formation of a mineral or soluble species. They are solved simultaneously, usually using the Newton-Raphson method, and predict which solid is controlling the soluble quantities of a mineral component and the solution speciation of that component (Sposito, 1981). The data required to use this type of model are given in Table I1 along with the major assumptions and limitations of its use. Commonly, the activities of H+, AP+, and H4Si04 in a soil solution are compared to those values which are in equilibrium with certain well-defined minerals such as gibbsite, kaolinite, or muscovite. Solubility diagrams are constructed to estimate whether a particular mineral is controlling soluble (A13+). Equations defining the solubility product constant in terms of the ion activity product can be rearranged to express (Al3+) in terms of Ksf. For example, Eq. (2) becomes:

log (A13+) = 8.04 - 3 pH,

(4)

where log Ksf = 8.04. If log(A13+)is plotted versus pH, we achieve the variation in (A13+) with pH when Al(OH), (with log Ksf = 8.04) is in equilibrium with the solution phase. The activity of Al3+ and pH in a soil solution may be estimated analytically and then plotted on the above solubility diagram. If the data point falls on the solubility line then it could possibly be assumed that the mineral is controlling soluble AP+ , but this assumption is by no means unequivocal (Sposito, 1986). The soil solution would be undersaturated or oversaturated with respect to Al(OH), if it falls under or above the line, respectively. If the soil solution data points are in close proximity to a solubility line, it is sometimes assumed that that mineral is controlling (Al3+) in solution. Small variations in equilibrium constants, errors in pH measurement, and small divergences of the actual data point from a solubility line can lead to large errors in predicting the soluble activity of A13+ (Tables III-V). The apparently small divergences are a result of the logarithmic representation of data in solubility diagrams obscuring

Table 11

Thermodynamically Based Models for Predicting A1 Quantities in Solution Thermodynamic basis

Model

Emphasis

Data required

Assumptions/limitations

Equilibrium (Carrels and Christ, 1965; Lindsay, 1979)

Solution composition

Equilibrium mass balance, conservation of neutrality

Solution concentrations, pH, 1, redox potential, mineral phases present

(i) (ii) (iii) (iv) (v) (vi)

Equilibrium achieved All solution species have been identified Experimental accuracy of data Choice of equilibrium constants Kinetics and surface morphology not considered Factors 2, 5 , 1 I , 13-23 in Table I not accounted for

Quasi-equilibrium (Helgeson, 1968)

As above

Equilibrium mass balance, irreversible mass balance

As above

(i) (ii) (iii) (iv)

Partial equilibrium achieved for intermediary phases Equilibrium for final secondary mineral Only one reaction pathway As for iii-vi

Nonequilibrium (Hemingway, 1982)

As above

As above

As above

(i) As for iii-vi for first model (ii) Does not recognize irreversible reactions explicitly (iii) Only applies to closed systems

MINERAL DISSOLUTION/PRECIPITATION

67

Table 111

(w)

Variation in the Activity of AIJ+ with Error in the Slope of the Solubility Curve for Gibbsite (pAl = 3pH - 8.04)

PH

Error in slope (%)

Value of slope

4.013

4.188

4.214

4.246

4.347

4.68

0 I 3 10 15

3 2.91 2.90 2.70 2.55

100

I32 253 I603 6410

30.0 40.0 78.5 540 2294

25.0 33.5 66.0 459 1969

20.0 26.8 53.2 376 1629

10.0 13.5 21.2 20 1 902

1.38 2.90 25.4 128

Table IV Variation in the Activity of A13+ ( p M )Predicted by Theoretical Gibbsite Solubility Caused by Errors in pH of kO.1 Unit (Log K , Gibbsite = 8.04)

Error in pH

PH

0

+o. I

-0.1

4.188 4.214 4.347

30 25

15.0 12.5

59.1 49.5 19.9

5

10

Table V Variation in the Activity of (A13+)Caused by Errors in the Estimation of Log K , for Gibbsite Dissolution

Log

+5 +2 +I 0 -1

-5 - 10

K,

4.347

4.246

4.214

8.442 8.201 8.120 8.040 7.960 7.638 7.236

253 14.5 12.1 10.00 8.32 3.91 1.57

50.6 29.0 24.1 20.0 16.7 7.94 3.15

63.2 36.3 30.2 25.0 20.8 9.91 3.93

1

.oo

68

G. S. P. RITCHIE

the large deviations in the predicted activity caused by the limitations and assumptions in Table I1 and are discussed in the previous section. Another approach to deducing the phase-controlling soluble Al using equilibrium thermodynamics is to estimate the relative saturation (RS) or the saturation index (SI) for different Al-containing minerals. Inferences that RS = 1 and SI = 0 indicate that a particular mineral is controlling soluble Al, suffer from the same limitations as mentioned earlier for deductions made from solubility diagrams. Similarly, it may be tempting to say SI = 0.2 is close enough to zero to represent equilibrium. However, it is equivalent to a 0.2 change in Ksp which results in approximately a 50% change in (Al3+) (Table V). The size of the errors in predicting (A13+)has important implications for plant growth. In nutrient solutions, activities of A13+ as low as 2 phl are toxic to barley (Cameron et al., 1986). In the field, estimates of toxic Al for the subsoil of yellow earths of Western Australia show that A1 >30 p l 4 measured in a 0.005 M KCI extract are toxic to wheat (Cam et al., 1991). A 1% error in slope (Table III), a 0.1 unit error in pH (Table VI), or a 2% error in the log Ksp value (Table V) can result in an erroneous prediction of toxicity in the pH range of 4.2-4.25 if one assumes gibbsite is controlling A1 solubility. A 1% error in the slope of the solubility line at pH 4.0-4.35 is equivalent to a 2.5-3% error in log(A13+)which is often assumed to be accurate enough for the purpose of predicting (A13+) in soils. However, a 2.5-3% variation in log(AP+) is equivalent to a 33-35% error in (AP+) which at pH 4.2 is sufficient to change (A13+) from a nontoxic to a toxic value for the yellow earth soils studied by Carr et al. (1991). The size of these errors increases as pH decreases. The inaccuracies in predicting (A13+)that arise from not identifying all the complexing ligands in solution depend on the pH, the equilibrium constant (log K") for Al reacting with the unknown ligand (L) and the activity of the unknown ligand relative to that of Al3+ (Figs. 9 and 10). The concentrations of A1 species in Figs. 9 and 10 were estimated using an equilibrium program, TITRATOR (Cabaniss, 1987), assuming a hypothetical case in which A1(OH)2+,AI(OH)2+, Al(OH),, ALL, and HL were the species formed in solution. Log K" values were taken from Lindsay (1979) except for ALL and HL (log K" = 3); ionic strength was set at zero and the total concentration of A1 was 30 phl. The ligand concentration was 30 phl unless the L:Al ratio varied between 1 and 3.3. At pH 4.5 (Fig. 9a), as the log K" for Al-L increases from 3.2 (a weakly complexing ligand) to 6.98 (a strongly complexing ligand), (A13+)decreases from 50 to <5% of total soluble Al, Al,, when equivalent concentrations of Al, and L are present. At pH 4.0 (Fig. 9b), the trend is similar except that (A13+)is 84% of Al, at the initial log K" of 3.2. Increasing the ratio of a strongly complexing ligand and A1 from 1 to 3.3 has little effect on (A13+) because it is a minor proportion of Al,. At the other extreme, increasing the L:AI ratio for a weakly complexing ligand also does not change (Al3+) by more than 7% simply because of the low

Table VI Kinetically Based Models for Predicting Al Quantities in Solution Theoretical basis Model

Emphasis

Thermodynamics

Kinetics

Datarequired

PaEes (1978)

Solution coinposition

Equilibrium (E) Quasiequilibrium (QE) Irreversible (IR)

Crystal growthdifferential rate law (CG-DRL) Transport (T)

Solution concentrations, PH, I

Van Straten et al. ( 1984)

Precipitation

E

CG-DRL nucleation

As above

Stumm and Wieland (19%)

Dissolution

E

CG-transition state theory (CG-TST)

As above measured before and after adsorption of H/OH and ligands

Nagy and Lasaga ( 1992)

Dissolution/ precipitation

E, QE, IR

CG-TST, N

As for first model

Steefel and Van Capellan (1990)

Dissolution/ precipitation

E, QE, IR

CG-DRL, N

As for first model

(N)

Assumptionsllimitations A11 solution species have been identified Experimental accuracy of data Choice of equilibrium constants Factors 2, 5, 11, 13-23 in Table I not accounted for Surface morphology not considered Formation of an activated complex not explicity recognized Does not include nucleation, epitaxy, or crystal ripening All solution and surface species have been identified Does not include crystal ripening All solution and surface species have been identified As for ii-iv and vii for first model System is far from equilibrium Surface area does not change Active sites are instantly regenerated Active sitedtotal sites 6 1 As for i-iv for first model Does not include epitaxy or crystal ripening explicitly As for i-iv for first model

G. S. P. RITCHIE

70 30 r

h

25

.-Y

Y U

2

20

C

.-0

5

lo

c

c Y

E

5

U

0 3 .O

4.0

5.o

6 .O

7.0

6.0

7.0

log K' f o r AI- L

) .

c

.-

15

c

c

*

c

Y

"

7.5

0 U

0 3.0

4.0

5.0

tog K' for A ( - L

Figure 9 The variation in concentration of A P + , AI(OH)'+, AI(OH),+, and AI-L with log K" for ALL at pH 4.5 (a) and pH 4.0 (b). Al, = LI. = 30 JLM.

log k? (data not shown). The biggest effect of L:Al ratios on (Al3+) occurs when ligands with medium binding strength (log k? = 5) are present (Fig. 10). In this case, (Al3+) decreases from 41 to 13%of Al, as the L:Al ratio changes from 1 to 3.3.

2. Quasi- and Nonequilibrium Thermodynamic Approaches The limitations of equilibrium thermodynamics even within a closed system led some workers to consider irreversible reactions, partial equilibria, and meta-

MINERAL DISSOLUTION/PRECIPITATION

0.8 3

1.46

2.09 L : Al ratio

2.12

71

3.35

Figure 10 The variation in concentration of A13+ and AI-L with the ratio of L:AL at pH 4. Al, = 30 phf and log K" for AI-L = 5.00.

stable solids. Helgeson (1968) considered that the dissolution of a mineral in a soil or rock to be an irreversible reaction resulting in the formation of one or more minerals in an equilibrium state. Before the final stage is reached, a series of compounds may form which only achieve partial equilibrium in the system, but each state is reversible with respect to the next. The major assumption of such a model is that there is only one reaction pathway and that it is deduced from the initial and final states of the system. In addition, the model does not describe the process over long enough time periods for equilibrium to be achieved for some final phases, e.g., systems described by this model are often supersaturated with respect to quartz, even if equilibrium has been achieved between the dissolving phase (pyrophyllite) and another final phase, kaolinite (Helgeson, 1968). The models also have limitations in common with equilibrium models (Table 11). In reality, the formation of stable secondary minerals is irreversible and the composition of a solution depends on the irreversible reactions as well as the equilibrium mass balance equations for reversible reactions that produced metastable minerals. This approach to describing equilibrium precipitation and dissolution processes was formalized by Ostwald as long ago as 1897 (Hemingway, 1982). The law of successive reactions quantifies the common observation that unstable forms of minerals frequently precipitate before a stable form. The law points out that when a mineral dissolves, the first new phase to precipitate will be the mineral that has a free energy neurest to that of the dissolving mineral rather than the solid phase that has the lowest free energy. If several mineral phases exist with intermediary values of free energy, then each one will precipitate successively, in order of decreasing free energy, with the most thermo-

72

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dynamically stable mineral precipitating last. Ostwald’s observation is not a universal law. It simply recognizes that intermediary phases with simple structures tend to form before more complex minerals with a lower free energy. This law represents only one of many possibilities. Attempts to derive it from a rigorous thermodynamic standpoint are inappropriate because there is no evidence that the pathway of precipitation has to include several metastable phases (Morse and Casey, 1988). The extent to which intermediary phases form and persist depends on the initial species in solution and the relative rates of formation of all the metastable and final phases in relation to their free energy state. This model can be qualitatively applied to explain why the numerous studies of A1 precipitation differ in their conclusions as to which mineral phase is formed and to explain why solutions can remain supersaturated with respect to gibbsite (the most stable Al-H,O mineral) for months or even years (Hemingway, 1982). From theoretical considerations, one would expect A1 to precipitate out of solution to successively produce the following mineral phases: amorphous Al(OH), diaspore,

+ bayerite --5, nordstrandite +=

boehmite + gibbsite

-

assuming the thermodynamic equilibrium constants quoted by Lindsay and Walthall (1989) are appropriate. In published experiments, however, each phase is not necessarily observed, because varying experimental conditions change the A1 species in solution, which may favor the formation of one phase more than another. For example, anything (e.g., pH, temperature) that promotes the formation of A10, linkages as opposed to Al(0H); bonds will favor the formation of AlOOH minerals rather than Al(OH), phases. In reality, mineral phases that are not the most thermodynamically stable are observed in soils (e.g., boehmite; Hsu, 1989) and weathered rock, and soil solutions and water samples are often found to be supersaturated. Both these observations point to the long times (tens of thousands of years) required for progression down the pathway from the irreversibly dissolving mineral grain to the precipitation of the most thermodynamically stable secondary mineral.

B. KINETICAPPROACHESTO MODELING There are many kinetic approaches to modeling precipitation and dissolution (Sparks, 1989). In the context of this review, chemical kinetics refers to the rate of chemical reactions where transport is not limiting. Nonchemical kinetics refers to the rate of transport of reactant and products in the bulk solution or at the solid-solution interface. In soils, both types of kinetics occur simultaneously and are not necessarily differentiated appropriately in some research (Skopp, 1986). The rate of precipitation or dissolution may be surface controlled, transport

MINERAL DISSOLUTION/PRECIPITATION

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controlled, or a combination of both (Lasaga, 1990). In the geochemical literature, movement of reactants or products toward and away from a surface is considered to be so rapid that it is not usually the rate-determining step and tends to be ignored in models. The rate of reaction and detachment at the surface are considered to be the major mechanisms controlling dissolution and precipitation (Stumm and Wollast, 1990). However, in soil acidity research, we need to be more circumspect because one exception to this general behavior is the dissolution of gypsum which can be used as an ameliorant for acidic subsoils (Berner, 1978). Chemical kinetic models may be based on general differential rate laws, transition state theory, crystal growth, or nucleation (Walton, 1967; Sparks 1989). Nonchemical models include parabolic rate laws that can describe diffusion near a precipitating or dissolving surface as the rate-limiting step and algorithms that consider water flow through soil profiles (Skopp, 1986; Sparks, 1989). Empirical rate laws describe the rate of dissolution or growth in terms of the extent of saturation of the solution and a rate constant or coefficient, k. The mechanism(s) may be inferred from the shape of the relationship and from changes in the shape brought about by varying the initial driving force for the reaction. In their simplest form, they apply to elementary reactions in which the reactants A and B combine to form a product C without precursors being formed in an intermediary phase (Sparks, 1989):

A f B - C This interpretation assumes that k is a rate constant that only varies with temperature and pressure. Unfortunately, reactions in soils appear to be far from elementary and hence empirical rate laws may not be strictly appropriate for describing A1 solubility in soils. This drawback also applies to transition state theory. However, a more flexible approach is to look upon k as a coefficient which can also depend on factors such as surface area, the free energy change of the reaction, or the concentration of precursors (Nagy and Lasaga, 1992). Alternatively, some of these parameters can be stated explicitly in the rate equation (Paces, 1978; Steefel and Van Capellan, 1990; Nagy and Lasaga, 1992). The applicability of these types of models involves assumptions and limitations common to other approaches (Table VI) but varies quite widely, depending on how many parameters that affect k are stated explicitly in the model. A general rate law can be used to model soluble A1 by assuming that both the dissolution of the primary mineral and precipitation of the secondary mineral are irreversible reactions and that metastable intermediary mineral phases may also form (PaEes, 1978). The model can apply to both closed and open systems and therefore recognizes that the solution composition will be influenced by the speed of percolating water in the soil profile. As pointed out by the author, most published data contain insufficient detail to test the model and therefore Paces

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(1978) was only able to use it to consider the nature of reversible metastable solids that could be controlling A1 and Si in natural waters. Using data from 152 sets of field and laboratory data, he postulated that the presence of a metastable aluminosilicate was best able to explain soluble A1 in the natural waters being studied. The form of the mineral varied with pH such that the solubility product, K = 10-5.89 + 1.59pH. SP Implicit in the Ostwald law of successive reactions is that the rate of reaction of each successive step is slower. The total free energy of a reaction (AG,) has three components: i. The standard free energy of a reaction in an ideal system, AG,"; ii. the surface-free energy of precipitate growth; and iii. the surface-free energy of nucleation. The surface-free energy may represent a major proportion of the total free energy of a reaction and hence the equilibrium thermodynamic contribution becomes less important and supposedly metastable phases precipitate first before stable phases. Hence the Ostwald law may also be modeled from a kinetic point of view that considers morphological features of the surface (Table VI) (Van Straten et al., 1984). Accordingly, the kinetic rule of stages predicts the sequence of mineral precipitation from the steady-state rate of nucleation which increases with the number of collisions by which a critical nucleus evolves from monomeric species and decreases with the free energy difference, AGc, that measures the reversible work required for the formation of a nucleus. The free energy difference increases with increasing interfacial tension and decreasing saturation of the solution with respect to a particular mineral. The induction time as well as the rate of nucleation needs to be considered when attempting to explain or predict a precipitation sequence. The induction time is the period before a steady state is reached for the nucleation rate, and it increases as the free energy of formation of a surface, AGc, increases. The more disordered the surface structure (i.e., more amorphous) the shorter the induction time because the roughness of the surface enhances the probability of a monomer sticking to the surface. Hence, an amorphous material may precipitate before a crystalline phase because it has a smaller induction time even though the crystalline phase may have a higher nucleation rate due to the solution being more supersaturated with respect to the crystalline phase. Van Straten et al. (1984) used this model to explain the sequence of precipitation of aluminum hydroxide phases from potassium aluminate solutions that varied in their extent of supersaturation but had the same pH. At high supersaturation (pH pAl 5 I2), poorly crystalline boehmite precipitated before bayerite because the former mineral has a less-ordered structure and hence a lower induction time. Bayerite is thermodynamically less stable than poorly crystalline boehmite and would be predicted to precipitate first if one considered supersaturation without

+

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the effect of induction times. When supersaturation was low (pH + pAl > 12.53, only bayerite formed presumably because the induction times were similarly large. The parabolic rate law predicts that the concentration in solution of mineral components is linearly related to the square root of time. However, the application of the law has been the subject of much debate [see Velbel(1986) and Sparks (1989), and references therein] because it implies that the rate of dissolution is transport controlled and this process is not considered to be a major mechanism by many workers (Stumm and Wieland, 1990). A linearity between concentration and the square root of time has been observed for Al-containing minerals such as feldspar but the correlation can be explained by mechanisms other than diffusion control (Nielsen, 1986; Velbel, 1986; Sparks, 1989). A parabolic relationship can result from the formation of a surface layer that increases in thickness with time; the adherence of finer particles onto mineral surfaces after grinding is used in sample preparation; nonstoichiometric dissolution; the linear release of mineral components followed by their nonlinear precipitation as secondary minerals; changes in parameters (e.g., pH, CO,) thought to be constant; and a surface spiral mechanism of dissolution. Irrespective of the differences in interpretation of this law, the limitations and assumptions of its use given in Table VI indicate that it is of limited value for predicting solution composition unless combined with other kinetic or thermodynamic approaches. The chemical kinetic rate law may also be written in terms of the Gibbs free energy of the reaction, AG, (Nagy and Lasaga, 1992). In this approach, the rate of dissolution or precipitation is not only proportional to the rate coefficient and activities of the species in the rate-determining step but also is proportional to a function of AG, f(AG,). The function can depend on defect properties and densities, precipitation of intermediary phases, the irreversibility of the reactions, and changes in surface and solution speciation. The rate coefficient was assumed to be dependent on temperature, pressure, the reactive surface area, and the concentrations of reactants and other unaccounted effects of the properties of the solution. For elementary reactions, dissolution becomes independent of AG, far from equilibrium but for reactions with more than one step, the variation of the rate of dissolution or precipitation with AGr is more complex. Much of the research and modeling of dissolution and precipitation rates into aluminosilicates and aluminum hydrous oxides have been carried out on systems far from equilibrium, and it has been assumed (implicitly or explicitly) that the mechanism controlling precipitation or dissolution does not vary with the extent of saturation of the solution, i.e., proximity to equilibrium. The model developed by Nagy and Lasaga (1992) takes this possibility into account by the inclusion of AGr in the rate equation. Their work has shown that the values of AGr can vary enormously with the saturation state of the solution and hence illustrates the dangers of estimating rates of dissolution or precipitation in sys-

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76

tems far from equilibrium and assuming that they are still applicable for solutions near to equilibrium. An important outcome of this study was the observation that transition state theory (surface complexation models) cannot be inferred as controlling precipitation and dissolution over a wide range of saturation values if the experimental evidence was only collected over a narrow range, and for dissolution or precipitation alone. The differential rate law has been used as the basis for a model that predicts solution composition in time and space during dissolution reactions while recognizing the influence of metastable phases, irreversible reactions, nucleation, epitaxy, and crystal ripening (Steefel and Van Capellan, 1990). The model differs from others in several ways. It recognizes that the formation of the most thermodynamically stable mineral is an irreversible reaction, but does not make any a priori decisions about the pathway for dissolution because it is completely kinetically based. Hence the sequence of precipitation of intermediary metastable phases is based on their rate of nucleation and factors that can affect nucleation (in particular, epitaxy and the interfacial tension of the solid phases considered). Simulations of granite dissolution using this comprehensive model highlighted the importance of nucleation in controlling the rate of dissolution and solution composition. Slow nucleation may be a major cause of the persistence of thermodynamically unstable mineral phases and the occurrence of solutions supersaturated with respect to the most thermodynamically stable mineral. Transition state theory explicitly recognizes the formation of an intermediary phase or precursor (AB*)when reactants A and B combine to form a product C (Sparks, 1989): A

+ B + AB*

+C

In this case, the rate of reaction is defined by the rate constant and the concentration of the precursor rather than the reactant concentrations. Stumm and coworkers (Stumm and Wieland, 1990, and references therein) have used this approach to model the rate of dissolution of oxides in terms of protonation/hydroxylation of the surface as well as specific adsorption of ligands. They hypothesize that the rate-limiting step is the detachment of a metal center from the surface and that the precursor concentration is proportional to the surface concentration of protonated, deprotonated, or ligand bound metal ions. The effects of H+ ions and ligands on dissolution rate are considered to be additive which implies that they adsorb at different sites. Such a condition is not obvious in their schematic representations of oxide surfaces (see Fig. 5 ) . Apart from some limitations inherent in other models (Table VI), the model only applies to systems far from equilibrium in which the surface area does not change, and active sites are a small proportion of total sites and are regenerated instantly (Stumm and Wieland, 1990). Its application is also dependent on intrinsic equilibrium constants for surface reactions being available to estimate surface concentrations.

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The model has also been applied to minerals that contain constant charge surfaces as well as variable charged sites (e.g., clay minerals). Schott et al. (1989) expanded the TST approach to allow for the rate of dissolution being a function of the formation of the precursor or activated complex at both perfect and dislocated surfaces. They found that increasing the surface area of dislocations in comparison to the perfect surface area did not increase the dissolution rate very much because the effect of increasing surface area was countered by a decrease in surface strain energy as dissolution proceeded. According to Schott (1990), dislocation densities are negligible in comparison to surface sites and hence it may be erroneous to assume that the number of active sites is related to the number of defects. In addition, it is unlikely that active sites would be instantly regenerated (as is assumed in TST models) at dislocation sites because the strain energy decreases as the hollow cores open up and dissolution would tend to become transport controlled as etch pits deepen (Schott, 1990). However, such modeling only applies for solutions far from equilibrium. Nagy and Lasaga (1992) have found that dislocation defects are important for systems near equilibrium. Monte Car10 simulations may also be used with kinetically based models to predict the energies (i.e., measures of bonding energy) within a two-phase system and the extent of supersaturation by using statistical sampling with random numbers (Blum and Lasaga, 1987; Lasaga, 1990). The equations can also be expanded to include changes in surface energy due to defects, the effect of saturation on ordered growth and etch pit formation, and the role of dislocation defects on dissolution rates and etch pit formations. The main benefit of such simulations is their use as a form of sensitivity analysis for identifying major factors that affect surface kinetics.

V. ALUMINUM IN ACIDIC SOILS: PRINCIPLES AND PRACTICALITIES The soil is an open system in which the solution composition is continuously changing in time and space in response to losses and gains by percolation into and out of a volume of soil, plant uptake and release, atmospheric deposition, evapotranspiration, application of amendments, and the removal of vegetation. Precipitation and dissolution in a soil are the net result of all the factors discussed in the previous sections as affected by the cycling of water and soluble components and vary in time and space. In the interest of clarity, the preceding discussion has considered the effect of solution properties on dissolution and precipitation at one moment in time. In reality, however, the activities of many ions vary both spatially and temporally. For example, the activity of H,SiO,

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G. S. P. RITCHIE

varies seasonally due to wetting and drying and may be influenced by plant uptake (Acquaye and Tinsley, 1965). Impeded drainage in micropores in aggregates or sandy topsoils above impervious clay horizons increases contact time between minerals and the soil solution which could result in higher activities of H,SiO, (Kittrick, 1969). The concentration of A1 may increase in some soils during a dry, hot summer due to a decrease in rainfall and an increase in evapotranspiration which would both decrease soil moisture content. On the other hand, A1 concentrations could decrease if the drier, hotter conditions speed up the formation of thermodynamically more stable minerals or result in coprecipitation of minerals. Inclusion decreases the solid-phase activity compared to the pure mineral solid and hence decreases A1 solubility. The importance of the effect of space and time on precipitation and dissolution has been recognized by several workers but has not received much attention partly because of the difficulty in acquiring data to test models. It has been recognized that equilibria in a soil may be very localized (Kitterick, 1969; Tardy and Nahon, 1985; Nahon, 1991; Steefel and van Capellan, 1990) and that the compositional changes in water flowing through a soil affect rates and extents of dissolution and precipitation (Kittrick, 1969; Pates, 1978; Steefel and van Capellan, 1990). Failing to acknowledge the three components of the framework shown in Fig. 1 and a hasty desire to develop invariant rules about the effect of solution and solid properties on dissolution and precipitation can lead to erroneous deductions of the mineral controlling A1 quantities in solution. At this stage, each situation needs to be considered separately and many observations need to be made under differing conditions before paradigms can be combined into a chemical principle. Many models have been proposed to predict the rate of dissolution and precipitation of minerals. It would appear that their application to realistic open systems is limited by the lack of appropriate data sets with which they may be tested (i.e., solution composition data collected through time and space) and the increased complication from acknowledging that a nonsteady state exists. The assumption of a steady state (i.e., A1 fluxin = A1 fluxout)is a pragmatic approximation that may be too limiting for topsoils and between soil layers where the control of soluble A1 changes from one phase to another (e.g., organic + mineral as water flows from organic to a clay-enriched horizon). In addition, the limitations and assumptions given in Tables I1 and V1 need to be considered. It would appear that kinetically based models have fewer assumptions than thermodynamic approaches and are more adaptable to incorporating the factors described in the previous section and in the framework shown in Fig. 1. In particular, kinetic models can address transitions involving metastable reactants and/or products. However, further research still needs to be carried out to ascertain the overall effect of assumptions that the reactive surface area is proportional to the total

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Table VII Suitability of Kinetic Models for Predicting Soluble Al over Different TFme Periods

Model

Time scale

Steefel and Van Capellan ( 1 990) Nagy and Lasaga ( 1992)

Years

Stumm and Wieland

Hours

( 1990)

Days-Years

Level of processes (model basis) Macroscopic (mechanistic) Microscopic (nonmechanistic) Microscopic (mechanistic)

Possible uses Long-term acidification (>5 years); soil formation Predicting A1 Toxicity to plants; medium term acidification (<5 years) Ascertaining molecular mechanisms of dissolution

surface area; that the density of defects is proportional to the reactive site density; and that models and mechanisms that are developed for systems far from equilibrium are applicable to systems near equilibrium. A balance between principles and practicalities is required for their application to acid soils. The ultimate choice of a model will depend on the time scale of interest and the reason for requiring A1 solubility predictions (Table VII). The kinetic models discussed in the previous section vary widely in their time scales. The mechanistic model of Steefel and Van Capellan (1990) can make predictions for open systems over many years and would be most appropriate for estimating soluble Al in the long term (>5-10 years) such as may be required for predicting longterm acidication rates or to ascertain how often a soil should be limed. Medium or short-term (days-years) predictions of A1 solubility could be made with the nonmechanistic model of Nagy and Lasaga (1 992) which uses macroscopic measurements of changes in free energy to predict dissolution rates. Very short-term predictions (hours-days) would be better served by the model developed by Stumm and co-workers (Stumm and Wieland, 1990). The latter model is mechanistically based and deals with molecular processes but is currently limited because measurements to test its validity can only be made at a macroscopic level.

ACKNOWLEDGMENTS This work was conducted while on sabbatical leave at the Department of Soil Science, University of California, Berkeley, and was partially funded by a gift from the ALCOA Foundation and a grant from the Kearney Foundation of Soil Science. I thank Erich Wieland, Gary Sposito, and Andreas Gehring for helpful comments.

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“Mineral-Water Interface Geochemistry” (M. F. Hochella and A. F. White, eds.), Rev. Mineral. 23, 133-176. Ritchie, G . S. P. 1989. The chemical behavior of aluminum, hydrogen and manganese in acid soils. In “Soil Acidity and Plant Growth (A. D. Robson, ed.), pp. 1-60. Academic Press, San Diego. Ritchie, G. S . P. 1994. Soluble aluminum in acidic soils: Principles and practicalities. Dev. Planr Soil Sci. (in press). Robson, A. D., ed. 1989. “Soil Acidity and Plant Growth.” Academic Press, San Diego. Schott, J. 1990. Modeling of the dissolution of strained and unstrained multiple oxides: The surface speciation approach. In “Aquatic Chemical Kinetics” (W. Stumm, ed.), pp. 337-366. Wiley (Interscience), New York. Schott, J., Brantley, S., Crerar, D., Guy, C., Borcsik, M., and Williams, C. 1989. Dissolution kinetics of strained calcite. Geochim. Cosmochim. Acra 53, 373-382. Skopp, 1. 1986. Analysis of time dependent chemical processes in soils. J. Environ. Qua/. 38, 23 I266. Sparks, D. L. 1989. “Kinetics of Soil Chemical Processes.” Academic Press, San Diego. Sposito, G . 1981. “The Thermodynamics of Soil Solutions.” Oxford Univ. Press, New York. Sposito, G. 1984. “The Surface Chemistry of Soils.” Oxford Univ. Press, New York. Sposito, 0 . 1986. Distinguishing adsorption from surface precipitation. In “Geochemical Processes of Mineral Surfaces’’ (J. A. Davis and K. F. Hayes, eds.), pp. 217-229, Am. Chem. Soc., Washington, D.C. Sposito, G. 1989a. “The Environmental Chemistry of Aluminum.” CRC Press, Bocd Raton, Florida. Sposito, G. 1989b. “The Chemistry of Soils.” Oxford Univ. Press, New York. Steefel, C. I . , and Van Capellan, P. 1990. A new kinetic approach to modelling water-rock interaction: The role of nucleation, precursors, and Ostwald ripening. Geochim. Cosmochim. Acra 54, 2657-2677. Stumm, W., and Wieland, E. 1990. Dissolution of oxide and silicate minerals: rates depend on surface speciation. In “Aquatic Chemical Kinetics” (W. Stumm, ed.), pp. 367-400. Wiley, New York . Stumm, W., and Wollast, R. 1990. Coordination chemistry of weathering: Kinetics of the surfacecontrolled dissolution of oxide minerals. Rev. Geophys. 28, 53-69. Tardy, Y. 1971. Characterization of the principal weathering types by the geochemistry of water from some European and African crystalline massifs. Chem. Geol. 7 , 253-271. Tardy, Y., and Nahon, D. 1985. Geochemistry of laterites, stability of Al-goethite, Al-hematite, and Fe3+-kaolinite in bauxites and ferricretes: an approach to the mechanism of concretion forrnation. Am. J. Sci. 285, 865-903. Tsuzuki, Y. 1967. Solubility diagrams for explaining zone sequences in bauxite, kaolin and pyrophyllite-diaspore deposits. Clays Clay Miner. 24, 297-302. Van Straten, H. A,, Holtkamp. B. T. W., and de Bruyn, P. L. 1984. Precipitation from supersaturated aluminate solutions. 1. Nucleation and growth of solid phases at room temperature. J. Colloid Interface Sci. 98, 342-362. Velbel, M. A. 1986. Influence of surface area, surface characteristics, and solution composition on feldspar weathering rates. I n “Geochemical Processes of Mineral Surfaces” (1. A. Davis and K. F. Hayes, eds.), pp. 615-634. Am. Chem. Soc., Washington, D.C. Walton, A. G. 1967. “The Formation and Properties of Precipitates.” Wiley, New York. Wieland, E., and Stumm, W. 1992. Dissolution kinetics of kaolinite in acidic aqueous solutions at 25°C. Geochim. Cosmochim. Acta 56, 3339-3355. Wollast, R. 1967. Kinetics of the alteration of K-feldspar in buffered solutions at low temperature. Geochim. Cosmochim. Acta 31, 635-648. Zawacki, S. J., Koutsoukos, P. B.. Salirni, M. H., and Nancollas, G. H. 1986. The growth of

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calcium phosphates. In “Geochemical Processes of Mineral Surfaces” (J. A. Davis and K . F. Hayes, eds.), pp. 650-662. Am. Chem. SOC., Washington, D.C. Zhang, J.-W., and Nancollas, G. H . 1990. Mechanisms of growth and dissolution of sparingly soluble salts. In “Mineral-Water Interface Geochemistry” (M.F. Hochella and A . F. White, eds.), Rev. Mineral. 23, 365-396.

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MANAGING PLANTNUTRIENTS FOR OPTIMUM WATERUSEEFFICIENCY AND WATERCONSERVATION Jessica G. Davis Department of Crop and Soil Sciences University of Georgia Coastal Plain Experiment Station Tifton, Georgia 3 I793

I. Introduction 11. Conserving Water Supply by Optimizing Water Use Efficiency A. Yield B. Evapotranspiration 111. Conserving Water Quality through Nutrient Management A. Sediment B. Nutrients C. Pesticides D. Organic Matter Interactions n! Needs for Further Research References

I. INTRODUCTION Nutrient contamination of surface and groundwater supplies is an issue of increasing importance and national attention. Organic and inorganic fertilizer sources must be managed to minimize nutrient losses and protect water sources. However, other impacts of nutrient management on water use and water conservation have largely been ignored. Nutrients can be supplied in such a way not only to maximize yield and minimize leaching and runoff losses, but also to conserve water by optimizing water use efficiency and to protect water quality by diminishing pesticide use and soil loss. In 1962, Frank G. Viets, Jr. authored a review of the influence of fertilizers on water use efficiency. The current emphasis on agricultural impacts on water quality makes it imperative to understand nutrient interactions with all other inputs and losses, particularly those which influence the conservation of water quantity and quality. 85 Advances in Agmnmny, hlume 53

Copyright 0 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.

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This review focuses on macronutrient deficiencies and toxicities, although micronutrients occasionally enter the discussion. Sometimes it can be difficult to separate nutrient effects because of their interactions. For example, an excess of one nutrient can induce a deficiency of another nutrient. It is precisely these interactions, including nutrient, organic matter, and pesticide interactions with water and with each other, which will be addressed here.

11. CONSERVING WATER SUPPLY BY OPTIMIZING WATER USE EFFICIENCY Water use efficiency (WUE) can be defined as: WUE

Y

ET ’

where Y is yield and ET is evapotranspiration. Nutrient deficiencies and toxicities can affect WUE by altering yield, evaporation, or transpiration. There are many examples of fertilization leading to increased WUE. Power et al. (1961) showed that P fertilizer increased WUE of wheat (Fig. 1). By increasing the slope of the line (Y as a function of ET), fertilizer increased the WUE. Nitrogen fertilization can increase WUE of native mixed prairie (Smika et al., 1965) wheat (Jensen and Sletten, 1965), and sorghum (Onken et al., 1991).

61 h

2 5-

\

Fl

2 428 3-

v

F

0

I

I

I

I

I

I

5

10

15

20

25

30

ET (cm)

1

35

Figure 1 Biomass yield of wheat as a function of ET and P fertilization. -, P added; --, no P added. (Modified from Power et al.. 1961.)

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A. YIELD Fertilizer application normally results in increased yield with diminishing returns until maximum yield is reached: thereafter, excessive fertilizer application can reduce yield. Yield response to fertilization vanes with crop, soil type, and other limiting factors. For example, in soils low in P, P fertilizer application increased millet dry matter yield and WUE (Payne et al., 1991). In some cases, fertilization may increase dry matter production but have no effect on harvested yield. For example, applying K fertilizer to K deficient peanuts can result in increased plant size and improved canopy cover without any subsequent effect on peanut yield (Table I). Therefore, increasing dry matter production alone does not increase WUE if harvested yield is unchanged. On the other hand, it is possible for fertilization to both increase yield and reduce harvest index (McNeal et al., 1971). Yield quality can also be affected by fertilization; for example, increased Mn application increases oil and decreases protein in the soybean seed and affects the oleic and linoleic acid contents as well (Wilson et al., 1982). Therefore, nutrients must be managed for optimum quality as well as yield, although yield quality is not reflected in WUE. Nutrients can influence yield through their effect on photosynthesis. Nitrogen, S, and Mg are constituents of protein and chlorophyll synthesis in chloroplasts, and Mg, Fe, Cu, S, and P are essential in the electron transport chain (Marschner, 1986). In addition, other nutrients (Mg, Zn, Fe, K, Mn, P) are important for their roles in enzyme activation and osmoregulation in the photosynthetic process. Plant nutrition can also affect yield through its influence on flower initiation, flower fertilization, and seed development. For example, flower initiation in apples increases with ammonium application due to the secondary effect of

Table I Effect of Potassium Fertilizer on Peanut Stands and Yield Peanut yield (kg/ha) K application (Kg K,O/ha)

Stand ratinga

Runner peanuts

Virginia peanuts

2.67 B b 4.17 A 4.25 A

3360 A 3829 A 3857 A

3255 A 3786 A 3734 A

~~

0 88 176

a Rating based on plant size and canopy cover. Stand rating ranges from 1 (stunted and unhealthy) to 5 (very healthy). Potassium application rates with common letters are not significantly different ( p I0.05) by least significant differences.

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ammonium on increasing cytokinin transport from roots to shoots (Buban et al., 1978). Phosphorus and K supply are also directly related to cytokinin level and flower number in many species (Marschner, 1986). Potassium deficiency can elevate abscisic acid levels in leaves, resulting in premature ripening and reduced seed size (Haeder and Beringer, 1981). Flower fertilization can be severely reduced in cereal crops with Cu deficiency due to inhibition of anther formation, decreased number of pollen grains, and nonviability of the pollen (Graham, 1975). In addition, Mo deficiency decreases the number of pollen grains, and B is essential for pollen tube growth and silk receptiveness to pollen (Marschner, 1986). Nitrogen application prior to flower initiation increases the number of seeds per plant (Steer et al., 1984).Application of N fertilizer to soybeans during flowering reduces pod drop and increases seed yield (Brevedan et al., 1978). Nutritional effects on flower and seed development can be direct (deficiencies) or indirect through the influence of a nutrient on a plant hormone (cytokinins).

B. EVAPOTRANSPIRATION Nutrients can influence ET use by crops by altering (1) the supply of water or (2) the demand for water. Nutritional effects on roots generally impact water supply by altering the potential for water uptake. Nutrient deficiency or toxicity symptoms which are expressed in the aboveground portion of the plant influence the demand for water by the crop. 1. Supply of Water Water supply can be increased by improving rooting characteristics or by altering the soil water balance. Rooting volume and surface area are characteristics of root systems which can be manipulated to maximize available water supply (Taylor, 1983). Rooting volume is primarily a function of rooting depth and the rate of root extension. Not many cases of nutritional effects on the length of the main root axis have been reported. Ritchey et al. (1982) determined that subsoil limitations to rooting in Brazilian Oxisols were related to Ca deficiency. Adams (1966) reported that the Ca/total cation ratio in soil solution was the determining factor in maximizing root length. Excessive Mn, B, Zn, Cu, and Fe can also result in diminished root length (Davis et al., 1993). Rooting depth is an estimate of rooting volume at one point in time. The rate of root extension controls the length of time required to achieve the maximum rooting depth. Calcium deficiency can limit root extension rates. Calcium plays a critical role in cell extension and cell wall structure; therefore, root extension ceases in the absence of Ca (Marschner and Richter, 1974). In addition to Ca, P (Davis et al., 1993) and B (Bohnsack and Albert, 1977) deficiencies can reduce

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root growth rates. On the other hand, toxic concentrations of ammoniacal N can reduce radicle elongation rates (Bennett and Adams, 1970). Water use can be increased within an established rooting volume by increasing surface area of roots or rooting density. Water uptake is influenced by root length density to a much greater degree than root weight due to the close relationship between root length density and surface area of roots. Davis-Carter (1989) found that application of N (urea) and P (simple super phosphate) to sandy soils increased the root count (No. roots/ 100 sq cm) of millet grown on those soils to a depth of 90 cm (Fig. 2). This increased rooting density resulted in decreased soil water content due to increased water uptake. Therefore, application of a deficient nutrient increased root density in this case: however, in other cases, nutrient deficiency increased rooting. For example, Anghinoni and Barber (1980) reported that the longer the period of P starvation in maize, the greater the root length and the smaller the root radius. This adaptation results in greater surface area available for P absorption. The number and length of root hairs make an important contribution to root surface area, which in turn influences water supply and uptake. Soil nutrient status can affect root hair formation and growth. For example, Fe deficiency leads to abundant root hair formation and enhancement of Fe uptake (Romheld and Marschner, 198 1). Phosphorus deficiency also results in increased number

Root count (t roots/lOO 3q cm) 0 10'

5' 1 0' 1 5' u .*I 2 5' 3 0' 3 5' 4 0 4 5 I

-

!

0

M-

i

i

Figure 2 Fertilizer (40 kg P,O,/ha as simple super phosphate and 30 kg N/ha as urea) effects on pearl millet root count (5 weeks after planting) and soil volumetric water content (at harvest) in a Fertilizer added; ..., Unfertilized. (Modified from Davis-Carter, 1989.) Labucheri sand. -t,

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and length of root hairs (Foehse and Jungk, 1983). The form of nitrogen influences the relative abundance of root hairs. High nitrate concentrations result in reduction in number (Munns, 1968) and length (Bhat et al., 1979) of root hairs, but ammonium leads to the formation of abundant, long root hairs (Bhat, 1983). Root function can also be affected by nutritional status. For example, P deficiency diminishes the hydraulic conductance of roots (Radin and Eidenbock, 1984). By reducing conductance, P deficiency effectively decreases the transport of water from soil through the roots to the leaves. Fertilization can alter the amount of water available for plant uptake and evapotranspiration through its influence on the soil water balance. This is particularly true for organic fertilizer sources; by increasing soil OM content, these sources can increase water infiltration and hence enhance the availability of water in the root zone (see Sections III,Al and II1,Dl). Gypsum application can also increase infiltration by reducing soil crusting (Shainberg et al., 1989). Nutrient supply can alter the water balance through its effects on plant characteristics as well. For example, increased rooting depth could reduce deep drainage of water out of the root zone thus increasing water supply, or increased rate of root extension could reduce storage of water in the soil for use later in the growing season.

2. Demand for Water Nutritional status can affect transpiration demand through its influence on leaf area and structure. Improvements in plant nutrition often lead to increased leaf area index. One example of this is the influence of N fertilizer on tillering of cereals; increased shoot density results in higher leaf area indices (Maizlish et al., 1980). Eavis and Taylor (1979) reported that transpiration of soybeans increases with increasing leaf area. Ritchie and Burnett (197 1) found that relative plant transpiration (T/ET) increases with leaf area index for both cotton and sorghum (Fig. 3). As leaf area increases, transpiration increases and evaporation from the soil surface declines because of shading and canopy closure. Therefore, nutrients influence transpiration demand through effects on leaf area index. Leaf area duration influences transpiration demand as it changes throughout the growing season. Leaf growth rates and senescence influence the time required to reach the maximum leaf area and the time for that leaf area to decline. When the nutrient supply is deficient, the growth rate of the leaves can be limited by insufficient cell expansion or low photosynthetic rates (Marschner, 1986). Nitrogen and phosphorus deficiencies can both cause reduced cell expansion which results in smaller leaves due to diminished leaf expansion. Application of N fertilizers can enhance new leaf growth (increased leaf area index) and delay plant senescence (increased leaf area duration), resulting in increased transpiration demand. Nutrient deficiencies often lead to more rapid senescence.

91

OPTIMIZING WATER USE EFFICIENCY 1.00,

.OO

50

1.00

1.50

2.00

2.50

3.00

Leaf Area Index Figure 3 Relative plant transpiration (T/ET) as a function of leaf area index in cotton and grain sorghum. (Modified from Ritchie and Burnett, 1971.)

Nutrient deficiency and toxicity symptoms are frequently reflected in leaf shape, angle, and color. For example, Mn toxicity results in leaf crinkling and cupping in soybeans. Although the leaf area itself is not altered, the effective leaf area and the demand for water are reduced. Zinc toxicity in peanuts results in horizontal leaf orientation (Davis and Parker, 1993) which could result in increased absorption of radiant energy, increased leaf temperatures, and increased evaporation from leaf surfaces. Nutritional problems can also influence leaf color. Chlorosis is a symptom common to many nutritional defects from Mo to Fe deficiencies. Phosphorus deficiency generally results in dark green leaves, and Mg deficiency can cause leaf reddening. These effects on leaf color are important in their influence on light absorption, leaf temperature, and photosynthetic rates. Nutrients may also influence leaf resistance to water loss through effects on the quantity and structure of hairs and cuticles on leaf surfaces. Nutrient effects on stomatal function can be direct, through the K + balance in guard cells, or indirect, through nutritional effects on abscisic acid production. The potassium ion is the major solute responsible for water potential gradients between guard cells of leaf stomata and surrounding epidermal cells (Zeiger, 1983). Potassium deficient plants have lower tolerance for water stress due to the role of K + in stomatal regulation and in plant cell vacuoles (Marschner, 1986). During periods of water stress, the abscisic acid level in leaves increases and stomatal closure occurs, thus reducing transpiration. Nitrogen deficiency enhances abscisic acid synthesis and increases abscisic acid concentrations in leaves and stems (Goldbach et al., 1975). Radin and Ackerson (1981) reported that N deficiency results in more rapid stomatal closure and increased leaf resistance to water vapor diffusion, thus diminishing transpiration rates. Phosphorus deficient plants also accumulate more abscisic acid in leaves than P sufficient

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plants, and the stomata close at higher leaf water potentials, thus reducing the demand for water (Radin, 1984). In addition, nutritional effects on leaf color, discussed previously, can also influence stomatal closure through altering light absorption and leaf temperature. Nutrition may also alter stomatal density and location.

111. CONSERVING WATER QUALITY THROUGH NUTRIENT MANAGEMENT It is of utmost importance that soil fertility levels be maintained in order to meet present needs without compromising the needs of future generations (Johnston, 1990). Currently, however, fertilizer additions are inefficiently utilized and significant amounts of nutrients are lost in surface runoff or leached into groundwater (La1 et al., 1988). Traditional agriculture has attempted to maintain soil fertility through use of mulches, crop residues, vegetative covers, and nutrient recycling (Marten and Vityakon, 1986). It is imperative that a balanced approach be used in nutrient management so that neither food production nor water quality are sacrificed in exchange for the other.

A. SEDIMENT Soil erosion has deleterious effects both where the soil is removed from and where it is deposited. Soil fertility is reduced by topsoil removal, and sediment deposition in lakes and streams diminishes surface water quality. Nutrient management can be a tool for minimization of soil erosion. The utilization of legumes, cover crops, and crop residues as nutrient sources also results in reduced runoff and erosion. Fertilizers (organic or inorganic) can play an important role in improving yields on productive land and thus reducing the use of marginal, erodible land for crop production and can also be utilized for remediation of eroded land.

1. Legumes and Crop Residues as Erosion-Minimizing Nutrient Suppliers It is well known that legumes play a vital role in stimulating yield gains to the succeeding or companion crop due to nutrient contributions and that legumes can also reduce runoff and soil loss (El-Swaify et al., 1988). Legumes are used as cover crops in a variety of cropping systems from rubber plantations on Sri Lanka (Jayasinghe, 1991) to no-till corn in Kentucky (Frye et al., 1985). Non-

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leguminous cover crops can also reduce soil erosion and indirectly (by maintaining soil fertility) improve subsequent crop yields; for example, oats serve as a good cover crop for potatoes in Western Australia (McFarlane et al., 1991). Soil cover provides the soil with erosion-protecting residues (Buchner and Vollmer, 1984) which decompose to recycle nutrients. Forage production can be rotated with row crop production and result not only in reduced soil erosion and water contamination but also in reduced fertilizer and pesticide use (Gustafson, 1991). Animal manure usage can also result in reduced erosion due to increased water infiltration rates, while also providing crop nutrients. The nutrient management effects on surface water quality are inextricably linked to the effects of nutrient sources on soil physical properties and water movement. Crop residues increase soil water storage and reduce runoff (Doran et al., 1984) and erosion (Gilley et al., 1987; Brown et al., 1990) and can also result in decreased wind erosion as well (Geiger et al., 1992). Crop residues can reduce evaporation losses (Todd et al., 1991), thus effectively increasing water supply for the crop which can result in increased yields (Lal, 1984). However, crop residues can also intercept rainfall and actually reduce soil water recharge by capturing the water in the residue layer (Zhai et al., 1990). Crop residues can be used to meet nutritional or water needs, but the implications for both factors must be considered. Management of the residues is critical to their usefulness; Cog0 et al. (1984) determined that partially incorporated residue was more effective in reducing soil loss than loose residue. Therefore, residues must be carefully managed to result in the desired effect. Although the benefits of crop residues are well known, there are many competing uses for residues, particularly in the developing world where residues are used as a fuel source, animal feed, and for housing construction (Unger et al., 1991). Alternative practices must be developed to ease demand for crop residues and increase supply of residues, possibly by increased production through nutrient management.

2. Fertilizers Protect Erodible Land As the human population continues to expand, greater pressure is put on marginal soils which are highly erodible (Sant’ Anna, 1985). Improved nutrient management of fertile soils with low erodibility can increase food production, thus minimizing the pressure on highly erodible soils. If more erodible soils could be taken out of production, then sediment levels entering surface waters would be diminished (Follett and Walker, 1989). Retiring erodible land and focusing food production on the best land also reduces the fertilizer and pesticide requirements for crop production (Boggess and Heady, 1992). In many tropical areas, including the semiarid regions, fertilizer and lime are

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required for intensive food crop production (Juo and Kang, 1989; Klaij and Hoogmoed, 1989). Erosion decreases soil productivity, lowers yields, and increases fertilizer requirements (Alt et al., 1989). On the other hand, proper nutrient management can also be used to remediate eroded lands, restore productivity, and minimize further soil loss. Cultivation of erodible land results in loss of the more fertile topsoil and requires increased nutrient inputs (Mokhtaruddin et al., 1984). The impact of erosion on deep, fertile soils is more gradual and less devastating than on soils with shallow A horizons (Daniels, 1987). In effect, the deeper the A horizon, the more buffering of the deleterious effects of erosion; however, given enough time, even deep, fertile soils are eventually depleted of topsoil. Fertilizer application has been shown to mitigate adverse effects of erosion in many instances and result in restored yields (McFarlane et al., 1991 ; Vittal eta!. , 1991; Siebert and Scott, 1990). Fertilizers can also be used to remediate sloping soils (Choi et al., 1991) and minesoils (Ping and Kaija, 1989; Cherrier, 1990). Other nutrient sources such as legumes (Vasileva, 1989), manures (Afel’der, 1988; Dormaar el al., 1988; Mbagwu, 1992), and sludges (Hansson, 1984; Wilson et al., 1985) can also be used to revegetate eroded land. However, there are also many instances where fertilizer could not restore productivity of eroded soils (Mbagwu ef al., 1984; Massee and Waggoner, 1985; Dormaar et al., 1986; Mielke and Schepers, 1986; Lal, 1987; Shafiq er al., 1988; Tremols et af.,1988). Therefore, nutrient management should be utilized primarily to reduce erosion, since alone it cannot readily compensate for erosion. Erosion results not only in chemical and biological changes in the topsoil, but also in physical changes. The primary physical effect of erosion is reduced water storage, which results in reduced water use efficiency (Lo, 1989). This limitation can be partially alleviated by applying appropriate fertilizers in some cases (Massee, 1990). The understanding of nutrient and water interactions is critical to improved water use and quality.

B. NUTRIENTS The most obvious and direct impact of nutrient management is on nutrient contamination of surface and groundwater supplies. There is a tremendous volume of material on the subject, which will not be reviewed here. However, a few ideas of potential importance will be addressed. 1. Nitrogen

Leguminous cover crops and rotation crops have long been known to provide N for succeeding crops. Sorghum grown after soybean with no N fertilizer in

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Nebraska yielded 85% more grain than continuous sorghum (Gakale and Clegg, 1987). Residual soil NO,-N was 50-60 kg N/ha more following soybean than after continuous sorghum. Varvel and Peterson (1990) showed that crop rotations on a Nebraska Mollisol can reduce inorganic N fertilizer needs and reduce the amount of N available for leaching, apparently due to immobilization by crop residues and soil organic matter. Research on sandy loam soils in Alabama showed that a reseeding crimson clover system, in combination with a soybeancorn rotation, provided a fertilizer equivalent for corn up to 159 kg N/ha (Oyer and Touchton, 1990). Ditsch and Alley (1991) described the use of nonlegume cover crops following harvest of summer annuals to recover residual N that might otherwise be lost to the environment and “trap” the N for use by the following summer annual. Nitrogen tissue concentration is related to the C/N ratio of the crop residues which influence the rate of N mineralization (Bruulsema and Christie, 1987). Nitrogen availability from manure has not been as widely studied as crop residues or cover crops. The addition of manure to soils alters the kinetics of N mineralization due to the superposition of two mineralization processes involving different substrates, manure and crop residue (Diaz-Fierros et al., 1988). Measurement of potentially available N from manures is the critical factor in developing manure application rates (Chescheir et al., 1986) so that manure N can be managed precisely. Manure will be discussed further in Section 111,B,2. Chemical fertilizer N sources can also be utilized to maximize uptake efficiency and reduce losses to the environment. Application methods such as fertigation hold promise for supplying N to coincide with N demand, thus reducing leaching losses. Placement and timing of N sources are critical factors in the reduction of nitrate leaching losses (Buchholz and Murphy, 1987). Reducing N losses not only protects water supplies, but also increases profitability. a. Soil Testing Soil testing for N is difficult because of the many forms of N present in soil systems and the dynamic nature of the interactions among N forms. Most states recommend N fertilizer based on crop type and yield potential, although use of irrigation and credits for rotation legumes are also used to adjust N application rates. In addition, the presidedress nitrate test (Fox et al., 1989; Magdoff, 1991) is being adopted for corn in the Northwest and Midwest (Binford et al., 1992; Eckert, 1991) and shows promise for other crops and regions. Residual NO,-N may also be useful in making fertilizer recommendations in dryland regions where leaching is limited. Its use in cotton in New Mexico (Cohacek and Kerby, 1991) and possibly in Arkansas (Miley et al., 1990) looks particularly promising. However, available soil N represents not only residual NO,-N in the profile but also N mineralized from the soil organic matter during the growing season

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(Smith et al., 1990). Mineralization, though it can provide a significant portion of a crop's N needs, usually constitutes less than 3% of the total soil organic N (Smith et al., 1990). Nitrogen mineralization potential can be estimated with aerobic (Stanford and Smith, 1972) or anaerobic incubations or by chemical indices. Hong et af. (1990) stated that a 0.01 M NaHCO, soil extract offered the possibility of improving the prediction of the N supplying capacity of heavily manured fields in Pennsylvania. Many other methods for estimating N availability have been tested (Keeney and Bremner, 1966). In addition, Reddy et al. (1979) stated the importance of including soil temperature and moisture in modeling mineralization of organic N. Development of a rapid and accurate procedure for analysis of N mineralization potential would be a major contribution to improving management and reducing N losses. b. Tissue Monitoring Tissue NO,-N monitoring can also be beneficial for predicting N needs and is a more accurate measure of plant nutrient status. Weekly NO,-N measurements in cotton petioles, based on research in Arkansas and Georgia, are useful in cotton production in regions from Florida (Lutrick et al.. 1986) to Australia (Constable et al., 1991). Karlen et al. (1987) determined maximum N accumulation by corn in South Carolina and suggested that the data could be used to prevent excess N fertilization. However, maximum uptake varies with climatic and edaphic conditions. Binford et al. (1990) used tissue NO3-N concentration in corn to characterize the degree of N excess. Stem NO3-N analysis is useful in predicting the N status of dururn wheat as well (Knowles et al., 1991). Quick tests for tissue testing allow for quicker results and more flexible N management. Plant sap can be extracted using a garlic press and analyzed in the field for NO,-N using an ion selective electrode meter or hand-held colonmeter. Test strips can be used for color development and compared with a color chart or inserted in a reflectometer. Scaife and Stevens (1983) showed that plant sap (cabbage) NO3-N measurements by ion selective electrode and test strips resulted in similar conclusions, although sample preparation affected concentration. A reflectometer improved precision of test strips on sunflower sap and soils of Australia (Schaefer, 1986). Jemison and Fox (1988) determined that quick test results with a reflectometer were highly correlated with laboratory results for both corn tissue and soil extracts in Pennsylvania. The reflectometer method was also very useful in evaluation of N nutritional status of corn in Germany (Geyer and Marschner, 1990). The sheaths of the two oldest, photosynthetically active leaves are the most suitable organs for the quick test. A chlorophyll meter can also be used to predict sidedress N requirements for rice ('lbrner and Jund, 1991), corn (Piekielek and Fox, 1992), and cotton (Edmisten et af., 1992). Quick tests must be calibrated to crop response in order to exploit their usefulness. Once calibrated, these tests will be very beneficial in supplying N to the plant when it needs it and in preventing excessive N application.

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2. Phosphorus Phosphorus contamination of surface waters is largely attributed to losses from agricultural land which can result in eutrophication of water bodies. A large proportion of P lost in runoff is bound to sediments unless the runoff event occurs soon after P application (Truman et al., 1993). Therefore, practices which reduce erosion will generally also reduce P losses. Animal manures are a potentially large source of P runoff contributions. In most cases, pollution from feedlots and barnyards is greater than from manure application sites or grazing areas (Ritter, 1988). However, this review focuses on land that is in row-crop production; therefore, the P discussion will also be limited to row crops. a. Conservation Tillage and Crop Residues Conservation tillage and crop residue management are common means for controlling runoff and erosion, which can also result in reduced P losses (Dillaha et al., 1988; Skoien, 1988; Yo0 et al .. 1988; Brown et al. , 1989; Prato et al., 1989; Tanaka, 1989; Johnsen, 1990; Sharpley et al., 1991). However, total P losses are not necessarily reduced to the same degree as the reduction in soil loss (Skoien, 1988). McDowell and McGregor (1984) found that P losses from conservation tillage corn reduced total P losses due to 92% reduction in soil loss. However, solution P concentrations in runoff were greater in the conservation tillage treatment. Thus, conservation tillage reduced P losses associated with sediments but increased solution P concentrations and losses; these effects resulted in an 80% reduction in total P losses. Langdale er al. (1985) studied six tillage/cropping systems on three watersheds over a 10-year period and reported that although both soluble and total P concentrations were higher with conservation tillage, total P losses were reduced by 50% or more due to reduced runoff volume. In addition, percentage of soluble P increased from 10 to 40% of total P losses with increasing crop residue levels. In both of these cases, conservation tillage reduced total P losses, although the mechanisms of loss reduction varied. Sludge application to land can reduce P losses due to increased infiltration and diminished runoff (Deizman et al., 1989). Increasing the amount of crop residues can have a significant impact on runoff and P loss control (Mostaghimi er al., 1988). Tillage and residues can be utilized to reduce P losses when the P source is land application of manures as well (Mueller et al., 1984; Andraski et al., 1985). Soil P and fertilizer P can also contribute to runoff losses, and these losses can be controlled with conservation tillage and crop residue management. McDowell et al. (1984) measured P runoff from a watershed (Sharkey silty clay) planted to continuous cotton for a 6-year period. Although no P fertilizer was applied because of lack of yield response, 7% of the total P losses were transported in solution with an average soluble P concentration of 0.26 mg/liter (1.4 kg/ha year) and an average sediment P loss of 18.3 kg/ha year. Smith et al. (1991)

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reported that reduced tillage of wheat in the Southern Plains resulted in reduced sediment and sediment-bound P losses. Losses were <1 kg/ha year for soluble P and 0.1 to 6 kg/ha year for total P, but soluble P levels exceeded recommended water quality levels in all treatments. Agronomic practices influence the aggregate size of soil lost in erosion, and this, in turn, may influence the quantity of P lost. Crop residues can reduce the percentage of large-size particles being removed in erosive events (Cog0 et al., 1983; Lopes et al., 1987). Bhatnagar et al. (1985) studied the influence of aggregate size on P losses from P fertilizer and manure sources. They reported that although most added P is associated with the clay size fraction, manure P was preferentially sorbed to larger-sized aggregates; therefore, if residues reduce losses of larger aggregates, they will also reduce P losses in the sediment-bound phase. b. Organic Manures Manure applications to land can clearly result in increased soil P levels. However, the form of that increased P level and its influence on P runoff susceptibility remain unclear. Beef cattle manure application increased Bray PI levels linearly with manure addition, but soil P intensity increased curvilinearly and reached a plateau (Vivekanandan and Fixen, 1990). Sharpley et al. (1984) found that feedlot waste applications increased total, inorganic, organic, and available P content and decreased the P sorption index of surface soil. They hypothesized that increased P contents will increase the potential for soluble and sedimentbound P to be transported in runoff. Increasing manure P application may increase the proportion of P which is desorbable (Bhatnagar et al., 1985). However, P is not the only nutrient level which increases with manure application. Soil nitrate levels can also become elevated and may pose a hazard for groundwater pollution (King er al., 1990). There is a current trend to regulate manure applications to land based on P needs of the crop rather than N needs. The practices described earlier to diminish runoff losses may increase nitrate losses in subsurface drainage (Ritter, 1988). Some studies suggest that sludges may reduce N and P losses compared to chemical fertilizers (Mostaghimi et al., 1992), but organic P sources are more frequently credited with being greater potential pollutants because of excessive application rates. Sharpley et al. (1986) contrasted the possible hazards from N and P in runoff to N leaching losses and concluded that P enrichment of surface runoff from agricultural lands was the greater hazard in the Southern Plains. Chemical fertilizers are not immune to the same hazards. Chemically fertilized corn grown in Missouri resulted in runoff which always exceeded the water quality standard for phosphate and sometimes exceeded the standards for nitrate and ammonium (Alberts and Spomer, 1985). It is critical that manure application recommendations and regulations be site

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specific. If the physical and hydrological characteristics of the soil are more susceptible to runoff then P should be the limiting criteria, and if the soils are highly leached, then nitrate leaching should be the primary concern. The chemistry of the soil should also be taken into account. Not only should current soil test P levels be studied, but also the potential of a soil to sorb (and desorb) P. Soil test P and manure-applied P alone will not accurately predict P runoff losses (Uhlen and Osterud, 1992). Increases in surface soil P levels due to manure application is primarily due to increased labile P levels (Sharpley et al., 1984). Sharpley et al. (1990) reported that the percent retention of applied manure P increased as P sorption capacity increased; however, as the P sorbed increased, the capacity to sorb further increases in soil P was reduced. Phosphorus sorption capacity information complemented with soil textural, soil test P, and manure P analysis information could be used to optimize manure applications to land. Soil and plant biology should also be taken into account in making manure P recommendations. Organic P mineralization rates influence P availability for crop use or runoff (Sharpley et al., 1984), and crop needs and uptake rates should also be calculated as part of manure application rate determinations (Ritter, 1988). Application timing should be utilized to manage manure nutrients for improved efficiency, just as is currently done with chemical fertilizers (Sharpley et al., 1986). Truman et al. (1993) found that soluble P losses declined exponentially as a function of time since fertilization, but sediment-bound P losses increased for about 30 days following application and then declined. McLeod and Hegg ( 1984) reported that nutrient runoff concentrations were more dependent on the number of rainfall events since application than on the quantity of rainfall or runoff. Timing should be scheduled in order to meet crop needs while minimizing runoff susceptibility. Avoiding application of manures during high rainfall periods is one way to achieve this goal. Another is splitting applications on high P fixing soils to maximize fixation and minimize losses. Confining manure application to the growing season could reduce P runoff losses substantially, but would involve considerable expense for farmers if extra storage had to be built (Johnsen, 1990). Manure spreading schedules can potentially reduce P loading by as much as 35% (Brown et al., 1989). The placement and method of application of organic manures and fertilizers is another management tool for minimizing P runoff losses. Application of fertilizer below the soil surface and below the crop residues can protect surface waters from excessive P and N runoff (Dowding et al., 1984; Dillaha et al., 1988). Surface applications of sludges or manures can result in higher P losses than incorporation (Mueller et al., 1984; Deizman et al., 1989). Banding fertilizers in conservation tillage systems can minimize P runoff losses, just as incorporating manure can diminish P losses (Mueller et al., 1983). The same principles apply whether the P sources are inorganic or organic.

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3. Potassium Potassium is not known to be a potential pollutant of water sources, and, therefore, discussion of K losses from soil to water are relatively unimportant from an environmental point of view. However, economically, increased K use efficiency and reduced losses lead to increased profits. In intensive cropping systems, leaching losses of K can amount to 50% of applied fertilizer K (Pieri and Oliver, 1988). Reduction of K losses is related to erosion control, maintenance of soil pH and organic matter levels, and efficiency of fertilizer use. Fertilizer application methods and chemical formulations (such as slow release sources) can be utilized to improve the crop’s use of K and minimize losses. For example, split applications and mulching of residues can reduce K leaching and runoff losses (Valentin, 1980). Improved fertilizer recommendations based on including subsoil K and taking K fixation and release into account can be an important step toward efficient fertilizer use.

C. PESTICIDES An integrated approach to pest management, including insects, diseases, and weeds, has been developed in the past few decades. However, integrated pest management has been studied exclusive of soil fertility effects on pests and pesticide efficacy. The main components of agricultural production systems are fertilizers, pesticides, tillage, and rotations (Edwards, 1989). Knowledge of how these production practices interact must be developed prior to the design of fully integrated, sustainable farming systems that minimize chemical inputs, produce good yields, increase farm profits, and reduce environmental problems. For example, conservation tillage utilizes crop residues to supply nutrients and reduce erosion; however, diminished erosion is effectively being traded for increased herbicide usage (Flach, 1990). Studying each production component separately will never lead to integrated farming systems. 1. Herbicides Nutrient management should be utilized to optimize the crop’s ability to comPete with weeds and, therefore, to minimize the need for herbicide application. Weeds compete with crops for water, nutrients, and light. This review focuses on water and nutrient interactions; therefore, root competition is most important. Clements et al. (1929) explained that a larger, deeper or more active root system enables one plant to secure a larger amount of the water available for growth, which decreases the amount obtainable by the other. Pavlychenko and Harrington (1934) showed that success in competition depends in part on a root

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system with a large mass of fibrous roots near the surface but with its main roots penetrating deeply. Competition for water and nutrients commences when crop and weed root systems overlap and manifests itself in retarded development of top growth (Pavlychenko and Hamngton, 1935). For example, barley is more successful in depressing weeds than wheat because of a larger number of seminal roots and crown roots. The greater the differences between species in root, stem, and leaf characteristics, the less severe the competition (Weaver and Clements, 1938). The implications of root competition are elucidated in the following example. Scott and Oliver (1976) reported that during soybean vegetative growth, soybeans produced roots at rates similar to those of tall morning glory; however, during the soybean reproductive stage, the root intensity of the weed increased at a much greater rate than that of soybeans. Therefore, morning glory had an advantage for extraction of water and nutrients during the critical reproductive growth stage of soybeans. Fertilizer can affect germination of weed seeds (Kim and Moody, 1989) as well as growth and dry weight of weeds. Espeby (1989) reported that moderate amounts of fertilizer often favor weed seed germination, whereas higher amounts inhibit germination and delay emergence of weed seeds due to osmotic effects. Fertilizer applications reduce weed growth in peanut (Hamada et al., 1988), pastures (Hsieh and Cheng, 1988; Rahman et a!., 1990), wheat (Prasad et al., 1991), alfalfa (Lee et al., 1990), rice (Raju and Reddy, 1989), apple and pear (Raese, 1990), and cotton (Singh et al., 1988) due to enhancement of the crop's growth and competitive ability (Radics, 1990). In other cases, fertilizer has no effect on weed competition (Pandey and Thakur, 1988; Lee and Moody, 1989). In some instances, fertilizer has led to increased weed dry weight in maize (Thakur and Singh, 1990), sugarcane (Chauhan and Das, 1990), and wheat (Wimschneider et ul., 1990). Fertility effects depend on the weed type as well as the crop type. For example, Florida beggarweed [Desmodium tortuosum (Sw.)DC] is more sensitive to low pH, K, and P than coffee senna (Cassia occidentulis L.) (Buchanan et al., 1975; Hoveland et al., 1976). Venkitaswamy et al. (1991) reported that N fertilizer decreased weed dry weight during the northeast monsoon in India, but increased weed weight in the southwest monsoon season because of seasonal changes in weed flora composition. Fertilization can result in shifts in weed dominance within the weed community (Ognjanovic, 1990). Withholding P fertilizer from pastures can change the pasture composition, with less clover and more weeds (O'Connor et al., 1990). Fertilizer placement can also be used to manipulate the crop/weed competitive balance. Espeby (1989) found that deep placement favored barley over weeds, but shallow placement promoted weed growth. Similar results were found for wheat in competition with downy brome (Malik, 1991). Weeds can affect nutrient uptake and crop response to fertilizers (Cooper et

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a l . , 1989). Herbicides or hand weeding can increase N and P uptake by rice (Pandey and Thakur, 1988) and nutrient uptake and nutrient use efficiency in cotton (Vireshwar et al., 1988). Yokota et al. (1989) found that autumn application of herbicides in apple orchards enhanced the effect of fertilizer application due to increased soil NO,-N levels and reduced nutrient uptake by weeds. Buresh et al. (1989) also reported increased soil NO,-N concentrations with reduced weed populations. Fertilizer alters the weed-crop interaction and the yield loss associated with weeds. Increasing N fertilizer application increased nutrient uptake by weeds in competition with cotton grown in India (Singh et a l . , 1988), but had no effect on weed competition in cotton in 2 out of 3 years in Alabama (Buchanan and McLaughlin, 1975). Evanylo and Zehnder (1989) determined that K fertilization increased snap bean yields only when full season weed competition occurred. The yield loss due to wild oats in wheat was higher at higher soil fertility levels (Wimschneider et al., 1990), partially due to increased yield potential. Although wild oats reduced the light available to the wheat by 30-35%, straw yield was primarily influenced by root competition, whereas grain yield was most affected by light.

2. Fungicides Optimization of plant nutrition enhances the plant vigor and increases its resistance to disease. Some specific examples of nutrient disease interactions are well-known, but this knowledge is rarely used to minimize fungicide requirements and thus diminish environmental impacts of fungicides as well as fungicide application costs. Good reviews on this subject have been written by Kiraly (1985) and Graham (1983). a. Nutrient and Disease Interactions i. IN FIELD. There are many examples of nutritional imbalances that make agronomic crops susceptible to disease. Some of these include Ca deficiency and pod rot in peanuts (Csinos and Gaines, 1986), K deficiency and verticillium wilt in cotton (Cassman, 1986), excessive P and Rhizoctonia solani in mungbean (Kataria and Grover, 1987), and excessive N and powdery mildew in barley (Oerke and Schonbeck, 1990). Generally, optimum plant nutrition reduces disease incidence; however, nutrient stress can also result in reduced aggressiveness of pathogens due to the reduced C supply for the pathogen (Arora et al., 1985). In the following discussion, two examples will be focused on: one nutrient (Si) and one disease (rust). Silicon is a plant nutrient which is rarely applied as a fertilizer due to plentiful supplies in most arable soils. However, besides its nutritional aspects, Si also acts as a disease suppression agent in many cases. For example, Si amendment

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suppresses Pythium ultimum in cucumbers (Cherif and Belanger, 1992) and increases resistance of cucumbers to powdery mildew (Adatia and Besford, 1986). Adatia and Besford (1986) showed that despite regular applications of fungicide, outbreaks of powdery mildew occurred on mature leaves of low Si cucumber plants. Without Si, fungicide applications were negligible in their efficacy against powdery mildew on cucumbers. However, Voogt and van Elderen (1991) found no evidence of any link between Si uptake and mildew in roses. Rice is the most important crop to which Si fertilizer is applied, and in addition to its nutritional benefits, Si also increases resistance of rice to blast disease (OsunaCanizalez et al., 1991). Proper use of Si amendments can reduce the need for fungicide sprays. Rust is a widespread disease on many crops which appears to be linked to nutritional imbalances. However, the exact interrelationship between plant nutrition and incidence of rust has not yet been elucidated. Anderson and Dean (1986) studied rust in sugarcane and found that certain nutritional conditions can favor the development of rust and, therefore, modifying the plant nutritional status may be a possible method for minimizing rust in sugarcane. The highest rust severities in sugarcane are associated with N, P, and Zn deficiencies and excessive K and Ca levels (Anderson et al., 1991). Zaiter et al. (1991) related rust pustule diameter on bean leaves (Phaseolus vulgaris) to leaf nutrient concentrations and determined that C1 and Mn concentrations were positively correlated with pustule diameter, but K concentration was negatively correlated with diameter. The incidence of rust disease has been related to plant nutrient status in a few cases, but the general relationship between nutrients and rust resistance remains unclear. ii. IN STORAGE.Nutrient management can also reduce disease incidence in postharvest storage. In a test comparing N rates and forms for cabbage production, Berard et al. (1990) determined that susceptibility of stored cabbage to grey speck disease and vein streaking was associated with nitrate-induced Mn deficiency at high N application rates. Therefore, N must be managed not only for optimum economic yield but also for its effects on yield quality, including postharvest disease incidence.

b. Organic Fertilizer Sources Organic wastes such as manures and crop residues are often used as nutrient sources for crops. As one manages these materials as nutrient sources, one must be aware of the other effects that land application of organic materials can cause. One of these possible impacts is the influence of organic fertilizers on plant disease severity and fungicide efficacy. i. PLANTDISEASES. Crop residues and animal manures can provide an environment where pathogens can flourish. For example, farmyard manure can in-

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crease Rhizoctonia sofani disease incidence in cotton and cowpea seedlings (Bandyopadhyay et al., 1982). Kataria and Grover (1987) reported that both farmyard manure and green mature aggravated mungbean seedling rot by R . solani. Crop residues in conservation tillage can support higher pathogen and insect populations and thus prompt increased pesticide use (Sojka et al., 199 I). Weeks (1993) applied crop residues (peanut hay and peanut hulls) to heavy metal-contaminated land to increase adsorption of the metals and reduce their uptake by peanut, but spring hay application increased late leafspot disease scores. Organic amendments to soil have many positive effects; however, they can also increase disease incidence, thus increasing the need for insecticide application. ii. FUNGICIDE EFFICACY.Weed scientists have long known to vary herbicide application rates depending on soil clay and organic matter levels because of their adsorptive properties (Shea, 1989). Soil organic matter directly interacts with any soil-applied insecticides. Farmyard manure has been found to nullify the disease (Rhizoctonia sofani) controlling potential of methoxyethyl mercury chloride (MEMC) and quintozene and to markedly reduce the efficacy of carbendazim (Bandyopadhyay et a f . , 1982). Green manure (Sesbania acufeara) also reduces the efficacy of MEMC. Kataria er al. (1988) studied the efficacy of nine fungicides against R. solani and determined that humic acid extracted from farmyard manure considerably lowered the activity of all the fungicides except chloroneb. Herbicide acticity follows a similar pattern (Stearman er al., 1989). Increasing soil organic matter is viewed as an enhancement of soil quality and nutrient supply for crops. However, one must be aware that when soil organic matter levels are increased, the necessary application rates of soil-applied pesticides that are susceptible to adsorption by organic ligands are also increased. This trade-off should be acknowledged so that management decisions can be made to improve agricultural sustainability, both economically and environmentally. c. Mycorrhizal Competition with Disease Organisms Mycorrhizae can be beneficial to crops grown in soils with low nutrient supply. Mycorrhizal associations can lead to increased nutrient concentrations in plant tissue, particularly nutrients (e.g., P and Zn) which are dependent on diffusion toward roots for uptake. Therefore, management of mycorrhizae is becoming a potential method for crop nutrient management, although there remain a number of practical problems in this application. In addition to nutritional benefits of mycorrhizal associations with plants, mycorrhizae may also compete with fungal pathogens for infection of plant roots. For example, Chakravarty and Unestam (1987) reported that ectomycorrhizal associations with pine seedlings (Pinus syfvesrris)reduced disease severity

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of the pathogens, Fusarium monilijorme and R . solani. Sharma et al. (1988) suggested that inoculation of rice with vesicular-arbuscular mycorrhizae (VAM) is a promising management technique for management of Khaira disease. VAM also reduces Fusarium wilt severity in cumin (Champawat, 1991). However, VAM infection does not reduce disease severity in every case. Wani et al. (1991) determined that although VAM increased nutrient accumulation in barley, it did not reduce disease incidence of common root rot. The potential remains, nonetheless, for using VAM to reduce the need for both fertilizer and fungicide inputs.

3. Insecticides Cover crops and crop rotations, including relay and companion cropping, are methods of supplying N for crop production from a companion crop while minimizing nutrient losses to erosion. These cropping systems, therefore, decrease the need for off-farm inputs such as purchased fertilizers (Gliessman, 1987). Cover crops can also diminish the need for insecticide application by attracting beneficial insects which act as predators of the economically damaging insects. Most of the research done on beneficial insects in cover crops has been done in pecan orchards. Tedders (1983) used vetch as a cover crop in pecan orchards to enhance populations of predatory insects for the control of aphids. Bugg et al. (199 lb) tested annual legumes and mixtures of annual legumes with grasses as cover crops in pecan orchards as a source of nitrogen-rich OM to improve soil fertility and to sustain ladybird populations to aid in biological control of pests. Cover crop management is complex in achieving the optimal timing of N release as well as manipulating predatory insect populations. Cover crops of Vicia villosa and Secale cereale in pecan orchards sustained increased densities of insects which feed on aphids (Bugg et al., 1991a). Bugg et al. (1990) concluded that both risky and promising relationships between cover crops and insects were found in pecan orchards, and in order to optimize the benefits of cover cropping, the effects on soil fertility, weeds, diseases, nematodes, and insects must all be considered. Some work has been done on cover crops in vegetable production systems (Phatak, 1992). In cantalopes, subterranean clover resulted in the highest population of Geocoris punctipes, an important beneficial insect, although crimson clover resulted in optimum production (Bugg et a l . , 1991~).The trade-offs are complicated, but the economic and environmental sustainability of the system must drive the decision-making process. A small amount of work has been done on beneficial insects and cover crops in agronomic crops. Hou et al. (199 1) intercropped pigeon pea with a host tree for a beneficial insect in China. Zhang et al. (1990) studied cotton-barley relay crop-

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ping in China and found that the relay system allowed beneficial insects to transfer from one crop to the next, thus resulting in significantly higher populations of predatory insects. We have failed to explore the fertility-insect interactions in cover cropped agronomic crops in the United States due to our primary focus on the reduced erosion that cover cropping provides. Such systems may be able to increase beneficial insect populations as they provide nutrients, thus reducing fertilizer and insecticide requirements.

D. ORGANIC MATTERINTERACTIONS This section deals with how organic matter (OM) influences pesticide losses to the environment through physical, chemical, and biological processes. It is dealt with here because of the relation between soil organic matter levels and N availability to crops. With increasing emphasis on organic materials as fertilizer sources and renewed interest in the role that OM plays in maintenance of soil quality, it is increasingly important to recognize the interaction between OM and pesticide movement.

1. Physical Effects Increased soil organic matter levels are often associated with improved soil structure leading to increased infiltration rates (Tisdale and Nelson, 1975). Therefore, pesticides moving in soil water would infiltrate further and potentially shift pesticide losses from runoff toward leaching. On the other hand, DavisCarter and Burgoa (1993) reported that an increased lagoon effluent application rate increased average runoff rate and decreased average leaching rate. This was evidently the effect of the Na and fine solids in the lagoon effluent. Nonetheless, increasing effluent application rates shifted atrazine losses away from leaching toward runoff due to the effect of the effluent on water movement. Therefore, the composition of an organic waste material alters its influence on water and pesticide movement. 2. Chemical Adsorption Using organic materials to supply plant nutrients can influence pesticide efficacy and losses to the environment. Adsorption is generally better correlated with OM content than with other soil properties (Singh et af., 1990). This relationship between OM and pesticide adsorption has been shown for chlordimeform (Maqueda et al., 1982), oxamyl (Gerstl, 1984), methyl bromide (Arvieu and Cuany, 1985), metolachlor (Wood et al., 1987), diuron (Fernandez et al., 1988), ethirimol (Romera-Taboada et al., 1989), dichloropropene (Muller-Wegener et af.,

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1989), and many others. The adsorption of chlorsulfuron by OM from a peat muck was 100-fold larger than adsorption by mineral soils (Shea, 1986). Braverman et al. (1990) reported that thiobencarb adsorption was highly correlated with soil organic C content. Amendments of waste-activated C, sewage sludge, and bovine manure reduce phytotoxicity of pesticides due to their adsorption capabilities (Guo et al., 1991). Adsorption of pesticides reduces their efficacy, which can result in greater pesticide application rates in order to achieve efficacy. However, adsorption of pesticides to OM can effectively block adsorption of the pesticide to clay (SanchezCamazano and Sanchez-Martin, 1988). Pesticides adsorbed to OM can be released back to the soil environment as the OM decomposes; thus, the OM may effectively cause binding of pesticide followed by slow release and continued efficacy over a longer period of time. Increased pesticide adsorption to OM also results in reduced leaching losses (Nordmeyer, 1991). Khan and Khan (1986) showed that mobility of organophosphorus pesticides was reduced when soil organic matter, particularly fulvic acids, increased. Hubbs and Lavy (1990) showed that as soil OM increased, adsorption of norflurazon increased and mobility decreased. Hatzios and Penner ( 1988) reported that leaching of metribuzin was retarded by increasing OM content. The use of organic materials to reduce pesticide mobility could result in reduced pesticide losses into the environment, but may also retard pesticide efficacy, resulting in increased pesticide application rates.

3. Microbial Degradation Organic matter not only influences adsorption of pesticides, but can also affect microbial degradation rates. Biodegradation of pesticides depends on microorganisms responsible for the biodegradation pathways (Bums and Martin, 1986). The influence of soil OM on degradation, therefore, depends on its composition, stage of decomposition, and the microorganism populations that it supports (Arvieu and Cuany, 1985). Soil application of easily degradable organic material increases microbial activity and can enhance pesticide degradation (Hurle and Walker, 1980). Doyle et al. (1978) showed that manure increased atrazine degradation, but sewage sludge inhibited degradation. Hance (1973) reported that manure accelerated atrazine degradation in only one of two soils studied. Therefore, the OM type and the soil type affect whether OM protects pesticides from degradation through adsorption or whether it accelerates degradation rates through enhanced microbial activity. Arita and Kuwatsuka (1991) found that the pyrazoxyfen degradation rate was negatively correlated with soil OM content. Hamaker (1972) suggested that an increase in OM might increase the herbicide degradation rate to a limiting value, above which the rate of loss would be retarded. This retardation could be due to

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adsorption of pesticide by OM, thus preventing its degradation (Hurle and Walker, 1980). For example, atrazine persistence in soil increased when straw ash was added (Hurle, 1978). Burkhard and Guth (1981) reported that as atrazine adsorption increased, atrazine half-life also increased; however, adsorption does not always protect a chemical from degradation (Hurle and Walker, 1980). In addition, degradation products of certain pesticides can also be adsorbed by OM and thus retained in soils (Khan, 1991). Pesticide adsorption is often positively correlated with soil OM content, but the pesticide degradation rate is positively correlated with microbial biomass and respiration (Walker et al., 1992). Therefore, OM can result in reduced pesticide leaching and efficacy due to increased adsorption and can increase or decrease pesticide degradation rates depending on the specific situation. When man manipulates soil to increase OM content to provide increased soil fertility, he is also altering the soil environment into which pesticides are applied and affecting the application rates required, the potential leaching losses, and the length of time that the pesticide will remain in the soil. These factors should be considered when aiming for sustainability of a farming system.

IV. NEEDS FOR FURTHER RESEARCH Lester Brown (1991) writes that “the potential for expanding the cultivated area profitably is limited and food for the expected additional 960 million people in the 1990s will have to come from raising land productivity.” Increasing fertilizer use, in conjunction with improved varieties, has been responsible for growth in world food output since the 1940s (Brown, 1991). In the United States, the move toward sustainability and environmental stewardship has resulted in reduced fertilizer application rates; however, care must be taken not to jeopardize long-term sustainability for short-term benefits (Wallingford, 1991). A balanced approach with long-term and worldwide vision is the key to sustainability. Some of the areas requiring additional research efforts include: 1. Nutritional influences on transpiration demand; 2. effects of OM on contaminant movement and pest populations; 3. nutrient interactions with weeds, diseases, and insects; 4. mineralization potentials for N and P in soil OM and manures; 5. measurable basis for site-specific manure recommendations; and 6. ranking of potential agricultural contaminants for decision-making purposes (for example, if applying N diminishes the need for a herbicide, which presents the greater hazard to the environment?).

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It is critical that we meet the need for a systems approach in agricultural research (Logan, 1990) so that the real issues can be addressed. One input can be used to minimize the need for another potentially more hazardous input, and, therefore, it is essential to integrate the components of agricultural systems fully so that their impact on other inputs is taken into account (Edwards, 1989). When a farmer spreads fertilizer or irrigates with lagoon effluent, he is not only supplying nutrients to his crop, he is altering the environment for pest populations and influencing the movement of water across the landscape. The more that scientists and farmers understand these interactions, the more capable we will be of making wise decisions regarding our stewardship of land and water.

ACKNOWLEDGMENT The author is deeply indebted to the late Dr. Howard M. Taylor (1924-1991) for many of the concepts and thought processes that led to this work. An earlier version entitled, “Nutritional Effects on Water Use Efficiency and Root Growth” (J. G. Davis-Carter and P. G. Georgen) was presented at the Rhizosphere Research Symposium in Dr. Taylor’s honor at the American Society of Agronomy meetings in Minneapolis, Minnesota, on November 2, 1992.

REFERENCES Adams, F. 1966. Calcium deficiency as a causal agent of ammonium phosphate injury to cotton seedlings. Soil Sci. Soc. Am. Proc. 30, 485-488. Adatia, M. H., and Besford, R. T. 1986. The effects of silicon on cucumber plants grown in recirculating nutrient solution. Ann. Bot. (London) 58, 343-351. Afel’der, L. I. 1988. Decrease in soil erosion processes under fertilization. Sov. Agric. Sci. 6,68-70. Alberts, E. E., and Spomer, R. G. 1985. Dissolved nitrogen and phosphorus in runoff from watersheds in conservation and conventional tillage. J. Soil Water Conserv. 40, 153-157. Alt, K., Osborn, C. T., and Calacicco, D. 1989. Soil erosion: What effect on agricultural productivity? Agric. Inf. Bull (U.S.Dep. Agric. No. 556. Anderson, D. L., and Dean, J. L. 1986. Relationship of rust severity and plant nutrients in sugarcane. Phytoparhology 76, 581-585. Anderson, D. L., Henderson, L. J., Raid, R. N., and Irey, M. S. 1991. Sugar cane rust severity and leaf nutrient status. Sugar Cune 3, 5-10. Andraski, B. J., Mueller, D. H., and Daniel, T. C. 1985. Phosphorus losses in runoff as affected by tillage. Soil Sci. Soc. Am. J. 49, 1523-1527. Anghinoni, I., and Barber, S. A. 1980. Phosphorus influx and growth characteristics of corn roots as influenced by phosphorus supply. Agron. J. 72, 685-688. A d a , H., and Kuwatsuka, S. 1991. Relationships between the degradation rate of the herbicide pyrazoxyfen and soil properties. J. Pesric. Sci. 16, 71-76. Arora, D. K., Filonow, A. B., and Lockwood, J. L. 1985. Decreased aggressiveness of Bipolaris sorokiniana conidia in response to nutrient stress. Physiol. Plant Pathol. 26, 135-142. Arvieu, J. C., and Cuany, A. 1985. Effects of organic matter on the biological activity and degradation of methyl bromide in soil. Bull. OEPP 15, 87-96.

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McDowell, L. L., and McGregor, K. C. 1984. Plant nutrient losses in runoff from conservation tillage corn. Soil Tillage Res. 4, 79-91. McDowell. L. L., Willis, C . H., and Murphree, C. E. 1984.Plant nutrient yields in runoff from a Mississippi Delta watershed. Trans. ASAE 27, 1059- 1073. McFarlane, D., Delroy, N., and van Vreeswyk, S . 1991.Water erosion of potato land in Westem Australia: I. Factors affecting soil loss and the effect of soil loss on productivity. Ausr. J . Soil Water Conserv. 4,33-40. McLeod, R. V., and Hegg. R. 0. 1984. Pasture runoff water quality from application of inorganic and organic nitrogen sources. J. Environ. Qual. 13, 122-126. McNeal, F. H., Berg, M. A., Brown, P. L., and McGuire, C. F. 1971. Productivity and quality response of five spring wheat genotypes. Triticurn aestivum L., to nitrogen fertilizer. Agron. J. 63, 908-910. Mielke, L. N., and Schepers, J. S. 1986.Plant response to topsoil thickness on aneroded loess soil. J. Soil Water Conserv. 41, 59-63. Miley, W. N., Maples, R. L., and Keisling, T. C. 1990.Use of soil nitrate tests for cotton nitrogen recommendations-An extension viewpoint. In “Nitrogen Nutrition in Cotton: Practical Issues” (W. N. Miley and D. M. Oosterhuis, eds.), pp. 65-76. Am. Soc. Agron., Madison, Wisconsin. Mokhtaruddin, A. M., Jamal, T., Sulaiman, W. H. W., and Anuar, A. R. 1984.Yield reduction due to loss in soil fertility through erosion. In “Soil Science as a Tool for Rural Development” (S. Panichapong, ed.), Proc. ASEAN Soil Conf., 5th I, E6.1LE6.7.Dep. Land Dev., Bangkok. Mostaghimi, S., Dillaha, T. A., and Shanholtz, V. 0. 1988.Influence of tillage systems and residue levels on runoff, sediment, and phosphorus losses. Trans. ASAE 31, 128-132. Mostaghimi, S . . Younos, T. M., and Tim, U. S . 1992. Effects of sludge and chemical fertilizer application on runoff water quality. Water Res. Bull. 28, 545-552. Mueller, D. H., Andraski, B. J., Daniel, T. C., and Lowery, B. 1983.Effect of conservation tillage on runoff water quality: total, dissolved and algal-available phosphorus losses. Pap. No. 83-2535,Am. SOC.Agric. Eng., St. Joseph, Minnesota. Mueller, D. H., Wendt, R. C . , and Daniel, T. C. 1984.Phosphorus losses as affected by tillage and manure application. Soil Sci. SOC. Am. J. 48, 901-905. Muller-Wegener, U., Ehrig, C., Ahlsdorf, B., Litz, N., Katona, B., and Milde, G. 1989. The translocation of pesticides in soils. Mitt.Dtsch. Eodenkd. Ges. 59, 433-437. Munns, D. N. 1968.Nodulation of Medicago sativa in solution culture. 111. Effects of nitrate on root hairs and infection. Plant Soil 29, 33-47. Nordmeyer, H. 1991. Spatial variability of soil characteristics and its influence on the transport of pesticides in soil. Mitt. Dtsch. Eodenkd. Ges. 66, 373-376. O’Connor, M. B., Smart, C. E., and Ledgard, S. F. 1990. Long term effects of withholding phosphate application on North Island hill country: Te Kuiti. Proc. N . Z . G r a d . Assoc. 51, 2124. Oerke, E. C., and Schonbeck. F. 1990. Effect of nitrogen and powdery mildew on the yield formation of two winter barley cultivars. J . Phytopathol. 130, 89-104. Ognjanovic, R. 1990. The influence of application methods and fertilizer rates on structure and dynamics of weed communities in maize crops. Rad. Poljopr. Fak. Univ. Sarajevu 38(42),2761. Onken, A. B., Wendt, C. W., Lascano, R. J., Sow, A., and Kouyate, Z.1991.Effects of soil fertility, crop genotypes, and available soil water on water-use efficiency of sorghum. In “TropSoils Technical Report, 1988- 1989,”pp. 318-320. North Carolina State Univ., Raleigh. Osuna-Canizalez, F. J., de Datta, S. K., and Bonman, J. M. 1991. Nitrogen form and silicon nutrition effects on resistance to blast disease of rice. Plant Soil 135, 223-23 I. Oyer, L. I . , and Touchton, J. T. 1990.Utilizing legume cropping systems to reduce nitrogen fertilizer requirements for conservation-tilled corn. Agron. J . 82, 1123-1 127.

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INTERPARTICLE FORCES: A BASISFOR THE INTERPRETATION OF SOIL BEHAVIOR PHYSICAL J. P. Quirk Department of Soil Science and Plant Nutrition School of Agriculture The Univenity of Western Australia Nedlands, Western Australia 6009, Australia

I. Introduction 11. Interparticle Forces A. Suction in a Porous Material: Capillary Condensation B. London-van der Waals Forces C. Osmotic Repulsive Forces: Diffuse Double-Layer Theory D. Ion-Ion Correlation Forces: Potential Energy Minima between Particles E. Structural Component of Disjoining Pressure: Water Structural Forces 111. Soil Water Relations: Swelling and Shrinkage A. Porosity and Structural States B. Residual Shrinkage C. Structural Porosity and Shrinkage W. Swelling of Sodium Clays A. Extensive Crystalline Swelling B. Swelling between Crystals V. Swelling of Calcium Clays A. Clay Domains and Quasicrystals B. Mechanisms of Clay Swelling VI. Surface Area and Pore Size A. Pore Size Distribution B. Intrinsic Failure C. Packing of Clay Particles VII. Water Stability of Soil Aggregates A. Stabilizing Substances B. Disposition of Organic Matter and Aggregate Stability VIII. Sodic Soils and the Threshold Concentration Concept A. The Threshold Concentration B. Physical Basis for the Threshold Concentration C. Application of the Threshold Concentration Concept IX. Concluding Remarks References 121

Advrrncu in Agronomy, Yolume 53 Copyright 0 1994 by Academic Press, Inc. All nghts of reproduction in any form reserved.

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I. INTRODUCTION Micromorphologists (see, e.g., Brewer, 1988) have made substantial efforts toward defining soil structure and have generated numerous terms to describe soil characteristics as observed by the microscope. Although such research has enlarged our knowledge of soils, particularly in relation to soil classification, the real challenge in defining soil physical behavior lies at a level of discrimination beyond that of the light microscope. Soil structure has been defined by Baver (1940) as “the arrangement of soil particles” which determines the arrangement and size of soil pores. Soil particles with an equivalent spherical diameter of < 2 pm are described as clay and are referred to as the “active” fraction of a soil; the principal component of this fraction is generally the layer lattice silicates which have been extensively described by clay mineralogists (Brindley, 1980). As a result there is a substantial body of information about the crystalline swelling of smectites and vermiculites in various homoionic forms. The clay fraction of a soil frequently contains minerals such as illite, which may be very fine grained, and kaolinite; both of these minerals have basal spacings which are fixed at all water contents. In the absence of crystalline swelling, the increase in volume on wetting these materials, or a clay soil containing them, takes place as a result of interparticle (crystal) interaction. Because of the irregular nature of the clay crystals themselves, especially the distribution of crystal thicknesses and terraced surfaces, the crystal interaction resulting in swelling cannot be thought of in exactly the same way as the interaction between the regular 10 A thick aluminosilicate layers which is the basis of intracrystalline swelling of smectites and vermiculites and which can be observed by X-ray diffraction. Furthermore, the crystals and lamellae do not exist as separate entities but are assembled into compound particles which have been described as clay domains (Aylmore and Quirk, 1960) and as quasicrystals of smectites which also assemble to form clay domains (Quirk and Aylmore, 1971). The swelling or water uptake by a clay or soil involves the complex interaction between clay domains, between crystals (or quasicrystals) within a domain, and between the lamellae within quasicrystals. These interactions are occasioned by the operation of a number of interparticle forces, some of which are attractive and oppose swelling, and others which are repulsive and facilitate swelling. The manner in which these forces vary with the distance of separation of contiguous surfaces is an important consideration in arriving at a fundamental basis for the swelling of soils and clays due to the interplay of these forces. For soils with a texture between clay loam and heavy clay, particle interaction is expressed as a measurable increase in volume. However, such particle interactions also occur within the interstices of lighter textured soils. This swelling is

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internally accommodated but continues to influence soil physical properties such as permeability. Another feature of the “active” clay particles is that their plate-shaped character results in the particles (crystals and quasicrystals) within a soil taking up positions of near parallel alignment. The pore sizes in such an assemblage of particles will reflect the thickness of the crystals or quasicrystals and there is, inevitably, a considerable overlap of particle surfaces to give close distances of approach. In addressing the operation of interparticle forces, it is necessary to know the interparticle distances. This has been achieved by the measurement of pore size distributions using low temperature nitrogen desorption isotherms and other means (Aylmore and Quirk, 1967; Sills et al., 1973; Murray et al., 1985). Natural aggregates, even from soils which swell appreciably when fully wet, have a mass ratio of solid to liquid of 2 or greater reflecting the fact that a soil should properly be regarded as a condensed system quite distinct from a suspension for which the so1id:liquid ratio may be 0.1 (a 10% suspension) or less. Bradfield’s (1936) statement that “granulation is flocculation plus” needs to be modified since the particle arrangement and distances of surface separation in a floccule are vastly different from those of condensed clay particles within a clay or soil. For regions of particle surface overlap within a Ca clay domain, the surfaces are within what colloid scientists refer to as the primary potential minimum, that is the association is face to face and the separation is small. Particles in a primary minimum are adhesive and are not readily separated. In contrast the dispersion-flocculation transition is the result of a secondary minimum, involves card-house type structures, and is readily reversible. The balance between the forces when Ca clay particles are within the potential minimum must be delicate since 15% of Na ions or even less on the exchange sites can dramatically alter the physical behavior of a soil. The “plus” referred to by Bradfield encompasses a range of substances which are capable of stabilizing a soil aggregate against the disruptive forces of slaking due to immersion in water or exposure of the soil surface to intense rain. These stabilizing substances are sometimes referred to as cements and are considered to include the clay particles themselves, iron and aluminium oxides, or their precursors, silica, calcium carbonate, and parts of the organic fraction of soils. The profound effect of organic matter in influencing the water stability of aggregates remains largely unexplained. However, the variation of stability for a particular soil type depending on management and period under an arable phase is well documented (Bradfield, 1936; Greenland et al., 1962). The site of action of these stabilizing agents within the porous matrix of a soil has received very little attention. Quirk (1978) has defined soil stabilization by means of additives as “the strategy of placing the most appropriate material at the most efficacious place within the soil structure or pore space so that the desired strength may be

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achieved, most economically for agricultural or engineering purposes.” Such an aspiration may be fulfilled with an increased knowledge of soils as porous materials, particularly the micropores, and with advances in polymer science in relation to organic and inorganic polymers. The establishment of pillared complexes (Frenkel and Shainberg, 1980; Pinnavaia, 1983) at strategic points within a soil may be one such approach; such research also has the potential to extend our knowledge of the basis for the stability of natural soil aggregates.

11, INTERPARTICLE FORCES Swelling is one way in which the reaction between clay and water is manifest. The forces which give rise to a greater separation of two interacting clay particles (swelling) are restrained by the suction in the soil water, by London-van der Waals forces, and by ion-ion correlation forces (Kjellander er al., 1988). The forces which facilitate swelling are osmotic repulsive forces which arise from the interaction of diffuse double layers between contiguous particles and the hydration of the exchangeable cations and their perturbation of the normal hydrogenbonded structure of water in the vicinity of the clay/solution interface (water structural forces).

A. SUCTIONIN A POROUS MATERIAL: CAPILLARY CONDENSATION The pressure deficiency, P,, across a curved meniscus is given by the YoungLaplace equation,

P,=

?(t+ A)

cos8

where y is the surface tension of the liquid, r , and r2 are the two principal radii of curvature, and 8 is the contact angle which for readily wet surfaces is zero so that cos 8 = I . Within a porous material, such as soil, P, is expressed as a suction which is transmitted throughout the soil water. For a cylindrical capillary for which r l = r2 the pressure deficiency is 2 y / r , and in a slit-shaped pore for which r2 %- r , , the pressure deficiency is given by yJr,.

The condensation of liquid into pores from the vapor phase (capillary condensation) is described by the Kelvin equation which gives the relative vapor pressure (PIP”) in equilibrium with a meniscus in a cylindrical capillary of radius r ,

125

INTERPARTICLE FORCES = exp

( - rRT) 2vy

(2)

Po

where p is the vapor pressure above the meniscus, po is is.e saturated vapor pressure of the liquid at temperature T (K), V is the molar volume of the liquid, y its surface tension, and R the gas constant. Equation (2) can be rewritten,

and since the height of rise, h, in a capillary above a free water surface is given by the relation p gh = 2y/r, the lowering of the vapor pressure can be expressed in terms of h ,

where M is the molecular weight of the liquid and g is the gravitational constant. For a slit-shaped pore r is replaced by d, the slit width, in Eq. (3). In Table I the values of pIpo and hydrostatic suction corrggonding to particular sizes of slitshaped pore have been calculated using Eqs. (3) and (4). Even for small pores it is assumed that the bulk properkdof water, density and surface tension, are applicable. Table I shows the magnitude of the pressure deficiency (suction) residing in the water in a porous material for a range of circumstances. This suction is transmitted to the pore walls and acts to draw the walls of pores closer together. The suction is thus rightly considered to act in concert with other attractive forces to resist swelling. These considerations provide the basis for soil water-suction relationships (the moisture characteristic). For a clay soil, as the suction is increased the interacting plates are brought close together and shrinkage takes place.

Table I Relative Vapor Pressure and Suction of Water at which Slit-Shaped Pores of Various Widths Begin to Empty at 298°K Slit width (nm)

Relative vapor pressure ( p l p , ) Hydrostatic suction (MPa)

1000

100

10

0.999 0.137

0.990 I .38

0.900 14.4

I

0.350 144

126

J. P. QUIRK

B. LONDON-VANDER WMS FORCES In 1930 London applied quantum mechanics to derive the force between two apolar atoms arising from their mutually induced polarization. The basis of the attractive force is that the fluctuating dipole of one atom polarizes the other one and consequently the two atoms attract each other. The frequency of the fluctuations is of the order of the electronic frequency and is taken as 3 X 1015 sec-I corresponding to the first ionization potential for the Bohr hydrogen atom. These forces are also referred to as dispersion forces because of their link to the dispersion of electromagnetic radiation. In 1937, Hamaker introduced the idea that, for conglomerates of atoms in two interacting macroscopic bodies, the London forces are pair-wise additive; that is the interaction of all atoms in both bodies contributes to the energy and force of interaction (Verwey and Overbeek, 1948; Mahanty and Ninham, 1976; Israelachvili, 1985). The constant, A, which governs such interactions is referred to as the Hamaker constant and is given by A = $ h u o Q .2r r 2 q 2 where h is Planck’s constant, a0 is the static polarizability of the atoms, uo is the characteristic electronic frequency, and q is the number of atoms per unit volume of interacting bodies. Because of the additivity principle the energy of interaction decreases much more slowly with distance than that between individual atoms which decays as the inverse sixth power of distance. This treatment is referred to as the microscopic basis for the Hamaker constant. The assumption of simple pair-wise additivity inherent in Eq. (5) ignores the influence of neighboring atoms on the interaction between any pair of atoms. The problem of additivity is completely avoided in Lifshitz’s macroscopic theory (Mahanty and Ninham, 1976) in which the atomic structure is ignored and the forces between large bodies, treated as continuous media, are derived in terms of such bulk properties as the dielectric constants and refractive indices. The attractive pressure for two semi-infinite, thick, flat, parallel plates is given by

A PA = 6nD3 where D is the distance of separation of the plates. The value of A has been determined experimentally for muscovite mica surfaces separated by air and water (Israelachvili, 1985); the values reported were 13.5 X 10-*0 and 2.2 X 10-20 J, respectively. The principal contribution to this later value is in the electronic frequency (3 X 10’5 sec- I), however, the Keesom (dipole-dipole), Debye (dipole-induced dipole), and other effects contribute less than 10% be-

INTERPARTICLE FORCES

127

cause the frequencies involved are in the microwave region (10" sec-I). This low frequency contribution is referred to as the static or zero frequency contribution and is affected by temperature (Israelachvili, 1985). For a plate of thickness, t, the van der Waals interaction energy per unit area is given by

"+

VAT-- A 121~ 0

1

2

(D

+ 2t)2

1

-

-

(D

+ t)*

(7)

and the pressure by (D

+ 2t)3

(D

+ t)3

These equations are applicable over distances of separation from 0.2 (the surface granularity) to 7 nm. Because of the finite time required for the propagation of the electromagnetic radiation the attractive energy is reduced when D approaches c/u, where c is the velocity of light. The forces are then said to be retarded and decline more rapidly with increasing D when the separation exceeds about 7 nm for the system mica-water-mica. Table I1 gives the van der Waals energy and pressure for varying distances of plate separation for t = 1 nm, the thickness of an elementary aluminosilicate layer in a montmorilloniteor vermiculite, and also for the circumstances when t is much greater than D. At close distances of approach (0.25 nm) the energy of interaction is about 10 mlm-2 (erg cm-2). This may be compared with 107 d m - 2 obtained by Bailey and Kay (1967) for the pristine cleavage of muscovite in water. McGuiggan and Israelachvili (1988) reported values of 7 to 10 mlm-2 for the adhesion energy between two molecularly smooth muscovite surfaces at Table II van der Waals Interaction Energy and Attractive Pressure between Surfaces for the System

Mica-Water-Mica in Relation to Distance of Separation ( D ) and Plate Thickness (Oa Surface separation (nm) 0.25

0.5

1.o

2.0

4.0

-8.7 -9.3

-1.9 -2.3

-0.36 -0.6

-0.05

-0.006 -0.04

~~~~~~~~~~~

Energy ( d m - * ) t=lnm T*D Pressure (MPa) t = lnm t%D a

74 75

8.7 9.3

The Hamaker constant for eqs. (7) and (8) is 2.2

-0.15

0.9 1.2 X

0.08 0.15

J.

0.005 0.018

128

J. P. QUIRK

“contact” in water. Quirk and Pashley (1991a) have discussed the nature of “contact” and have concluded that the mica surfaces in adhesive “contact” in aqueous solutions are probably separated by two layers of water; they also discussed the special role of H 3 0 + in enabling surfaces to be brought into “contact. From Table I1 it may be noted that the interaction energy and pressure, for surfaces separated by water, decrease markedly with increasing distance of separation and that beyond I-nm separation the magnitude of these quantities is relatively small. For distance of separation of 1 nm or less Table I1 shows that the energy and pressure of interaction are similar for t = 1 nm and t % D. Through the use of Eq. (8) it can be shown, particularly for small separations, that the attractive pressures calculated for a plate thickness of 5 nm are not very much less than when t D. Using Eq. (8) it can be calculated that the attractive pressure between two plates 5 nm thick at a distance of 5 nm is 7 kPa whereas when the same plates are at a distance of 0.5 or 1 nm the pressures are 9.3 and 1.2 MPa. These calculations can be considered in relation to the two separate sets of slit-shaped pores which would exist within a clay matrix; one set for which the surface separation would be similar to the crystal thickness, for example, 5 nm, and the other set in regions of crystal overlap for which the surface separation might be I nm or less. The attractive pressure in the smaller pores, as seen from these calculations, is several orders of magnitude greater than in the larger pores. From the information in Tables I and I1 it is possible to conclude that for slitshaped pores greater than 1 nm, the van der Waals forces make only a minor contribution to the forces resisting swelling in the usual suction regimes in soils. ”

C. OSMOTIC REPULSIVEFORCES: DIFFUSE DOUBLE-LAYER THEORY The DLVO theory,* for the stability of colloidal suspensions, combines Gouy’s original ideas on the diffuse distribution of counterions at particle surfaces in an aqueous environment with the London-van der Waals forces; the Gouy treatment leads to the osmotic repulsive force for two interacting surfaces (Langmuir, 1938). The DLVO theory incorporates a Stern layer as modification of the Gouy double layer to allow for the fact that the counterions, as point charges, can approach a surface without any limit and this gives rise to impossibly high concentrations. The double layer is divided into two parts, a Stern layer approximately two water monolayers thick (5.5 A) in which there is a rapid fall in potential to the value at the Gouy plane; the behavior of ions in the diffuse part *The DLVO theory is a result of the independent work of Derjaguin and Landau in the USSR and of Verwey and Overbeek (1948) in the Netherlands during World War 11.

INTERPARTICLE FORCES

129

of the double layer is governed by the Gouy potential. The total surface charge is balanced by counterions in the Stem layer and by the excess of counterions over coions in the Gouy layer. The diffuse double-layer theory involves two dimensionless parameters. One concerns the balance between the electrical forces attracting a counterion to the surface and the diffusion of counterions away from the surface. The balance between these two opposing tendencies is expressed as the ratio of the electrical and thermal energies, zeJl,lkT, in which z is the charge on the counterion, e is the electronic charge, +G is the electrical potential at the origin of the diffuse layer (Gouy potential), k is the Boltzmann constant, and T is the temperature (OK).The other dimensionless parameter concerns the product of half the distance of separation of the Gouy planes of the interacting surfaces taken as 2x and K from the Debye-Huckel theory for strong electrolyte solutions; K has the dimensions of a reciprocal length and is given by

where ni(o) is the number concentration of ions far from the surface, z is the dielectric constant, e is the electronic charge, and z is the valency of the ions in solution. At 25°C the magnitude of the Debye length (K-I) in A is 3 . 0 4 / G for a 1: 1 electrolyte, 1.76/< for a 2: 1 electrolyte, and 1.52/& for a 2:2 electrolyte; c is the molar concentration. The general equation for the case of interacting or overlapping diffuse double layers for symmetric electrolytes has been presented by Verwey and Overbeek ( 1948)

dYlh

( 2 cash YG - 2 cash U ) ’ Q

= K

(10)

where x is the distance from the midplane, Yc = z e q G / k T is the reduced or scaled electric potential at the Gouy lane, and U is the reduced potential at the midplane where dY/& = 0, Y = U ,and x = 0. Integration gives

1 I:” =

dY [2 cosh Yc - 2 cosh Ull/z

This leads to an elliptic integral of the first kind for which tables are available; it can therefore be solved numerically to obtain the potential distance curve and, in particular, the midplane electric potential for different values of Y,, of plate

separation, of concentration (contained in K), and of counterion valency. Kemper and Quirk ( 1970) have provided a nomogram which illustrates the interrelationship of these variables (see also Bresler et al., 1982). The starting point for the application of this theory is the relationship

130

J. P. QUIRK uG=

(7) 2nekT sinhT Yc

(12)

in which uGis the surface density of charge at the Gouy p.,ne and n is the number concentration of ions. It has been acknowledged for a long time that there was no satisfactory way of estimating 3rC from the crystallographic charge of a surface on account of the rapid fall in potential between the surface itself and the Gouy plane. Zeta potential values have not been considered satisfactory, except in a general way, because of the uncertainty of the position of the shear plane. When appropriate values of the Gouy plane potential are available then the midplane potential can be arrived at by the application of the just-mentioned theory. The equation of Langmuir (1938) can then be applied,

P, = 2 RTc ( C O SU~ - 1)

(13)

to obtain the swelling pressure P, due to the excess of ions at the midplane in relation to the bulk concentration, c; Derjaguin and Churaev (1989) refer to this swelling pressure as the electrostatistical component of the disjoining pressure.

1. Surface Potentials To a significant extent the difficulty concerning Yc [Eq. (12)] has been clarified (Chan et af.,1984) by the reinterpretation of the coion exclusion measurements of Edwards et al. (1965), for a montmorillonite and illite saturated with alkali metal ions. They derived an equation, based on double layer theory, which enabled the Gouy plane potential for a clay to be obtained from the measured volumes of coion (chloride) exclusion with respect to concentration. For an interface of area, A, the volume of exclusion is given by

Vex = A

2

[I - exp(ze+,/2kT)]

It may be noted that for Gouy potentials of - 150 mV the volume of exclusion is within 5% of 2 / multiplied ~ by the surface area. This is the circumstance to which Schofield’s (1947) equation was applied; he proposed negative adsorption (coion exclusion) as a measure of surface area. However, this approach is not justified as the Gouy potentials for clays are generally less negative than - 100 mV (Table 111). Equation (14) can be rewritten by noting that the slope of the plot of volume of exclusion against 2 / that ~ is Vex ~ / 2 has , the dimensions of a surface area which is denoted as A, so that

131

INTERPARTICLE FORCES

Thus, when the surface area of a clay is known the surface potential can be calculated by the application of Eq. (15). For a smectite the area, A, can be calculated from crystallographic parameters and chemical composition and for an illite or kaolinite the nitrogen surface area is used. Chan et al. (1984) have reported the surface potential values shown in Table 111. From the good fit of the experimental results to Eq. (14), that is a plot of Vex against 2 / for ~ a range of concentrations from 10-4 to 10-1 M of alkali metal chloride solutions (Edwards et al., 1965), it would seem that clay surfaces behave as constant potential surfaces. The authors described this finding as surprising because the surface charge of clays results from isomorphous substitution in the crystal lattice and thus a constant charge behavior for the double layer would have been expected. It was concluded that, over the above concentration range, there must be a potential-regulating mechanism for which there is currently no theoretical treatment. Miller and Low (1990) reported similar results to those of Edwards et al. (1965) and using Eq. (15) arrived at a similar conclusion. There must, of course, be some limit to the constant potential behavior, otherwise the Gouy layer charge would exceed the crystallographic charge at high concentrations. Kemper and Quirk (1972) have reported a considerable decrease in the negative electrokinetic potentials at concentrations above 0.1 M NaCl for Na clays. According to Eq. (12), for the potential to remain constant with increasing concentration, uGmust increase and hence fewer ions would be accommodated in the Stem layer. This does not accord with the Langmuir treatment, on which the Stem theory is based, which requires that more ions should be accommodated in the Stem layer with increasing concentration. Horikawa et al. (1988), using electrokinetic measurements, showed that there is some dependency of zeta potential on concentration. However, the observed behavior resembles that of constant potential more closely than constant charge. Table 111 Surface Potential (mV) for Fithian Illite and Wyoming Montmorillonite Saturated with Alkali Metal Ions [Eq. (15)] Ion Mineral

Li

Na

K

Rb

Illite Montmorillonite

-65 -90

-51

- 19

-5

-69

-44

-

CS 0 - 12

J. P. QUIRK

I32

There is only a limited amount of information for the surface potentials of Ca clays. The coion exclusion results of Edwards et af. (1965) for Ca-illite (Fithian) indicate a surface potential of - 1 I mV. From osmotic efficiency measurements on clay plugs, Kemper and Quirk (1 972) obtained values of zeta potentials for Ca clays at concentrations in the vicinity of 0.1 M CaCl,. Calcium-kaolinite and CaWillalooka illite had potentials of about - 10 mV; Fithian illite and Wyoming montmorillonite had potentials, respectively, of -20 and -25 mV. Horikawa et af. (1988) found zeta potential values for Ca-Wyoming montmorillonite around - 10 mV, and for Ca-Muloorina illite the potentials ranged from -6 to - 17 mV to M CaCl,. In general terms it would over the concentration range of seem that the surface potentials of Ca clays could be considered to be in the range of - 10 to -20 mV. That is, a reduced electrical potential at the Gouy plane (Yc) of 0.8 to I .6 which contrasts with values for Na clays of 2.0 to 3.0. 2. Swelling Pressures

In Table 1V a comparison is made of the swelling pressures obtained when the diffuse double-layer theory is applied for reduced Gouy electrical potential values of Yc = 1, Yc = 2 , and Yc = 3 with a Gouy plane separation of 20 8, in a 0.1 M solution of a 1: 1 and a 2:2 electrolyte. In making a comparison of the swelling behavior of Na and Ca clays two features need to be considered. First the Ca clays have a smaller negative potential than Na clays so that in Table 1V for YG = 1 and a 0. I M solution of a 2:2 electrolyte, the swelling pressure is 0.01 MPa but for Yc = 3 in the presence of a 1: 1 electrolyte and also at 0.1 M the swelling pressure is 0.67 MPa. Second, for a clay such as Na-Wyoming montmorillonite the whole surface of 750 m2g-1 is involved in the swelling process whereas the interactions for Ca-montmorillonite are between the external surfaces of the quasicrystals after the crystalline swelling to a d(001) value of 19 A is complete at a suction of about 10 MPa (see Fig. 6 in Section V). Table IV Calculated Swelling Pressure (Mh)for Na and Ca Clay Surfaces with a Separation of 20 between the Gouy Planes and with Varying Reduced Electric Potentials in 1:1 and 2:2 Electrolyte Solutions (0.1 M ) Reduced surface potentials Electrolyte type

Y, = I

YG = 2

Y, = 3

I:I 212

0.09

0.33

0.01

0.05

0.67 0.09

INTERPARTICLE FORCES

133

The information in Table IV is for the separation between Gouy planes of 20 A so that the actual surface separation is this distance plus twice the thickness of the Stern layer (5.5 A); the surface separation is therefore 31 A.At this separation the van der Waals attractive pressure would be about 0.02 MPa for 10 A plates and 0.04 MPa for thick plates [Eq. ( 6 ) ] .for Na clays with reduced surface potentials (Y,) in the range of 2 to 3 it may be concluded that at a distance of surface separation of 30 A or greater the contribution of the van der Waals forces is much smaller than the osmotic repulsive pressure. However, with the much lower repulsive pressures of the Ca clays with Yc in the range of 1 to 2 the van der Waals contribution is similar in magnitude to the osmotic repulsive pressure and as a result the Ca clays have a lesser tendency to swell than the Na clays.

D. ION-ION CORRELATION FORCES: POTENTIAL ENERGY MINIMABETWEEN PARTICLES 1. Theoretical Background

In 1985 Kjellander and Marcelja provided a theoretical adaptation of the primitive model for electrolyte solutions (see Vaselow, 1972) to the inhomogenous, two-dimensional situation of ions between charged plates. In the primitive model for aqueous solutions, which has been highly successful in predicting thermodynamic properties, the ions are treated as hard spheres with an assigned size. Both their electrostatic and core-core interactions are accounted for in the ion-ion correlation. The term primitive model is used because the water is considered to be as a dielectric continuum and the molecular structure is ignored. A feature of the Kjellander-Marcelja analysis is that the surface density adopted is that at the immediate surface; for a clay the crystallographic charge density is used. Another feature of their analysis is that the aqueous and solid phases have different dielectric constants. Thus the primitive model treatment differs in four respects from the DLVO treatment in that the actual surface charge is used, the ions have a finite size, there is a dielectric discontinuity at the solid interface, and ion-ion correlations are included. The thermodynamic feature of interest is the pressure between the charged plates as influenced by the surface density of charge, the separation of the surfaces, and the bulk solution concentration. Kjellander and Marcelja (1986) showed that ionic radii of less than 3 A has little effect on the pressure-distance curves generated. However, for larger ions, core-core interactions have a significant effect, especially for close distances of approach of surfaces which have a large surface density of charge. The most significant feature of the Kjellander-Marcelja theory is that, for close distances of surface approach (about 10 A or less) and when polyvalent

J. P. QUIRK

134

ions (radius <3 A) balance the surface charge, an attractive pressure is predicted. Guldbrand et al. (1984), in Monte Carlo studies of the electrical double layer, in which particle correlations are automatically accounted for, found that ion-ion correlations can cause the force between two equally charged surfaces in a coulombic or ionic fluid to be attractive. This attraction is because of ion fluctuations. The local decrease in concentration around each ion resulting from ion-ion correlation-a “hole” in the ion cloud-constitutes a charge depletion and can be regarded as a charge distribution of opposite sign. It extends across the midplane if the ion is located nearby. An ion and part of the “hole” on the other side of the midplane interact with an attractive force. The fluctuations or local change in ion density, due to thermal motion, in one half of the system gives rise to an electrostatic attraction between the two halves and thus is an extension of the static (“so called” zero frequency) term of the van der Waals force. In their theoretical treatment, Kjellander and Marcelja (1985) and Kjellander (1988) use the Ornstein-Zernike equation (McQuarrie, 1976) to separate the direct correlation function, c(r), from the other correlations which, together with the direct correlation function, encompass the radial distribution function. This definition of c(r) is used in the hypernetted chain (HNC) integral equation (McQuarrie, 1976) and enables the calculation of anisotropic ion-ion correlation functions, the concentration profiles, and all relevant thermodynamic quantities including pressure. The image charge interactions are included. This theory has been used by Kjellander et al. (1988) to calculate the attractive pressure between clay surfaces for close distances of approach when the surface charge is balanced by a divalent ion and in the absence of electrolyte in the bulk solution.

2. Application of Theory to Clay Particle Interaction The net internal pressure between two charged plates is given by pnet = Pionic

+

pvw

- Pbulk

(16)

in which Pbulk refers to the bulk solution, P,, is the van der Waals contribution, and Pionicis made up of a number of components thus, Pionic

=

kT

Ci ni(m) + Pel + Pcore + Pi,

(17)

where n,(m) is the number concentration of ions at the midplane which when Eq. (13) is applied gives the Langmuir pressure, Pel is the electrostatic force across the midplane due to ion-ion interactions (this is not zero since each ion on one side affects the ion distribution on the other side), PCore is the pressure component due to core-core interactions across the midplane, and Pi, is the electrostatic

INTERPARTICLE FORCES

135

interaction due to the difference in dielectric constant between water and the interacting particles. Pressure-distance curves for two interacting clay plates have been obtained for the following conditions: the charge on the 10 A thick clay plates is balanced by divalent counter ions of diameter 4.25 A, the dielectric constant of the clay is 3 and that of the water is 78, and there is no electrolyte in the bulk solution (counterions only). The results in Fig. la are for a surface with I unit charge per 135 A2 or 0.1 19 cme2 (Wyoming montmorillonite) and those in Fig. l b are for a surface with 1 unit charge per 60 A2 or 0.267 Cm-2 (Llano vermiculite, van Olphen, 1965). Figure 1 shows that the maximum attractive pressures are, respectively, 1.7 and 15 MPa for charge densities of 0.119 and 0.267 Cm-*. The position of the potential minimum is where the curves cross the abscissa (zero pressure) at 0.88 and 0.6 nm, respectively: these values convert to expected d(OO1) spacings of 18.1 and 15.3 A, respectively, if the silicate thickness is taken as 9.3 A. These predicted spacings compare with the measured 19 for Ca-montmorillonite (Wyoming) and 15 A for Ca-vermiculite (Llano). This agreement is reasonable as no account is taken of the molecular structure of water. It is probable that the attractive forces shown in Fig. 1 would be somewhat larger since there would be some degree of dielectric saturation in the interlamellar region, particularly in the presence of divalent ions.

10-

a

I

I

I

I

8-

-

6-

-

s!

4-

-

g!

2-

-

a" z

I

a

0. -2

25 20 b 15 -

I

-

a"

I

I

z

-

-

50 g -5a -10 $

1

-

10-

-

-

-20 0 Separation (nrn)

I

A

-15 I

'

1 2 Separation (nrn)

3

Figure 1 The pressure-distance relationship for two interacting surfaces with a divalent ion balancing the surface charge of 0.119 Cm-2 (a) and 0.267 Cm-* (b) corresponding, respectively, to 1 unit charge per 1.35 nm2 (135 A 2 ) and 0.60 nm2 (60Az)of surface. The pressure takes into account the electrostatic ion-ion correlation, core-core interaction of the exchangeable ions, the traditional van der Waals contribution, the osmotic component of disjoining pressure, and image forces. For distances between 0.5 nm (5 A) and 1.5 nm (15 A), the pressures are strongly attractive with the potential minima (zero pressure) at 0.88 and 0.6 nm, respectively. At large distances, the pressures become weakly repulsive, i.e., about 0.02 MPa at 5 nm (not evident because of the scale). At close distances of approach the pressures become strongly repulsive because of Born repulsion. (Modified from Kjellander er al., 1988.)

136

J. P. QUIRK

The approximate energy required to remove the surfaces from within the potential minimum is given by the J p.dv with the limits corresponding to the distances at which the curves cross the abscissa and the pressures become repulsive. For montmorillonite this energy is 1.2 mJm-2 and for vermiculite is 7.5 mJm-2; this latter value may be compared with the 1.9mJm-2 van der Waals contribution to the attractive energy at a separation of 0.5 nm (Table 11). It would be expected that the work required would exceed the van der Waals energy since it would also include that necessary to overcome the effect of ion-ion correlation forces. The significance of the ion-ion correlation force at small distances of approach is further illustrated by considering the magnitude of the total attractive pressure in Fig. 1 . For montmorillonite the point of maximum attraction is at 1.14 nm where the pressure is 1.7 MPa of which 0.6 MPa is due to the van der Waals contribution [Eq. (S)]. For vermiculite with a greater surface density of charge the ion-ion correlation contribution is larger since at the distance of maximum attraction (0.78 nm), the attractive pressure is 15 MPa, of which 2.1 MPa is due to the high frequency van der Waals contribution. In the early 1950s the existence of Ca-montmorillonite lamellae in a potential well, even in distilled water, was established (Quirk, 1952, 1968; Norrish and Quirk, 1954), but the nature and magnitude of the forces responsible for this phenomenon have only recently been described (Kjellander et af., 1988). Figure 2 illustrates the effect of surface density of charge on the presence and magnitude of the attractive force. When the surface charge is balanced by a divalent ion the force is repulsive up to a surface density of charge of 2.0 nm2 (200 A*) per unit charge when the interaction is relatively weak with an attractive pressure of only 0.05 MPa. This increases markedly with increasing surface charge density as can be seen from the magnitude of the attractive pressures in Fig. 1 for a montmorillonite and a vermiculite. The general appearance of Fig. 2 is similar to the plot of the van der Waals equation of state for a gas in which the resultant pressure is a balance between the van der Waals force between molecules and thermal motion. At low temperatures the van der Waals forces cause the molecules to be in a potential minimum and condensation to the liquid state occurs. At higher temperatures, kT exceeds the van der Waals contribution and the molecules are in the gaseous state. Figures 1 and 2 show that at large charge densities the pressure between the surfaces is attractive because of the large ion-ion correlation contribution. However, as the surface density of charge is decreased, this interaction is significantly diminished because of the greater separation of the exchangeable ions. For values of the charge density less than one unit of charge per 2.0 nm2 (200 A2) the pressure becomes increasingly repulsive. It must be emphasized that the usual situation in soils is that the clay particles are in a condensed state, as distinct from a flocculated state, as a result of ion-ion correlation forces operating at close distances of particle approach in regions of overlap. The condensed state corresponds to the

INTERPARTICLE FORCES

137

Separation (nm) FCgure 2 The effect of surface density of charge on the pressure-distance relationship. The attractive pressure is small for a surface density of charge of 1 unit per 2.0 nm* (200 A 2 ) and repulsive for smaller surface densities of charge. For surface densities of charge greater than 2.0 nmz per unit charge the attractive pressure becomes increasingly attractive as shown in Fig. 1. (After Kjellander e t a / . , 1988.)

primary potential minimum whereas the dispersion-flocculation transition involves the secondary potential minimum. The mathematical background in using the anisotropic hypernetted-chain approximation, in treating the coulombic fluid as a plasma, is complicated, and the numerical solution of the integral equations to provide the information presented in Figure 1 requires 5 to 10 hr on a VAX workstation. The theory also predicts a sharp rise in repulsive pressure as the surfaces approach closer than the position of the potential minimum; this can be seen in Figure I . For Ca-montmorillonite (Wyoming), Slade and Quirk (1991) found that to change the d(OO1) spacing from 19 to 15.5 A, that is from a separation of 0.97 to 0.62 nm if the aluminosilicate lamella is considered to be 0.93 nm thick, a pressure of 13 MPa was required. van Olphen (1969) found that Mg-vermiculite (Llano) changed from a d(OO1) value of 14.8 to 11.6 A at aplp, value of 0.02. This means that the change in separation from 0.53 to 0.23 nm requires a pressure of 53 MPa. This region of interaction is sometimes referred to as Born repulsion. The Kjellander-Marcelja theory, as it exists, does not provide for differences in the surface interaction of cation species of the same valency. For a 1:1 electrolyte a moderate attractive pressure is predicted only for a large surface density of charge and for concentrations of about 1 M. For surface separations beyond the attractive region the repulsive forces pre-

138

J. P. QUIRK

dicted by the Kjellander-Marcelja theory can be matched with the repulsive pressure predicted by the nonlinear Poisson-Boltzmann theory if an appropriate charge density is chosen for the Gouy plane in the DLVO treatment. This is more than a convenient procedure since Kjellander and Mitchell (1992) have shown that the primitive model can be reformulated in terms of quasi-particles constituting “dressed” colloid particles, where part of the ion cloud around each bare particle is included in the quasi-particle. The resulting exact theory for the dressed particles is virtually identical to the linear DLVO theory, where the bare particle charges in the DLVO theory are replaced by internal charge distributions of the quasi-particle.

E. STRUCTURAL COMPONENT OF DISJOINJNG PRESSURE: WATER STRUCTURAL FORCES The term and concept “the structural component of disjoining pressure” was proposed by Derjaguin in the early 1930s (Derjaguin, 1987) to describe the repulsive pressure between two particles as a result of the overlapping of the two structurally modified boundary layers of liquid. The other components of disjoining pressure are the pressure due to molecular interactions (van der Waals forces) and that due to ionic-electrostatic interaction (diffuse layer forces); these latter two forces constitute the basis of DLVO theory (Derjaguin and Churaev, 1989). A surface-induced perturbation of the water structure, which may be propagated to distances of 2.5 nm from each surface or greater in some cases, is the basis of the water structural forces. In recent years considerable information on structural forces, observed in the interaction of muscovite mica surfaces in aqueous electrolyte solutions, has accumulated as a result of the work of Israelachvili and Adams (l978), Pashley (1981a,b, 1982), and Pashley and Israelachvili ( 1984). The technique used to obtain experimental force-distance curves is due to Israelachvili and Adams (1978). It involves the measurement of the force between two molecularly smooth muscovite mica surfaces, in cross-cylindrical configuration, as the surfaces are moved closer. This force can be converted to the energy of interaction per unit area for two parallel surfaces by using the Dejaguin approximation. One of the mica surfaces is fixed and the other is mounted on a calibrated cantilever spring which bends in response to the interaction of the surfaces as they are moved toward one another in a controlled manner by means of a micrometer in conjunction with a spring. The deflection of the calibrated cantilever spring during this process provides the force. The two facing sheets of mica, with the surfaces away from the solution silver coated, produce interference fringes of equal chromatic order (FECO) from a white light source. Analysis of these parabolic fringes in a visible light spectrometer allows

INTERPARTICLE FORCES

I39

the determination of the mica-mica separation to an accuracy of about 2 0.2 A. Both attractive and repulsive forces can be measured by this technique. When the forces become large the fringes show a flattened region so that it is not possible to estimate the energy of interaction. However, the pressure arising from the interaction can be calculated from the area of the flattened region of the fringes. Pashley (1981b) reported two types of behavior for the interaction of muscovite mica surfaces in alkali metal chlorides: 1. Below a critical concentration (e.g., about 1 X 10W2M NaCl and 5 X M CsCl), which was different for each cation, the DLVO theory was followed

from large distances of separation (1000 A), with surface potentials in the range of -70 to -130 mV, down to separations of about 20 A at which point the surfaces jumped into “contact.” 2. Above the critical concentration the DLVO theory is again followed from large distances of separation to about 30 A separation when the applied force needed to bring the surfaces closer increased rapidly in a monotonic fashion; this repulsive force was considered to be due to the influence of the surfaces on the structure of the intervening water, and the phenomenon was described as being due to hydration forces, secondary hydration forces, or water structural forces.

The reason for this contrasting behavior at close distances of approach is not directly due to the concentration of the electrolyte itself but is caused by the special role of the proton in competing for surface exchange sites. As the surfaces are brought close together, there is a tendency for the exchangeable ions to move into the Stem layer; this circumstance causes a proton to move to a water molecule at the surface to balance the negative site associated with the triad of surface oxygen atoms immediately above the position where Si4+ is replaced by A13+ in the tetrahedral layer of muscovite (Pashley and Quirk, 1989). The nearest that other cations can approach the surface is at a distance of about the diameter of one water molecule which forms part of the primary hydration shell of the ion. Even without this special behavior of H30+, the DLVO theory predicts the dominance of attractive forces at close distances of approach. The magnitude of the exchange advantage enjoyed by the proton is indicated by the fact that the critical NaCl concentration at pH 5.7 is about 1 X M NaCl. Pashley (1985) envisages that when the surfaces jump into contact below the critical concentration, all other ions are displaced and the surface charge is balanced by H30+ ions. Although the critical concentration decreases with increasing ion size in the alkali metal series (Pashley, 1981b), in approximate terms each critical concentration seems to correspond to 30% of the exchange sites being occupied by an alkali metal ion. When the solution concentration is greater than the critical concentration the measured structural force increases with increasing concentration because of the increased number of Li+, Na+, K + , and

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J. P. QUIRK

Cs+ ions balancing the surface charge; this is akin to an increase of the surface charge density. It seems reasonable to conclude that H30+ ions are so strongly bound that they do not perturb the water structure even though their hydration energy is much greater than the other monovalent ions. The unique role of the proton is only revealed in these experiments because less than 1 cm2 of muscovite mica (charge 3.6 X 10-7 meq/cm-2) is exposed to a reservoir of about 300 cm3 at pH values around 5.5. This reservoir problem is not encountered in considering the behavior of clays since the surface area is more than six orders of magnitude greater and the amount of solution is so much less, i.e., 0.5 g H20g-l clay. In the simple ideal unit cell formula for muscovite-(Si,A1,)lV (AIJV' 02,(OH),-K2-the lattice charge due to the replacement of Si4+ by Al3+ in the tetrahedral layer is balanced by 2 mol of K+ . Since the unit cell parameters are a = 5.19 A and b = 9.00 A and as there are two faces of the unit cell in the ab plane the unit cell area is 93.4 A2. Thus there is one K+ per 46.7 A 2 which can be expressed as 0.343 Cm-2 Also, from the unit cell weight (796 g) and the area per unit cell it can be calculated that the area of the fully cleaved muscovite is 706 m2/g. It is assumed that when the fully cleaved mica is immersed in water or an electrolyte solution that all, instead of most, of the ions are amid two layers of water (5.5 A)at the surface. On the basis of this assumption, there is a concentration of about 6.5 M for the K+ such a two-dimensional solution at a cleaved mica surface; this would also be the concentration at the external surface of an uncleaved crystal. It seems reasonable to conclude that the large number of cations, in the two-dimensional solution, is the cause of the perturbation of the water structure for some distance from the mica solution interface.

Oscillations and Pressures With refinements in the technique, Pashley and Israelachvili (1984) were able to detect oscillations in what was previously thought to be a monotonic forcedistance curve. Within force-distance curves, which exceeded DLVO predictions of repulsive pressure for distances of separation up to 40 A,they obtained seven oscillations in the last 20 A of approach in I M and 10-3 M KCl. The periodicity of these oscillations was 2.5+ 0.3 A which is close to the diameter of the water molecule (2.76 A). The pressures in 1 M KCI at separations of 5 and 10 A were in the vicinity of 15 and 10 MPa and it was not possible to force the surfaces into contact; the surface sites occupied by K + were 95% (Pashley and Israelachvili, 1984). These oscillations appeared as vertical discontinuities in the pressure-distance curves. In M KCI when 40% of the exchange sites were occupied by K + , the measured pressures were less than half those for the molar solution at the same separations but the oscillations were more clearly defined.

INTERPARTICLE FORCES

14 1

The last three oscillations had minima which were attractive and the final minimum was adhesive, that is, it required work (2 to 5 rnJm-,) to separate the surfaces from within what is probably a primary minimum. Using an entirely different technique of adsorption of water vapor onto a freshly cleaved K-muscovite surface at a range of pIpo values and measuring the corresponding film thicknesses by an ellipsometric technique, Beaglehole and Christenson (1992), using Eq. (4), obtained disjoining pressures of 40 and 10 MPa for films of 5 and 10 A thick, respectively. The central consideration here is that water structural forces extend to separations approaching 40 and that the repulsive pressures existing between the surfaces are at least an order magnitude greater than osmotic repulsive forces (Table IV). The magnitude of pressures seems to increase with increasing surface density of charge. The result of Beaglehole and Christenson indicates that pressures in excess of 30 MPa would be required to move K-muscovite surfaces from a separation of 10 to 5 A. Figure 3 shows the force-distance curve between Ca-muscovite surfaces immersed in 2 M CaCl, (Quirk and Pashley, 1991b); this concentration is necessary to ensure that the exchange sites are almost entirely occupied by Ca*+ ions. Within the force-distance curve there are nine almost vertical discontinuities with a periodicity of 3.4 A;these discontinuities are the equivalent of the oscilla3.0 2.5

6

2.0

- 1.5

Y

:0:

5M

2 1.0 0.5

0.0

0

1

2

3

4

5

Separation [nml Figure 3 The measured force-distance relationship for the interaction of two molecularly smooth muscovite mica surfaces immersed in 2 M CaCI,. The numbers on the curve are the repulsive pressures between the surfaces, expressed in MPa, due to the structural component of disjoining pressure (water structural pressure). (After Quirk and Pashley, 1991b.)

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tions reported by Pashley and Israelachvili (1984) for K-muscovite. However, because of the magnitude of the water structural repulsive force there are no attractive minima. Because the FECO fringes are flattened it is possible to calculate a repulsive pressure by expressing the force relative to the flattened area. The repulsive pressures shown in Fig. 3 are large, being 10 MPa at a separation of about 45 A and rising to about 40 MPa at close distances of approach. Although there may be some uncertainty introduced in estimating the flattened areas it is clear that the repulsive forces are large and are several times greater than those for K-muscovite in 1 M KCl. This may relate to the fact that, although the number of Ca2+ ions is half that of K+ ions at the surface, the hydration energy for Ca2+ ions (410 kcal/g ion) is four times that of K + ions (94 kcal/g ion). The hydration energy for the Ca2+ ion can be expressed in relation to the muscovite surface as 2.7 Jm-2. This is a thousand times greater than the adhesive energy overcome when two water layers separate the surfaces. This emphasizes that the major part of the ion-hydration energy is used in forming the primary hydration shell within two water layers at the surface; the term secondary hydration force is not, therefore, an inappropriate description of the water structural forces discussed here. It is reasonable to assume that if sufficient pressure is applied, two muscovite surfaces can approach one another so that each surface has its set of counterions in two layers of water; that is, the surfaces are separated by four layers of water or 10 to 12 A. The Ca2+ counterion concentration at each surface would be about 3 M so that to bring the surfaces into the primary minimum would require not only the displacement of two water layers, but also the work required to counter the electrostatic resistance to the transfer of the cations from one surface into the two layers of water at the other surface to achieve an interlamellar concentration of 6.5 M. For high surface density of charge this may require higher pressures than it is possible to achieve in Israelachvili’s apparatus using muscovite, but the results suggest that this may be achieved at lower surface density of charge. Force-distance measurements were also made in 0.1, 0.15, and 0.4 M CaCl, (Kjellander et al., 1990) for which the surface site occupation varied from 40 to 80% (Claesson et al., 1986). The results were erratic reflecting the difficult nature of the experiments. However, in each case oscillations were found and the repulsive pressures were much lower than in 2 M CaCl,. Perhaps the most definite expression of water structural forces is the considerable swelling which occurs for Willalooka illite in 4 and 1 M CaC1, (Aylmore and Quirk, 1962) as there is no other repulsive force to overcome the combined ion-ion correlation and van der Waals attractive forces in this material.. The surface density of charge for Willalooka illite is 0.260 Cm-2 and that for Llano vermiculite in Fig. lb is 0.267 Cm-* so that the attractive pressure within the illite in regions of crystal overlap would be about 15 MPa.

INTERPARTICLE FORCES

143

111. SOIL WATER RELATIONS: SWELLING AND SHRINKAGE

A. POROSITY AND STRUCTURAL STATES It has been customary to subdivide the total porosity of a soil into macropores, which are drained at 10 kPa suction (corresponding to an equivalent cylindrical pore radius r = 15 Fm), and micropores, the remainder of the porosity. Marshall (1959) referred to these two components as structural and textural porosity. Haines (1923), using remoulded soil blocks, eliminated the structural porosity and investigated the change in the textural porosity, resulting from shrinkage, of clay soils during drying. He designated a normal shrinkage and residual shrinkage regime. In the former, as drying proceeds, the change in the volume of a soil block is equal to its change in water content for a two-phase clay-water system; over the range of this regime the clay is pore space saturated. On the completion of normal shrinkage air enters the system and as a result the change in the water content of the blocks with increasing suction exceeds the change in volume. Holmes (1955) found that the normal shrinkage of clay blocks of a redbrown earth subsoil extended from small suctions to a suction of about 12.5 MPa which corresponds to a slit-shaped pore width of 12 nm. It is important to stress that the compressed clay blocks of Urrbrae loam subsoil had a similar moisture characteristic to that of the natural aggregates (Holmes, 1955). Also, Aylmore and Quirk (1967) found that the pore size distribution of the dry compressed clay cores were almost congruent in the less than 50 8, pore size range for the cores and the natural aggregates. In the most general terms the swelling of a clay can be considered as the movement apart of the parallel plate-shaped clay crystals so that a greater amount of water is adsorbed as the suction restraining water uptake is reduced. The forces responsible for this swelling, or for resisting it, are the major focus of this review. Holmes (1955) reported that compressed clay blocks of the B-horizon of a redbrown earth soil took up about 30% water at a low suction, and as the surface area of the clay is about 120 m2g-' the average film thickness indicated by a parallel plate model would be 25 A or if regarded as a distance of surface separation it would be 50 8,. This is, of course, a vast oversimplification; the central problem in understanding clay-water systems is to separate the water which is simply enmeshed within a gel structure and retained by capillary forces from the water held at the clay solution interface by the interplay of the forces described earlier. Croney and Coleman (1954), in their classic paper, showed that a clay (Lon-

144

J. P. QUIRK

don clay) can exist in a series of structural states depending on mechanical disturbance and sample history. That is, the amount of water retained by clay at a given suction can vary considerably depending on the amount of enmeshed water which increases with disturbance, such as rapid wetting of a dry soil or mechanical working of the soil at low suctions (puddling). They reported that London clay, when puddled in preparation for engineering testing, retained 105% water at a suction of I kPa and that as the suction is successively increased the water content suction curve followed is the normal consolidation curve. Each point on this curve is a different structural state since a reproducible hysteresis loop is produced on decreasing and then increasing the suction to that at the starting point on the normal curve. When the suction reached the normal shrinkage limit (suction 6.3 MPa) the reproducible hysteresis loop traveled between water contents of 18 and 30% (suction 1 kPa); this final hysteresis loop is referred to as the overconsolidated curve and approximates closely the water content-suction curve (moisture characteristic) of the soil in its natural or undisturbed state. When a soil in its natural overconsolidated state is disturbed it tends to move toward the suction-water content relationship of the normal consolidation curve if water is freely available. If the water content is held constant, then the suction increases in response to applied work. Soil aggregates which are stabilized by the presence of organic matter, sesquioxides, and other materials resist this change to varying degrees and this can be regarded as one manifestation of structural stability. The limit of shrinkage occurs when particles come into contact. Camontmorillonite (Wyoming) sustains a d(OO1) value of 15.5 li over the pIpo range 0.91 (13 MPa) (Slade and Quirk, 1991) (Fig. 6, Section V) to 0.10 (Mooney et al., 1952). Magnesium-vermiculite (Llano) sustains a spacing of 14.8 8, from immersion in water to a pIpo value of 0.02 (van Olphen, 1969) when the two-layer state within the crystal starts to transform to the one-layer state. It is concluded that contact involves the separation of overlapping crystal surfaces by two layers of water. The air-entry value is determined principally by the pore size which in turn is related to the thickness of the clay crystals. For a slit-shaped pore of 50 width, the air-entry value would be 28.8 MPa and for pores 300 8, across the value would be 4.8 MPa; these pore sizes would be expected in a fine-grained illite and kaolinite. The swelling results of Holmes (1955) can be related to the swelling of the soil profiles (Aitchison and Holmes, 1953). A red-brown earth subsoil followed normal shrinkage over the water content range from 30 to 17%; with a particle density of 2.65 g cm-3, this represents a volume change of 24%. Blocks of a hydromorphic black earth followed normal shrinkage over the water content range 39 to 16%which corresponds to a volume change of 44%.These soils exist

INTERPARTICLE FORCES

145

in a region of winter rainfall and summer drought, and from the end of summer to the end of winter when the profile is fully wet the vertical movement measured is 4.4 cm for the red-brown earth soil and 7.9 cm for the black earth (Aitchison and Holmes, 1953).The horizontal expression of swelling along the other two axes is via extensive cracking, especially for the black earth; cracks are an important avenue for the wetting of subsoils. Both these soils have an almost identical clay content (64%) but their respective air-entry values correspond to pores sizes of 114 and 46 8, which reflects the finer particle size and hence greater swelling for the black earth.

B. RESIDUAL SHFUNKAGE Table V shows that the residual shrinkage of the kaolinite (N2surface area of 36 m2/g) and the fine-grained illite (160 m2/g) is relatively small, being the difference between the water content at the point of transition from normal to residual shrinkage and the final porosity (oven dry). The pore sizes at which air enters the clay, as shown in Table V, have been calculated using the relation pgh = 2 y / r ,given earlier, for slit-shaped pores. The pore size for air-entry reflects the relative particle thicknesses for kaolinite and illite. Expressed on a volume basis the illite has a residual shrinkage of 2% which contrasts with the rnontmorillonite which has a residual shrinkage of 15%; this large value is due to the removal of the interlamellar water and possible particle bending.

Table V Water Content and Suction at the Transition from Normal to Residual Shrinkage+ the Slit-Shaped Pore Size Corresponding to the Suction, and the Porosity Prior to Swelling for Ca Clay Coresa Materialb

Water content (g H Z O K ' )

Kaolinite Illite Montmorillorite

0.21 0.20 0.21

Suction (MPa) 5.1

31 50

Pore size

(4

260 40 30

Dry porosity (cm3 g-I) 0.185 0.188 0.135

Experimental information is taken from Aylmore (1960). Rocky Gully kaolinite is from the pallied zone of a laterite and Willalooka illite is from the B horizon of a solodized solonetz. The nitrogen surface area of these three clays is, respectively, 36, 160, and 38 m2g-1. a

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J. P. QUIRK

C. STRUCTURAL POROSITY AND SHRINKAGE The change in structural porosity of clay soil aggregates with increasing suction has been investigated by Lauritzen (1948) and Stirk (1954); they found that there was an initial stage of shrinkage for naturally structured (undisturbed) soil aggregates for which the decrease in volume of the soil was less than the change in water content and concluded that this was because of air entering large pores and cracks. This structural porosity extended to a suction of 0.03 MPa after which the shrinkage became normal. Structural pores are obviously critical in relation to water entry and ready drainage after rainfall. For many practical purposes the soil water content attained a few days after the cessation of rainfall or irrigation is sensibly stationary, and for a freely draining soil this water content is referred to as field capacity. The attainment of field capacity depends on a high probability of continuity of macropores or structural porosity since, if these pores were randomly distributed throughout the soil mass, the approach to equilibrium would be very slow. Millington and Quirk (1961, 1964) have discussed permeability in terms of the probability of continuity of pore size classes. Although the total water content of a soil increases when it is puddled, the permeability decreases markedly because the structural pores are destroyed or lose their continuity by being randomly distributed throughout the soil mass. It is for this reason that rice soils are puddled to reduce percolation.

W. SWELLING OF SODIUM CLAYS Considerable attention has been given to the extensive crystalline swelling, d(001) >40 A, of Na clays because such swelling can be readily followed by low-angle X-ray diffraction and also because of the possible implications that such research has in relation to sodic soils. Sodic soils have been defined (Richards, 1954) as soils with an exchangeable sodium percentage in excess of 15; however, adverse physical behavior may be encountered at lower percentages (McIntyre, 1979). The physical behavior of a soil may be more difficult than anticipated from the exchangeable sodium percentage because of ion segregation. For instance, Ca2+ absorption on the internal surfaces of montmorillonite is favored, and as a result the exchangeable sodium percentage on external surfaces of qu8sicrystals is greater than the average for the whole material. Another reason why the swelling of Na clays is considered important is that advanced by Clapp and Emerson ( 1965) who proposed that the swelling pressure of Na-saturated soils could be used as “a chemical hammer” to measure the effect

INTERPARTICLE FORCES

147

of organic matter in conferring different degrees of stability on soil aggregates in water. This method has been used to assess the stability of soil aggregates of the surface soil of a red-brown earth which had experienced varying periods in pasture and in arable phases (Greenland et al., 1962).

A. EXTENSIVE CRYSTALLINE SWELLING Norrish (1954) and Norrish and Rausell-Colom (1963) measured the X-ray spacing of oriented flakes of Na-montmorillonite (Wyoming) and single crystals of Li-vermiculite (Kenya) as a function of NaCl and LiCl concentration in the absence of a restraining pressure; that is, under conditions of free swelling since there was no applied suction to restrain the swelling. The following relationships were obtained between the basal spacing and the electrolyte concentration, c, for Na-montmorillonite and Li-vermiculite: d(001) = 11.3 c-0.5

d(OO1)

=

23.8

c-0.5

+ 21 + 22

M)

( c from 0.3 to ( c from 0.15 to 2

X

10-2M)

(18) (19)

This strong dependence of swelling on electrolyte concentration is indicative of diffuse double layer behavior and especially so as the slope of the lines is related to c-0.5 that is K - I in diffuse layer theory. The difference between the two minerals is not due to the saturating cation since the slope of Li- and Namontmorillonite is almost the same (Norrish, 1954). The intercepts shown in Eqs. (18) and (19) were interpreted as arising from the aluminosilicate layer thickness (10 A) and the two layers of water (5.5 A) associated with each of the opposing surfaces; this is, in effect, evidence for the existence of Stem layers. Since K - I is 3.04 c - O . ~ for a 1:l electrolyte, the slope of 23.8 [Eq. (19)] converts to 7.8 K - I which is the separation of the Gouy planes at a given concentration. Norrish and Raussel-Colom (1963) also obtained X-ray spacings for a Livermiculite crystal immersed in 0.03 M LiCl as a function of applied pressure. Their results are shown in Fig. 4 in which the half-separation of the Gouy planes $[d(OO1)-21] varies with applied load (pressure). Also shown in the figure is the expected pressure-distance relationship for reduced Gouy electric potentials of 2, 3, 4, and 6 . If the full crystallographic charge density of 5.8 X 104 esu cm-2 (0.193 Cm-2 or one unit charge per 83 A2) is substituted into Eq. (13) for the concentration of 0.03 M ,the reduced electric potential calculated is 5.9, corresponding to a surface potential of - 150 mV. Figure 4 shows that the spacing-pressure relationship is reversible. Over the

148

J. P. QUIRK 102

10'

<-

decreasing pressure increasing pressure 1 5A spacing also present calculated repulsive force

46 \/

-

100 m

n

r2

v) 3

I

h 10'

10.2

103

0

10

20

30

40

50

60

half distance between Gouy planes

70

80

(A)

Figure 4 The pressure-distance relationship as measured for Li-vermiculite in 0.03 M LiCl (Norrish and Rausell-Colom, 1963). The increasing and decreasing pressure points show that the relationship is reversible. The lines shown are for reduced electric potentials at the Gouy plane of 2, 3, 4, and 6. The points are close to the line Y, = 4 indicating a Gouy potential of about -90 mV.

pressure range of 0.2 to 0.0035 MPa the points are either between Y , = 3 and Y, = 4 lines or actually on the line Yc = 4 so that -90 mV would be a reasonable estimate of the surface potential for this Li-vermiculite. From the force-distance curve for Li-muscovite (0.344 Cm-*), Pashley (1981b) derived a surface potential of - 110 rnV in a 0.03 M LiCl solution. It was indicated earlier that suction or applied load acts in the same sense as an attractive force. When the van der Waals attractive pressure is calculated, fore the actual separations of the surfaces, over the pressure range in Fig. 4 it was found that it contributes less than 1 % of the pressure needed to balance the Langmuir repulsion [Eq. (13)]. The applied load is the force which balances the osmotic swelling pressure.

INTERPARTICLE FORCES

149

Viani et af. (1983) measured the X-ray spacing of Na-montmorillonite (Wyoming) in equilibrium with M NaCl and pressures within the range of 0.7 to 0.03 MPa and obtained results similar to those for Li-vermiculite (0.03 M LiCl), even though their electrolyte concentration was much less. On the basis of their measurements and conventional double-layer calculations, they decided that, as the calculated double-layer pressures were so much smaller than the applied suction, the repulsive pressure balancing the applied suction must be because of water structural forces (see also Low, 1987). The surface separations reported by Viani et al. (1983) varied from 30 to 90 A and in this respect it is noteworthy that Pashley and Israelachvili (1984) and McGuiggan and Pashley (1988) considered that the hydration or structural forces were operative for separations up to 30-40 8, for K- and Na-muscovite which has a surface density of charge about three times that of Wyoming montmorillonite. On this basis it would appear that the influence of structural forces in Namontmorillonite is virtually expended at a separation of 33 A [d(OOl) = 43 A] at which distance and beyond diffuse double-layer behavior governs free swelling in NaCl solutions of decreasing concentration when no applied pressure is involved (Norrish, 1954). Aylmore and Quirk (1962) found that at a suction of 1 MPa there was no effect of NaCl concentration on the swelling of Na-montmorillonite cores in the overconsolidated state; that is, the swelling at that pressure is associated with the effects of cation hydration. The solution uptake for all concentrations at 1 MPa suction was less than that for M NaCl at 1 kPa suction when the d(OO1) value was 19 A. At a pressure of 0.1 MPa the effect of electrolyte concentration was evident. However, the difference in water content between 10-2 M NaCl and distilled water was hardly discernible. The swelling pressures measured by Viani et af. (1983) were in the range of 0.03 to 0.7 MPa and were thus in the transition region between cation hydration forces, responsible for limited crystalline swelling, and diffuse double-layer behavior. This conclusion is supported by the presence of d(OO1) spacings of 19 A together with larger spacings at pressures of 0.3, 0.5, and 0.7 MPa in their experiments. Since there is hardly any difference in the swelling of Na-montmorillonite cores between 10-2 M NaCl and distilled water, the expected swelling pressure at 10-2 M NaCl for Gouy plane separations of 60 and 80 A using a reduced Gouy potential of YG = 2.75 (-70 mV) was calculated. Pressures of 0.067 and 0.036 MPa were obtained using Eq. (13) which may be compared with the experimental values of 0.1 and 0.05 MPa observed by Viani et al. (1983) using M NaCl. Thus to a considerable extent the swelling of Na-montmorillonite at these pressures is explicable in terms of diffuse double-layer theory. The matter at issue here is not the presence or absence of water structural forces but their magnitude, their range of operation, and their significance relative to DLVO theory. On the matter of relevance to soil behavior, the immediate

150

J. P. QUIRK

surface soil is subject to flooding under irrigation or during intense rain, and furthermore the suction in the transmission zone during water entry is of the magnitude of 1 kPa (Bodman and Colman, 1944; Marshall and Stirk, 1949). In these circumstances the free swelling behavior of a clay [Eqs. (18) and (19)] would be the appropriate reference and hence the behavior of sodic soils should be considered in these terms. For cores of Na-montmorillonite (Wyoming), Aylmore (1960) reported a solution content of almost 4.8 cm3 g-I clay in equilibrium with 0.1 M NaCl at a suction of 1 kPa. The slope of the relationship between the X-ray spacing and c-0.5 for montmorillonite is 11.3 [Eq. (18)] and this would indicate a separation of Gouy planes of 36 A at 0.1 M NaCl. If 2 X 5.5 A is added for the Stem layer on each surface the actual surface separation is 47 A. From this, using 750 m2g-I for the interlamellar area (an overestimate), an expected solution content of 1.76 cm3 g-l clay can be calculated. The solution content values, attributable to double-layer interactions, are appreciably less than the measured solution content. The additional solution is enmeshed in a gel structure and held by capillary forces. By the adroit use of scanning and transmission electron microscopy, Tessier (1990, 1994) has revealed the nature of the sponge-like structure of Namontmorillonite gels at small suctions and their collapse with increasing suction as well as the organization of the aligned lamellae constituting the walls of the pores. For Na-Wyoming montmorillonite the transition from limited to extensive crystalline swelling takes place at a concentration of about 0.3 M NaCl and the flocculation-dispersion transition in a suspension occurs at 0.01 M NaCl. This illustrates the difference between the primary and secondary minimum and emphasizes that before particles within a soil disperse they first have to be moved from within the primary minimum. Studies of the behavior of Na-smectites are important in that they provide a basis for appreciating more fully the behavior of Ca clays and sodic soils. The transition from limited to extensive crystalline swelling for montmorillonite (Wyoming) in dilute solutions occurs over the range of 20-60% exchangeable sodium (Shainberg and Otoh, 1968; Shainberg and Kaieserman, 1969; Bar-On et al., 1970) and at the stage that this process has begun soil containing such a montmorillonite would have long since become intractable. Slade et al. (1991) investigated the swelling of Na-smectites; the smectites conformed to the definition of having a 17.6 A spacing in glycerol when Mg saturated. The smectites examined include the following for which the total charge and that arising from isomorphous replacement in the tetrahedral layer are given in parentheses: Wyoming (0.74, 0.08), Otay (1.04, 0.06), Nibost (1.06, 1.02), and Drayton ( I . 14, 0.74). Neither of the beidellites (Drayton and Nibost) exhibited extensive crystalline swelling in water and in fact their d(O01) spacings did not exceed 16 A. Their behavior is thus more akin to vermiculites and

15 1

INTERPARTICLE FORCES

emphasizes the importance of the structural origin of the charge. Otay montmorillonite, with a similar total charge as the beidellites, gives a 19 8, spacing in 0.25 M NaCl at which concentration the Wyoming montmorillonite quasicrystals have expanded to a d(OO1) value of about 43 8, (Table VI). With increased surface density of charge the expansion of Otay quasicrystals is delayed to lower concentrations of NaCl.

B. SWELLING BETWEEN CRYSTALS Figure 5 shows the effect of NaCl concentration on the swelling of compressed cores (overconsolidated condition) of Na-Willalooka illite (Alymore and Quirk, 1962); this sample of Willalooka illite has a surface density charge of 0.260

2'41

2.2

I WElTING DRYING

0.0 ! 0.001

I 0.01

0.1

1

10

Suction (MPa) Figure 5 Solution content-suction relationship for Na-Willalooka illite with respect to NaCl concentration for compressed clay cores in the overconsolidated condition. (After Aylmore and Quirk, 1962.)

152

J. P. QUIRK

Cm-* (area, 160 m2g- I) and is not interstratified as indicated by the agreement between the surface areas measured by nitrogen, water vapor, and cetyl pyridinium bromide adsorption (Greenland and Quirk, 1962). The solution contents measured were the result of the interaction between crystals. The clay cores were wet slowly in stages and Fig. 5 shows the effect of electrolyte level on wetting to 1 kPa and the subsequent effect of increased suction on the amount of solution retained by the core. The increase in solution uptake with decreasing NaCl concentration is in general conformity with diffuse double-layer principles; between concentrations of 0.1 and 0.01 M NaCl there is a doubling of the solution content. At 1 kPa the distilled water uptake was 8.4 crn3 g-1 and the solution uptake of 1 M NaCl was 0.48 cm3 g-l. This latter uptake is only a little greater than the uptake of CaCI, solutions at the same suction, indicating, as expected, that in 1 M NaCl the crystals are still within the potential minimum. With increasing suction the effect of electrolyte concentration is progressively diminished, being only slightly evident at 1 MPa suction when the solutions uptake, for all concentrations, is less than the uptake of 1 M NaCl. Similar studies of the water uptake at I kPa suction by Na-kaolinites have revealed that the interaction between these crystals is affected very little by electrolyte concentration and that their swelling is restricted since the solution uptake is the same as that of Ca-kaolinites (Aylmore and Quirk, 1966). Rocky Gully kaolinite from the pallid zone of a laterite was the same clay as was used by Schofield and Samson (1954). This clay, with a surface area of 36 m2g-', increased in volume by 3 1% from the dry state to equilibrium with 1 kPa suction. The water content of the Na-kaolinite (Rocky Gully) in distilled water was 0.36 cm3 H20 g-l. The swelling after pretreatment of the clay with Na tripolyphosphate to nullify the effect of positive edge charge was unaffected. The dramatic effect of increased pH values on the behavior of kaolinites in suspension (Schofield and Samson, 1954) contrasts markedly with the absence of any effect on the swelling of the Na-kaolinite following the nullifying of the edge charge. These results emphasize the hazard involved in extrapolating from the behavior of suspensions to that of overconsolidated soils and clay.

V. SWELLING OF CALCIUM CLAYS

A. CLAYDOMAINS AND QUASICRYSTALS The results shown in Table VI reveal that Na- and Ca-montmorillonite (Wyoming) have similar permeabilities at high concentrations of electrolyte. The permeability of Ca-montmorillonite is sustained in distilled water but the permeability of Na-montmorillonite becomes virtually zero below a concentration of

153

INTERPARTICLE FORCES Table VI Permeability and X-Ray Spacing for Na- and Ca-Montmorillonite (Wyoming) in NaCl and CaCI, Solutions

Solution 0. I M CaCI,

< 10-5 M CaCI, I .O M NaCl 0.5 M NaCl 0.3 M NaCl

Permeability

X-ray spacing

(105 cm sec-1)

(A)

I .7 1.4 I .8 0.4 ca.0

19.0 19.0 18.7 18.9 43

0.3 M NaCl (Quirk, 1952; see also Quirk, 1968). Mering (1946) considered that the large macroscopic swelling of Na-montmorillonite was not due to the expansion of the crystals. However, Quirk (1952) concluded that the dramatic decrease in permeability was due to the onset of extensive crystalline swelling as a result of the development of diffuse double layers on all the opposing surfaces. This interpretation was confirmed by X-ray measurements (Table VI) which showed that the basal spacing jumped from 18.9 in 0.5 M NaCl to 43 A in 0.3 M NaCl. Furthermore, Norrish and Quirk (1954) also reported that Ca-montmorillonite maintained a basal spacing of 19 8, even in distilled water; the lamellae of the montmorillonite remain in a primary potential minimum and are separated by 9 A of water or about three molecular “layers.” The swelling (solution content) of compressed cores of Na- and Camontmorillonite (Wyoming) and Ca-illite (Willalooka) in relation to electrolyte concentration at 1 kPa suction (Aylmore and Quirk, 1962) is shown in Table VII. Since the material is saturated, the volume of the solution taken up is a measure of the swelling since the initial porosities were similar. It may be noted that the solution content of Ca-montmorillonite changes to some extent over a wide concentration range, whereas those of the Na-montmorillonite increase markedly as a result of the development of diffuse double layers on opposing lamellae in dilute solution. The characteristics of the swelling of Na-illite are essentially similar to Na-montmorillonite in that the solution content attained at a given suction is greater the more dilute the NaCl solution used to wet the clay (Fig. 5). Although the swelling of Na-illite and Na-montmorillonite is qualitatively similar the solution contents attained by the illite are less than those for montmorillonite because the surface area of the illite is much less, being about 160 m2gg1 compared with 750 m2g-1 for the montmorillonite (Table VII, Fig. 5). The Ca-illite solution contents (Table VII) are almost constant, differing little between a concentration of 1 M CaCl, (K-I = 1.76 and distilled water. Clearly diffuse double-layer theory cannot be used to describe the swelling

A

A)

154

J. P. QUIRK Table VII Solution Content (g . g-1 Clay) at 1 kPa Suction for Compressed Cores of Na- and Ca-Montmorillonite (Wyoming) and Na- and Ca-Illite (Willalooka) in Relation to the Concentration of NaCl and CaCI, Solutions Montrnorillonite

Illite

Concentration

(M) 1 .o

0. I 0.01 <10-5

Na

+

0.95 4.9 7.7 18.0

CaZ+

Na

0.58 0.68 0.71 0.71

0.48 0.75 I .60 8.4

+

Ca2+ 0.45 0.46 0.47 0.47

between crystals of Ca-illite and the intracrystalline swelling of Camontmorillonite. The similarity in behavior of the two systems led Aylmore and Quirk (1960, 1962) to conclude that contiguous Ca-illite crystals at regions of overlap reside in a potential minimum similar to that which gives rise to the fixed 19 8, basal spacing for Ca-montmorillonite. These observations, together with electron microscopy and pore size distribution measurements, are the basis on which Aylmore and Quirk (1959, 1960, 1962, 1967), Quirk (1968), and Quirk and Aylmore (1971) have described the assemblages of Ca-illite crystals and the assemblage of montmorillonite quasicrystals as clay domains. The principal basis for the stability of these compound particles in water is the operation of ion-ion correlation forces at areas of overlap between crystals of illite and quasicrystals of Ca-montmorillonite; as a result the swelling of Ca clays is almost insensitive to electrolyte concentration. It is noteworthy that Terzaghi (1956) referred to “clusters of clay particles” as consisting of a “great number of crystals.” He also drew attention to the need to establish “the seat and nature of the molecular forces which prevent the clusters or piles floating in a suspension from being dispersed by the osmotic pressure acting on their elements.” Bolt and Koenigs (1972) have also discussed the significance of potential barriers at regions of particle overlap. Olson and Mitronovas (1962) studied the compression of Ca-Fithian illite and concluded: Electrolyte concentration had almost no effect on the position of the rebound swelling curves. The most important effect of electrolyte concentration seemed to be its effect on the geometric arrangement of particles that were sedimented from dilute suspensions. The geometric arrangement of particles seems to be a more significant variable than the osmotic repulsion between the particles.

155

INTERPARTICLE FORCES

Ben Rhaiem ef al. (1987) examined the structure of Ca-montmorillonite (Wyoming) in equilibrium with a range of suctions by means of high intensity, lowangle X-ray diffraction (see Table VIII). It may be seen that, over the pressure range 100 MPa to 3 kPa on the rewetting cycle, the average number of lamellae (Ei) in a quasicrystal changes from 1 1 to about 8, indicating that these entities are relatively stable. The average number of lamellae in an assemblage of quasicrystals (M)vanes from 400 to 90, indicating that the clay domains break up to form smaller domains on the wetting cycle; this is part of the swelling process. If it is assumed, for the sake of illustration, that it is permissible to divide by E then the number of quasicrystals per clay domain would vary from 36 to 11. The average thickness of the clay domain, obtained by multiplying the 4001) value by varies from 0.76 to 0.17 pm. Tessier ( 1990) concluded, from electron microscope observations, that the pores between the quasicrystals within a clay domain of montmorillonite are 3040 A across and that during wetting the clay domains split at some of these sites to give several domains as is indicated by the decrease in (Table VIII). Murray et al. ( 1985) examined the microstructure of the natural aggregates of 12 Queensland vertisols containing smectite by applying the general B.E.T. equation to the low temperature nitrogen adsorption isotherms. They concluded that their results were consistent with the presence of pores with widths of 14 to 28 A in the dried materials. The surface areas of these materials varied from 100 to 200 m2g-I when expressed on a clay basis (<2 pm). Using 750 m 2 g 1as a reference area for montmorillonite, then a value of ii of 11 (Table VIII) would indicate an area of 68 m2g-1. Aylmore and Quirk (1962) reported a nitrogen surface area of 38 m2g- 1 for Ca-montmorillorite (Wyoming). The surface area obtained using E, an average, would include areas of overlap, between quasicrystals, which would not be accessible to nitrogen.

a

a,

a

Table VIII Characteristics of Quasicrystals and Domains of Ca-Montmorillonite(Wyoming) Influenced by Applied Suctionu

100 1 0.003 a c'

15.6 18.6 18.6

400 170 90

11

8.4 8.2

36 21 11

0.62 0.3 0.17

Low-angle X-ray diffraction results are from Ben Rhaiem ef al. (1987). The average number of elementary layers in a clay domain of montmorillonite The average number of elementary layers in a quasicrystal. The average number of quasicrystals in a flay domain ( Z / E ) . The average particle thickness [d(001) x MI.

0.26 0.60 1.10

156

J. P. QUIRK

As well as order in the c axis direction there is also a long-range order in the ab plane (Laffer et d., 1969; Fitzsimmons et d., 1970; Greene e t a / ., 1973); these authors prepared single lamellae of Ca-montmorillonite in a virtually electrolytefree solution. Optical birefringence developed after shaking a 0.3% suspension for a period of I month, and electron microscope observation of the particles, adsorbed onto a positively charged grid, indicated that single lamellae of about 0.3 pm across had condensed to give quasicrystals of more than 5 pm in the a-b direction with a thickness of about 50 8,. The kinetics of the condensation was hastened if the single lamellae were flocculated with 0.1 M CaCl,; birefringence developed after a few days shaking and the quasicrystals were much thicker. The stability of quasicrystals with respect to the degree of sodium saturation has been investigated by Shainberg and colleagues (Shainberg and Otoh, 1968; Shainberg and Kaiserman, 1969; Bar-On et al., 1970). They found that as the percentage of exchangeable sodium increased up to 20% the electrokinetic velocity also increased but that the light scattering showed that the particle size was unaffected. They therefore concluded that the Na+ ions were adsorbed on the external surface of the quasicrystals. Quasicrystals progressively disintegrated as increasing amounts of Na+ ions, between 20 and 60% exchangeable sodium, occupied the interlamellar exchange sites. Within a swollen mass of Ca-montmorillonite there are pores between the individual lamellae within a quasicrystal, pores between the quasicrystals assembled in a clay domain, and pores between the clay domains themselves. The definition of the clay domains and quasicrystal entities is primarily energetic in that these entities represent relatively stable groups of particles connected by overlapping areas in an adhesive potential minimum and have a near parallel alignment because of their plate-shaped character. Within a clay soil matrix these assemblages must be in random array to be consistent with the observed isotropic behavior of the swelling of clay cores and clay soil aggregates. Because two lamellae in contiguous quasicrystals of montmorillonite can conform more closely than two crystals of illite, perhaps 50 8, thick, the stability of a quasicrystal of montmorillonite and an assemblage of quasicrystals would be greater than an illite domain with the additional possibility that individual lamellae could be shared between contiguous quasicrystals. Where there is a high degree of conformity between illite surfaces the greater surface density of charge would give rise to a strong attraction because of ion-ion correlation forces. The distribution of particle thicknesses in illite may be an important feature contributing to the stability of a domain as the thin particles would interleave with coarser ones. The less than 0.2-pm fraction of Willalooka illite has a nitrogen surface area of 227 m2g-' (Alymore and Quirk, 1967), indicating that there would be some very thin particles since the average thickness would be less than four elementary silicate sheets thick. Because of their flexibility these thin particles could act as

INTERPARTICLE FORCES

157

an aggregating agent and could play a significant role in determining long-range order in the a and b directions in a clay domain. The stability of an illite domain within a soil would be considerably enhanced by the large suctions developed during drying. Aylmore and Quirk (1962) have shown that clay domains of Willalooka illite, composed of individual crystals of about 50 8, thick and 700 8, in lateral extent, have a lateral extent of about 5 km. Lebron et al. ( 1993), using photon correlation spectroscopy, investigated the nature of clay domains of Silver Hill illite which has 1.5 units of charge per unit cell formula (Sposito and LeVesque, 1985); this is a similar surface density of charge to that of Willalooka illite. They found that the Ca-saturated illite had a much larger particle size than the Na-saturated illite. They also reported that the Ca clay domains broke down to give smaller particles at SAR values in the range of 10 to 15 and also that the particle diameter decreased by a factor of 2 to 3 , indicating a large decrease in particle size since this would vary as the cube of the measured diameter.

B. MECHANISMS OF CLAY SWELLING 1. Crystalline Swelling of Calcium Smectites SIade and Quirk (1991) reported d(OO1) spacings in distilled water for the following smectites for which the total charge and tetrahedral contribution to the charge are shown in parentheses: hectorite, 20.7 8, (0.70,0.04);Wyoming, 19.1 8, (0.74, 0.08); Otay, 18.5 8, (1.04, 0.06); Nibost, 17.8 8, (1.14, 0.74); and Drayton 16.7 A (1.06, 1.02). Thus the extent of the crystalline swelling of Casmectites is influenced by the magnitude of the charge and also by its structural origin. Figure 6 shows the effect of CaCl, concentration on the transition from the spacing in water to about 15.5 8,. Posner and Quirk (1964a,b) reported that this transition is reversible with respect to concentration and took place between 1 and 2 molal for Li, Na, Mg, and Ca chloride solutions and that the spacing did not decrease appreciably at higher concentrations. Since the electrolyte did not penetrate the interlamellar space until the transition had taken place, the transition is an osmotic phenomenon. The osmotic pressure of the solutions is indicated in Fig. 6. Figure 6 shows that the 15.5 8,spacing for Wyoming montmorillonite is attained at a pressure of I3 MPa, the transition for Otay montmorillonite occurs in two stages with the first at 3.4 MPa and the second at about 13 MPa, the transition for Nibost beidellite is from 17.8 8, to about 15 8, and also occurs in two stages but at lower pressures. Each 3 8, increase in spacing contributes about 0.10 g H,Og-l clay if 650 m2 g-I is taken as the internal area. The different behavior of the smectites shown in Fig. 6 for pressures less than

J. P. QUIRK

158

Total

18.5 0 L

Tetrahedral

Wyoming 0.74

\

0 may

1.04

0.06

A Nibost

1.14

0.74

s

u 16.5

F -

5 4 1

00

05

10

15

2.5

20

Molality of CaCI,

5

10 15 20 Osmotic Pressure (MPa)

25

30

Figure 6 The d(001) spacings for Ca smectites as intluenced by the total charge, the structural origin of the charge, and CaCI, concentration. Since salt does not enter the interlamellar spaces until the 15.5 8, spacing is attained, the transition is in response to the osmotic pressure of the solution. (After Slade and Quirk, 1991 .)

3.4 MPa (0.5 molal), the range of principal interest for agriculture, is such that Wyoming montomorillonite cannot be regarded as representing the smectites generally. It should be noted that Drayton, a subsoil clay is classed as subplastic (Nomsh and Tiller, 1976). 2. Clay Swelling

The swelling of Ca clay cores in an overconsolidated state has been investigated with respect to applied suction and electrolyte concentration by Aylmore (1960) and Aylmore and Quirk (1962, 1966). The swelling to equilibrium with 1 kPa suction and 1 M CaCI, for Ca-kaolinite (Rocky Gully), Ca-illite (Willalooka), and Ca-montmorillonite (Wyoming) expressed as a percentage increase in the original volume of the core is, respectively, 3 1,45, and 85%. However, if the solution uptake by montmorillonite is corrected for the interlamellar water (0.30 cm3 g-'), then this last figure becomes 43%. The original porosity and surface area of these clays are, respectively, 0.185, 0.188, and 0.135 cm3 g-'

INTERPARTICLE FORCES

159

and 36, 160, and 38 m* g-1. This information reveals that swelling cannot be interpreted simply in terms of the nitrogen surface area involved. The work of Ben Rhaiem et af. (1987) shows that quasicrystals separate to give a larger number of quasicrystals and thereby increase the external surface area of Wyoming montmorillonite during swelling. The swelling in a I M CaCl, solution ( K - ' = 1.76 A) and other evidence indicates that diffuse double-layer forces make little contribution so that swelling must result from a balance between attractive forces (ion-ion correlation and van der Waals) and repulsive forces (water structural). The suction in the soil water acts to restrain swelling, and as the suction is reduced the balance of the forces favor swelling. A simple three plate model can be used to illustrate the interplay of the forces; in this, two plates in parallel alignment are separated by a third plate in such a way that a pore is formed with a surface separation similar to the thickness of the middle plate and in the areas of overlap the surfaces are virtually in contact. With the adsorption of water these contacting surfaces may be separated by distances up to 10 A. Water structural forces are operative in the slitshaped pore between the separated plates, and in the pores, generated by overlap, ion-ion correlation forces, together with van der Waals forces, act to restrain swelling. The available evidence indicates that the magnitude of the water structural forces is influenced by surface density of charge and that the ion-ion correlation forces are a function of both the surface density of charge and the electrolyte concentration. The surface density of charge of the kaolinite, illite, and montmorillonite, referred to earlier, are, respectively, 0.106, 0.260, and 0.119 Cm-2; 1 unit of charge per 152, 62, and 135 A 2 of surface. Figure 3 indicates the large repulsive pressures which can be generated by water structural forces in relation to the distance of separation of surfaces for slitshaped pores. For surfaces with a smaller surface density of charge these pressures would be lower. To quantitatively appreciate the operation of these forces it would, ideally, be appropriate to have pore size distribution measurements before swelling takes place and at various stages of the swelling process. For a sample of Urbrae loam B horizon (80% clay, surface area of 116 m2g-l), Murray and Quirk (1980a) measured the pore size distribution of the dry clay material and in equilibrium with a suction of 10 kPa; the modal pore sizes were 36 and 63 A. The swollen porosity was preserved by the displacement of water by increasing the concentration of dioxane in dioxane-water mixtures, followed by the displacement of dioxane by liquid C 0 2 which was then removed at its critical point. Features of the pore size distributions could reasonably be interpreted as arising from variable crystal thicknesses. The nitrogen desorption isotherm enabled pore sizes less than 300 A to be measured. Pore sizes larger than this value could not be measured because the pressure involved in Hg injection porosimetry caused collapse of the pores and this, as well as the preservation of the swollen porosity, indicates that the original swollen structure was, at least partially, maintained.

160

J. P. QUIRK

Although the initial porosity was preserved by freeze-drying techniques the pore size in the < I 0 0 A range was identical with the oven-dry material because the water migrated and was frozen elsewhere in the clay matrix. In considering the application of information such as that given in Fig. 3 to the swelling of a clay-water system, the range of surface separations (pore sizes) in the system arising from the range of crystal thicknesses, needs to be taken into account. The clay mass is thus more complex than the three plate model used earlier, as can be seen from Fig. 8 in which the presence of wedge-shaped pores is evident; however, the description of near parallel alignment is still apposite. Another contribution to the swelling process is the interaction between the clay domains. Since the domains are in random array and swell unidimensionally they will compete for space and thereby enlarge the porosity as the suction is decreased. The swelling results of Aylmore and Quirk (1962) given in Table VII show that the effect of CaCI, concentration on swelling is greater for montmorillonite than for illite; the surface charge of illite ( 1 unit of charge per 62 A) is in the vermiculite range. The variation of ion-ion correlation attractive pressure with CaCI, concentration has been considered by Kjellander et al. (1990) who for a charge density of 1 unit charge per I35 A 2 calculated that the pressure increased from I .7 to 5.5 MPa over the concentration range from zero (counterions only) to 2 M CaCI,. For 1 unit of charge per 60 A2 the pressure increased from about 15 to 30 MPa over the same concentration range. The hysteresis between the wetting and drying curves for these materials is accentuated with increasing CaCI, concentration; this would be consistent with the operation of ion-ion correlation attractive forces on the wetting cycle. On the drying cycle the solution content decreases only slowly to a suction of 0.1 MPa and then falls more rapidly at greater suctions. This is thought to be associated with the strength of the structure formed on the wetting cycle. On the drying cycle the water structural forces between surfaces removed from the potential minimum would be operative and, furthermore, considerable work would be required to bring opposing surfaces into the potential minimum because the two sets of ions associated with each surface have to be shared at the midplane in the potential minimum. Calcium ions at clay surfaces have a dual role; they are responsible for the attractive ion-ion correlation pressures at regions of close approach of surfaces, and in larger pores the Ca ions, because of their large concentration at the clay solution interface, perturb the normal structure of water which is the basis for the repulsive pressures described as the structural component of disjoining pressure or secondary hydration forces. It is also noteworthy that these countervailing forces each increase considerably in magnitude with increasing surface density of charge. The balance between the two sets of forces is obviously a delicate

INTERPARTICLE FORCES

161

one since 10 to 20% Na+ ions, or even less, can give rise to adverse physical behavior at low electrolyte levels. If it were not for the presence of ion-ion correlation forces within a clay matrix or soil aggregate it would be virtually impossible to use soils since water structural pores would dominate. Aylmore and Quirk (1966) have reported on the swelling of four Ca-kaolinites: Mercks I1 ( 1 1 m2g-1, 0.18 Cm-z), Malone (17 m2g-1, 0.24 Cm-,), Rocky Gully (36 m2g-', 0.11 Crn-,), and New Zealand (40m2g-', 0.08 Crn-,). The initial porosity of these materials was similar and the percentage increases in volume, from the dry state, for the clay cores in equilibrium with 1 M CaCI, and I kPa were, respectively, 13, 22, 3 I , and 50; when these percentages are expressed in relation to the surface area of each clay the following results emerge: 1.18, 1.29, 0.86, and 1.25. These ratios may be compared with that for Willalooka illite of 0.3 which suggests that the kaolinites behave as single crystals. The basic swelling mechanism must be water structural forces accompanied by a general relaxation of the clay as the suction is reduced.

VI. SURFACE AREA AND PORE SIZE The B.E.T. equation and the Kelvin relationship [Eq. (2)] have been applied to low temperature nitrogen adsorption-desorption isotherms to obtain, respectively, surface area and pore size distribution of clays and soils (Aylmore and Quirk, 1967; Murray and Quirk, 1980a,b, 1990a,b; Murray et al., 1985). Murray et al. (1985) have critically assessed the assumptions used in the B.E.T. and Kelvin equations as applied to clay systems. In applying the Kelvin equation, allowance is made for the thinning of multilayers on the surface of pores already emptied by capillary evaporation on the desorption isotherm (Aylmore and Quirk, 1967). The procedure adopted is to obtain the volume of capillary evaporate removed over a series of intervals @lpo values) on the desorption isotherm. Since the Kelvin equation provides the average slit width for each vapor pressure interval, this value, together with the change in volume in the vapor pressure interval, is used to yield a surface area. The sum of the surface areas for all of the intervals provides the desorption isotherm surface area. The pore size distribution is also obtained and the procedures involved are set out by Aylmore and Quirk (1967) and Murray el al. (1985).

A. PORESIZEDISTRIBUTION Figure 7 shows the N, sorption isotherms for Ca-Willalooka illite (a separate sample from that quoted earlier with an area of 160 m2g-') and the natural

J. P. QUIRK

162 0.08-

a F

.

.

7

8

0.06.

b, c)

0 .2

.4 .6 PIP0

.8 1

0 . 2 . 4 .6 .8 1 PiPo

2 4 6 810 pore width (nm)

2 4 6810 pore width (nm)

Figure 7 Nitrogen sorption isotherm and derived cumulative pore size distribution (a) natural aggregates of a vertisol (total porosity, 0.13 cm3g-I) and (b) Ca-Willalooka illite-compressed cores (total porosity, 0.23 cm3g-I). (After Murray er al.. 1985.)

aggregates of a Queensland vertisol (No. 10). It can be noted that the plot of cumulative volume versus pore size indicates modal values of about 4 and 3 nm respectively. The slope of the desorption isotherm for Willalooka illite indicates a mixture of slit- and wedge-shaped pores. The isothern for the vertisol shows a large hysteresis and its shape indicates a predominance of slit-shaped pores. There is a considerable variation in the isotherms for the other vertisols studied with some having a sloping desorption isotherm. Figure 7 also shows that the pore size in these materials is predominantly < 10 nm. For the vertisol, 50% of the pore space is in pores less than < 3 nm and 95%

INTERPARTICLE FORCES

163

of the surface area is in these pores. For the Willalooka illite, 85% of the pore space and virtually all of the surface area available to nitrogen are in voids less than 5 nm. From these and other observations it can be concluded that these pore sizes, which have been almost totally neglected, are a common feature of clay soils. Sills et al. (1973) have studied the movement of the pore peak as illite is added in increasing proportions to kaolinite. As the percentage of illite increases the large voids between kaolinite crystals are filled with illite particles and as a result the pore sizes decrease substantially. From this information it can be readily understood that pore sizes for finegrained illites and montmorillonites in the dry state are predominately less than 5 nm which thus allows for the effective interplay of interparticle forces. Murray and Quirk (1994) have discussed the conditions for particle bending on the drying cycle of a clay when very large forces are operative for close distances of approach of the surfaces. The release of this mechanical energy would assist the early stage of the swelling process; however, as noted earlier, for most practical purposes the surfaces of clay particles within a soil are always separated by at least two layers of water.

B. INTRINSICFAILURE Table IX shows the magnitude of the surface areas of clay particles. The significant feature is the close agreement between the surface areas obtained from both arms of the sorption isotherm. The B.E.T. and Kelvin equations involve entirely different sets of assumptions (Murray et al., 1985; Murray and Quirk, Table IX Comparison of the Specific Areas Obtained from Nitrogen Adsorption Isotherms (B.E.T. Equation) and the Desorption Isotherms (Kelvin Equation)u

Sample Redhill montmorilloniteb Willalooka illite ( < O . 1 Rocky Gully kaoliniteb Queensland vertisol No. 1 I Urrbrae B aggregatesb

Adsorption isotherm (m2g- I)

Desorption isotherm (m2g-I)

99 227 36 154 94

94 210 37 145 90

The clay cores were Ca saturated and the vertisols and Urrbrae loam B horizon were aggregates as sampled from the field. From Aylmore and Quirk (1967). From Murray er al. (1 985).

164

J. P. QUIRK

1990b) and hence the agreement between the surface areas derived from both arms of the sorption isotherm is noteworthy. This must mean that there are no “ink bottle” pores or restrictions hindering the access of nitrogen to the whole surface area and its removal on the desorption isotherm. Virtually every pore within clay domains has access to the sample surface via at least one path which contains no finer voids or restrictions. That is, domains or regimes of common particle orientation are bounded by an extensive network of larger voids or cracks. These flaws or discontinuities must be an intrinsic feature of clay materials (Murray and Quirk, 1990a,b, 1994) and have been described as intrinsic failure (Quirk, 1978). The intrinsic failure pores are obviously sites of potential weakness in the structure. Williams et al. (1967) obtained a reduction of the nitrogen surface area and porosity of Urrbrae loam B horizon to about two-thirds of the untreated material by the adsorption of 0.035-g polyvinyl alcohol per gram of soil. The molecular weight of the PVA was 25,000 and the effect was much less when a PVA with a MW of 70,000 was used. The authors describe the effect as being due to “peripheral pore occupation” by the polymer, preventing access of N2 molecules to pores and surfaces within clay domains. In the light of more recent work it seems a reasonable hypothesis that the polymer modifies access to the whole surface by affecting access via the set of intrinsic failures. Although such a system of discontinuities does not contribute significantly to either the porosity or the surface area of the clay (Murray and Quirk, 1990a), it imposes a limit on the strength of the clay matrix. The intrinsic failure pores are probably the precursors of cracking in general and the network of flaws probably influences the slaking and dispersion of clays or collections of domains (microggregates) (see model in Williams et al., 1967). It also seems reasonable to conclude that intrinsic failure pores are involved as a source of weakness leading to failure of sodic soils at low electrolyte concentrations. The actual sites of weakness may be the connections, between the walls of these pores, which must exist at periodic intervals. Thus the intrinsic failure pores can make a contribution to physical behavior which is quite disproportionate to their actual volume. The presence of intrinsic failures within a clay mass which swells isotropically is preordained by the presence and nature of clay domains since the clay domains themselves provide a discontinuity. The swelling of the domains, although unidimensional for domain, create a competition for space since each domain swells in different directions because of their random array. This contributes to the swelling process by increasing the volume of the intrinsic failure pore space.

C. PACKING OF CLAY PARTICLES The interleaving of lamellar particles produces slits and wedge-shaped pores with maximum widths which reflect the thickness of the particles. Additionally

INTERPARTICLE FORCES

165

there are surfaces which are essentially in adhesive contact but which separate on hydration; most of the total surface area of smectites is disposed in such regions of close approach. Figure 8 shows a two-dimensional structure generated by stacking lamellar particles with random lengths of 0-1000 units and random, but discrete, thicknesses of 1-10 units. The porosity (0.36 cm3 ~ m - ~surface ), area (73 m2g-l), and pore size distribution derived from this artificial assembly are very similar to the experimentally determined values obtained for a relatively coarse illite (Quirk and Murray, 1991). The relative abundance of slits and wedge-shaped pores, and the acuity (i.e., the dihedral angle) of the wedge-shaped ones, must depend on a number of factors, including the aspect ratio and pliability of the particles, the forces between the particles, and the history of the material (Murray and Quirk, 1994). Obviously the aspect ratio of particles is important. Thin particles lead to an abundance of slits while “blocky” particles produce inferior orientation. True slits can only arise in two situations. The first of these occurs when two particles are propped apart at two or more points by particles of equal thickness; this is relatively improbable. The second occurs when two particles are propped apart by a single, flat, third particle; this is a more probable event. However, the pliability of particles acts to reduce the abundance of true slits, especially of long, narrow ones. These expectations of clay microstructure are supported by experimental measurements of pore size distribution.

I

8

100nm F’igure 8 A randomly packed but oriented assembly of lamella particles with random lateral extents (
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J. P. QUIRK

VII. WATER STABILITY OF SOIL AGGREGATES SUBSTANCES A. STABILIZING Baver (1940) remarked, “The colloidal material is responsible for the cementation of primary particles into stable aggregates.” He considered that the clay particles themselves, oxides of iron, aluminium and silicon, and the organic colloids were significant with respect to cementation. To obtain a more complete appreciation of the relative importance of different colloids a variety of methodologies have been used to assess stability. These include the examination of a large number of soils with a view to establishing correlations between water stability and a particular soil component; the major investigation by Kemper and Koch (1966) is an example of this methodology. Another involves the effect of the addition of various substances such as organic materials, lime, and silicates to assess the role that such substances may have in conferring stability. One issue of “Soil Science” (73,No. 6, 1952) was devoted to the role of synthetic polymers in stabilizing soil aggregates. Yet another approach involves subtractive techniques whereby a chemical reagent specific for a particular soil constituent is used to remove that constituent to determine its role in stabilizing aggregates. There is an exceedingly large body of literature concerned with these topics; however, in this review attention is given to a few pieces of work which illustrate the importance of microstructure and the disposition of cements or stabilizing materials. Greenland er al. (1962) examined the permeability of prewet aggregates of a red-brown earth (alfisol) after treatment with alkaline sodium periodate. The reduction in permeability was small for samples from plots which had been under pasture for many years, but large for samples which had been under pasture for 4 years or less. They concluded that aggregate breakdown as a result of the periodate treatment is probably due to the disruption of polysaccharide and polyuronide moieties in the soil and that stability under cropping and young pastures is due to relatively ephemeral materials. This is consistent with the correlation established by Chesters et al. (1957) between stability and polysaccharide content. The nature of the organic matter in virgin soil samples or accumulating over time under pasture remains largely unresolved but several possibilities exist such as a greater aromatic content or the slow accumulation of clay-polyvalent metalorganic matter complexes as suggested by Edwards and Bremner (1967) with respect to the formation of aggregates less than 250 pm. Desphande et al. (1968) investigated a number of red soils for which iron oxides were thought to exert a favorable effect on soil physical properties; these included krasnozem (oxisols), terra rossa (rhodic calcixerolls) and related rendzina (lithic calcixerolls), and a lateritic red earth (Ultisols) soils. The removal of

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167

iron oxides by sodium dithionite treatment did not result in any changes in the permeability of a bed of aggregates as compared with the control treatment, sodium sulfate. The changes in specific area following the removal of iron oxides indicated that, in all but one instance, the iron oxides are principally present as discrete particles with relatively large surface areas. They qualified their conclusion by suggesting that it was possible that minor amounts of iron oxide may be present as active-binding agents. However, Desphande et al. (1968) also reported that acid treatment (0.1 M HCI), which removed silicon, aluminium, and minor amounts of iron, as compared with the 2-15% by weight removed by dithonite treatment from the soils studied, mostly produced larger changes in physical properties. Because of the effects of the removal of relatively small amounts of aluminium on the results for permeability, wet sieving, mechanical analysis, and swelling determinations, Desphande et al. (1968) considered that reactive aluminium in the form of interlayers or islands between contiguous crystals or clay domains could confer stability. The authors observed that ferric oxides are not known to form chloritic type interlayers, although chlorites containing high proportions of Fez+ in the brucite layer are well known. The small quantity of silicon and aluminium may arise from very small particles with a composition similar to the clay minerals. These considerations suggest that the various empirical approaches to assessing aggregate stability are limited and are not capable of unequivocal interpretation with respect to the basis of stability. Virtually no attention has been given to the disposition of stabilizing agents in the porous matrix of the soil or indeed to the nature of the porous matrix itself. The actual forces involved in cementation need definition. It should be emphasized again that a soil is a condensed particle system to which the excellent studies of heterocoagulation between clays and oxides (Tama and El-Swaify, 1978) may have limited applicability in considering the role of oxides or their precursors in stabilizing soil aggregates. The basic questions which need attention are: What is the nature of the forces between differently charged particles at close distances of approach? What is the relative role of van der Waals forces in such an interaction? It seems timely to address this difficult problem since considerable information on the surface chemistry of oxides has accumulated in recent decades (Bowden et al., 1980; Goldberg, 1992).

B. DISPOSITION OF ORGANIC MATTER AND AGGREGATE STABILITY The outstanding feature of soil structural behavior is the profound influence that organic matter, or rather some moiety of it, can have on the water stability of

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soil aggregates. If variations in stability can be attributed to different levels of organic matter in soils, then the questions which need to be addressed are: What part of the organic matter is operative? What is its disposition in the soil? What is the mechanism of stabilization? Using sintered glass funnels Quirk and Panabokke (1962) studied the rate of wetting, under suction, of aggregates ( 1 cm') of a red-brown earth soil taken from an area with no previous history of cultivation (virgin) and from an adjacent area of continuous wheat-fallow (cultivated). At suctions of 0.2 and 1 kPa the cultivated aggregates took up water much faster and attained a larger water content than the virgin aggregates. The mechanical compositions of the two soils (20% clay, 28% silt) were almost the same but the levels of organic matter were sufficiently different (virgin 2.7%, cultivated 1.3% organic carbon) so that the variations in the rate of wetting could reasonably be attributed to organic matter. This effect does not seem to be due to a larger finite contact angle for the virgin soil because when the cultivated aggregates are wet slowly in stages (10+3+ 1+0.2 kPa) the water content of the aggregates is virtually identical to that of the virgin aggregates. Furthermore, the total porosity of the virgin and cultivated aggregates was 25.9 and 26.7 cm3/ 100 g, respectively, and the rate of uptake for a nonpolar liquid was the same for each soil so that the different wetting behavior does not appear to result from different porosities, pore size distributions, or pore continuity. In a period of 5 min at a suction of 0.2 kPa the virgin and cultivated aggregates reached water contents of 18 and 33 cm3/ 100 g; thus the original porosity of the cultivated aggregates was exceeded, and with a Ca soil of this texture the additional uptake could not be attributed to swelling so the additional water uptake must be due to failure within the wetting aggregate. The cultivated aggregate, wet rapidly at 0.2 kPa suction, slaked when placed in water whereas when the aggregates were wet in stages to 0.2 kPa they did not slake. This observation suggests that the aggregate failure is caused by rapid wetting and Quirk and Panabokke (1962) used the term incipient failure to describe the phenomenon. The points of weakness in a porous material would be expected to be the coarse pores so Quirk and Panabokke (1962) measured the strength of the virgin and cultivated aggregates shown in Table X . It can be seen that, for a wide range of water content values, the virgin aggregates were appreciably stronger than the cultivated aggregates. It is also notable that the remolded cores of each material had almost identical strengths under a range of conditions, suggesting that the presence of organic matter itself did not impart increased strength and that the disposition of the organic matter was important. Quirk and Panabokke (1962) advanced the hypothesis that the strengthening of coarse pores by an organic matter moiety is the basis for aggregate stabilization by organic matter. By introducing polyvinyl alcohol [(CH,CHOH). MW 70,000] into different pore classes of the cultivated aggregates, Quirk and Williams (1974) showed that the pores which drain between 3 and 10 kPa were significant in this respect. This

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169

Table X Mechanical Strengtha (Load in Kilograms) of Virgin and Cultivated Aggregates and Cores Virgin

Cultivated

Suction (MPa)

Aggregates

Cores

Aggregates

Cores

0.01 1 .o 4.0

0.54 1.11 I .88

0.44 1.70 4.80

0.3 1 0.17 1.18

0.42 1.67 4.69

'' Determined using an Atterberg balance. same group of pores is responsible for the rapid attainment of field capacity and hence must have a high probability of continuity within the aggregate. These pores thus have a dual function as they are also the sites where organic matter strengthens the aggregates so that they are stable to rapid wetting. The different suctions used in this study are significant in that they determine the rate at which water is transferred to the aggregates. The amount of water taken up by the cultivated aggregates in 5 and 10 min at 0.2, 1, and 3 kPa suction corresponds to rainfall rates of 5 cm/hr for 5 min, 2.2 cm/hr for 10 min, and 1 cm/hr for 10 min. They concluded that for rainfall rates significantly greater than 1 cm/hr for 10 min that appreciable structural damage, due to incipient failure, would occur for red-brown earth soils in rotations in which the cultivation phase exceeds about three years. The soil samples were obtained from plots at the Waite Institute where the probability of a shower at 3 cm/hr for 15 min is once a year and for a shower of 2 cm/hr for 30 min is also once a year. These results have implications with respect to the general concept of water stability which needs to be defined and understood in more precise terms than those afforded by observing the degree of slaking of soil aggregates on immersion in water; an assessment of water stability would, of course, need to include the effect of raindrop action which would be enhanced by incipient failure.

VIII. SODIC SOILS AND THE THRESHOLD CONCENTRATION CONCEPT A. THE THRESHOLD CONCENTRATION The behavior of saline and sodic soils has been reviewed from different points of view by Richards (1954), Rhoades (1982), Bresler et al. (1982), Shainberg and Letey (1984), and Oster el al. (1994); specific attention will be given here to the threshold concentration concept.

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170

The Gapon equation, which has been widely used in research concerning saline and sodic soils (Richards 1954), has been discussed by Sposito (1981) in relation to the more conventional cation exchange equations. In the Gapon equation the ions are shown as reacting in equivalents: NaX (s)

+ 0.5 CaC1,

(aq)

=

Ca0,,X(s)

+ NaCl(aq)

(20)

and the Gapon equilibrium constant is:

which may be rearranged to give

where E N , and Ec, are the equivalent fractions of sodium and calcium ions on the exchange complex and the square brackets refer to concentrations in solution (mmol liter-1) rather than activities. The ratio of ion concentrations is of a similar magnitude to the corresponding ratio of ion activities over the concentration range common to salt-affected soils, even though the activities themselves vary considerably. In considering K , values for individual soils it is realistic to recognize that soil clays are frequently a mixture of clay mineral types and contain four principal cationic species: sodium, potassium, magnesium, and calcium. The ratio EN,/Ec, is referred to as the exchangeable sodium ratio (ESR), and the ratio, if only sodium and calcium ions are present, E,,IEN, + Eca, expressed as a percentage is the exchangeable sodium percentage (ESP); more generally the ESP is the percentage of exchangeable sodium relative to the exchange capacity. The ratio (Na+)/(Ca2+)0.5is referred to as the sodium adsorption ratio (SAR) of the equilibrium solution, and in practice Mg2+ is coupled with Ca2+ in the denominator. The Gapon equation has two interesting features. First, if a soil is equilibrated with a solution having a given SAR value, and it is desired to percolate the soil with a more dilute solution and at the same time maintain the ratio of sodium to calcium on the soil colloid surfaces constant, then if the sodium ion concentration in the solution is reduced by a factor then the calcium ion concentration must be reduced by the square of that factor. Second, if water being used to reclaim a sodic soil is simply diluted, this dilution decreases the SAR value and thus favors the adsorption of calcium on the soil surfaces, so the exchangeable sodium percentage is reduced and reclamation is assisted. Difficulties encountered in the irrigation of sodium-affected soils or in the reclamation of saline-sodic soils can be circumvented by controlling the electrolyte level in the irrigation water; this prevents the deterioration of soil structure

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and sustains permeability (Quirk and Schofield, 1955; Quirk, 1957, 197 1, 1986). The threshold concentration simply expresses the minimum level of electrolyte required to maintain the soil in a permeable condition for a given degree of sodium saturation of the soil colloids. In practice a soil should be irrigated with water which contains a level of electrolyte above the threshold concentration and preferably has a SAR value which favors calcium adsorption onto the exchange complex. To determine the threshold concentration for various degrees of exchangeable sodium saturation, Quirk and Schofield (1955) brought Sawyers 1 (an illitic Rothamsted loam soil) to equilibrium with relatively concentrated mixed solutions of NaCl and CaCl, having different SAR values. When exchange equilibrium was established between a particular solution and the soil surfaces then the permeability to a series of successively more dilute solutions was determined; the dilutions were carried out in accordance with the Gapon equation so that each solution in the series had the same SAR value. The threshold concentration is taken as the concentration at which there is a 15% decrease in permeability; the concentration is expressed in milliequivalents per liter (meq liter- I ) by adding the concentrations of sodium and calcium in the threshold concentration solution. Figure 9 shows a plot of the threshold concentration in relation to the SAR value of the percolating water (Quirk, 1971). It may be noted that if the soil permeability is to be sustained, the electrolyte level in the water should be to the right of the line, delineating decreasing permeability and unfavorable physical behavior, from stable permeability; this situation is exacerbated the further the concentration of the percolating water is removed from the threshold concentration. The relationship between the threshold concentration, cT, and SAR is cT = 0.56 SAR

+ 0.6

(23)

The relationship covers the range of ESP values from 0 to 35. It may be noted that Eq. (23) does not contain any parameters which are characteristics of the soil solid phase. The SAR value of the saturated extract of a soil, through the Gapon equation, provides the exchangeable sodium percentage. If a soil is irrigated with water of such a quality that the ESP value of the soil increases over time, then Eq. (23) can be used to assess the threshold concentration as the soil approaches equilibrium with the SAR value of the water. The SAR value on the ordinate in Fig. 9 can thus refer to the saturated extract of a soil or to the SAR value of an irrigation water. A stylized version of Fig. 9 has appeared in work done by Oster er al. (1984). Quirk and Schofield (1955) foreshadowed that materials such as oxides and organic matter would protect a soil from the adverse effects of raised levels of exchangeable sodium and envisaged that the threshold concentration concept should be especially significant with respect to the irrigation of arid and semiarid soils. McNeal and Coleman (1966) and Rhoades (1982) have presented results,

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40

-

30

-

20

-

$5 SAR

10-

io

20

Electrolyte concentration (meq I-’) Figure 9 The relationship between the SAR (NaKao.5) for an irrigation water or saturated soil extract and the electrolyte level required to maintain a stable permeability-threshold concentration. (After Quirk, 1971 ,) Equation (23) describes the relationship.

for soils from the western United States, in reasonable agreement with Eq. (23) up to SAR values of about 30. Most of the soils investigated by McNeal and Coleman (1966) contained a montmorillonitic component and thus the very variable behavior for the permeability above a SAR value of 30 is probably due to the crystalline swelling of the montmorillonitic materials in these soils.

B. PHYSICAL BASISFOR

THE

THRESHOLD CONCENTRATION

Quirk and Schofield (1 955) noted that dispersed particles did not appear in the percolate from permeameters until the electrolyte level was about one-quarter of the threshold concentration. The concentration for dispersion, cD, is given approximately by the relationship, cD = 0.14 SAR

+ 0.20

(24)

The amount of dispersed material in the percolate increased markedly with increasing exchangeable sodium percentage at the dispersion concentration and lower. These observations emphasize that factors other than dispersion are pri-

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173

marily responsible for decreases in permeability which occur for sodium-affected soils between the threshold and dispersion concentrations. Rowel1 et al. (1969), working with a brown loam, corroborated the findings of Quirk and Schofield ( 1955) and established correspondence between concentrations at which increased swelling of oriented flakes of the clay extracted from the soil is observed and the threshold concentration. They concluded that (a) the permeability begins to decrease at the same concentration as additional clay swelling begins, (b) the changes in permeability are directly controlled by the swelling of the clay until clay dispersion and movement begins, (c) the concentration at which clay disperses depends on the mechanical stress applied, and (d) large mechanical stresses may disperse more of the clay even at small exchangeable sodium percentages. This behavior is reminiscent of the movement, described earlier, of a soil from the overconsolidated condition toward the normal consolidation curve. The failure that occurs within a soil at low velocities in a permeameter could be described as sodic failure and furthermore the presence of exchangeable sodium assists any mechanical work applied in altering the structure by removing particles from primary potential minima and the disruption of the clay domains. Frenkel et al. (1978) concluded that particle dispersion and pore blocking were the cause of decreased permeability of sodium-affected soils. This conclusion has lead to comprehensive studies of the dispersion-flocculation transition (Goldberg et al., 1991) which have considerably advanced our knowledge of soil colloids. However, this review has stressed that interacting clay particles have to first be removed from within potential minima before extensive swelling takes place and this is followed by spontaneous dispersion if the solution is sufficiently dilute. The evidence presented in this review is that, within a soil clay, particles reside in potential minima when the soil is dry; even for Na-montmorillonite the X-ray spacing is 19 A at 1.5 MPa suction. The crucial issue is that the structure of a clay domain is very different from that of a floccule. There is an intermediate stage between swelling and dispersion which precedes dispersion. A soil, as a result of the additional swelling which takes place when some particles are removed from within potential minima, fails internally and this leads to pore size reduction or even blocking. This failure also provides surfaces to yield dispersed clay particles. These processes are probably concomitant as the electrolyte level is progressively reduced. Internal failure is considered to be a prerequisite to copious dispersion. The introduction made reference to the fact that the balance between forces when Ca clay particles are within a potential minimum must be delicate since 15% sodium or even less on the exchange sites can dramatically alter the physical behavior of a soil. Particle in this sense could refer to the interaction between quasicrystals or crystals in a clay domain or between the domains themselves. In this latter case because of ion segregation, whereby Ca2+ is adsorbed within the domain, there will be an accumulation of Na+ on the surface of intrinsic failure

174

J. P. QUIRK

pores. As a result, the interaction between surfaces will be strong where these pores are relatively narrow or form a nexus between the walls of the pore. The protective role that organic materials can have is best illustrated by Emerson (1954) who showed that soil from a pasture maintained a high permeability to low electrolyte levels even when Na saturated. It seems that the organic matter has prevented the failure but would not have been expected to decrease the particle interaction since large polymers would not be expected to enter pores much less than 100 A across, bearing in mind the free form of polymers in aqueous solution. Results presented by Quirk and Schofield (1955) show that organic matter or moieties of the organic matter enhance the repulsive forces which prevent flocculation of soil suspensions and that it is necessary to add higher concentrations of electrolyte to achieve flocculation in the presence of adsorbed organic molecules. This is illustrated by reference to surface and subsurface soils at Rothamsted. Sawyers I, a surface soil, required 300 meq/liter to flocculate the suspension of the Na-saturated soil whereas only 20 meq/liter was required to flocculate the Na-saturated subsoil with essentially the same mineral composition; this latter value is characteristic of the minerals Na-montmorillonite and illite. Thus organic molecules have a negative effect on soil structure when the particles dispersed by the action of raindrops lead to surface seals by the slow sedimentation of fine particles and in-washing into pores (McIntyre, 1958). There is thus a paradox. The presence of organic matter stabilizes aggregates against slaking and the presence of organic molecules adsorbed on the clay serves to prevent flocculation (peptization) and thus assists in the formation of surface seals. Rengasamy et al. (1984) examined the dispersion behavior of 138 samples taken from the surface and subsurface of Australian red-brown earths. They reported that 30 of the samples did not disperse after 1 hr of end-over-end mechanical shaking and that 28 samples dispersed spontaneously. For this last group they obtained the following relationship between the total cation concentration (meq/liter-1) and the sodium adsorption ratio, TCC = 0.16 SAR + 0.14, which is remarkably similar to Eq. (24) although the methods used are quite different. They also found that for surface soils which were mechanically shaken the boundary between dispersion and flocculation was defined by TCC = 1.21 SAR + 3.3. For subsoils the corresponding relationship was TCC = 3.19 SAR - 1.7 indicating the possible presence of inorganic peptizing agents because the organic matter content of the subsoils is very small. Rengasamy et al. (1 984) considered that the shaking procedure represented the mechanical effect of raindrop impacts. The large difference in the coefficients for SAR for the spontaneously dispersed soils, the surface soils, and the subsoils is noteworthy and reveals that factors other than the mineral species involved can have a major influence on flocculation concentrations.

INTERPARTICLE FORCES

175

C. APPLICATION OF THETHRESHOLD CONCENTRATION CONCEPT An example of the importance of the threshold concentration concept is gained by considering the behavior of Cajon sandy loam in Arizona. This soil was usually irrigated with Colorado River water, but little river water was available during the years 1946 to 1948, and as a result it was necessary to rely on underground water. This water contained 50 meq/liter of Na and 8 meq/liter of Ca (plus Mg). After irrigating with this water for 3 years the ESP value was reported as varying between 19 and 34, with 25 as the mean. Such a soil would require a concentration of 1 1 meq/liter of irrigation water to maintain a stable permeability (Fig. 9). The underground water, although it would have provided a very saline environment, exceeded this concentration. However, the river water contained 3.9 meq/liter, which is considerably less than the threshold concentration. McGeorge and Fuller (1950) reported that when river water was again available in 1949 its use caused the soil to become impermeable and to develop unsatisfactory physical conditions: the soil “froze up.” If the river water had been modified by the addition of 8 meq/liter of Ca2+ ions or, alternatively, if as little as one-seventh of the underground water had been mixed with the river water, this problem could have been avoided. This would have given an electrolyte concentration of about 11.6 meq/liter, which could have been progressively reduced by monitoring the ESP value until it fell to a level that would have allowed the use of river water alone. Because it would not have been economical to add large quantities of gypsum to soils used for pasture development in western New South Wales, Davidson and Quirk (1961) added gypsum to the irrigation water to assist pasture-plant establishment on Riverina clay. This soil had an ESP value of 20 and the irrigation water had an electrolyte concentration of about 1 meq/liter, and as a result dispersion occurred when the soil was irrigated; water entry was poor and very few pasture plants emerged because of the hard surface crusts that developed between imgation events [see photographs in Davidson and Quirk (1960)l. These difficultieswere overcome when 10 meq/liter of Ca2+ as gypsum (approx. 0.6 t ha-1) was added to the irrigation water. It was only necessary to add gypsum to the first irrigation to achieve satisfactory pasture establishment and development. Reeve and Bower (1960) remarked that “the use of waters of poor quality, that is those having a high salt content and a high proportion of sodium, for reclamation has been unthinkable in the past, and at first thought such use would seem to be a questionable practice.” These authors used water from the Salton Sea mixed with water from the Colorado River to reclaim soil (ESP = 37) in the Coachella Valley, California, to sustain the permeability by ensuring that the electrolyte

176

J. P. QUIRK

level in the manufactured irrigation water was always above the threshold concentration. As a result the reclamation time was shortened and was assisted by the “valence-dilution effect” which, as discussed in relation to the Gapon equation, favors the exchange of Ca for Na as progressively more dilute waters are used as reclamation proceeds. Oster et af. (1984) have discussed the threshold concentration in relation to the reuse of drainage waters for irrigation in the San Joaquin Valley in California as a means of obtaining more water for irrigation.

IX. CONCLUDING REMARKS Dexter (1988) discusses the hierarchical order within soil aggregates as The lowest hierarchical order is the combination of single mineral particles, such as clay plates, into a basic type of compound particle such as a domain. The next higher order is larger compound particles such as a cluster of domains. The next higher order is when a number of clusters are combined to form microaggregates. Compound particles of a lower hierarchical order are more dense than those of higher hierarchical order. This is because the order excludes the pore spaces between the particles of the next higher order“porosity exclusion principle.” Compound particles of a lower hierarchical order have a higher internal strength than particles of higher hierarchical order. Dexter also observes that not all soils possess all hierarchical orders. Quasicrystals, crystals, clay domains, and intrinsic failure are all features of the hierarchical order as are the pores which readily drain in attaining field capacity. To extend our knowledge of the hierarchical order which underlies physical behavior, more attention should be given to the soil as a porous system, particularly for soils of varying textures. We should be concerned not only with pore sizes, their interdependence, and their probability of continuity but also the disposition within the pore space of organic matter and other materials which promote aggregation. Over the past decade or so there has been substantial progress in the theoretical and experimental approach to interparticle forces, and although much remains to be achieved, it seemed timely to provide a bridge between mainstream colloid and surface chemistry and soil science in which field we have, through the activity of clay mineralogists and others, an unusually detailed knowledge of the surface chemistry of clays and oxides. The simple fact remains that soils and the clay materials in them are condensed

INTERPARTICLE FORCES

177

systems and it is the particle interaction at short range which holds the key for an understanding of clay-water interactions and their effect on the various aspects of the hierarchical organization within a soil. Two simple facts are relevant. First, even unstable aggregates do not slake when immersed in nonpolar liquids and second, a clay material such as Willalooka illite which increases in volume by 50% from the dry to wet state must, even in the wet state, have the predominant part and its surface area in fine pores (
ACKNOWLEDGMENT It is a pleasure to acknowledge the role that Dr. R. S. Murray of the Waite Agricultural Research Institute has played in the refinement and development of some of the ideas presented here; I am also grateful for his helpful advice on this review.

REFERENCES Aitchison, G. D., and Holmes, J. W. 1953. Aspects of swelling in the soil profile. Aust. J. Appl. Sci. 4, 249-259. Aylmore, L. A. G . 1960. Hydration and swelling of clay mineral systems. Ph.D. Thesis, Univ. of Adelaide. Adelaide.

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Aylmore, L. A. G . , and Quirk, 1. P. 1959. Swelling of clay-water systems. Nature (London) 183, 1752-1 753. Aylmore, L. A. G., and Quirk, J. P. 1960. Domain or turbostratic structure of clays. Nature (London) 187, 1046-1048. Aylmore, L. A. G . , and Quirk, J. P. 1962. The structural status of clay systems. Clays Clay Miner. 9, 104-130. Aylmore, L. A. G., and Quirk, J. P. 1966. Adsorption of water and electrolyte solutions by kaolin clay. Soil Sci. 102, 339-345. Aylmore, L. A. G . , and Quirk, J. P. 1967. The micro-pore size distribution of clay mineral systems. J . Soil Sci. 18, 1-17. Bailey, A. I., and Kay, S. M. 1967. A direct measurement of the influence of vapour, of liquid and of oriented monolayers on the interfacial energy of mica. Proc. R. Sor. London, Ser. A 301, 4756. Bar-On, P., Shainberg, I . , and Michaeili, I. 1970. The electrophoretic mobility of Na/Ca montmorillonite particles. J. Colloid Interface Sci. 33, 471-472. Baver. L. D. 1940. “Soil Physics.” Wiley, New York. Beaglehole, D., and Christenson, H. K. 1992. Vapour adsorption on mica and silicon: Entropy effects, layering and surface forces. J . Phys. Chem. 96,3395-3403. Ben Rhaiem, H., Pons, C. H., and Tessier, D. 1987. Factors affecting the microstructure of smectites: role of cation and history of applied stress. Proc. Int. Clay Conf., Denver, 1985 (L. G . Schultz, H. van Olphen, and F. A. Mumpton, eds.), pp. 292-297. Clay Miner. Soc., Bloomington, Indiana. Bodman, G . B., and Colman, E. A. 1944. Moisture and energy conditions during downward entry of water into soils. Soil Sci. Soc. Am. Proc. 8, 116-122. Bolt, G. H., and Koenigs, F. F. R. 1972. The physical and chemical aspects of the stability of soil aggregates. Symp. Fundam. Soil Conditioning (M. de Boodt, ed.), No. 37, pp. 955-973. Fac. Agric. Sci., State Univ. Ghent, Ghent. Bowden, J. W., Posner, A. M., and Quirk, J. P. 1980. Adsorption and charging phenomena in variable charge soils. I n “Soils with Variable Charge” (B. K. G. Theng, ed.), pp. 147-166. N. Z. SOC.Soil Sci., DSIR, Lower Hutt. Bradfield, R. 1936. The value and limitation of calcium in soil structure. Am. Soil Surv. Assoc. Bull. 17, 31-32. Bresler, E., McNeal, B. L., and Carter, D. L. 1982. “Saline and Sodic Soils.” Springer-Verlag. Berlin. Brewer, R. 1988. “Soil Structure and Fabric.” CSIRO, Div. Soils, Adelaide. Brindley, G. W. 1980. “Crystal Structures of Clay Minerals and Their X-Ray Identification.” Mineral. SOC.,London. Chan, D. Y.C., Pashley, R. M.,and Quirk, J. P. 1984. Surface potentials derived from co-ion exclusion measurements on homoionic illite and montmorillonite. Clays Clay Miner. 32, I3 1138. Chesters, G., Attoe, 0. J., and Allen, 0. N. 1957. Soil aggregation in relation to various soil constituents. Soil Sci. Soc. Am. Proc. 21, 272-277. Claesson, P. M.. Herder, P., Stenius, P., Eriksson, J. C., and Pashley, R. M. 1986. An ESCA and AES study of ion-exchange on the basal plane of mica. J . Colloid Interface Sci. 109, 3 1-39. Clapp, C. E., and Emerson, W. W. 1965. The effect of periodate oxidation on the strength of soil crumbs. I. Qualitative studies. Soil Sci. Soc. Am. Proc. 29, 127-130. Croney, D., and Coleman, J. D. 1954. Soil structure in relation to soil suction (pF). J. SoilSci. 5,7584. Davidson, J. L., and Quirk, I. P. 1961. The influence of dissolved gypsum on pasture establishment on irrigated sodic clays. Aust. J . A@. Res. 12, 100-1 10.

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Index A Abscisic acid, water stress effects, 91 Agricultural commodity programs, crop rotation, 34-36 Alfalfa, nitrogen scavenging, 14- 15 Allelopathy, in crop rotation, 21-22 Aluminum in acidic soils, 77-79 activity variation, 65-67 dissolution effects of space and time, 78 steady state assumptions, 78 dissolution modeling, 64 differential rate law, 76 equilibrium thermodynamic, 65-70 Gibbs free energy, 75 kinetic approaches, 72-77 Monte Carlo simulations, 77 nonequilibrium thermodynamic, 70-72 parabolic rate law, 75 rate law, 73 transition state theory, 76-77 effect on clay stability, 167 precipitation induction time, 74 kinetic approaches, 72-77 mineral phases, 72 modeling, 64-77 nonequilibrium thermodynamic, 71-72 nucleation rate, 74 sinks, 48 soil forms, 48 solubility inaccuracies in predicting, 68 models for predicting, 68, 69, 79 plant growth, 68 toxicity, 48, 68 Aluminum-containing material dissolution carbon dioxide effect, 59 components influencing, 50 factors affecting, 51-64 inclusion effect, 56,

ion adsorption affect, 54 ionic strength effect, 56, 58 organic ligand effect, 58 particle size effect, 62 pH effects, 54, 56 polydispersity effect, 59 rate, factors affecting, 63 saturation state, 52-54, 61 solid properties, 61-64 structural changes affecting, 63-64 surface area affect, 61 theories, 48-49 water activity and, 59-60 leached layers, 63 precipitation, component adsorption effect, 62-63 components influencing, 50 factors affecting, 51-64 inclusion effect, 56 ionic strength effect, 56, 58 rate controlling step, 53-54 structural changes affecting, 63-64 saturation index, 68 solubility product constant, 52 solution properties, 5 1-60 surface morphology changes, 56 Aluminum hydrous oxide, precipitation, pH effects, 58 Aluminum hydroxide, precipitation, pH effects, 56 Ammonium, cytokinin transport, 88 Anorthite, aluminum dissolution, 54 Atrazine, degradation, 107, 108

B Bahiagrass, soil pores, 23 Barley allelopathic effects, 21 rotation, 2 Bayerite, precipitation, 56, 72 Beans, crop rotation, 2 Beidellites, swelling, 150 Bioenergy, crops for, 32 185

INDEX

186 Biological diversity alternative land uses, 32 crop rotation, 30-32 Biopores, cropping system effects, 22 Blast disease, 103 Boehmite dissolution, 56, 58 precipitation, 72 water activity influence, 60 Boron deficiency, rooting, 88 Bromegrass, soil structure, 23

C Cajon sandy loam, threshold concentration concept, 175 Calcium crop rotation effect, 16 deficiency, peanut pod rot, 102 effect on rooting, 88 hydration energy, interparticle forces, 142 Carbendazim, manure effect, 104 Carbon dioxide, aluminum-containing material dissolution, 59 Chemical kinetics, and nonchemical kinetics, defined, 72 Chemical thermodynamics, defined, 50 Chlordimeform, adsorption, 106 Chlorophyll meter, in tissue NO,-N monitoring, 96 Chlorosis, nutrient deficiency, 9 I Chlorosulfuron, adsorption, 106 Clay aggregate mechanical strength, 168-169 stability, 167- 169 calcium domain, 152-157 electrolyte effect, 154, 160 ion-ion forces, 154, 160 nitrogen surface area, 155 pore size, 155 pore size distribution, 159- 160 quasicrystals, 152-157 stability, 156 surface potential, 132 swelling, 158-161 defined, 122 interparticle forces, see Interparticle forces

intrinsic failure pores, 163- 164 packing, 164-165 particle alignment, 123 particle interaction, 134-138 pore size distribution, 161- 163 surface area and, 161-165 shrinkage and swelling, 143-146 sodium crystalline swelling, 147-151 swelling, 146-152 swelling pressure, 132- 133 stabilizing substances, 123, 166-167 structural porosity, 146 structural states, 144 surface area, pore size and, 161-165 surface potential, 130-132 swelling, 122-123 mechanisms, 157-161 pressures, 132- 133 water structural forces, 138-142 wetting, organic matter effect, 168 Clover, rotation, 6 Coion exclusion measurement, surface potential, 130 Conservation tillage, phosphorus contamination, 97-98 Copper deficiency, crop yield, 88 Corn grain, crop rotation effect, 12 Corn rotation, 6, 7, 11, 17 alfalfa rotation with, 15 crop yield effects, 12 in disease prevention, 20 insect control, 19 nematodes, 2 1 nutrient uptake, 15-16 phosphorus concentration, 16 strip intercropping, 3 1 weed competitiveness, 18 Cover crops crop rotation, 6-7 effects on predatory insects, 105 Cowpea, crop rotation, 5 , 7 Crop residue management, crop rotation, 12 phosphorus contamination, 97-98 Crop rotation agricultural commodity program impact, 34-35 allelopathy, 21-22

INDEX benefits, 35 biological diversity, 30-32 cover crops, 6-7 crop residue management, 12 decline, factors, 10 disincentives, 36 economics, 32-33 effects on crop yield, 11-13 disease and pest interactions, 16-22 nutrient use efficiency, 14-16 organic matter, 28-30 soil aggregation, 24-25 soil bulk density, 25 soil erosion, 26-27 soil structure, 22-23 water use efficiency, 13-14 experiments, 5-6 incentives, 36 industrial nitrogen, 10 and irrigation, 9 legumes, 3, 5, 9 manure, 3 monoculture cropping, 10 nematode control, 20-21 nonfarm policies, 35 Norfolk, 3 origin, 2-5 policy impacts, 33-36 Roman, 2 six year, 5 soil quality effects, see Soil quality temporal and spatial diversity, 13 three year, 6 tillage losses, 28-29 20th century pre-World War 11, 5-9 post-World War 11, 9-10 21st century outlook, 10-11 United States, early, 3, 7 water infiltration and retention, 26 wildlife and, 31 crop sequencing, nitrogen movement, 14 Crop yield, see Yield Crystal dissolution, processes involved, 6 1-62 interaction, clay soil, 122 ripening, 61 Cultivated crop, defined, 7

187

Cutworms, crop rotation control, 19 Cytokinin ammonium effect, 88 phosphorus and potassium effects, 88

D 2.4-D [(2,4-dichlorophenoxy) acetic acid], 17 Dakota vetch, rotation, 6 Debye-Huckel theory, 126, 129 Dichloropropene, adsorption, 106 Disease, crop yield effects, 19-20 Disjoining pressure, interparticle forces, 138 Dispersion-flocculation transition, clay soil, 123 Diuron, adsorption, 106 DLVO theory colloidal suspension stability, 128 water structural forces, 138, 139 Drayton clay, swelling, 158 Durra,rotation, 9

E Economics, crop rotation, 32-33 Electrokinetic potential, clay, 13I Emmer, crop rotation 2, Ethirirnol, adsorption, 106 Evapotranspiration water demand, 90-92 water supply, 88-90 water use efficiency, 88-92 Exchangeable sodium ratio, threshold concentration, 170, 174

F Farm policy, crop rotation impact, 33-36 Feldspar, dissolution, pH and, 58 Fertigation, nitrogen utilization, 95 Fertilizer effects on yield, 87-88 erodible land protection, 93-94 manure interaction, 30 plant diseases, 103- 104 water supply effect, 90 weed-root competition, 100-101 Fithian illite, surface potential, 131 Flower initiation, yield, 87-88

188

INDEX

Foxtail, control of, 18 Fringes of equal chromatic order, I38 Fungicide efficacy, 104 nutrient management, 102- 105 Fusarium moniliforme, mycorrhizal competition, 104

G Gaeumannomyces graminis, crop rotation, 19 Gapon equation. threshold concentration, 170171 Geocoris puncripes, insecticide, 105 Geometric mean diameter, soil aggregation, 24 Gibbsite, 52, 72 dissolution rate, 54 equilibrium thermodynamics, 65 precipitation, iron and, 58 solubility, theoretical, 67 supersaturation, 72 water activity influence, 60 Goethite, 60 solubility, 62 Gouy treatment, interparticle forces, 128-1 33 Gray leaf spot, crop rotation, 20 Gypsum dissolution, 73 threshold concentration concept, 175

H Hamaker constant, interparticle forces, 126 Hematite, 58, 60 solubility, 62 Herbicide crop rotation, 17-18 nutrient uptake, 102 Heteridera glycines, 20 Hoplolaimus Columbus, 2 1

I Illite, 122 coion exclusion measurement, 130 dry porosity, 145 fithian, surface potential, 131 pore size, 145 residual shrinkage, 145 suction, 145

surface density charge, 159 surface potential, 131 water content, 145 willalooka. see Willalooka illite Insecticides, nutrient management, 105- 106 Insects predatory, effects of cover crops, 105-106 susceptibility to crop rotation, 19 Intercropping, 3 I Interparticle forces, see also Ion-ion correlation forces capillary condensation, 124-125 London-van der Waals, 126- 128 osmotic repulsive, 128- 133 suction factor, 124- I25 surface potential, 130-132 water structural forces, 138-142 oscillations, 140- 142 pressures, 140- 142 role of proton, 139- 140 Ion activity product, 52 Ion-ion correlation forces clay particle interaction, 134- 138 energy minima, 135- 136 ion attraction, 134 ionic radius, 133 pressure-distance relationship, 135 surface density of charge, 136 theory, 133-134 Iron aluminum-containing material precipitation, 58 deficiency, root hair formation, 89 Iron oxide, clay stability, 166-1 67 Irrigation, crop rotation, 9

J Jefferson, Thomas, crop rotation, 3-4 Johngrass, control, 17

K Kaolinite, 122 dry porosity, 145 equilibrium thermodynamics, 65 nitrogen adsorption isotherms, 163- 164 nonequilibrium thermodynamics, 71 pore size, 145 precipitation, 52

INDEX residual shrinkage, 145 suction, 145 surface density charge, 159 swelling, 161 electrolyte effect, 152 water content, 145 Kelvin equation, interparticle forces, 124-125 Khaira disease, 105 nutrient and disease interaction, 102-103 Kjellander-Marcelja theory, 133 Krasnozem, iron oxide effect, 166-167

L Langmuir equation, interparticle forces, 130 Lateritic red earth, iron oxide effect, 166-167 Leaf area, effect on transpiration, 90 Leaf senescence, nutrient deficiency, 90 Legume, 3, 5 nitrogen contribution from, 15 20th century popularity, 5 Ley farming, 3 Lifshitz's macroscopic theory, 126 London-van der Waals forces, interparticle, 126-128

M Magnesium crop rotation effect on, 16 leaf color, 91 Manganese toxicity, effect on leaf morphology, 91 Manure application time, 99 atrazine degradation, 107 effect on fungicides, 104 fertilizer interaction, 30 nitrogen availability from, 95 phosphorus water contamination, 98 plant disease association, 103-104 regulation, 98-99 Meloidogyne, crop rotation, 20 Metal oxide, proton-promoted dissolution of, 54.55 Methoxyethyl mercury chloride, manure effect, 104 Methyl bromide, adsorption, 106 Metolachlor, adsorption, 106 Metribuzin, leaching, 106

189

Mica force-distance relationship, 14 I water structural forces, 138-139 Microcline weathering, aluminum solubility during, 52-53 Millet crop rotation, 2, 6 Mineralization, organic matter, 29 Molybdenum deficiency, crop yield, 88 Monoculture cropping crop rotation and, 10 negative consequences, 11 Monte Carlo simulations, aluminum dissolution modeling, 77 Montmorillonite, see also Clay, calcium; Clay, sodium aluminum dissolution, 54 coion exclusion measurement, 130 crystalline swelling, 147-151 electrolyte effect, 149 flocculation-dispersion transition, 150 solution content, 150 dissolution rate, 64 dry porosity, 145 nitrogen adsorption isotherms, 163- 164 particle interaction, 135, 136 permeability, electrolyte effect, 152- 153 pore size, 145 pressure-distance relationship, I37 suction, 145 surface density charge, 159 surface potential, 131 swelling, electrolyte effect, 153 water content, 145 X-ray spacings, 147, 149 electrolyte effect, 152-153 Muscovite equilibrium thermodynamics, 65 surface separations, 149 Mycorrhizae crop rotation, 16 plant disease, 104-105

N Nematode, control by crop rotation, 20-21 Newton-Raphson method, 65 Nitrate alfalfa removal, 14 rooting effects, 90 tissue monitoring, 95

INDEX

190

Nitrogen, see also spec$c forms availability from manure, 95 deficiency, effects on abscisic acid synthesis, 91 leaf growth, 90 effect on yield, 88 fertilizer replacement, 15 fixation, discovery, 4-5 legume contribution, 15 mineralization, 94-95 movement, effect of crop sequencing, 14 pea contribution, 15 rooting effects, 89, 90 soil testing, 95-96 soybean scavenging, 14 tissue monitoring, 96 use efficiency, crop rotation effects, 14- 15 water contamination, 94-96 No-till systems, organic matter loss, 29, 30 Nordstrandite, dissolution, 72 Norfolk rotation, 3 Nutrient contamination nitrogen, 94-96 potassium, 100 Nutrient management and disease resistance, 102-105 and herbicide use, 100-102 mycorrhizal associations, 104 postharvest storage effect, 103

0 Oat cover crop, 93 rotation, 7 Opportunistic cropping, 13 Organic matter content, soil structure, 23 crop rotation length effect, 28 commercial agriculture effect, 29 effects on water movement, 106 interactions with fertilizers and manure, 30 pesticides, 106- 108 mineralization effect, 29-30 soil wetting, 168 tillage loss effect, 28-29 water infiltration, 29 Osmotic repulsive force, interparticle forces, 128-133

Ostwald law of successive reactions, 71 free energy, 74

P Pea, nitrogen contribution, 15 Pest control, crop rotation, 16-22 Pesticide-organic matter interaction, effects on chemical adsorption, 106- 107 microbial degradation, 107- 108 Phosphorus crop rotation effects, 15-16 deficiency, effects on disease susceptibility, 102 leaf color and growth, 90-91 root hair growth, 89-90 water transport, 90 runoff, control by conservation tillage and crop residue management, 97-98 manure application, 98 sludge application, 97 soil retention, 99 Plant disease fungicide efficacy, 104 mycorrhizal competition, 104-105 nutrient management effects, 102-105 Potassium crop rotation effects, 16 deficiency, effects on crop yield, 88 rooting, 88 Verticillium wilt, 102 hydration energy, interparticle forces, 142 role in stomata1 function, 91 water contamination, 100 Potato rotation, 6 Powdery mildew, silicon effect, 102, 103 Pressure deficiency, interparticle forces, 124 Primary potential minimum, clay soil, 123 F'yrazoxyfen, degradation, 106, 107- 108 F'ymphyllite, nonequilibrium thermodynamics, 7 1 Pyrhium ulrimum, silicon suppression, 103

Q Queensland vertisols nitrogen adsorption isotherms, 163- 164 nitrogen sorption isotherm, 162 pore size, 155, 162 Quintozene, manure effect, 104

INDEX

R Rape rotation, 15 Reflectometer, tissue nitrate, 96 Rhizocroniu soluni, manure effect, 104, 105 Rice, silicon application, 103 Ring-neck pheasant, biological diversity, 31 Roots depth and volume, nutritional effects, 88-90 weed competition, 101 Rust, and nutritional imbalances, 103 Rye allelopathic effects, 21 rotation, 6

S Saline seeps, crop rotation, 13 Shattercane, control, 17 Silica, solubility, 62 Silicon aluminum bonding, 58 disease suppression, 102- 103 kaolinite activity, 53 water activity effect, 60 Sludge, application, phosphorus contamination of water, 97 Smectite crystalline swelling of calcium, 157- 158 packing, 165 spacings, 157-158 swelling, 150-151 Sodium-affected soil, threshold concentration, 170-171 Soil, see also specific types bulk density crop rotation, 25 soil impedance, 25 erosion crop residue effect, 93 crop rotation effects, 26-27 forage production, 93 legume effect, 92-93 nutrient management, 93-94 water quality, 92-94 interparticle distances, 123 interparticle forces, basics, 122-124 porosity, structural states, 143-146 quality, 2 aggregation, 24-25 bulk density, 25

191

soil erodibility, 26-27 soil structure, 22-23 water infiltration and retention, 26 sodic, threshold concentration, 169-176 structure defined, 122 organic matter content, 23 photosynthate, 23 swelling, 122-123 Soil aggregates hierarchical order within, 176- 177 mechanical strength, 168-169 stability, crop rotation effects, 24 Solid, dissolution of, 51-52 Solubility, surface area affect, 61 Sorghum urundinaceum, allelopathy, 22 Sorghum rotation, 13 effect on nitrogen retention, 95 length effect, 28 Soybean nitrogen leaching, 95 transpiration, 90 Soybean rotation, 11, 13, 17 as intercrop, 6 length effect, 28 in nematode control, 20, 21 nitrogen contribution, 15 nitrogen scavenging, 14- 15 phosphorus concentration, 16 strip intercropping, 3 I weed competitiveness, I8 yield, factors affecting, 20 Spatial diversity, crop rotation, 30-31 Stern layer, interparticle forces, 128-129, 131 Stomata, nutrient effects, 91 Straw ash, pesticide persistence, 108 Strip intercropping, 31 Structural component of disjoining pressure, defined, 138 Summer fallow, crop rotation, 14 Supersaturation, nucleation, 63 Switchgrass, bioenergy crop, 32

T Temporal diversity, crop rotation, 30-3 1 Terra rossa, iron oxide effect, 166-167 Threshold concentration application, 175-176 concentration for dispersion, 172 Capon equation, 170

INDEX

192 Threshold concentration (conrinued) irrigation, 17 I - 172 organic material protection, 174 physical basis, 172-175 sodium adsorption ratio relationship, 17 I sodium saturation, 171 soil permeability, 173 Tillage loss, crop rotation, 28-29 Tissue monitoring, nitrogen, 96 Tobacco rotation, 7 in nematode control, 20 Transpiration, crop residue effect, 93 Transpiration demand, see Water demand Tritium, interparticle forces, 128 Thmip crop rotation 2,

U Urea, rooting effects, 89 Urrbrae B aggregates, nitrogen adsorption isotherms, 163-164

V van der Waal's interaction energy, micawater-mica, 127 Vermiculite crystalline swelling, 147-151 particle interaction, 135, 136 pressure-distance relationship, 137, 147148

surface density of charge, 142 swelling, 144-145 van der Waal's energy, 127 X-ray spacings. 147

W Washington, George, crop rotation, 4 Water activity, aluminum-containing material dissolution and, 59-60 Water demand nutritional status effects, 90-92 Water infiltration, effects of crop rotation, 26 organic matter, 29 Water quality fungicides, 102- 105 herbicides, 100- 102 nitrogen contamination, 94-96

nutrient contamination effect, 94 pesticides, 100-102 phosphorus contamination, 97-99 potassium contamination, 100 soil erosion effect, 92-94 Water retention, crop rotation, 26 Water structural forces, clay, 138- 142 Water supply conservation, water use efficiency, 86-92 crop residue effect, 93 nutrient alteration, 88-90 root system manipulation, 88 Water use efficiency crop rotation, 13-15 defined, 86 evapotranspiration. 88-92 fertilizers, 86 nutrient effect, 89-90 rooting depth, 89 root-weed interaction, 100- 101 yield alteration, 87-88 Weeds allelochemical control, 21 crop competitiveness, 18 crop root competition, 101 crop rotation, 17-19 Wheat rotation, 7 allelopathic effects, 21 weed competitiveness, I8 Wildlife, effects of crop rotation, 31 Willalooka illite nitrogen adsorption isotherms, 163- 164 nitrogen sorption isotherm, 161 nitrogen surface area, 156-157 pore size, 162-163 swelling, electrolyte effect, 151, 153 water structural forces, 142

Y Yield copper deficiency effect, 88 crop rotation effect, 11-13 soil structure and, 23 disease effect, 19-20 dry matter production and harvest, 87 flower initiation, 87-88 molybdenum deficiency effect, 88 nitrogen effect, 88 nutrient effect, 87

INDEX potassium deficiency effect, 88 water use efficiency and, 87-88 weed effect, 17-19 Young, Arthur, 3 Young-Laplace equation, interparticle forces, 124

193 Z

Zinc toxicity, leaf morp~ology,91