Advances in Agronomy, Volume 13

ADVANCES IN

AGRONOMY VOLUME 13

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ADVANCES IN

AGRONOMY Prepared under the Auspices of the AWXUCANSOCZETY OF AGRONOMY

VOLUME 13

Edited by A. G. NORMAN The Uniuersity of Michigan, Ann Arbor, Michigan

ADVISORY BOARD D. G. ALDRICH,JR. C. 0.GARDNER E. G. HEYNE W. H. ALLAWAY J. E. DAWSON F. L. P A ~ ~ S O N R. W. PEARSON

AN ACADEMIC PRESS REPLICA REPRINT 1961

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CONTRIBUTORS TO VOLUME 13

RUSSELLS. ADAMS,JR., Graduate Assistant, Department of Agronomy, University of Illinois, Urbana, Illinois T. J. ARMY,Soil Scientist, Soil and Water Conservation Research Division, Agricultural Research Seruice, United States Department of Agriculture, Fort Collins, Colorado

K. P. BARLEY,Senior Lecturer, Agronomy Department, Waite Agricultural Research Institute, Adelaide, South Australia JOHNC. BROWN,Soil Scientist, Mineral Nutrition Laboratoy,Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland

ROSCOEELLIS,JR., Professor of Soils, Department of Agronomy, Kansas State University, Manhattan, Kansas P. F. Low, Professor of Soils, Department of Agronomy, Purdue University, Lafayette, Indiana

T. M. MCCALLA,Microbiologist, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, Lincoln, Nebraska F. H. W. MORLEY,Geneticist and Agronomist, Division of Plant Industry, Commonwealth Scientific and Industrial Research Organization, Canberra, A.C.T., Australia ALEX.Mum, Head, Soil Survey of England and Wales; and Head, Pedology Department, Rothamsted Experimental Station, Harpenden, Herts., England

W. F. ROCHOW,Plant Pathologist, Crops Research Division, Agricultural Research Service, United States Department of Agriculture, and Associate Professor of Plant Pathology, Cornell University, Ithaca, New York

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PREFACE It is the function of this series to review research progress in soil and crop science and developments in agronomic practice. The central theme, of course, is soil-plant relationships, but there may be excursions from time to time beyond the generally recognized margins of soil and crop science. Frontiers change as new fields are explored. The final test which the editors employ is whether the material reviewed is likely to be of interest and use to a group of reasonable size. Although science is international, there are aspects of agronomy that are regional. Some practices that are highly effective and economic in one area may not be suitable or acceptable elsewhere. From time to time reviewers have complained that it is primarily North American agronomy that is dealt with in these volumes, and although this may be so, the test of usefulness is the one that should be applied. In the following pages only one article, the comprehensive account of the practice of stubble-mulching, by McCalla and Army, is directly identified with North American agriculture. The practice, found effective in soil and water conservation in our midcontinent states, may well be capable of adaptation or modification, to areas with similar conditions in other parts of the world. Another article, which might be considered to be of a regional nature is that by Morley on subterranean clover in Australia, but the agronomic developments that have followed the widespread use of this legume are so dramatic as to transcend purely regional considerations. Australian workers have approached their peculiar soil and land-use problems with great effectiveness. The remaining chapters cannot be collectively described; all have been prepared by specialists who have become deeply involved in the topics which they treat. The subject matter diversity is so great that there should be something of interest to all.

A. G. NORMAN Ann Arbor, Michigan

July, 1961

Vii

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CONTENTS Page

........................................ PREFACE .......................................................... CONTRIBUTORS TO

VOLUhlE 13

V

vii

THE PODZOL AND PODZOLIC SOILS

BY ALEX. Mum I. I1. I11. IV . V. VI . VII . VIII. IX . X. XI . XI1.

Introduction ................................................ The Recognition of the Podzol as a Soil Formation . . . . . . . . . . . . . . . . Ortstein and Bleisand ........................................ The Recognition of an Illuvial Horizon .......................... Degree of Podzolization ...................................... The Sod-Forming Process of Soil Formation ...................... Geographical Variants ........................................ The Western Contribution .................................... What Is Podzolization? ....................................... Micromorphology ............................................ The Characteristics of the Russian Podzol and Podzolic Soils ....... Summary and Conclusions .................................... Addendum .................................................. References ..................................................

1 2 11 19 23 24 30 33 38 46 47 51 52 53

SUBTERRANEAN CLOVER

BY F. H. W. MORLEY I. I1. I11. IV. V. VI. VII. VIII . IX . X.

Introduction ................................................. Taxonomy .................................................. Physiological Variation ........................................ Nodulation and Nitrogen Fixation .............................. Ecology .................................................... Agrotechnology .............................................. Diseases and Pests ........................................... The Estrogen Problem ....................................... Agronomic Improvement ...................................... Potential Usage .............................................. References .................................................. ix

58 61 66 72

77 90 103 105 107 113 117

X

CONTENTS

STUBBLE MULCH FARMING BY T. M. MCCALLAAND T. J . ARMY I. I1. I11. IV. V VI . VII . VIII . IX. X XI . XI1 XI11. XIV. XV . XVI. XVII.

. . .

Introduction ................................................. Historical ................................................... Wind Erosion Control ........................................ Infiltration .................................................. Water Erosion Control ........................................ Moisture Conservation ........................................ Soil Temperature ............................................ Soil Structure and Other Physical Properties ..................... Chemical Effects ............................................ Microbial Activity ............................................ Tillage Equipment and Use ................................... Weed Problems .............................................. Insects and Plant Diseases ..................................... Crop Yields ................................................. Mulch Maintenarice .......................................... Use With Other Conservation Practices ......................... Summary and Conclusions .................................... References ..................................................

Page 126 129 132 135 138 144 150 154 159 165 170 175 177 179 187 191 191 194

CONTAMINATION OF SOILS BY PETROLEUM HYDROCARBONS BY ROSCOEELLIS. JR., I. I1. I11. IV. V

.

AND

RUSSELL S. ADAMS. JR .

Introduction ................................................. Effect on Microbial Activities .................................. Effect on Soil Properties ...................................... Effect on Plant Growth ...................................... Summary ................................................... References ..................................................

197 198 203 210 214 215

THE BARLEY YELLOW DWARF VIRUS DISEASE OF SMALL GRAINS BY W . F . ROCHOW I. I1. I11. IV V. VI . VII . VIII . IX.

.

Introduction ................................................ History and Distribution of the Disease ......................... Crops Affected and Importance of the Disease .................... Symptoms of the Disease ..................................... Aphid Vectors of the Virus .................................... Biological Nature of the Virus .................................. Factors That Affect Outbreaks of the Disease .................... Control of the Disease ........................................ Summary and Conclusions .................................... References ..................................................

217 218 220 226 228 231 235 238 244 245

CONTENTS

xi

THE ABUNDANCE OF EARTHWORMS IN AGRICULTURAL LAND AND THEIR POSSIBLE SIGNIFICANCE IN AGRICULTURE BY K . P. BARLEY I. I1. I11. IV. V.

Introduction ................................................ Earthworm Populations under Crops and Pastures . . . . . . . . . . . . . . . . The Influence of Earthworms on Soil Fertility .................... Effects of Earthworms on the Yield of Crops and Pastures . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References ..................................................

Page 249 250 256 262 266 266

PHYSICAL CHEMISTRY OF CLAY-WATER INTERACTION BY PHILIPF. Low

. . . . . . . . . .

I I1 I11 IV V VI VII VIII IX X

Introduction ................................................ Nature of Ice and Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nature of Ionic Solutions ...................................... Mechanisms of Clay-Water Interaction .......................... Specific Volume of Clay-Adsorbed Water ........................ Viscosity of Clay-Adsorbed Water .............................. Dielectric Properties of Clay-Adsorbed Water .................... Supercooling and Freezing of Clay-Adsorbed Water . . . . . . . . . . . . . . Thermodynamic Properties of Clay-Adsorbed Water . . . . . . . . . . . . . . A Working Hypothesis ....................................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

269 269 277 284 287 295 303 306 314 322 323

IRON CHLOROSIS IN PLANTS BY JOHNC . BROWN I. I1. I11. IV. V. .VI.

Introduction ................................................ Earlier Findings on Iron Chlorosis .............................. Factors Involved in Iron Chlorosis .............................. Chelating Agents as Iron Carriers .............................. Chelating and Other Mechanisms in Plants ...................... The Future of the Iron Chlorosis Problem ....................... References ..................................................

329 331 334 351 360 365 366

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

371

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

383

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THE PODZOL AND PODZOLIC SOILS

Alex. Muir Soil Survey of England and Wales, Rothamrted Experimental Station, Harpenden, Herts., England

Page I. Introduction ...................................... 11. The Recognition of the Podzol as a Soil Formation . . . . . . . . . . . . . . . . 111. Ortstein and Bleisand ........................................ IV. The Recognition of an Illuvial Horizon . . . . . . . . . . . . . . . . . . . . . . . . . . V. Degree of Podzolization ................................... VI. The Sod-Forming Process of Soil Formation . . . . . . . . . . . . . . . . . . VII. Geographical Variants ........................................ VIII. The Western Contribu .............. IX. What Is Podzolization? ....................................... X. Micromorphology . . , . ............................... XI. The Characteristics of t Podzol and Podzolic Soils . . . . . . . . XII. Summary and Conclusions ................................ References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 11 19

30 33 38 48

47 51 53

1. Introduction

When foreign words are adopted into a language there is quite commonly a change or an extension of meaning which is often signified in dictionaries by little danger signals as a guard against mistranslation. It is a pity that the earlier (and also some more recent) writers on soils did not use a similar system to warn their readers that the descriptions being used as illustrations did not necessarily conform to the original definitions. This is perhaps as true for podzols and podzolic soils as it is for any other soil group. Each successive generation of writers has tended to build on the results of immediate predecessors without considering the possibility of some subtle and even serious change in the meaning of the terms. When one is dependent on translations through a second foreign language the potential difficulties multiply considerably. Although the general history of the development of ideas on podzolic soils has been reviewed in the past, little attention has been given to the earliest work, particularly the Russian; and it is just this that provides some clues to the divergent opinions on this group that still exist. In fact it is not inappropriate to quote some remarks by Dokuchaiev from his “Cartography of Russian Soils”: “As the podzol is a formation so far I

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comparatively little studied, and therefore controversial, it is undoubtedly true that the term has different meanings in different parts of Russia, so that in discussing the problem of this soil it is probably best to consider it historically and, to begin with, to take account of exclusively factual data.” It is certainly not true now to say that the podzol has been little studied, but it is still a controversial topic. Glinka (1923) listed some 160 publications in which soils of the Russian podzolic zone were discussed, and this figure could no doubt be trebled or quadrupled by subsequent work. This review deals primarily with the Russian evidence as far as it is accessible to me, but touches on the Western European and American literature to bring out the divergences in usage and point to their origin. The translation of Russian words derived from podzol offers minor difficulties in the presentation of the review. I have consistently rendered podzolistii as “podzolic” and opodzolennii as “podzolized.” As I understand the latter word it means that the soils in question have undergone the process of podzolization but may not qualify for the term podzolic in the Russian sense. I have also used it for soils that would commonly be called podzols in Western Europe or North America. It. The Recognition of the Podzol as a Soil Formation

Since Dokuchaiev is commonly given priority for naming the podzol, it is perhaps as well to give in abbreviated form some extracts from his principal writings on the subject. It is convenient to begin with his ”Cartography of Russian Soils” ( 1879). “It is first of all evident that in various parts of Russia the name podzol is given to by no means identical or even similar formations.. , , [A certain number] are peaty deposits with more or less mineral matter; all have been more or less burned; almost all contain fresh-water shells; all are near rivers and some are covered with river deposits. Such deposits are found everywhere on the alluvial banks of our rivers . . . . Their place among other rocks and their period of formation has been elucidated and it is therefore superthous to retain for them the special name of

poazois. . . .

“The remaining examples of podzols are more or less uniform and correspond in general to the description given by Mr. Solov’ev.” “These are their characteristics. “They lie indifferently in lowlands, at the bottom of slopes, on slopes, or on the tops of hills. * Said by Dokuchaiev to have given the “first sufficiently detailed description of the external features of the podzol” in his “Agricultural Statistics of the Smolensk Government.”

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3

“They generally occur in forests, on the sites of former forests or near bogs into which they sometimes gradually merge. “The podzol occurs sporadically, here and there as small islands, and only rarely occupies large areas. “The podzol lies either on the very surface , , . when it is often used as arable land, or it is covered by a thin layer (several inches) of vegetable earth, generally of forest or bog origin when it plays the part of subsoil. “In either case its depth is not great: 6-12 inches, deepening only in places as it approaches bogs. “Podzols are underlain by clays, loams or sands: in all these cases the podzol appears to pass gradually into the underlying rock. “As regards the composition of podzols, all those investigated so far are of a sandy type and contain 8344% SiOz.. . . “Microscopic analysis shows that the examples examined so far consist of phytoliths, diatoms, soft vegetable residues, quartz grains, silty particles, etc. “External characteristics of the podzol are: an ashy colour, generally with yellowish, bluish or greenish tones; it consists of very small particles so that when dry it breaks down into dust, in the wet state it is slimy like clay and because of this the podzol readily holds water and is therefore sometimes the cause of the appearance of bogs. “Such are the general and most important characteristics of the podzol . . . .” [Dokuchaiev explains the origin of the term podzol as arising from the custom of “slash and burn,” a form of shifting cultivation, still practiced in his time. According to him the name was applied to the infertile layer of soil, gravel, sand or heavy clay, which underlay the ashes resulting from burning the forest cuttings. Vosmer (Russisches Etymologisches Worterbuch) states that the element pod is cognate with Ger. Boden, Gr. pedon, Lat. peda; thus podzol simply means ashy soil.] “There are 4 possible modes of formation of the podzol. “1. . , . that it is due to fires . , . . That cannot be agreed to . . . . “2. . . . due to the burning of p e a t , , , does not apply to podzols. “3. From microscopic evidence it might be considered a simple accumulation of vegetable residues, e.g., phytoliths, but this view . . . cannot be upheld. It does not explain ( a ) the exclusive occurrence of the podzol on the surface or under a thin layer of plant remains; nor ( b ) the striking constancy in thickness (6-8 inches); ( c ) it contradicts the gradual transition of podzols to parent material and finally ( d ) phytolith formations may arise by chemical means alone. “4. These considerations (excluding the last) and at the same time

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its position in depressions and on the slopes of hillocks, (e) its small thickness, ( f ) the strange discontinuity of the podzol in one and the same peasant’s field, and finally, ( g ) its content of organic, sometimes structured, material-all these taken together will not allow us to accept the conclusion of Krylov that our podzol can be regarded as a kind of chemical precipitate, as a kind of rock, related by its composition (but the clayey podzol?) and micro-structure to foreign siliceous plant bioliths . , . . “It seems to us that all the characteristics (from a to g) of our podzol compel the conclusion that we are here dealing not with rock . . but with soils . . . “It is evident that these sandy, and rarely loamy, podzols are soils of a mixed, vegetable-bog character; all their differences, in mode of formation, from the common northern dry-Zand vegetable loamy and sandy soils, consist in the fact that the latter, in general, are formed in drier places with free access of air and light and the predominance of a dryland herbaceous vegetation, while podzols are formed mainly in forests with a significant participation of bog and forest vegetation. Here, evidently, there was more moisture, less light and probably less access of air to the soil. That is why podzols occur predominantly in forests and near bogs; why they not infrequently pass gradually into these latter formations; why they are coloured with bluish, greenish (vivianite and ferrous iron) and yellowish tints; why one occasionally finds in them ore concretions and still undecomposed plant remains. “It stands to reason that in the formation of different varieties of podzol special conditions operate . . . . “As regards loamy podzols, to my mind, they are none other than the ordinary northern loamy soils, only changed not by dry-land grass vegetation but predominantly by forests and small bogs . . . .” These extracts show clearly enough that the word podzol was restricted to the ashy gray, often superficial mineral layer and had little reference to what was below. This restricted use of the word podzol continued for many years in Russia and is well exemplified by the following quotation from an account by Sibirtzev (1884) of the soils of the Arzamas district (Nizhni Novgorod) in which he describes soils, with the appearance of podzols, occurring in hollows and depressions of some 60-100 meters diameter, in the chernozem plateau. “A Greyish white, slight yellow tinge; when moist fairly compact; on drying becomes an extremely fine grained, loose, light mass, 1-1sft. or more in thickness. B Gradual transition to brownish subsoil, 3 in. C Yellowish brown or yellowish grey clay.”

.

.

THE PODZOL AND PODZOLIC SOILS

5

Locally the A horizon may appear on the surface, but it is often covered by “black e a r t h to several inches or even by peat, when it contains diatoms and phytoliths. “Therefore we feel that it is simplest to regard podzols, in particular the ‘white land’ of Arzamas district, as vegetable-bog soil, in the formation of which both the subsoil brown clay and its moist peaty, boggy cover took part. The brown clay underwent changes in the moist medium, without access of air, in the presence of carbonic acid and acid bog humus: it is not surprising, therefore, that it lost, by leaching, a considerable part of the soluble material, lost its uniform brown colour, became enriched in silica; the presence in the light grey mass, so formed, of diatom shells and phytoliths is in this way understandable; the yellowish or orange spots (in the A horizon) indicate the original parent material of the podzol . . .” (Sibirtzev, 1884). In all this early work the podzol was very carefully distinguished from the so-called “northern loams and sands,” which were considered as “dry-land vegetable soils” and formed the fifth group of the classification used for the Nizhni Novgorod survey. However, it was becoming recognized that there were soils in this “northern loam” category that had some characters of the podzol. A distinction was therefore introduced between the normal and the “podzolic” varieties, and Dokuchaiev quotes several descriptions in his final report on the Nizhni Novgorod survey ( 1886). “From the data of Sibirtzev and Ferkhmin the structure of the normal (northern) sandy loams in their natural state may be represented by the following schematic section: Horizon A: either ordinary sod or forest mat, s-1-2 in., consisting of leaves, roots, twigs, etc., and under it a grey sandy mass, cemented by clay particles; thickness . . . about 3 in. and not more than 5 in. Horizon B: greyish white, sometimes with yellowish brown and reddish veins and spots, fine sandy loamy mass, without nutty structure or only a slight indication of it, in wet weather it is fairly sticky, in dry it is powdery (thickness 2-3 and up to 7 in.). Horizon C: greyish brown, sticky when wet, consisting of sand cemented by reddish brown particles of clay; on rubbing, numerous colourless, reddish and yellowish quartz grains are exposed; in a word this is a sandy clay or sandy loam; imprints of roots fairly frequent.” The podxolic sandy loams have the following structure according to Amalitzky, one of Dokuchaiev’s colleagues (in Dokuchaiev, 1886) :

“A. Soil horizon of a greyish bluish colour, sometimes with a yellowish

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tint; the basic mass is very fine, floury to the feel, dust-like, contains sharp, easily visible quartz grains; in the wet state it is sticky and difficult to work. When dried, immediately following rain, it is hard and cracks, but after prolonged droughts it is powdery and dusty, average thickness about 5-7 in. B. Transition horizon: it is still lighter and unevenly coloured; contains residues of roots and clayey granules of a dark grey colour, occasionally porous; thickness 3-5 in. C . Distinctive light ashy podzolic sandy loam, uncoloured by humus, becoming yellower towards the bottom and then effervesces with acid. There are occasional hard granules of reddish brown colour that are only rubbed down with difficulty, also pores in which there are occasional root residues; towards the bottom it gradually passes into the underlying rock; thickness 0 to 2 ft. C1. Most frequently red-brown sandy loam, sometimes stony; in its upper horizons it has the nutty structure so characteristic of the forest loams; below it is the ordinary boulder clay , . . .” The reference to effervescence with acid is unusual and no explanation is given. On the basis of his description and a chemical analysis of the bleached horizon, Amalitzky concluded that the podzolic sandy loam soil just described is very close to the podzols of Smolensk, referred to by Dokuchaiev. During the Nizhni Novgorod survey a close connection was noted between the thickness (the A horizon of today’s nomenclature) of such podzolic sandy loams and steepness of slope provided there was no change in parent material; the depth of soil increased downslope and movement of material was suggested. ”. . . the material from which this soil [a sandy loam] was formed, because of its occurrence on present or former slopes, has undergone alteration by eluvial processes, mainly resulting in the loss of clay and lime and enrichment in sand . . .” (Dokuchaiev, 1886). “The material (podzol) sorted in this way, consisting of quartz and various silicates, poor in clay, will come under the influence of the different atmospheric agents and vegetation-( in this case forest) and undergo new and further weathering. Quartz will remain unaltered, the silicates will decompose forming amorphous silica which will be left; the clay would be carried by flowing water (slope)” (Amalitsky); the result would be an enrichment in silica. It was considered that amorphous silica formed the cement that made the podzol hard when it first dried out and also conferred its coldness in wet weather. That erosion occurred on these soils is mentioned by Dokuchaiev, who says it was sometimes so severe that it became necessary for peasants

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to abandon their fields to forest. Elsewhere he remarks that erosion on slopes has given rise to a “particular fine, floury, podzolic rock,” which increases in thickness from 4 to 6 inches at the top of the slope to 3 to 4 feet at the base. Georgievsky (1888), who has been cited by Glinka (1914, 1931), provides further interesting information on the podzol as it occurs in the former St. Petersburg and Novgorod governments. On the watershed between the rivers Syas and Tikhvinka (Novgorod government) the following types of soil occur: northern loam, sandy loam, sand, and podzol. “Referring in particular to podzols, we see that they nowhere occupy extensive areas, but are everywhere encountered only as bare spots, broken strips, etc.; in addition the following very characteristic feature of their distribution is observed. “Lying on level, relatively high situations or on slopes the formation we are considering nowhere occurs as a layer of constant, definite, thickness but always shows the well known discontinuity, even over the smaller areas: frequently over a distance of 1-2 arshins ( 2 8 5 6 in.) the podzol wedges out several times; sometimes over a whole peasants’ field it appears only at a few points, a search for it being necessary, the more so in that frequently it cannot be determined if there is podzol present or not at any given point either by the colour of the upper soil horizon ( A ) or by any noticeable change in relief. “It is characteristic that in those cases in which the podzol areas are bounded by adjacent swampy ground, the discontinuity of the podzol decreases in the direction of the swamp and its thickness increases. Thus the surroundings of bogs and various kinds of low places and depressions, even though barely noticeable, yet forming more or less wide swamps, are the most usual sites of the podzol in this area” (Georgievsky, 1888, PP. 8-91. Georgievsky divides his podzolic soils into three types according to their position in the relief. In the first type (Fig. 1, D) on the higher elements of relief, the “upper soil horizon A” of about 6 inches comes immediately under the sod (Russian dern) and passes gradually into the “podzolic horizon B” which may be anything up to 10 to 12 inches thick, but more commonly 3 to 4 inches. Frequently the latter horizon tongues and wedges into the subsoil “(horizon C)”. The second type occurs in depressions and moist spots. In this case the upper soil horizon ( A ) , 3 to 4 inches, takes on a whitish tone and in its properties approximates to the material of the true podzol horizon “B”; the latter is much finer grained and floury when very dry. Thirdly a profile from a bog is described, in which, under a layer of 12 inches or so of black bog earth “A,” the podzolic horizon “B” is of a bluish tint with reddish ochre veins

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and spots; it gradually passes into the subsoil “C.” The thickness of the podzolic horizon in this case may be 28 inches. In the examples given above the soils have generally been derived from what were called sandy loams (Russian a p e s ’ ) ,and it is surprising that no comment was made about any illuvial horizon. As far as can be seen from these early writings such a horizon had not been clearly recognized. Amalitzky (cf. p. 6 ) comes near it with his reference to a nutty structure in the “C1”horizon; and Georgievsky, after describing some sandy podzols with ortstein, says that “if the subsoil of the podzol is loamy . . . it passes into the country rock very gradually, separated from it by a yellowish loam; the latter occasionally appears as an independent horizon, replacing as it were the podzol.” With regard to the examples from near St. Petersburg referred to by Glinka (1914, pp. 68-69, 1931, pp. 341-342) and quoted by others (e.g., Robinson, 1949, p. 306), the descriptions are given rather differently by Georgievsky ( 1888, pp. 14-16). Near the river Luga, Georgievsky found on a sandy hillock white, fine, quartz sands underlain by loose, mobile reddish yellow or yellowish “diluvial” sand. “They [the white sands] always appear almost at the surface of the soil and are covered with only a thin layer of dark grey earthy sand, they are discontinuous and occasionally, in the form of long root-like salients, wedge into the subsoil; their thickness is 4 to 7 in., reaching 22 in. in the centre of depressions. In this case two features deserve particular attention: 1 ) on passing up-slope the white sands thin out and finally disappear completely; 2 ) under the white sand lies ortstein, occurring in all stages of development, but wedging out at the top of the slope . . . . The structure [of the ortstein] in a dry depression has the following appearance: “Above lies a dark grey sandy (A) soil (thickness 3 to 4 in.); below comes a white, not always uniformly coloured, sand in which white patches constantly alternate with darker; still deeper the dark colour intensifies and the sand is gradually replaced by ortstein; the upper horizon of the latter, in the form of a thin crust (in sections it appears as a black band which closely follows every undulation of the white sand), separates the overlying sand from the lower compact ortstein rock the colour of which is generally pale yellow (but nevertheless contains 0.65% humus), but there are also dark coloured patches of similarly compacted material. In addition it should be mentioned that the degree of cementation of the ortstein as it passes into the loose yellow subsoil sand always gradually decreases. “As to the depth of the ortstein, it is variable and apparently is not closely related to relief, being about 8 to 28 in., and reaching (in the

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centre of depressions) 34 in., but in the latter case the ortstein is only weakly cemented. It should also be noted that the surface of the ortstein horizon is extremely uneven . . . .’’ Another example is described from the edge of a bog where the podzol overlies, indifferently, either red boulder clay or sands. ‘When it overlies sand the following interesting section can often be seen.

“A1 Mossy cover-1-2 in. “A Dark-whitish soil, contains 1.7% organic m a t t e r 4 in. “B Sandy mass of a white colour, consisting of sandy fine earth with an admixture of a moderate number of coarse quartz grains; nevertheless hardens on drying; humus content-O.12%; thickness-12 in. “B1 Sandy loamy mass, stained by humus (up to 2.5% ) to a chocolate colour and gradually passing into the subsoil; on ignition first of all lightens in colour and then reddens; contains some concretions strongly cemented by iron oxide which confers the hardness on this horizon. Thickness 7-9 in. “C Yellowish sand with boulders.

“On the clayey subsoil, immediately at its boundary with the podzol, there was also observed a quite peculiar layer of red, hard ferruginous clay, concretions of which occurred in the lower parts of the podzolic horizon. The podzol in this case appeared as a compact mass which, on prolonged drying turned into absolute flour.” Georgievsky’s conclusions on the occurrence of podzols are similar to Dokuchaiev’s: “The influence of climate is shown by the geographical distribution of the podzol. Judging by the available literature this white earth . . . . in contrast to the chernozem, which is characteristic of the open steppe, is encountered (at least in its typical form), exclusively in the north and north west of Russia, i.e. in places with relatively high rainfall, and with an abundance of both forests and bogs. Such a geographical distribution is not fortuitous. It should not be forgotten that just such an abundance of moisture is one of the essential conditions for podzol formation as this soil is most frequently encountered in bogs, on the edges of bogs, in fields adjoining them-in general everywhere that water can stand for long” ( Georgievsky, 1888, pp. 35-36). It is quite obvious from these various descriptions that the idea of the podzol and its variants was confined exclusively to the bleached or A2 horizon of later nomenclature. This is clearly stated by Dokuchaiev

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in a “Short Programme for the Investigation of Soils” (1887-1891) when discussing soil structure: “podzolic, floury structure; the whole mass consists of the finest, dusty, fine earth, almost exclusively quartz, whitish, light yellowish or ashy grey colour; the given structure is especially clearly expressed in the B horizon of typical podzols.” This general attitude to podzolic soils continued up to the turn of the nineteenth century and is summarized by Sibirtzev (1900) in his book “Pochvovedenie.” After contrasting podzolic soils and chernozems and listing the factors influencing podzolization (climate, parent material, etc.) he goes on: “Thus the soils of this type fall naturally into subtypes :

I

Soddy soils, weakly (sometimes not visibly) affected by podzolforming processes. I1 Podzolic soils proper, with a podzolic horizon clearly separated and sharply distinct from the upper horizon. I11 Podxols or soils strongly podzolised (frequently to the surface).” The use of the term “sod-podzolic” is considered to be not altogether appropriate “as the majority of the soils are formed under mixed forest , . . ; but we retain the conventional term ‘soddy’ or ‘sod-podzolic type’ in order to indicate that the degree of podzolization is quite variable (Sibirtzev, 1896). The morphology of the ordinary podzolic soil is summarized thus:

. . .”

“A Upper soddy horizon, from 3 to 5 in. thick, usually structureless, coloured light grey, light cinnamon, or grey. “B Under this lies the podzolic horizon proper, whitish, sometimes almost white, sometimes yellowish and bluish; thickness variable, from 2 in. to a foot or more. “C The podzolic horizon gives way to the subsoil material, e.g. brown clay, brownish bouldery sandy loam, yellowish clayey sand, orange coloured loess-like rock and so on. Frequently the podzol penetrates into the subsoil in ‘pockets and spots,’ i.e. the change to the subsoil is gradual.”

“. . , The common new formation of podzolic soils-ortstein-occurs as segregations of varying size, grains and veins . . .” It is surprising that in none of the writings on soil morphology of the “fathers of pedology” do there appear any clear suggestions that the C horizon of their nomenclature might be the illuvial horizon and the recipient of at least some of the material lost from the bleached layer. Although there is frequent reference to ortstein in the early works, it is

.

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only in connection with the formation of this layer in sandy soils that the possible fate of the leached substances is considered at all fully. 111. Ortstein and Bleisand

Soils were often described by competent observers in the seventeenth and eighteenth centuries, but none seems to have noticed or recorded the hard humous or ferruginous layer that is so common in heath and moorland soils particularly. The earliest reference given by Glinka is to a paper by Kindler (1836), who observed the bleaching effect of decomposing roots on ferruginous sands. Following this there appears to have been quite a crop of observations in various countries. Thus, Daubrhe ( 1845 ) confirmed Kindler’s observations and accepted Berzelius’ suggestion of crenic acid as the active agent. Sprengel’s description of Bleisand corresponds roughly to that of the early Russian workers of their podzol, i.e., he is concerned only with the bleached layer: “When, under the fine quartz sand, there is so much carbonaceous, hardened, or very resinous, humus that it has a blue-grey colour, it is known in certain localities, e.g. Liineburg area, as Bleisand. This sand is so very infertile that in the Luneburg area it is reckoned the worst of all soils. It is formed both in heaths and in pine forests in which the carbonaceous, resinous humus is formed (Sprengel, 1837; 1844, p. 154). Here we have reference to the bleached layer only. Later in his discussion of ferruginous loams Sprengel refers to “an earthy layer that is very rich in ferric and ferrous oxides; the iron coming partly from the cultivation implements and partly collected from the soil” (Sprengel, 1844, p. 175). Although some analyses of soils and subsoils are given to illustrate their chemical composition, no attempt is made to link them with the soil descriptions. Following Sprengel, a British writer (Gray, 1839) gave a description of “moorband pan” but had no comment to make on a bleached layer. It is not until we come to Senft (1862) that we get a reasonable description of a German podzolic soil. Senft was primarily interested in the influence of humic matter on the formation of limonite deposits, and ortstein therefore figures prominently in his account. His description of a heath podzol is as follows (Senft, 1862, p. 181). “This Ortstein or Ortsand generally occurs in heath soil, especially on the Luneburg Heath. The plant cover consists either of Calluna vulgaris and other Ericaceae or of Nardus stricta, etc. and Vacciniaceae. Directly under the plant cover there generally occurs a 6 to 8-inch-thick layer of sand of leaden-grey to smoky-brown colour due to the heath humus, or a sand-rich layer of brown loam. This layer is followed by one of variable

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thickness, of white colour grading into yellowish or greyish white sand, frequently mixed with rounded boulders and small erratic blocks, often cemented with a little loamy material. Below this whitish sand at 2 to 5 ft. depth is a 10-inch to 4$-foot-thick layer of Ortstein distributed in nests or islets or elsewhere in a continuous formation. It is notable that this Ortstein, according to a written communication from Mr. Schultze, never occurs in fully forested areas of the investigated region, but appears some time after careless deforestation is followed by a period of noncultivation. Furthermore, Ortstein never occurs on the ridges of higher hills, but it does occur on slopes even as steep as 35”.” Elsewhere Senft mentions that many ortsteins consist of sand coated with a skin of ferruginous clay or clayey ochre. Of all the early writers on podzolic soils the greatest is undoubtedly P. E. Miiller, whose name frequently appears in reference lists but who appears to have been little read by soil scientists. His work on natural humus forms is a model of scientific observation, and he provides the most detailed account of a podzolic soil given by any nineteenth century writer. In the following paragraphs the word Torf is translated as raw humus, which seems appropriate to Muller’s definition of his use of the term. “The soil in a rather dense beech forest with a raw-humus layer is only scantily covered with the vegetation mentioned above [i.e., Aira flexuosa, Trientalis europea, and mosses]; it carries a poor vegetation; the surface is covered with small branches, twigs and some fallen leaves between the moss and the few and insignificant phanerogamic plants. The loose leaf-layer always covering the mull is conspicuous by its absence from the raw humus soil. The soil is compact and yields underfoot no more than a thick felt layer over a solid basis. The surface is so compact that rainwater will sometimes collect in puddles on the loose sandy soil where the soil is covered with a raw humus layer; if this layer becomes wetted by continued rainfall, it becomes saturated like a sponge, while the underlying soil remains dry. “When the soil is dug, there first appears a tough blackish brown humus layer, the raw humus. Underneath more or less sharply differentiated from the raw humus is a generally loose sand lacking the ochreous colour common in transported formations. Its colour varies between greyish white and grey or dark grey and usually becomes lighter with depth. Beneath is a dark layer of reddish brown or brown colour, which is underlain by sandy loam, sand or an intermediate form of these two. “These layers [Fig. 1, A and B] are of very different thickness and also show other differences. The greyish sand layer, designated Bleisand, under a thin layer of raw humus may occur either as a very fine band

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FIG. 1. Schematic representations of podzolic soils. A-C after Miiller (1887); D after Georgievsky (1888); E after Afanasiev (1930).

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hardly one inch thick with indistinct boundaries, or as a sharply delimited, four-inch thick sandy layer of almost pure white colour, such as is often encountered in forest land adjacent to the North Sea; layers of six inches thickness are less frequent, though in forests on the transported sands in Jutland may attain a thickness of a couple of feet. On a flat terrain, the thickness of the peat and Bleisand layers often show some correlation in so far as the thicker layers of raw humus occur over thicker layers of sand. On more varied relief there are many exceptions to this trend where, in depressions, very thick layers of Bleisand may occur under thinner raw humus layers. The sand is generally very loose, but at the boundary with the underlying layer it sometimes becomes more compact and even assumes the appearance of sandstone in some places [Fig. 1, B]. “The brownish layer underlying the Bleisand, designated as red earth or Ortstein, varies both in thickness and consistency. Ortstein and Bleisand are generally the same thickness when the latter occurs as thin bands hardly one-inch thick, but under thicker layers of the sand the Ortstein may be four to six inches or even eighteen inches thick. The total thickness of the three layers can therefore vary between about four inches and two and a half feet. Thin layers of Ortstein are always loose and earthy, and even deposits of four to six inches often have a similar consistency. But thicker layers of Ortstein are generally compact and develop either into a sandstone-like formation or into the true Ortstein, well known on heathland, such as in the Silkeborg Forests in Jutland.. . . “The subsoil, like that under mull, is thus often compact where the Bleisand and red earth layers are thicker; otherwise the subsoil is very variable and may consist of transitional forms between poor sands low in clay, occurring in bouldery formations, and plastic and micaceous loam characteristic of lignite formations. When the subsoil is sandy or gravelly, the two overlying layers differ little except in thickness; on the other hand, where the subsoil is loamy, three different forms of these layers may be distinguished. “The first form is exemplified by the profile at Store Hareskov (Seeland) [Fig. 1, C]. In a depression and under a not particularly firm raw humus layer 21/, in. thick there was a loam layer 5 in. thick having a uniform very light colour below which was a similar thickness of a uniformly reddish brown loam layer resting on a subsoil of irregularlycoloured ochreous yellow sandy loam. “The second form is shown in a profile occurring at the Teglstrupper Gehege ( Seeland). The profile is in a recently dug ditch in loam on an elevation, falling on three sides by 6-8 feet at distances of about 100-200 feet, Under a 4-inch thick extremely tough layer of raw humus there is a

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6 or 7 inch thick Bleisand layer of whitish grey colour, underlain by a 16-inch-thick sandstone-like layer of light greyish brown colour with dark brown veins and streaks; a vein of the overlying Bleisand ran downwards through this layer. The subsoil consists, as at the previously described place, of ordinary sandy loam with gravel, sand and glacial boulders. “A third form is shown in the profile of the soil of a high slightly inclined plateau in Laven-Skow, near Silkeborg (Jutland). The very tough raw humus layer of more than 4-inch is underlain by 3 to 4-inches of fine, whitish, micaceous sand, the lowest part being a darker colour and compacted into a very hard but thinner layer. Immediately below this layer is a slightly plastic micaceous clay without any glacial boulders, which undoubtedly belongs to a lignite formation. Though the overlying Bleisand covers the clay in the form of a dense mass without any cracks or fissures, the uppermost part of the clay is full of cracks which divide the soil at many points into larger or smaller clods. The cracks are filled with a blackish brown powdered material which also covers the clods, so that a clean section of the soil appears to be marked with brown veins. If the brown powder covering the clay is scraped off, the surface of the latter is seen to have a greyish colour and a Bleisand appearance and contains less clay; however, at a greater distance from the brown surface (up to 1 inch) the mass is plastic and ochreous-yellow coloured.” After an account of the microscopic appearance of the beech raw humus, Miiller continues: “Mechanical analyses show that the three layers below the raw humus, like those under mull, decrease in stone content with increasing depth, but the clay content may either increase or decrease with depth. In this respect it is particularly noteworthy, that the clay in all profiles in elevated positions increased very markedly with depth, while in the only profile in a very low position the highest amount of clay occurred immediately below the raw humus. It will be seen later that this is due to the effect of water in translocating clay from higher to lower layers of the soil or to lower-lying parts of the relief. In order to ascertain that the process does not take place in the reverse direction and that the sand is not moved by water, as some practical men believe whenever they see a whitish band of sand near the soil surface, a profile about 300 yards long was examined at Store Hareskov. Here the Bleisand was of variable thickness throughout the entire line, but careful levelling showed that it behaved similarly on hilltops, small plateaux and in depressions, showing no removal whatsoever of sand by water. “Further examination of the peculiar structure of Bleisand and Ortstein shows that the former is composed mainly of pure mineral frag-

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ments, especially quartz, intermixed with the other components of granite. The humus-like particles, which confer a dark colour on the layer of Bleisand directly underlying the raw humus, as well as containing fine root fibres, consists mainly of small black humus-particles embedded between the grains of sand. Ortstein possesses a quite different constitution: each grain, each solid particle, is surrounded by an apparently structureless substance; each is encrusted with the brown material which confers its colour on the layer, making the entire mass so similar to the topsoil below mull that the uniform colour prevented any differentiation between the structure of Ortstein and of this layer (i.e,, topsoil) when examined under the microscope. A similar incrustation, though less complete and of another composition, occurs in the loamy subsoil.” In a general discussion of forest soils with mull and raw humus, Miiller points out (1887, p. 72) that the whitish gray layer that can occur under mull is not to be regarded as Bleisand for it by no means lacks the encrusting substances characteristic of mull. In other words, we appear to have the first reference to a soil akin to the Parabraunerde or sol lesSiOh.

The heath soil with its ortstein or moorband pan, which provided problems both for the forester and the agriculturalist, gave rise to an extensive literature in Germany and Scandinavia during the later nineteenth century and the early part of the twentieth century. Much of the work was largely repetitive and few adequate soil descriptions were given. Of the various writers, Emeis (1876) and Muller were the most productive and both gave good descriptions with diagrams of heath podzols to which modern horizon terminology can be readily applied. The morphology of the “typical heath soil” is summarized by Muller (1887, pp. 139-140) as follows: “The soil is covered with a layer of compact and tough raw humus, consisting mainly of organic components and residues of heathland plants, densely interwoven with heather roots, fungal mycelia, occasional moss thallus, and roots of other plants. The lowest parts of the heath layer usually contain mineral components, mainly sand, from the subsoil, in amounts increasing with depth, and it assumes the character of a sandy soil containing plant roots strongly intermixed with plant residues, rather than resembling the true raw humus formation on the original soil. Drying, however, also hardens this part of the surface, often making it rock-hard and not sharply distinguishable from the raw humus on the top. The heath layer is underlain by Bleisand in a white layer mixed with humus particles and showing tints of snow white to a greyish black mull-like colour according to the amount of admixture. The boundary between the Bleisand and the heath

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layer is sharper in proportion as the above-mentioned lower layer of the raw humus is less developed; sometimes this boundary is quite obliterated, but often it is very marked. The layer below the Bleisand is coloured by humic acid and its compounds and is called Ortstein or ‘red earth.’ The mineral constituents of the soil in this layer are covered with material of a humic nature. The layer is usually very thick, permitting only a very slow penetration of water. The Ortstein layer is never sharply delineated at its lower edge; transition to the ochreous sand of the subsoil is indistinct and usually very irregular, so that the dark to blackish brown layer penetrates the subsoil in tongues. The rich variety in the forms of this transition is mainly due to differences in the movement of water. Emeis published very fine drawings of heath profiles, showing all these characteristics. The boundary between the Ortstein and Bleisand is often very sharp, but also frequently indistinct due to accumulation of humic matter in the lowest layers of the sand.” In view of the importance of ortstein Muller gives a detailed account of its nature and origin which is the fullest to be found in the nineteenth century literature. His recapitulation of its nature and occurrence is very complete and does not appear to have been superseded (Muller, 1887, __ pp. 222-224) : “Nature A. Ortstein formed by deposition. 1. Clay Ortstein. A more or less porous, solid and hard mixture of sand and clay of a rather uniform greyish colour. Does not change its consistency on treatment with alkali or dilute mineral acids. 2. Peat-like (Torfige) Ortstein. Dense, earthy to hard, blackish brown to black or bluish black accumulation of carbonized humus containing humic acids and their salts, and more or less mixed with white sand-grains lacking ochreous films. Acid because of the humic-acid content. On treatment with sodium hydroxide easily decomposes into white sand and amorphous humose mud in a black solution. Disintegrates when exposed to air. (Many fine heather roots often occur. ) B. Ortstein formed by absorption. 3. Humus Ortstein. Dense, earthy to hard, yellowish brown to blackish

Occurrence

On mull-covered sandy loam soils it forms continuous layers 8-200 cm. from the surface; it remains even after the soil surface has become covered with raw humus and has consequently undergone changes. Below ,the leached surface of coarse sandy soils on heathland and in forests, best developed and forming continuous layers in fresh to moist or wet soil: immediately overlies the true humus Ortstein from which it is not clearly differen,tiated.

Always occurs in continuous layers, more or less following the relief of the surface,

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ALEX. MUlR

Nature brown sandstone, consisting of the skeletal components of the soil (especially quartz sand) which are covered with, and cemented together by, humic acids and their salts. Easily decomposes on treatment with sodium hydroxide into sand and fine soil which contains some amorphous humose mud in a black solution. Is only slightly affected by dilute mineral acids, and disintegrates when exposed to air. a. Ferruginous humus Ortstein containing larger amounts of iron than the subsoil below the layer. b. Iron-deficient humus Ortstein (red earth) containing smaller amounts of iron than the subsoil below the layer. C. Ortstein formed by concretion. 4. Iron-sandstone. Dense, hard sand-

stone of yellowish brown colour, consisting of the skeletal components of soil (especially quartz sand) coated and cemented with iron hydroxide (according to Senft intermixed with some lower iron oxide and other substances). Does not disintegrate when exposed to air, or treated with alkali solution, but decomposes on treatment or boiling with dilute hydrochloric acid.

5. Bog iron (Raseneisenstein) . Porous, slag-like ore of a blackish colour and containing 80-95% iron hydroxide (Senft). Behaves like 4 towards air, alkali and acid.

Occurrence

150 cm. below the uppermost leached layer of the top soil (Bleisand) in forests and heathland with raw humus.

In layers, in the uppermost part of the subsoil in sandy soils very poor in clay. In layers, in the topsoil, at variable depths from the surface, of sands richer in clay or of loamy sands and sandy loams. Occurs in the uppermost layer of raw humus covered sandy soil in the form of aggregates of different sizes and shapes. Probably f;orms larger continuous masses where humus Ortstein occurs in moist heath soils.

Slag-like masses occurring in moors and bogs as well as in clay Ortstein occurring near the humose surface and overlying an impermeable subsoil; also in many other partidly or temporarily anaerobic places in the uppermcost layers of soil affected by humose material especially in sands.”

A little earlier than Miiller, Burkhardt (1870) and Wessely (1873) put forward interesting points about ortstein. Burkhardt stated that it was formed only in sandy soils and did not occur even in sandy loams. Wessely asserted that ortstein was always associated with a heathy vegetation, so that heath humus was a condition of its formation and the thicker the ortstein the harder it was cemented. This idea was taken

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up by Pavlinov (1887) in Russia, who suggested that the phenomenon might be termed “callunism” and the ferruginous deposit (ortsand and ortstein) “callunite.” This paper of Pavlinov’s is also interesting in that, apart from a footnote reference, the word podzol nowhere appears and no attempt was made to connect the soil studied with the podzolic type described by Dokuchaiev and others. Following the German writers, he argues for a movement of clay from the surface to the subsoil, but the “clay fraction” (<0.01 mm.) in the ortstein layer is less than the total sesquioxides extracted by strong acids and his argument is not convincing. Many papers continued to appear dealing with heath and other podzolic soils, but the interest in the Western countries was so concentrated on ortstein that no real progress was made in characterizing the group. This is well seen in Sibirtzev’s treatment already referred to: even when considering the fate of the substances lost from the surface horizons of podzolic soils no mention is made of the possibility that they might be deposited in the “C horizon.” In fact, in referring to ortstein as the common accompaniment of podzolized soils, he states that it “lies in the podzolic horizon, usually in its lower part or directly under it. Small pellets of ortstein sometimes appear in the upper horizon of podzolicloamy or light loamy soils.” IV. The Recognition of an llluvial Horizon

The idea of downward translocation of substances from the upper layers of Bleisandboden to the lower was clearly enough brought out by the earliest West European writers on ortstein and was recognized to be highly probable for this formation by the Russian workers. In general, the transference was thought of as occurring in solution. It has already been mentioned that Pavlinov (1887) suggested the movement of clay particles, and some of Muller’s data would support this argument, but as field estimates of texture were probably somewhat rough and mechanical analyses were rarely extended below 20 p, any textural differences were not recognized. The more general concept of illuviation in the pedological sense was put forward by Vysotzky (1899), who defined illuvium as the material washed out of the surface soil and deposited below to form a definite horizon of accumulation (i.e., the illuvial horizon). He used eluvium, however, in the sense of lateral removal of dissolved substances. It was some years before a clear concept of eluviation and illuviation was established. The problem, from the point of view of morphology, was most acutely felt in dealing with podzolic soils that showed bleaching in patches

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(either an incipient bleached layer or the beginnings of degradation in the illuvial horizon). According to Zakharov (1906) it was common to find writers who would relegate to the subsoil the patchy bleached layer of a weakly or soddy podzolic soil. He strongly advocated its recognition as part of the podzolic horizon and proposed that the following letters should symbolize the various horizons: A, upper humous horizon; B, transitional or podzolic-eluvial horizon; C, illuvial (ortstein ); D, parent material. This particular usage of symbols continued among many Russian workers until about 1930, when the form earlier proposed by GlinkaA, eluvial; B, illuvial; C, parent material-with the addition of numeral subscripts for subdivisions became the standard. Once this solution of the division of the podzolic soil profile was accepted, profile descriptions became rationalized and at the same time very much improved. It was recognized that ortstein as defined by Senft and later writers was mainly characteristic of sandy soils whereas in podzolic soils of finer texture rounded concretions in the Az and B horizons were the commoner forms of iron segregation and deposition. Tumin (1912) pointed out that ortstein is confined to the A-B boundary, while concretions were disseminated in all horizons. For a podzol from the Smolensk district he gave the following figures for concretion content: Al, 1.15%; Az, 0.75%; B, 0.29%; C, 0.10%. He found that there tended to be an increase in the number of concretions near depressions, the maximum occurring in the upper 10-15 cm. of the B horizon, but in the soils of depressions no concretions occurred, owing to strong reducing processes. The translocation of clay from the surface to the subsoil had been suggested but inadequately established by early writers. The active agent in podzolization was commonly thought to be crenic acid. However, Glinka (1924, 1931) put forward a strong case for his belief that “in general the podzolic process is basically none other than the leaching from the upper horizons of mobile humus sols of low calcium saturation and, under their protective influence, of fine mineral suspensions, and the deposition of these suspensions in the B horizon together with a small amount of humus.” He rejected the theory that crenic acid was the active decomposing agent in podzolization, but in doing so went still further and assumed that no decomposition of the clay fraction took place in soils of the podzolic type (Glinka, 1931, p. 343), because chemical analyses had shown that there was little “colloidal silica” in the bleached horizon (e.g., Georgievsky, 1888; Gedroiz, 1926) and little free alumina in the B horizon (Gemmerling, 1922). In his discussion he incidentally mentions that the “silica powdering” of degraded loams is

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nothing more than tiny quartz grains freed from their coatings of soil colloids. This was later confirmed by Rod6 and Feofarova (1955), who also showed that feldspars and small clay aggregates were important constituents of the powder. Good descriptions of the clay coatings on the peds of the B horizon were given by Rod6 (1930) and Gemmerling (1930), who provided the first analyses of the ”crusts,” as he termed them (see below). Rod6’s description of a strongly podzolized soil on varve clay from Lisino Forest, near Leningrad, under a pine-spruce stand (typical Pinetum oxalidostcm) is as follows: “A0

0-1 cm.

Bedding of forest floor, loose, little decomposed. Immediately below lies the principal mass of roots.

‘‘A1

1-9 cm.

Grey, rather pale, medium loam. Above with irregular clods 5-7 mm. in diameter; below the clods increase in size, become angular and acquire a certain schistosity in their disposition. When dry, clods are scarcely perceptible; fragments of soil do not exhibit any apparent tenacity. Rapidly merges into subjacent horizon, but the boundary is indistinct, being diffuse and broken on account of wedges of humus.

“A2

9-26cm.

Whitish with a greyish yellow tint. Medium loam, smears much. In dry condition falls into small and unstable pieces, manifesting an evident tendency toward lamination, well exhibited when moist. Contains an appreciable number of dark brown small ore grains (about 1 mm. in section), on the walls of the section usually appearing in the form of commas (smudged by the spade). Rapidly passes into the subjacent horizon, without, however, showing any distinct boundary line, and in penetrating into that horizon to the depth of 8-10 cm. forms tortuous tongue-like projections and streaks.

“B1 26-35cm.

Pale pinkish yellow heavy loam. When dry, compact and tenacious, of indistinctly expressed usual polyhedral structure with aggregates gradually increasing in size with depth. The aggregates are separated from one another by very fine layers of clay particles of a somewhat paler tint than the aggregates themselves, showing as a kind of “polish” on the surfaces of the latter. In the upper parts contains in some quantity

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small ore grains. Gradually passes into the following horizon. ”B2 35-63cm.

Yellowish brown (when fresh darkens to crimsonbrown). In other respects similar to the preceeding. Aggregates are larger. Gradually passes into the subjacent layer.

“C 63-115cm.

Varve clay consisting of summer bands, loose, silty, much staining, of a light yellowish grey colour, 6 7 cm. thick, and of winter bands 1-1.5 cm. thick, dark, crimson-brown with distinct fine laminar structure, clayey, very tenacious, apparently consisting of very fine particles.

“The soil is a strongly podzolized soil on varve clay with all the typical podzol features. The humus horizon Al is not very thick, light coloured, although containing above 4 p.c. of humus. The podzol horizon A2 is fairly thick and has a characteristically friable structure. The presence of ore grains should be noted. In horizon B we meet with the polyhedral structure . . , consisting of angular aggregates. It may be mentioned that the faces of the aggregates are paler, but-a very exceptional case within the forest-they do not exhibit any traces of reduction processes; this is evidently due to the fairly satisfactory aeration of the soil” ( RodB, 1930). Gemmerling’s description of a loamy podzol is from near Moscow under young deciduous forest. “A/O-turf. Depth 6 cm. “A/l-no sub-horizon-if a very narrow greyish stripe, merging with the turf is not considered. “A/2,-thick, whitish, mealy structure. In the upper part a lamellar unstable structure is preserved. Iron concretions occasionally are found (very few), the small ones readily disintegrate, the larger grains are very solid. Penetrates pocket-like into horizon B. Depth 6-23 cm. Pockets up to 33 cm. “B/1-light brown, nutty structure, the nuts throughout are podzolized, dense, the edges of the nuts are covered with a brown crust. Depth 33-45 cm. “B/Sbrown colour, breaks into prismatic units, the edges of which are covered with a reddish crust, dense. Depth 45-90 cm. ”B/&yellowish brown colour, breaks into large prismatic units, the edges of which are covered with a reddish blue-grey crust. In the

23

THE PODZOL AND PODZOLIC SOILS

upper part of the sub-horizon are rather many black spots of manganese, very dense. Depth 90-169 cm. “B/&sandy lentil of yellowish colour. Its depth is over 40 cm.” The characteristic features of these soils are the tendency to a platy structure; the frequent concretions in the A1-AP horizons; the tonguing of the A2 into the B and the nutty or blocky structure and clay skins of the B horizon. The tonguing of the A2 is illustrated in Fig. I, E, which has been drawn after a picture in Afanasiev (1930). The concretions in the A2 are also indicated. The mechanical analyses given by these two workers show strong apparent clay shift, even allowing for small variations in the texture of the drift parent materials. Data given by Gemmerling for the composition of the “crusts” are recorded in Table I. TABLE I Composition of Clay Skins and Peds in Loamy Podzola B3

B2

Component SiO, Fe203

A1203

MgO

+

p205

Si02/A1203(approx. ) a

Clay skin

Ped

Clay skin

Ped

(%) 55.58 7.17 24.27

(%)

(%I

(%I

73.45 5.30 13.46 1.70

56.28 6.88 22.29 2.04

75.48 5.18 13.03 1.33

-

3.8

-

4.5

-

Data from Gemmerling (1930).

Gemmerling states that the thickness of the ”crusts” or clay skins is 0.5 to 2 mm. and “they are much better developed along the vertical than the horizontal sides of the structural units. These crusts are formed from the products, which are washed out from horizon A. In view of the greater density of horizon B, the soil solutions penetrate into this horizon chiefly along the fissures between the structural units; the fine suspensions . . . are gradually deposited on the sides of the units.” While the data do not prove that the material came from above, its composition is obviously close to that of soil clay. Such analyses of more carefully separated clay from clay skins was later reported by Yarilova and Parfenova (1957). Their results were very close to those of Gemmerling, the Si02:A1203 values for podzolic soils being between 3 and 3.5. V. Degree of Podzolization

As mentioned above, Sibirtzev (1900) recognized three main subtypes within the sod-podzolic type: soddy soils, podzolic soils, and pod201s (or strongly podzolic soils). The first attempt to define more closely

24

ALEX. MUIR

the various degrees of podzolization was made by Zakharov (1910,1911). Starting with the relation of soils to relief in which the sod-soils occur on upper parts of slopes and the strongly podzolic toward the foot, Zakharov transposed the concept to a time scale. The position is then: (1)in soddy soils (i.e., the youngest) there is a slight development of an accumulation horizon shown as a graying and browning of the upper layer of the parent material. ( 2 ) As organic matter accumulates the conditions become suitable for the appearance of podzolization, which shows as whitish spots and patches that merge to form a layer-the eluvial horizon; at the same time the brownish ortstein horizon forms. (3) As the podzolic horizon becomes sufficiently developed and thickens, there appears a subdivision of the ortstein horizon into an upper layer, darker red brown when wet, and a lower layer lighter in color. The upper darker part Zakharov considered to be due to humus leached down after the loss of iron from the eluvial horizon. This subdivision for fine-textured materials may be tabulated (Zakharov, 1931) thus: Subtypes Soddy (or cryptopodzolic) Weakly podzolic Podzolic Strongly podzolic Podmls

Degree of development of podzolization Not noticeable As spots Whole horizon (i-e., A, horizon) Whole horizon occupies half the total A horizon Occuuies almost all the A horizon

Such definitions were highly desirable, for the word podzol was loosely used from the start and even long after Zakharov’s proposal. For instance, Glinka (1923, p. 34) describes a “clay podzol” in which the Al horizon was 13-15 cm. thick as against 8-13 cm. for the Aa horizon. The above subdivision, which is applicable to all classes of podzolic soils, is still in general use with minor modifications. Thus: slightly podzolic, Al > 2A2; medium podzolic, Al rr A,; strongly podzolic, A, < % A,. VI. The Sod-Forming Process of Soil Formation

As has been mentioned above, a group of so-called soddy soils was recognized in which little or no podzolization had occurred, Sibirtzev (1896) remarked that “the term sod-podxolic is not altogether appropriate, as the majority of such soils are formed under mixed forest,” but he used the term sod (Russian dernina) and its adjective in the general sense of turf. “In the northern loamy [i.e., podzolic] soils, where moisture is near, the roots and rhizomes of grasses form a shallow, but dense turf horizon” which may contain some 3% organic matter as against about

THE PODZOL AND PODZOLIC SOILS

25

0.5% in the succeeding whitish horizon. However, the influence of the ground vegetation on the amount of organic matter was considered only in relation to forest versus steppe, and variation within the zone of podzolic soils does not seem to have received much attention. The first worker really to emphasize the importance of woody versus grass-herb vegetation seems to have been V. R. Williams. His full treatment appeared in his textbook on pedology, and his ideas were well disseminated by his colleagues at the Timiriazev Institute, Moscow. From the pedological viewpoint, Williams’ theory is so overlain by irrelevant matter and supported by such fragmentary data that, not surprisingly, it found little favor. The botanical aspects received a slashing attack from V. N. Sukachev (1916), a distinguished ecologist, who concluded that the theory lacked any factual basis. Neustruev (1927) remarked that the description of the processes “is dogmatically stated in such a way as to impede critical judgement” although he admits that some instances of the “soddy” process may be observed. In fact, the Dokuchaiev school more or less rejected the theory and little more than formal mention of it is to be found in textbooks of the 1930’s. Reduced to its simplest form the theory states that on opening up a closed-canopy forest with podzolic soils, a grass-herb ground cover may, in course of time, become so dominant that the woody vegetation cannot regenerate. The grass phase will also occur in cutover woodland used for hay or for grazing. As the herbaceous litter is much richer in bases and undergoes (partly for physical reasons) a different type of decomposition, there is an increase in humus content and base status of the soil. Similar views, without reference to Williams, were expressed by Vysotzky (1927), who also recommended practical measures for the reclamation of forest lands to agriculture. Williams (1940, p. 118) gives the following generalized description of a soddy-podzolic soil, this stage being best developed when “loosetufted grasses” ( e.g., Dactylis glomernta, Poa nemoralis, P . sterilis), together with legumes, reach their heyday in the succession. Four horizons are clearly differentiated: “1. Soddy horizon, coloured brownish black from the top by the presence of humin. With depth the colour changes to brown as ulmin predominates below. In its upper part the horizon consists of loosely held stable clods, all interstices between which are filled with living and dead roots and rhizomes, binding the clods into a sod. “With increasing depth the amount of organic residues and amorphous humus gradually decreases; similar changes take place in the degree of clodiness, stability of the clods and colour of the soil, until the upper horizon merges into the second, podzolic horizon.

26

ALEX. MUJR

“2. Podzolic horizon of grey colour, with a much smaller content of organic residues. Cloddiness in the podzolic horizon gradually changes to an aggregate state. Thanks to this the plasticity of the podzol is strongly decreased and the whole horizon assumes a loose loess-like character. “The podzolic horizon gradually passes into the ortstein horizon. “3. The ortstein horizon may have a great variety of tints of yellow, red, brown, black and grey colours. This horizon may or may not effervesce on moistening with acid. Its structure may likewise be most diverse. In the ortstein horizon there is frequently an abundance of roots, alive and dead. “4. Below this, again with a gradual transition comes the gley horizon of grey colour . . . [in it] there are living and dead roots of legumes only and dead roots of trees. “In its lower part the gley horizon begins to break up into angular nutty fragments which are frequently gleyed on the surface but internally retain the colour of the parent material. The surface of the aggregates is covered with a silica dust and coloured with a brown film of ulmin. [With increasing depth the gleying decreases and the transition to the parent material takes place.]” Although Williams had a low regard for soil morphology (“a children’s disease” he called it), his continued emphasis on the sod-forming process had resulted in the recognition by Russian morphologists of a group of soils that had previously been regarded as ordinary podzolic soils, i.e., those now known broadly as sod-podzolic soils. The &st attempt to show that a grassy vegetation following forest would produce at least some of the effects postulated by Williams was that of I. V. Tiurin (1935). In a comparison of two profiles from the Lisino Forest, near Leningrad, one under an old pine-spruce stand, the other in an adjoining old meadow (at least 90 years old) with scattered trees, Tiurin showed that there was a marked increase in the thickness and humus content of the Al horizon under grass and of pH and exchangeable bases in both the Al and Az horizons. Earthworms were extremely active in the meadow soil. No data were given on the over-all changes in the total chemical composition of the soil or its clay fraction. There is thus no evidence of the establishment of an independent soil type; we simply have forest and meadow phases of the same podzolic soil. The descriptions, however are worth quoting:

THE PODZOL AND PODZOLIC SOILS

27

Profile I, under pine-spruce forest (approximately 90 years) with bilberry, cowberry, grasses, and mosses. “Aol Ao”

0 . 5 cm. 0.53 cm.

AoAl

3-5cm.

A1

5-10 cm.

A1/A2 10-16 cm. A2

16-25cm.

B1

25-40cm.

B2

40-80cm.

Undecomposed litter. Brown, partly decomposed litter with fungal mycelia. Transition layer from the litter to A1; grey with a predominance of mineral matter. Light grey with slight yellowish tint, small cloddy, abundant tree roots, loose; gradual transition to Light grey with more definite yellowish tint, more compact than A1. Fewer roots. Whitish podzolic horizon, very compact, small ore grains. Chocolate-brown, very compact, coarse cloddy structure; gradual transition to More uniform, brown, in the lower part some lighter patches; very compact; clayey. At 80 cm. large boulders.”

Profile 2, under meadow adjoining the forest, 150 yards from Profile 1. Grasses include Deschampsia caespitosa, D . flexuosa, Agrostis sp.; herbs and mosses are present. “A0

All

A12

A2

B1 B2 C

0-1 cm. 1-6cm.

Litter of dead leaves. Upper (soddy) part of humus horizon, strongly permeated by grass roots. Grey humose horizon, fairly sharply separated from 6-14cm. lower podzolic layer into which it tongues; well developed small cloddy structure. In spite of drought the horizon is moist, more so than under forest; large number of earthworms. 1424cm. Whitish, with small ortstein concretions; brown spots in the lower part; more compact than Al; wormholes. 2435cm. Brown, with whitish spots; weakly fissured; coarse cloddy; merges into 3Ei55cm. Brown, compact, clayey, illuvial horizon. 55-100 cm. Brown loamy till.”

Although till was encountered in the second pit, it was assumed that both soils were developed in varve clay.

28

ALEX. MUIR

The changes in soil morphology described by Tiurin can be seen in a comparison of an old grassland soil (Park Grass) at Rothamsted (Guide, 1959) and a woodland soil of the Chilterns (Batcombe silt loam; Avery, 1958). There has been an increase in organic matter in the A horizon of the grassland soil, 3.8% organic C as against about 1.5% for the Al of the woodland soil; there is no change in base status, and both the grassland and woodland soils have a pH range of 4 . M down the profile. It appears that the only source of lime and other bases, the underlying Chalk, is completely out of reach of the roots of the sward and of the moderate earthworm population (Satchell, 1953). In Tiurin’s example, roots and invading worms were presumably able to reach the till of higher base status and thus bring nutrients to the surface. The socalled sod-forming process, therefore, does not necessarily lead to an increase in fertility insofar as mineral elements are concerned. In his book on pedology written for foresters, Tiurin (1933) began his discussion of podzolic soils with what he called the “pre-podzolic” or cryptopodzolic stage, which is obviously equivalent to the sod soils of Sibirtzev, Zakharov, and others, and equated them with the forest brown earths of Ramann. Tiurin considered that under primary grassland within the forest zone it was possible for podzolization to occur and, as grass vegetation may invade and colonize a deforested area, one could have two series of podzolic soil which he illustrated thus: Grassland

Forest Cryptopodzolic

Soddy soil

L

(forest brown earths) Weakly podzolic

hlediuni podzolic Strongly podzolic

Sod-weakly podzolic Sod-medium podzolic

&

Sod-strongly podzolic

Podzols

The arrows indicate the possible changes that can occur following changes in the vegetation. The sod-podzolic group as described by Tiurin appear to be essentially what were also called meadow podzolic soils (Glinka, 1923, 1931). Whether, in fact, podzolization can take place under meadow grassland, as distinct from heath grassland, seems not to have been adequately demonstrated.

THE PODZOL AND PODZOLIC SOILS

29

It is probable that some of the soils described in the older literature as meadow podzolic or peaty podzolic are probably essentially surfacewater gley soils and related to pseudogleys; some appear closer to humic gleys. The following description from Glinka (1931, p. 344) illustrates the type from the Amur region.

‘‘A1

“Az

“B1 “C

Almost black when wet, dark grey towards the bottom; structureless. Contains many undecomposed organic remains. Thickness 25 cm. Olive-grey, unevenly coloured; numerous tongues of humus penetrate from upper horizon. Indistinct lamellar structure; porous; hard ortstein concretions of dark brown colour. Thickness 20 cm. Olive-yellow; lamellar stricture breaking down to flat shiny aggregates with pores. Large number of hard ortstein concretions. Thickness 25-30 cm. Brownish yellow sticky clay.”

The dull colors of this profile suggest infiltration of humus and a certain amount of gleying due to impeded drainage. The soil could be described as a humic gley. In proposals for a new soil classification, Gerasimov et al. (1939) included under gray forest soils, brown earth (burozem) and podzolic soils, a subgroup of “sod soils” which was defined as “soils with a greater than typical accumulation of rather saturated humus mostly under grasslands or forest with a well-developed grassy cover.” In other words the sod process was thought of as secondary. This scheme was attacked in principle by S. P. Yarkov (1942), a student and colleague of V. R. Williams, who remarked that “certain pedologists do not recognize the sod-forming process as an independent one.” Following Williams, he considered that the true podzol was found only under close-canopy forest, allowing for variations due to relief and parent material. He considered that the podzolizing process was most vigorous when swamping of the upper layers occurred, but this gave rise to “false podzols” which should be separated from the normal type. The sod-podzolic soils arose as described by Williams, and various subtypes could be distinguished by the thickness of the A2 horizon: weakly ( < 10 cm.), medium (10-20 cm.), and strongly ( > 20 cm.) podzolic, straw-colored podzolic ( Afanasiev ) , and yellow podzolic. However, Yarkov, like Williams, provided no evidence that would enable a satisfactory division to be made between podzolic and sod-podzolic soils apart from the nature of the ground vegetation.

30

ALEX. MULR

As mentioned above, Tiurin (1935) was able to show that there was an increase in total organic matter in the meadow compared with the forest soil, particularly in the Al, but also to some extent in the A2, horizon. This increase was largely due to the more vigorous root system of the grasses. Quantitative differences showed in a smaller content of uronic acids and a higher content of the lignohumus fraction in the meadow soil. He regarded the grassland soil as showing a slight similarity to the chernozem. In more recent work by Kononova (1951) it has been shown that there is a distinct difference in the proportions of humic and fulvic acids in the podzolic and sod-podzolic types (Table 11). TABLE I1 Humic and Fulvic Acid Contents of Podzolic Soilsa Humic acid Soil Strongly podzolic soil (Komi A.S.S.R.) Sod-podzolic soil (Yaroslav region)

Humus A,,

(I)

Fulvic acid (11)

Ratio 1:II

%

%

S 1 2 c m . 2.31 15-20m. 3.49

0.23 0.59

0.55 1.16

0.56 0.51

6.93 1.69 1.00 0.52

1.55 0.25 0.14 0.07

1.96 0.45 0.24 0.14

0.79 0.58 0.57 0.50

B,, A,, 4- 7cm. A,A,, 7-15 cm. A,A,, 15-23cm. A,,, 23-38 cm.

Data from Kononova (1951),

Both soils occurred in spruce forest, that from Komi with an undergrowth of spruce, birch, and cedar and a well-developed raw humus ( 8 cm.); that from Yaroslav with a mixture of birch and ash, and ground cover of herbs and grasses (no mention of an A0 horizon). The differences are quite striking, particularly in the apparent mobility of the fulvic acid in the Komi soil, which is probably texturally similar to the sod-podzolic soil, both being formed on silt-loam drifts. Such differences are considered to be due to the presence or absence of the grassy ground vegetation, but no data are available for a comparison of open- and closedcanopy forest in the same region. The ratio of fulvic and humic acids has been suggested by Duchaufour (1957b) as a criterion to distinguish lessiuS and podzolized soils. VII. Geographical Variants

Perhaps we may take as the first recognition of the fact that there might be geographical variants of “podzolic” soils, the suggestion by Glinka (1911) that the Ramann “Braunerde” was really a slightly podzolized soil that had through cultivation lost its bleached layer. The

THE PODZOL AND PODZOLIC SOILS

31

latter, according to Glinka, was unlikely to be strongly developed mainly because in western Europe, with its higher temperature, longer growing period, and higher rainfall, the decomposition of organic residues would be more energetic. He considered that the “Braunerde” of western Europe was, “as it were, the last stage of the podzolic (acid) type of weathering, intermediate between the latter and the more southern krasnozems and terra rossa.” Glinka compared these “Braunerden” with the soils of the Novo-Alevandrovsk Experiment Station which were undoubtedly podzolic in character both from morphology and total chemical composition. The first to provide some evidence for Glinka’s theory was Afanasiev (1926a), who, in discussing the soils of Bielorussia (White Russia), pointed out that the strongly podzolic soils on clays could be distinguished from those to the north and east of that country. “The most important and interesting characteristics of these soils . , . are (1) the straw-yellow tint of the podzolic horizon; ( 2 ) obvious and deeply penetrating tongues of siliceous formations and ( 3 ) weak development of the underlying, illuvial B, horizon; in comparable soils to the north-east (outside Bielorussia) the podzolic horizon is usually light grey and whitish and the B horizon sharply defined.” Afanasiev, like other before him (e.g., Zakharov, 1910, 1911; Tumin 1912), felt that it was too much to expect uniformity of morphology over the whole of the enormous area shown as podzolic soils on the soil map: as well as variations by latitude, there must also be longitudinal changes. He suggested that the Moscow region might be taken as the central area, with variants to a still more continental climate toward the east and to a more oceanic type toward the west. The latter set of variants he named yellow podzolized in his account of Russian soil classification ( Afanasiev, 1927), and a visit to Czechoslovakia confirmed him in his view (Afanasiev, 1926b). After the 1st International Congress of Soil Science, when an opportunity was afforded of viewing the soil changes from Canada to the southern United States, he could write: “If we compare the colours of the forest soils of temperate latitudes with examples from Western Europe and the United States, it is evident, as should have been expected, that the forest soils of these latitudes are, in the colour of their A horizons, transitional; the grey tones on passing to the south take on a brownishness, a straw-colour, a yellowing, changing very gradually into zhetozems” ( Afanasiev, 1930). He equated his “straw-coloured podzolic soils with the gray brown podzolic soils. This point was not taken up at the time by his colleagues although he showed on the soil map of Bielorussia (1926a) quite large areas of “straw-coloured podzolic soils in the vicinity of Senno (south of Vitebsk), to the east of Vitebsk, and to

32

ALEX. MUIR

the west of Slutsk. On the recent soil maps of the U.S.S.R. this soil type has been merged in the sod-podzolic group (e.g., the 1:4M soil map of 1954), perhaps with some justification, as Nogina (1952) has shown that the evidence in favor of Afanasiev’s suggestion is contradicted by other facts. In her view these soils are simply variants of the sod-podzolics that occur in other regions. However, the geographical aspect of the group of podzolic soils had been raised by Zavalishin (1944) during consideration of podzolic soils of the Trans-Ural region. He pointed out that two groups were in general recognized: (1) Typical podzolic or peaty podzolic soils with a raw humus or peaty horizon. These had a higher acidity in the upper part of the profile, the maximum being usually in the A0 or AoAl horizon. Leaching had been strongest in the upper horizon (i.e., A1). This soil type is associated with “the root system and leaf-fall of spruce forests and their moss cover.” ( 2 ) The sod-podzolic soils had a more or less humic Al horizon and a great profile thickness due to the development of this horizon, a depression of the level of the podzolic A2 horizon, and an increase in thickness of the B horizon. They were also acid, but the maximum acidity was usually at a depth of 30-40 cm.from the surface. The most impoverished horizon was not the Al but the Az. This type occurred in the southern taiga with mixed and broad-leaved forest together with a grassy ground flora, as well as mosses. Zavalishin pointed out that there is no sharp boundary between the two types and asked: “Can one regard strongly podzolic raw humus soils of north-eastern European Russia or Trans-Ural as having the same degree of podzolization as a sod-strongly podzolic soil at the southern border of the podzolic zone under broad-leaved forest. Or is it more correct to regard them as independent series of varying degree of podzolization increasing in some or other directions (e.g., to the north or to the south)?” The point made by Zavalishin about the differences in the way in which the acidity varied down the profile was also discussed by Rod6 (1944), who concluded that the differences were of degree rather than of kind. However, some previous results obtained by Zavalishin for the pH values of soils in the Kuznetz basin, and quoted by Rod6, show that in the more strongly podzolized soils the minimum pH occurred in the B horizon. In soils from Lisino (near Leningrad) the minimum p H was in the surface layer. The type of variation in acidity referred to by Zavalishin is shown in Table I11 where pH data for various kinds of podzolic soils are given. The gradual decrease in acidity with depth is characteristic of sandy podzols described in western Europe and North America. The first two soils in Table I11 are sandy clay loams in the surface and loams and clay

33

THE PODZOL AND PODZOLIC SOILS

loam in the subsoil; the sod-podzolic soils have sandy loam surface soils over loam subsoils. From an examination of the field data provided by Ufimtseva (1955) and Vadkovskaya (1955) it appears that the soils with close-canopy forest, whether it be coniferous or broad-leaved, tend to be most acid in the A, horizon; in more open forest, particularly with a grassy sward, the pH is at a minimum in the A2B or B horizon. TABLE I11 Variations in Acidity in Podzolic Soils A, soil (pH) Podzolic soil (Leningrad) 4 4.0 3.4 Podzolic soil (Archange1)b S o d - p d d i c soil ( IVZUIOV)~ 5.6 5.4 Sod-podzolic soil (Vo1ogda)b Sod-podzolic soil (Moscow)~ 4.3 Gray brown podzolic soil6 6.3 Gray-wooded soile 6.5 0 Rod15 (1937). b Uht se v a (1955). 0 Vadkovskaya ( 1955 ) . d Brown and Thorp (1942). e Newton et al. (1959).

A2

B,

B2

B,

CorD

(pH) 5.0 4.5 5.3 5.4 4.8 5.9 5.8

(pH) 5.1 4.5 4.9 6.3

(pH) 5.4 4.8 4.9 6.9 5.2 5.2 4.8

(pH)

(pH) 6.9 7.2

4.9

5.5 5.6

-

4.1

-

6.4 5.3

-

7.3 7.4 7.6 7.3

Zavalishin (1954; Zavalishin and Nadezhdin, 1957) returned to this question of distinguishing the main subtypes of soils in the forest zone of European U.S.S.R. by a study of the Baltic region. He regarded the sod process as the “transformation” of soils under the intluence of a “meadow grassy meso-hydrophyl vegetation” with a consequent concentration in the accumulation horizon (A,) of humified organic matter, more or less saturated with bases and sesquioxides. However, he concluded that the sod process in the western Russian plain was stimulated by calcareous conditions in the ground-water and parent material. Some of the soils described were extremely acid throughout even under oak-hornbeam forest; cultivated variants showed both apparent clay shift and a minimum pH in the B or BC horizon. However, no adequate data were given to enable a separation of podzolic and sod-podzolic soils to be made. VIII. The Western Contribution

In the nineteenth century, theories regarding the problems of the podzol stemming from investigations in Russia and elsewhere developed concurrently in many parts of the world, although with differences in nomenclature. The Russians quite early recognized the similarity of their

34

ALEX. MUIR

podzols to the Bleisand of Senft and Muller, but the reverse process seems to have been a much slower one. When the Russian ideas and the name first spread further is not clear, but in the main it took place through Germany. This is not entirely surprising in view of the proximity of the two countries and the attraction of German laboratories for Russian workers. The first of the Dokuchaiev school to study in Germany appears to have been V. R. Williams, who carried the Russian ideas to the Wollny laboratories at Munich. It is to Ramann ( 1911), however, that we are most indebted for adopting the Russian term podzol and equating it with Bleisandboden, which he thought was a misleading name. Bleicherde he suggested should be used as a group term to cover podzol soils, Grauerden, and others that were depleted in iron and had become bleached. The Russian ideas were adopted surprisingly slowly although information about them had been widely disseminated. V. R. Williams had organized the Russian agricultural exhibit at the Chicago Fair in 1894 for which Dokuchaiev wrote an account in English of Russian soils. Later, Sibirtzev presented a paper in French on the soils of Russia to the 7th Geological Congress and Dokuchaiev’s soil map of the world was shown at the Paris exhibition in 1901. About the same time an excellent English translation of one of Sibirtzev’s accounts of Russian soils appeared in the Experiment Station Record ( Fireman, 1901) . Although in Germany the Bleisand with its associated ortstein was taken as equivalent to the podzol, little consideration was given to the possibility of the “Molkenboden” being similar to the more commonly found heavier-textured podzols of the Russians. Sibirtzev ( 1898) had pointed out that the Russian term podzol nearly corresponds to the German expression Bleisand, but is also applied to loamy and clayey soils if they are clearly affected by the chemical leaching processes characteristic of Bleisand. However, German workers devoted most of their attention to the ortstein layer as it was undoubtedly a major hindrance to soil utilization, particularly when cementation had taken place. The use of the term podzol was thus limited to what w e n nssentially tbsandier varieties. The influence of the German emphasis on ortstein and orterde was reflected in a lecture by C. F. Marbut (1928) when he distinguished the Lakewood, Cecil, and other soils as being podzolic soils developed under the influence of a relatively weak operation of the podzolic process. Although they had light-textured A horizons and heavier-textured B horizons, the latter differed from the ortstein of the “true podzol” in having a percentage of organic matter no higher than was present in the A horizon and in not being indurated as a rule, A similar statement could equally well have been made of many of the true podzols of the Russians.

35

THE PODZOL AND PODZOLIC SOILS

A selection of data for humus and nitrogen contents of Russian podzolic soils is given in Table IV. Only the peaty sandy podzol shows B humus-illuvial horizon, and the figures for the strongly podzolic soil under pine forest are similar to those for the sod-podzolic soil. TABLE IV Organic Matter and Nitrogen in Podzolic Soils

Horizon Soil Strongly podzolic soil on till under pinea Strongly podzolic soil on till under meadowa Peaty sandy podzol, humus illuvial; forestb Clayey podzol on till0

Sandy straw-colored podzolic soild a

b 0

d

Constituent

*I

Humus ( % ) N (%I C:N Humus ( % ) N (%I C:N Humus ( % )

3.81 0.14 15.7 4.84 0.18 15.6 37.55 0.99 22 7.92 0.32 14.3 2.0

N

(%I

C:N Humus ( ”/o ) N (%I C:N Humus ( % )

A2 0.81 0.04 11.7 1.22 0.09 8.0 0.36 0.02 10 0.61 0.031 11.4 1.4

Bl 0.41 0.05 4.8 0.41 0.05 4.8 0.92 0.04 9.6 0.42 0.035 7.0 0.7

2.41

0.06 17.4 0.43 0.048 5.2 0.3

Tiurin ( 1935). Rod6 ( 1937). Kossovich and Krasiuk (cited from Rod6, 1937) : first layer A,A,; last layer C,. Nogina (1952): layers 3 and 4, A,B and B,, respectively.

The advent of Glinka’s “Die Typen der Bodenbildung (1914) stimulated interest in the main groups of soils, but its description of podzolic soils and podzols on clay and loess did little to alter the prevalent idea that such soils were confined to sandy parent materials: the later work of Frosterus (1914) and Tamm (1920) rather confirmed it, although Tamm (1930) described a “mull podzol” which, from its morphology, could equally well have been called a sod-podzolic soil. During the 1930’~~ both in the United States and Britain the analysis of the clay fraction of soils became a standard procedure in soil studies and at once provided a fairly certain distinction between podzols and brown earths (including gray-brown podzolic soils). This aspect received strong emphasis by Robinson (1930, 1949), who showed that in the Welsh podzols there was a marked change in the composition of the clay in passing from the A to the B horizon in comparison with the relative constancy in the whole profile of the brown earths. This received confirmation in the work of Anderson and Byers (1931), who examined

36

ALEX. MUIR

the clay fraction from American podzols, gray-brown podzolic, and redyellow podzolic soils. There were even more marked differences in the clays from their podzols than in Robinson’s, whereas in the other soils the composition was almost constant. The use of the clay fraction in defining soil groups was extensively used in Britain (e.g., Kay, 1934, 1939; Muir, 1934, 1935; Muir and Fraser, 1940; Robinson, 1935). Muir showed that the separate ratios of alumina to iron oxide and of silica to alumina were of more value than the ratio of silica to sesquioxides, as the marked differentiation of iron and aluminium in podzols was a characteristic feature (Table V ) , The method for distinguishing between podzols and brown earths using acid-oxalate extraction introduced by Tamm (1920) was applied by him and by Aaltonen (1935) in Finland to elucidate the rate of podzol development. Tamm’s method was used by Lundblad (1934) to distinguish brown earths and podzols and, together with clay analyses, was used by Muir (1935) to separate podzols and a variety of brown earth common in Britain. The method is not satisfactory for heavier textured soils (cf. Tamm, 1934), but this difficulty has been overcome by the use of dithionate (Deb, 1950; cf. Duchaufour, 1956). These variations in clay fraction ratios, however, give no indication of the mineral constituents of the various horizons. MacEwan (1948) showed for some Scottish podzols that there generally appeared to be an increase in the illite content at the expense of biotite, hydrobiotite, and chlorite. Tedrow (1954) showed an increase in the quartz content of the clay from Az horizons of podzols; the dominant minerals in the soils were interstratified 2:l layer silicates and illite. Recent studies by Jackson and his co-workers (e.g., Brown and Jackson, 1958) have shown that in some sandy podzols montmorillonite may form in the A2 horizon at the expense of a vermiculite-chlorite that is present in the B and C horizons. None of these workers produced evidence for mineral synthesis in the B horizon. In the case of the gray-brown podzolic soils, McCaleb (1954) found that illite was the dominant mineral in all horizons; and in the more base-saturated profiles, some vermiculite and montmorillonite. The more acid soils had some kaolin. In soils of a similar character Avery et d. (1959) showed a tendency to increase in vermiculite content-possibly at the expense of clay mica. On the morphological side it was generally accepted that raw humus was characteristic of the podzols while mull was one of the principal features of brown earths in the widest sense (Robinson, 1949). The latter soils lacked any marked deposition of iron oxides in the subsoil, the profile having a much more uniform appearance, although in the more strongly leached varieties there might be a clear textural and structural

TABLE V Variation in Clay Fraction Ratios of Podzols and Podzolic Soil Soil Brown eartho Soil (England) Podzol ( Scotland)b

Podzol (U.S.A.)O

Ratio

A,

A2

A3

Si02/A1203

3.17 2.81

2.94 2.90 3.03 5.85 3.22 5.72 3.15 3.98

-

d2°3/F%?03

Si02/A1203 A1203/Fe203

-

M2O3/Fe2O3 Si02/A1203 A1203/Fe203

-

sio,/&o3

Cray-brownd Podzolic (USA.) a Kay (1939). Muir (1934). 0 Anderson and Byers ( 1931). d Brown and Thorp (1942).

*

-

3.23 3.86

-

3.11 3.64

Horizon Bl

3.04 3.00 2.41 1.58 1.85 0.84 3.05 3.66

+I

2 B,

B3

C

3.10 3.05 1.80 2.58 1.83 2.40 3.70 3.12

-

3.38 3.02 1.38 5.90 2.12 3.71 3.21 3.10

1.03 4.07 -

3.13 3.13

8

F 5 8

3

0

E n

cn

;

v1

38

ALEX. MUIR

change with depth. Such varieties were represented by the gray-brown podzolic soils and what were later to be called sols lessiue’s by French workers (cf. Duchaufour, 1956, 1960) and the Purubraunerde of the German workers (cf. Altemuller, 1956; Kubiena, 1956). The microscope had often been used in soil mineralogy, but its applications by Kubiena (1938,1953) to the examination of soil fabrics was a revolutionary stage in the study of soil morphology. Kubiena’s techniques, particularly that of thin sections, have now been widely used and, as applied to the groups of podzols and podzolic soils (lessive’, etc.), have provided more definite morphological criteria for their separation. However, although Kubiena gives Dokuchaiev credit for priority in the use of the term podzol, it is certain that many of the soils described by Dokuchaiev and his colleagues would not qualify for inclusion in Kubiena’s group of podzols. IX. What Is Podzolization?

The discussion of the geographical variants of podzolic soils has shown that, while there was a general realization of differences in both morphology and chemistry between podzolic and sod-podzolic soils, the kind of data that it had been customary to collect did not satisfactorily distinguish between the two types if they were to be regarded as more than variants. This is not entirely surprising, for from the beginning the terms podzol and podzolic had been loosely used. Both Dokuchaiev and Sibirtzev criticized this tendency, but with little effect. Thus, in an excellent account of the aspen groves of the Voronezh steppe, Popov (1914) repeatedly refers to podzolized solonetz. Such usage, at least in the earlier period, was not unnatural as the folk term referred solely to the bleached layer and neither Dokuchaiev nor Sibirtzev had ever clearly distinguished an illuvial horizon except in the sandy varieties. However, following Vysotzky’s definition of the illuvial horizon the terms podzol and podzolic were normally restricted to the soils found in the taiga zone. From the time of Senft and Dokuchaiev it was agreed that the basic process involved was the decomposition of the primary minerals with subsequent removal of sesquioxides together with more or less humus from the surface soil. Residual quartz and secondary silica formed the gray or white surface layer. The analyses quoted by Dokuchaiev and his co-workers were almost all confined to the bleached layer, and they invariably showed an extremely high silica content. Early Russian data for the other horizons are scarce, but analyses given by Sibirtzev (1898) are shown in Table VI.

THE PODZOL AND PODZOLIC SOILS

39

TABLE VII Podzol from Tikhvin: Total Chemical Analysis5 Sample Soil covering to pOdZ0l Podzol Subsoil of the

wdzol 5

Loss on ignition

Hygr. water

Humus

CaO

MgO

5.86 0.9

1.22 0.24

2.8 0.3

1.192 0.79

0.378 0.24

2.4

0.97

-

1.03

0.34

Data from Georgievsky (1888).

‘%O3

SiU,

p205

1.84 0.67

7.032 4.79

81.02 90.68

0.085

1.62

7.21

84.5

Trace

Fe203

0.059

w

5

B

41

THE PODZOL AND PODZOLIC SOILS

Later Russian workers carried out more complete chemical analyses, and many hundreds, if not thousands, must be in print. They were usually supplemented by determinations of exchangeable bases and organic matter and sometimes by analyses of the HC1, H2S04,KOH, and water extracts. Mechanical analyses when made were only rarely extended below 20 p. Bulk chemical analyses, of course, provided clear evidence of apparent movement of material from the surface layer to the subsoil, and the distribution of exchangeable bases showed the leaching of the more mobile ions. The KOH extraction seems first to have been used by Georgievsky (1888) as an improvement on the soda extraction introduced by Il’enkov (1869) in an attempt to study the state of the silica in the “podzol.” Il’enkov and later Amalitsky had considered “amorphous” silica the cause of the adverse agricultural properties of the soil. A selection of Georgievsky’s data for a sandy loam podzolic soil is given in Tables VII and VIII. Georgievsky concluded from his results TABLE VIII Analysis of KOH Extracta Sample SiO, Fe,O, Podzol 0.112 1.45 Subsoil of podzd 0.99 0.06 a Data from Georgievsky (1888).

A1,0, 0.56 1.52

CaO 0.05 0.02

MgO Trace Trace

that, although some amorphous silica might be present, the removal of alumina, etc., suggested that aluminosilicate materials had been attacked in the treatment. He thought that the small amounts of silica extracted could not appreciably affect the soil properties, as had been suggested. The 5% KOH extraction was later used by Gedroiz (1926) to distinguish between podzolic and solodized soils. The latter invariably showed a large excess of extractable silica over the amount required to form kaolin with the alumina simultaneously extracted; podzolic soils showed little or no excess. No development in the laboratory study of podzolic soils was made until in the early 1930s A. A. Rod6 began to publish the results of his researches on the soils of the Lisino Forest near Leningrad. These results, together with others, formed the basis of his monograph on the podzol-forming process (Rode, 1937), which was partly an extension and partly an amplification of 0. Tamm’s work on Swedish soils, with a wider survey of the available literature. In addition to detailed total chemical analyses of podzolic soils, Rod6 also gave a range of clay analyses, which if they had been more extensive might have provided

42

ALEX. MUJR

the basis for a distinction between the ‘‘true” and the “soddy” podzolic soils. Rod6 (1944) concluded that “the nature of the podzol-forming process, as regards the mineral part of the soil, consists in the complete breakdown of all minerab of the parent material, except quartz, and the removal of all products of this decomposition from the upper layers of the parent material.” He considered the most characteristic feature of the process to be the instability of the secondary clay minerals. In a later account, Rod6 (1955) again stressed these aspects and in regard to the sod-forming process remarked that indications of it can be found in all three zonal subdivisions of podzolic soils, even in the absence of a grassy vegetation. He therefore, concluded that there is no strict division between podzolic and sod-podzolic soils, a view that would not find general acceptance with Russian pedologists. Rod6s definition of podzolization implies differentiation of the clay fraction, but this is not so in many sod-podzolic soils which thus cannot be regarded as “true” podzolic soils (see p. 44 and Table XI). Yarilova and Parfenova (1957) suggested that clay is resynthesised in the B horizon of podzolic soils, a view to which Rod6 also subscribes. This, however, is not substantiated, although not excluded, by their data, which could equally well be given by clay that had been moved from the A horizon. Other Russian pedologists (Minashina, 1958; Fridland, 1958; Karpachevsky, 1960) accept clay movement. TABLE IX Clay Fraction Data for Lisino Podzolic SoiP Fraction

Fraction 2.5-0.25 p

< 0.25 p

Amount (%)

SiO,/ A1,0,

A1,0,/ Fe203

Amount

Horizon

(%)

SO,/ A1,0,

A1,0,/ Fe,03

A1 A2 B C

26.5 29.2 33.0 39.5

4.32 4.12 3.79 3.89

5.63 5.26 3.89 4.46

1.7 1.3 23.2 17.7

2.26 2.04 2.49 2.49

3.74 4.70 3.01 2.94

(1

Data from Rod6 (1937).

Clay fraction data obtained by Rod6 are given in Table IX. No standard clay fraction has ever been adopted, so that results by different authors are not always strictly comparable. Rode’s coarser clay probably contained quartz, which makes the Si02/A1203 value for it somewhat high, particularly in the upper layers. The relative uniformity of the si1ica:alumina ratios does not suggest alteration of the clay to the extent indicated by the data for the podzolized soils given in Table V. The differentiation of iron oxide and alumina is, however, clear in both coarse

THE PODZOL AND PODZOLIC SOILS

43

and fine fractions. The ratios for the fine clay shows that it is the more aluminous and ferruginous and is the main fraction that is migrating. However, on the clay fraction criteria mentioned above, this soil cannot be regarded as strictly comparable with the sandy podzols of the United States and Great Britain. Clay mineral analyses have been made on Russian podzolic soils with results that are probably similar to those obtained elsewhere for this general group. The results obtained by Rod6 and Sedletzky (1939) for sandy podzols indicate an interstratified mineral or vermiculite as the dominant constituent of all horizons, with little variation with depth. Yarilova and Parfenova (1957, 1959) have published similar data on B horizon samples from Russian podzolic soils. Skrynnikova ( 1958 ), from a consideration of pedogenic processes in the forest zone, concluded that podzolization is no longer an active process, except perhaps in some northern podzolic soils under conifers. She believes that podzolization was initiated at the end of the Ice Age under a colder and moister climate than the present. The advent of broad-leaved forest in the south of the forest zone has caused a reversal of the earlier processes, leading to “regradation.” She does not clearly define podzolization, but implies decomposition and leaching of mineral matter so that the Al is the most impoverished horizon. Under broadleaved forest and meadows there is not only an improvement in base status in the A horizon, but an increase in clay content in the A1 horizon following decomposition of mineral-rich litter. This higher clay content in the Al horizon is very common in what have been called sod-podzolic soils, and in many it is a very striking feature. A few examples are given TABLE X Clay Content of some Podzolic Soils ( ”/o of Air-Dry Soil)@

I I1 I11 IV V VI A, 4.47 3.39 14 12 9 15.9 5.13 0.04 9 6 8 15.5 A2 13.65 15 8 6 16.4 -42B 9.80 16.39 20 13 22.7 Bl B2 21.87 14.45 28 13 36.9 C 19.34 11.97 21 14 14 21.3 5 I, Strongly podzolic soil, Leningrad ( Rod&, 1937 ) : < 2 c~ clay. 11, Loamy podzol, Moscow (Gemmerling, 1930) : < 1 ~1 clay. 111, Podzolic soil, Archangel ( Ufimtseva, 1955) : < 1 P clay; last layer CD. IV, Sod-strongly podzolic soil, Vologda ( Ufimtseva, 1955) : < 1 p clay. V, Straw colored sod-podzolic Bieloxussia (Nogina, 1952) : < 1 clay: last layer CD. VI, Gray-brown podzolic (Brown and Thorp, 1942) : < 2 p clay; third layer is Horizon

’43.

44

ALEX. MUIR

in Table X with “true” podzolic and gray-brown podzolic soils for comparison. Some western European soils of the lessive’ type also show a somewhat higher clay in the A1 than in the A? horizon (cf. Duchaufour, 1957a; Avery, 1958) although none of these is as strongly bleached as the Russian soils. Results more comparable with those for sod-podzolic are given by Muckenhausen (1957) for a pseudogley. The same feature is seen in the gray-wooded soils of Canada (Newton et al., 1959), but it does not seem to occur in the gray-brown podzolic type. The distribution of podzolic soils with lower clay in the A2 horizon is not clear, but it is possible that they represent cases of extreme degradation of former “chernozemic” soils as described by Glinka (1924). From data given by Zavalishin and Firsova (1960), it appears that in the zone north of the 60th parallel podzolic soils should not show this feature, but exceptions occur as can be seen from the data for the Leningrad and Archangel soils in Table X. These presumably reflect purely local conditions. There is no very clear connection between the occurrence of this very depleted A2 horizon and vegetation, and it may occur in the presence or absence of raw humus. Of the soils listed in Table X, I and I11 occur under a raw humus layer; Zavalishin and Firsova (1960) describe a “strongly podzolic raw humus soil” from Komi which shows a decrease in clay in the lower A?. Russian ideas on podzolization have undergone a change since Fridland (1958) introduced the term and concept of “illimarization,” which is the exact equivalent of the “lessivage” of Duchaufour. The process of illimarization he considers to operate in brown forest soils, yellow podzolic soils, etc., and partly in the sod-podzolic group, whereas podzolization is the effective process in “typical podzolic” soils, gley podzolic, and others. The distinguishing feature is the presence or absence of differentiation in composition of the clay fraction down the profile. Fridland also recognizes that there are intermediate phases in which podzolization may accompany illimarization. The question of what are “typical podzolic” soils is left unanswered. The issue was somewhat confused by Gerasimov (1959) when he stated that “soils of a gley-pseudopodzolic type have been called sols lessiue’s and pseudogleys in Western Europe. Thus they have been rightly distinguished from the true podzolic soils characteristic of Eastern Europe, but have been wrongly regarded as genetic formations contrasting with and of equal status to that group.” Gerasimov does not define his “true podzolic soil” but he accepts Rodd‘s definition of podzolization which, as we have seen, implies a marked differentiation in the clay complex. A selection of Russian clay fraction data is given in Table XI which shows that there is no great differentiation in

45

THE PODZOL AND PODZOLIC SOILS

these sod-podzolic soils. The Al horizon tends to be somewhat more siliceous than the A2 horizon, which may indicate a certain breakdown of the clay minerals, but this is also shown by some soils that would be called Zessiue' by Duchaufour's definition. The difference could equally well be, and is more probably, due to the removal of the finest clay, as indicated by Rode's data. The figures in Table XI given by Karpachevsky TABLE XI Variation in Clay Fraction Ratios for Podmlic Soils Horizon

soil Sod-podzolic Moscow regiona Moscow regiona Bielorussiab Bielorussiab Moscow regionc Moscow regiond Podzolic Kama Rivers a b

e

Ratio

A,

A2

B

C

SiO,/Al,O, A1,O3/Fe,O3 Si0,/A1,03 A1203/Fe203 Si02/A1,03 Al2O3/Fe2O3 SiO,/AI,O, A120,/Fe,03 SiO,/AI,O, A1203/Fe203 SiO,/AI,O, A1,03/Fe203

4.98 4.32 4.26 3.70 3.72 2.58 4.56 2.96 3.26 3.26 4.7 4.5

4.13 3.90 3.09 4.43 3.59 2.61 4.12 2.64 3.63 3.44 4.0 3.5

4.08 3.68 3.00 4.23 3.20 2.80 3.90 2.34 3.12 3.05 3.5 3.5

3.99 3.72

3.33 2.67 4.4 2.5

Si0,/A1203 AI2O3/Fe2O3

-

5.55 5.45

2.47 3.84

2.87 6.75

Morozov (1940). Nogina (1952) Vadkovskaya (1955). Karpachevsky (1960). Rod6 and Sedletzky ( 1939 )

-

-

.

(1960) for a soddy medium podzolic soil of loamy texture under oak show a more marked differentiation of iron oxide and alumina between the A1 and A2 horizons than most of the other results. He nevertheless believes that Rode's argument for the breakdown of secondary minerals is wrong and that translocation of unaltered clay minerals can occur in both light- and heavy-textured podzolic soils. A more extreme example is that of the Kama River soil which contains only about 1.5% clay in the B horizon and is thus comparable with the American podzols referred to in Table v. Thus, on the available evidence of clay composition no clear distinction can be made between the two main Russian classes of podzolic soil. They both show a marked apparent transfer of clay to the subsoil, but

46

a E X . MUIR

without any strong differentiation of its constituents. This does not preclude transformation from one clay mineral to another (e.g., possible increase in vermiculite or illite in the A horizon), but it does not support RodGs contention unless one assumes equal rates of removal of the main weathering products and that the new minerals formed in the subsoil have the same chemical composition. The very high losses, particularly of silica, from podzolic soils shown by Rod6 (1937) could well be accounted for by the breakdown of primary minerals. There would inevitably be some loss of the weathering products before resynthesis of clay was complete. In any case the change from, say, feldspar (SiO2/ A1203 = 6 ) to a clay mineral (SiOz/A1203 = 2-4) involves a significant loss of silica. It would seem that, on the basis of the evidence adduced so far, the Russian podzolic soils on heavier textured materials have much in common with the sols lessive‘s (and Parabraunerde) of western Europe as well as the gray-brown podzolic and gray-wooded soils of North America. As many of the first group were formerly considered to be brown earths, Glinka’s suggestion that they formed the western end of a sequence through from the podzolic soils of eastern Europe was nearer the truth than at one time seemed to be the case. So too with Afanasiev’s equation of gray-brown podzolic soils and his straw-colored podzolics. Thus, it appears that the concept of podzolization as it applies to the great majority of Russian soils described as podzolic is simply an extreme expression of “lessivage.” X. Micromorphology

The use of thin sections to investigate micromorphology has only recently begun in Russia, and little information on soil fabrics is yet available. Karpachevsky ( 1960), Minashina ( 1958), Kundler ( 1959), and P. Bullock ( 1960, unpublished) have described the micromorphology of some podzolic soils in sufficient detail for comparisons to be made with Kubiena’s and other criteria. Yarilova and Parfenova (1957) printed excellent photographs suggesting clay deposition in B horizons, but gave no indication of the soils from which the specimens came. Two sandy soils, one from Leningrad (Bullock) and one from Moscow (Karpachevsky), showed all the features of Kubiena’s humus or humusiron podzols. P. Bullocks description is as follows:

A2 B1

Largely clear quartz grains, no evidence of iron oxide or humus; little clay. Largely quartz sand; little clay; a few iron oxide concretions. Quartz grains coated with humus which forms bridges be-

THE PODZOL AND PODZOLIC SOILS

BC

47

tween the grains and shrinks to form a network of fissures; local accumulations of humus pellets. Mainly quartz grains with little iron oxide; some humus coatings locally.

Sod-podzolic soils were described by all the authors mentioned; the description by Kundler is as follows: Al

A2

B

Porous crumbly fabric; finely dispersed organic matter mixed with minerals, many fine plant remains; frequent small dark brown concretions; the clay is unoriented. Consists of a series of roughly parallel layers (0.5-3 mm. thick) of a light yellow tint; the fine material is “unoriented; conspicuous dark brown concretions. Soil mass in section is yellowish brown and interspersed with a network of strongly birefringent platy minerals and mineral aggregates; the skins on the peds and linings to pores show layering and strong birefringence.

Bullock’s descriptions of a loamy podzol and a loamy strongly podzolized soil from the Moscow region correspond quite closely to that of Kundler, and they both show much strongly birefringent clay in the channels and pores of the B horizon. The schistosity in the A2 mentioned by Kundler was not seen in Bullock‘s specimen. It is unfortunate that the heavier-textured podzolic soils so far examined in thin section are those occurring in the sod-podzolic zone. However, Karpachevsky considers his sandy soil to be a sod-podzolic. It thus appears that sandy soils in the sod-podzolic zone can have the micromorphology of the western and American podzols. The heaviertextured podzolic soils have all the attributes of strongly developed sols lessiv6s or bleached Parabraunerde. Whether an examination of heaviertextured soils from areas shown on the soil map as unqualified podzolic soils will prove that they are likewise of the lessive‘ type remains for the future. XI. The Characteristics of the Russian Podzol and Podzolic Soils

Gorshenin ( 1958) recommended that the podzolic and sod-podzolic soils should be recognized as independent soil groups at the highest category of classi6cation. Tiurin (1960) has written that soils called ilovka and gley soils by Sibirtzev “can now without difficulty be recognized as pseudogleys (or surface gley podzols) .” He also mentions that Sibirtzev had commented on “the yellowish colour of the podzolic horizon

48

ALEX. MUIR

in sod-podzolic soils on the loess-like loams of Bielorussia and the plains of Central Europe. Such soils are now known as sod-straw-yellow podzolic (or sols lessivds).” However, this statement does not go far enough, for Nogina’s data showed that there was really little difference between the ordinary sod-podzolic and the straw-yellow variety. It has been shown above that in the absence of an adequate definition of a podzolic soil the present course seems to be to make a subdivision as already done in the West: i.e., the sandy soils of the north (and perhaps in the southern taiga also) correspond to the humus and iron-humus podzols of Kubiena whereas the podzolic soils on medium and heavytextured materials belong to the lessive‘ or Parabraunerde group. On the available data it is possible to distinguish two types within this broad group which correspond to Russian division into podzolic” and “sodpodzolic,” but it is doubtful if they are independent groups as Gorshenin suggested. They are, rather, expressions of degree of development ( RodB, 1944) or alteration due to human interference, as indicated by Tiurin (1935), or to difference in age. The sod-podzolic perhaps should have a subdivision for those soils that show a relic humus horizon in the lower Az. Kubiena (1953) has suggested that his Molkenpodzol is equivalent to Georgievsky’s clayey podzol although the description of the latter could well apply to soils that are probably of the lessive‘ type but are influenced by surface waterlogging. This effect of surface-waterlogging can be recognized in many of the heavier-textured Russian podzolic soils and podzols and has been commented on by many Russian workers. Some, like Yarkov (1954, 1956), have put considerable emphasis on anaerobic reduction processes in the formation of podzolic soils. There is no doubt that these must be responsible in part for concretion formation in the Az horizon, but there is no evidence that they are essential for the translocation of ferruginous clay. Once the textural and structural B horizon is well formed, temporary waterlogging in cracks and fissures can readily occur and give rise to the bluish gray colors that are sometimes seen on clay skins. It may be that waterlogging in the surface contributes to the breakdown of the structure aggregates at the top of the lower B horizon (cf. Grossman et al., 1959, p. 75) which results in the powdering that is such a common feature of the well-developed Russian podzolic soils (cf. p. 20). Soils showing the more extreme effects of surface waterlogging are usually separated as a subtype which approximates to the pseudogley (cf. Tiurin, 1960, quoted above ) In the description of the characters of the various Russian groups of podzolic soils attention has been concentrated on the better-drained varieties. Russian workers recognize some 20 subtypes and varieties of

.

THE PODZOL AND PODZOLIC SOILS

49

podzolic soils, some of which have a large literature devoted to them. The range of chemical analyses made is very wide, and only a selection of results that appear to be useful for characterization is mentioned in the following paragraphs.

A. Sandy podzolic soils, The podzol is probably the commonest variety. Usually associated with pine woods or heaths. 1. Field morphology. Raw humus, or moder, forms the A. horizon; the thickness is variable, 5-10 cm. The Al horizon is not well developed and if present is normally only slightly humic; its structure is weak. The A2 horizon is variable, 10-20 cm. thick; structure weak or absent. Usually sharp change to B horizon which may be more humic in its upper part ( Bl ); usually well cemented. 2. Micromorphology . Depending on the degree of development of the soil the sand grains in the A horizon may be coated to some extent with humic matter or be quite clean. The B horizon is characterized by the pronounced coating of the grains with humus or humus-iron complexes. These coatings tend to crack and flake off. Segregations of iron oxide may be present. 3. Chemistry. Sandy podzols are commonly strongly acid, this acidity decreasing gradually with depth; the minimum pH is usually in the A, or A2. Clay translocation may be shown by mechanical analysis, but in very sandy varieties this can be accounted for by “free” sesquioxides. Clay fraction shows marked differentiation of silica and alumina as well as alumina and iron. Organic matter content in B horizon normally higher than in A2 and has a high C:N ratio ( > 10). B. Loamy podzolic soils. This group appears to consist mainly of strongly podzolized soils and podzols, i.e., the A, horizon is poorly developed or absent. They occur under coniferous forest often with an ericaceous and mossy ground flora. 1. Field morphology. Raw humus or moder forms the A. horizon: the thickness varies and some varieties have been termed peaty. The bulk of roots occur in the raw humus and the Al, if present. Most of the humus in the Al is infiltrated and not due to decomposing roots. The structure of the A1 is weak, but sometimes shows a platiness; when dry breaks down to floury consistency. The A2 horizon is well developed and may be up to 15-20 cm. thick and shows a platy structure which is unstable when dry, Concretions of iron oxide are common. Humus content normally low but may reach 2 3 % . Frequently this horizon tongues into the illuvial layer, residues of which may form an AZB transition, which in part consists of a “powdering” on the B horizon peds. The B horizon is, however, clearly defined. It shows a strong increase

50

ALEX. MUIR

in clay content and has a well-developed blocky structure, the size of the aggregates increasing with depth. Clay skins on the peds are common. Concretions are usually present in the A2B, but may be absent below. Toward the bottom the structure becomes massive and passes into the parent material (varve clay, till, loesslike loam). Soil faunal activity is low. 2. Micromorphology. No data. 3. Chemistry. The soils are usually strongly acid with the minimum pH in the Ao, or Al horizon when present: the pH rises steadily with increasing depth. Maximum humus in Al horizon, decreasing rapidly below, but a few soils have been described in which there is a second accumulation at the top of the B horizon ( 2 Molkenpodxol). When this second humus horizon is present it has a high fulvic acid content. The clay shows a fairly uniform Si02/A1203 ratio, but the A1203/Fe203values suggest differentiation of the sesquioxides. The finer clay fraction is the more sesquioxidic. No data for “free” iron oxide. C. Sod-podxolic soils. This group shows the whole range of development from slightly podzolic to strongly podzolic. Podzols have also been described in which the Al horizon is vestigial, but the soils have the other properties of the group. 1. Field morphology. There may or may not be a raw humus layer, depending on the density of the forest canopy. When present it may be up to 2 inches thick; under more open conditions leaf fall is rapidly mineralized. The Al horizon usually contains abundant roots forming the “sod” which gives its name to the group. It is medium to dark gray in color, has a well-defined crumbly structure, and shows evidence of earthworm activity. Concretions are scarce. The transition to the A2 horizon is often gradual, allowing an AlA2 horizon to be separated. The A2 horizon i s commonly a slightly yellowish gray color when wet but dries very light gray. It is usually clearly lighter textured than the A1 and contains numerous concretions. The structure is usually platy. Tongues of Az penetrate in the B, the upper part of which shows a “powdering” on the peds. The powdering may extend down fissures into the B horizon. The B horizon is brown to strong brown or reddish brown in color and shows a marked increase in clay content. The subangular blocky structure is well developed, the peds increasing in size with depth until the structure is massive. Clay skins on the peds are well developed and sometimes thick. Concretions are uncommon, but segregations of manganese oxide may be present. In some soils the peds break horizontally to form coarse platy aggregates. Soils in the southern taiga (broad-leaved and mixed forest) often

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show a patchy second, “relic,” humus horizon in the middle or lower part of the A2. 2. Micromorphology. In the less strongly developed soils there may be some humus staining of the sand in the Al horizon and some unoriented clay. Concretions or segregations of iron oxide are present in both A1 and A2. The latter may show schistosity. The B horizon in thin section is yellowish brown with much birefringent material and oriented clay films in fissures and pores which decrease in frequency with depth. No information is available on the fabric of the second humus horizon. 3. Chemistry. The soils are moderately to strongly acid, the pH of the A1 horizon commonly being higher than that of the A2. The minimum pH is in the A2B or B in the more typical examples. The humus content is highest in the Al and usually falls sharply in the A2, decreasing still further with depth. The content of fulvic and humic acids is proportionate to the humus content. The fulvic acid is less mobile than in the podzolic soils. The clay has fairly uniform silica:alumina ratio throughout the profile and there is little differentiation of iron oxide and alumina. Determination of so-called “mobile” iron by the Kirsanov (1937) method (extraction in cold with 0.05 N HC1) gives lower amounts in the A2 than in the Al, the values of the latter being similar to those for the B horizon. The humus content of the second humic horizon is about 1% (cf. Ufimtseva, 1955) and is said to have a humic acid content suggesting its origin from earlier grassland conditions. XII. Summary and Conclusions

It has been shown that in the early Russian work the term podzol was applied solely to the bleached horizon that occurred at or near the surface of soils in the taiga zone. The word was by no means restricted to soils that were well drained. The appreciation of “illuviation” as an important feature of soils with bleached surface horizons led to the application of the word podzol and its derivatives to a wide variety of soils. The distinctions between many of these podzolic soils have never been clearly made, but two main groups of the better-drained varieties have been recognized: podzolic and sod-podzolic. The podzolic soils can be subdivided into those with and those without a humus B horizon. The former are commonest in sandy soils but apparently may occur in heaviertextured materials. From the available descriptions, the sod-podzolic soils may or may not carry a raw humus or moder cover. On the basis of laboratory data the sandy podzolic soils are similar to what have been called podzols or humus-iron podzols, etc. in western Europe and North America. The podzolic soils of heavier texture and

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the sod-podzolic soils show a relatively uniform clay composition which, together with thin section evidence, suggests that the textural B horizon results from clay movement. In the sod-podzolic soils of the sprucemixed forest zone, however, there is usually a distinctly lower clay content in the A2 than in the Al horizon. The latter feature might serve to distinguish soils that are close to pseudogleys from those that are strongly bleached sols lessivds or Parabraunerde. The apparent disintegration of the B horizon is clear in both cases. The following tentative correlations can be suggested, although further work is essential before they can be considered as established. Russia

Sandy podzolic soils Sandy podzolic soils with illuvial humic horizon Podzolic soils Podzolic soils with illuvial humic horizon Sod-podzolic soils

W. Europe and N . America Iron or iron-humus podzols Iron-humus or humus podzols Sols lessivds or sols podzoliques (bleached Parabraunerde)

Molkenpodzol Bleached Parabraunerde, pseudogleys, or gray wooded soils

ACKNOWLEDGMENTS

I wish to thank Dr. I. Sladits for making the translations from the German, Mr. D. V. Jones for making the diagrams, P. Bullock for the thin sections, and Dr. D. A. Osmond for much helpful discussion.

Addendum

In their textbook on Soil Science, published at the end of 1960, Gerasimov and Glazovskaya (1960) accept the idea of lessiuage, i.e., translocation of clay with little or no decomposition, but consider that gleying is an important accompaniment and suggest that the complex of processes giving rise to sols lessiv6s and related soils should be called “pseudopodzolization” (cf. Gerasimov, 1959). They contrast with this podzolization-“the podzolic process”-which, they say, “is accompanied by intense decomposition of the mineral part of the soil under the influence, principally, of humic materials, and the removal from the upper layers of the soil mass of the most diflicultly mobile products of weathering and soil formation-the hydrated sesquioxides.” Following leaching of the readily mobilizable bases as humates and fulvates, there takes place “the dissolution of substances entering into the nucleus of the colloidal particles. In particular, colloidal hydrates of iron, aluminium and manganese oxides are dissolved by reaction with fulvic acids” which also attack minerals coarser than clay. “Gradually from the upper horizons,

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lying directly under the forest litter, all the mobile products of the exchange between the mineral part of the soil and organic acids are leached out; the layer becomes impoverished in colloids and sesquioxides. Quartz, being most resistant to decomposition, accumulates as the residual product.” The more mobile constituents may be removed from the soil completely, “the less mobile fulvates (or crenates) of iron and aluminium (and also colloidal solutions of hydrated silica) precipitate from solution somewhat below the podzolic horizon and form a distinct ore-like or illuvial horizon.” It is considered that the various hydrated oxides may crystallize or combine to form secondary alumo- and ferrisilicates in the illuvial horizon. In discussing the podzolic soils of the taiga forests (predominantly coniferous), in the middle zone of which the typical podzol is developed, Gerasimov and Glazovskaya repeat the above arguments, but unfortunately use data for a sod-strongly podzolic soil to illustrate the typical podzolic profile. “The sod-podzolic soils are formed by the superimposition on the podzolic soil forming process of a more or less developed process of humus accumulation.” The explanation of this process is similar to that given by Williams and others, but one is left with the impression that sod-podzolic soils are in the main simply a phase or variant. REFERENCES Aaltonen, N. T. 1935. Conimuns. Inst. forest. Fenn. 20, No. 6. Afanasiev, J. N. 1926a. Zapiski Belaruskai DzarzhaunaK Akad. ScZ’skae i Lyasnoe Gaspadurki No. 1, 92-126. Afanasiev, J. N. 1926b. Pochvovedenie No. 2, 84-89. Afanasiev, J. N. 1927. Rzcss. Pedol. Invest. No. 6. Afanasiev, J. N. 1930. “Basic Outlines of the Earth‘s Soil Cover.” (Bielarusskaya Akademiya Navuk, Minsk). AltemuUer, H.-J. 1956. 2. P@nzenem?hr. Diing. Bodenk. 72, 152-157. Anderson, M. S., and Byers, H. G. 1931. U. S. Dept. Agr. Tech. Bull. 228. Avery, B. W. 1958. J. Soil Sci. 9, 210-224. Avery, B. W., Stephen, I., Brown, G., and Yaalon, D. H. 1959. J. Soil Sci. 10, 177195. Brown, B. E., and Jackson, M. L. 1958. Clays and Clay Minerals, Proc. 5th Natl. Conf. 1956, Washington, pp. 213-226. Brown, I. C., and Thorp, J. 1942. U. S. Dept. Agr. Tech. Bull. 834. Burkhardt, H. 1870. “Saen und Pflanzen.” (4 Aufl. Hannover). Quoted from Pavlinov ( 1887). DaubrBe, A. 1845. C m p t . rend. mad. sci. 20, 1775-1780. Deb, B. C. 1950. J . Soil Sci. 1, 212-220. Dokuchaiev, V. V. 1879. Cartography of Russian Soils. In “Collected Works,” Vol. 2, p. 226 et seq. ( Acad. Sci. U.S.S.R., Moscow.) Dokuchaiev, V. V. 1886. Materials for the evaluation of the lands of Nizhny Novgorod government. In “Collected Works,” Vol. 5, p. 510 et seq. (Acad. Sci. U.S.S.R.,Moscow.)

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Dokuchaiev, V. V. 1887-1891. Short programme for the investigation of soils. In “Collected Works,” Vol. 7, p. 318 et seq. Duchaufour, Ph. 1956. “PBdologie: Applications Forestihres et Agricoles” ( Bcole Nat. des Eaux et For&, Nancy). Duchaufour, Ph. 1957a. “Tableaux descriptifs et analytiques des sols” (Ecole Nat. des Eaux et For&, Nancy). Duchaufour, Ph. 195713. Compt. rend. ucad. sci. 246, 1154-1157. Duchaufour, Ph. 1960. “Prbcis de PBdologie.” Masson, Paris. Emeis, C. 1876. “Waldbauliche Forschungen und Betrachtung” Berlin. Fireman, P. 1901. Expt. Sta. Record 12, 704, 807. Fridland, V. M. 1958. Pochvovedenie N o . 1, 27-38. Frosterus, B. 1914. Geologiska Kommission i Finland. Geoteknisk Meddeknden No. 14. Gedroiz, K. K. 1926. Nosovsk. Se1.-Khoz. Opyt. Stu. Otdel ugron. No. 14. Gemmerling, V. V. 1922. Rws. Pochvoved No. 4-6; quoted from Glinka (1924). Gemmerling, V. V. 1930. Guide-book for the Excursion of the 2nd Intern. Congr. Soil Sci., Leningrad 11, 40-46. Georgievsky, A. 1888. Materialy PO izuchenie russkikh pochv, No. 4. Gerasimov, I. P. 1959. Izvest. Akad. Nauk S.S.S.R., Ser. Geogruf. No. 3, 20-30. Gerasimov, I. P., and Glazovskaya, M. A. 1960. “Fundamentals of Pedology and the Geography of Soils.” Gosudavstvennoye Izdatel’stvo Geograficheskoi Literatury, Moscow (in Russian), Gerasimov, I. P., Zavalishin, A. A., and Ivanova, E. N. 1939. Pochvovedenie No. 7, 10-43. Glinka, K. D. 1911. Pochvovedenie 13, 17-48. Glinka, K. D. 1914. “Die Typen der Bodenbildung.” Borntraeger, Berlin. Glinka, K. D. 1923. “Soils of Russia and Adjacent Countries.” State Publishing House, Moscow (in Russian). Glinka, K. D. 1924. “Disperse Systems in Soils.” Kultumo-Prosvetitel’noye Trudovoye Tovarishchestvo “Obrazovaniye.” Leningrad ( in Russian). Glinka, K. D. 1931. “Pochvovedenie” (text-book), 4th ed. State Agric. Publishing Office, Moscow. Gorshenin, K. P. 1958. Pochoovedenie No. 9, 114. Gray, R. 1839. Prlze Essays G Tram. Highland and Agr. SOC. Scot. 12, 286-287. Grossman, R. B., Stephen, I., Fehrenbacher, J. B., and Beavers, A. H. 1959. Soil Sci. SOC. Am. PTOC. 23, 73-75. Guide, 1959. Guide to the Experimental Farms. Rothamsted Experimental Station, Harpenden. Il’enkov, -, 1869. Russkoye Sel’skoye Khozuistuo 2, 118 (quoted from Georgievsky, 1888). Karpachevsky, L. 0. 1960. Pochvmedenie No. 6, 43-52. Kay, F. F. 1934. Univ. Reading Fuc. Agr. Bull. 48. Kay, F. F. 1939. Univ. Reading Fac. Agr. Bull. 62. Kindler, A. 1836. Ann. Physik. [NF] 37, 203-206. Kirsanov, A. T. 1937. Trudy Pochvennogo Inst. im. V. V. Dokuchaeva, &ad. Nauk. S.S.S.R. 14, 5-29. Kononova, M. M. 1951. “Problems of Soil Humus.” Acad. Sci. U.S.S.R., Moscow (in Russian). English translation by T. Z. Novakovsky to be published by Pergamon Press. Kubiena, W. L. 1938. “Micropedology.” Iowa Collegiate Press, Ames.

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Kubiena, W. L. 1953. “The Soils of Europe.” Murby, London. Kubiena, W. L. 1956. Eiszeitalter u . Gegenwart 7, 102-112. Kundler, P. 1959. 2. Pftanzaemiihr. Dung. Bodenk. 86, 16-36. Lundblad, K. 1934. Soil Sci. 37, 137-155. McCaleb, S. B. 1954. Soil Sci. 77, 319-333. MacEwan, D. M. C. 1948. Velre et silicates i d . 13, 41-46. Marbut, C.F. 1928. Soils, their genesis, classification and development. U . S. Dept. Agr. Mimeograph. Minashina, N. G. 1958. Pochvovedenie No. 4, 90-96. Morozov, S. S. 1940. Pochvovedenie No. 8, 43-66. Muckenhausen, E. 1957. Wiss. Schriftenreihe des Auswertungs- und Informations dienst, Bad Godesberg. Hft. 14. Muir, A. 1934. Forestry 8, 25-55. Muir, A. 1935. Forestry 9, 116-123. Muir, A., and Fraser, G. K. 1940. Trans. Roy. SOC. Edinburgh 60, 233-341. Muller, P. 1887. “Studien uber die naturlichen Humusformen.” Springer, Berlin. Neustruev, S. S. 1927. Russ. Pedol. Invest. No. 3. Newton, J. D., Bentley, C. F., Toogood, J. A., and Robertson, J. A. 1959. Univ. Alberta, Fac. Agr. Bull. 21 (5th rev. ). Nogina, N. A. 1952. Pochvovedenie No. 2, 132-144. Pavlinov, N. 1887. Alaterialy P O izuchenie russkikh pochv, No. 3. Popov, T. I. 1914. Trudy Dokuchaievskogo Pochvennogo Komiteta No. 2. Ramann, E. 1911. “Bodenkunde.” Springer, Berlin. Robinson, G. W. 1930. J. Agr. Sci. 20, 618-639. Robinson, G.W. 1935. Empire Journal of Experimental Agriculture 2, 258-273. Robinson, G. W. 1949. “Soils, Their Origin, Constitution and Classification,” 4th ed. London. (First published in 1932.) Rod&, A. A. 1930. An excursion to the Lisino Experimental Forest. I1 Congress Guide: Dokuchaiev Inst. Soil Sci. Leningad. Rod&, A. A. 1937. “The Podzol-forming Process.” Moscow-Leningrad, Acad. Sci. U.S.S.R. (in Russian), Rod&, A. A. 1944. Pochvovedenie No. 4-6, 159-179. Rod&, A. A. 1955. “Pochvovedenie” ( text-book ) . Goslesbumizdat, Moscow. Rod&,A. A., and Feofarova, I. I. 1955. Pochvovedenie No. 9, 58-60. Rod&,A. A., and Sedletsky, I. D. 1939. Trudy Inst. Pochv. Dokuchueva 19, 81-141. Satchell, J. E. 1953. Studies on Earthworms. Ph.D. Thesis, University of London. Senft, F. 1862. “Die Humus-, Marsch-, Torf- und Limonitbildung.” Engelmann, Leipzig. Sibirtzev, N. M. 1884. Soils of the Arzamas distriot. In “Selected Works” Vol. 2, p. 109 et seq. Izdatel’stvo Sel’skokhozaistvennoi Literatury. Moscow. Sibirtzev, N. M. 1896. Soils of the Previslan Region. In “Selected Works,” Vol. 2, p. 363 et seq. Izdatel’stvo Sel‘skokhozaistvennoi Literatury. Moscow. Sibirtzev, N. M. 1898. Zap. Novo-Alexandrov. In&. Se1.-Kh. 11, vyp. 3; also in “Selected Works,” Vol. 2, p. 308 et seq. Izdatel’stvo Sel’skokhozaistvennoi Literatury. Moscow. Sibirtzev, N. M. 1900. “Pochvovedenie” ( text-book), Quotations from 2nd ed. 1909, Fralova, St. Petersburg; also in “Selected Works,” Vol. 1. Skrynmkova, I. N. 1958. Pochvovedenie No. 4, 1-13. Sprengel, C. 1844. Die Bodenkunde oder die Lehre vom Boden, Nebst einer vollershdigen Anleitung zur chemischen Analyse der Ackererden ( 1st ed. 1837). Muller, Leipzig.

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Sukachev, V. N. 1918. Pochvovedenie No. 2, 1-28. Tamm, 0. 1920. Medd. Statens Skogsforsoksanst, 17, 49-300. Tamm, 0. 1930. Svenska Skogsvdrdsforen Tidskr. 28, pp. 1-30. Tamm, 0. 1934. M e d . Statens Skogsforsoksanst. 27, 1-20. Tedrow, J. C. F. 1954. Soil Sci. SOC. Am. PTOC. 18, 479-481. Tiurin, I. V. 1933. “Kurs Pochvovedeniya.” Gosydarstvennoye Izdatel’stvo Kolkhoznoi i Sovkhoznoi Literatury, Moscow. Tiurin, I. V. 1935. “Jubilee Volume for Academician V. R. Williams,” pp. 263-287. Sel’Khozgiz, Moscow. Tiurin, I. V. 1960. P o c h v o v e h b No. 7, 1-2. Tumin, G. 1912. Zhur. Opyt. Agron. 13, 321-348. Ufimtseva, K. A. 1955. Trudy Pochvenmgo Inst. im. V. V. Dokuchaeva, Akad. Nauk S.S.S.R. 46, 5-77. Vadkovskaya, 0. A. 1955. Trudy Pochvennogo Inst. am. V. V. Dokuchueva, Akad. Nauk S.S.S.R. 46, 78-135. Vysotzky, G. N. 1899. Pochvovedenie N o . 1, 36-44. Vysotzky, G. N. 1927. Byull. Pochvoveda pp. 5-13, 74-82, 104-119. Wessely, J. 1873. “Der Europaische Flugsand und seine Kultur.” Wien. Williams, V. R. 1940. “Pochvovedenie” (text-book), 4th ed. Sel’Khozgiz, Moscow. Yarilova, E. A., and Parfenova, E. I. 1957. Pochvovedenie N o . 9, 37-48. Yarilova, E. A., and Parfenova, E. I. 1959. Pochvovedenie N o . 2, 75-79. Yarkov, S. P. 1942. “Memorial Volume to V. R. Williams,” pp. 99-119. Acad. Sci. U.S.S.R., Moscow. Yarkov, S . P. 1954. The formation of podzolic soils. Comm. to V Intern. Congr. Soil Sci. (Russian and French). Acad. Sci. U.S.S.R., Moscow, Yarkov, S. P. 1958. Pochvovedenb No. 6, 30-44. Zakharov, S. S. 1908. Zhur. Opyt. Agron. 7 , 388-477. Zakharov, S. S. 1910. Pochvovedenie, 339-366. Zakharov, S . S. 1911. Pochvovedenie, 49-72. Zakharov, S . S. 1931. “Kurs Pochvovedeniya” 2nd ed. Gosudarstvennoye Izdatel’stvo Sel’sko-Khozaistvennoi i Kooperativnoi Literatury, Moscow. Zavalishin, A. A. 1944. Pochvovedmb No. 4-6, 180-204. Zavalishin, A. A. 1954. Sbornik Rabot Centralnogo Muzeya Pochvovedeniya 1, 101159. Zavalishin, A. A., and Firsova, V. P. 1980. Shornik Rabot Centralnogo Muzeya Pochvovedeniya 3, 7-95. Zavalishin, A. A., and Nadezhdin, B. V. 1957. Sbornik Rabot Centralnogo Muzeya Pochuovedeniya 2, 27-56.

SUBTERRANEAN CLOVER

. . .

F H W Morley Division of Plant Industry. Commonwealth Scientific and Industrial Research Organization. Canberra. Australia

I. Introduction ................................................. I1. Taxonomy .................................................. A . Distribution ............................................. B. Morphology ............................................. C . Cytogenetics ............................................. D. The Breeding System ..................................... I11 Physiological Variation ........................................ A. Embryo Dormancy ........................................ B . Physiology of Flowering .................................. C . Growth Rate ............................................ D . Chemical Composition .................................... IV. Nodulation and Nitrogen Fixation .............................. A . Nitrogen Economy under Sub. Clover ........................ B . The Symbiotic Relationship ................................ V . Ecology .................................................... A . Maturity or Flowering Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Seed Conservation ........................................ C . Seed Size ................................................ D. Competition ............................................. E . Weed Control .......................... ....... F. Limits to Distribution in Australia . . . . . . . . . VI . Agrotechnology .............................................. A . Establishment ............................................ B . Management of Sub. Clover Pastures ........................ C . The Role of Sub. Clover in Ley Farming .................... D . Sub. Clover and Soil Conservation ........................... E . Seed Production ......................................... VII . Diseases and Pests ........................................... A . Fungous Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Virus Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Pests ................................................... VIII. The Estrogen Problem ........................................ IX. Agronomic Improvement ...................................... A . The Varietal Scene ....................................... B. Hybridization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C . Polyploidy ..............................................

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D. Induced Mutation ........................................ E. Evaluation .............................................. X. Potential Usage ............................................. References .................................................. 1.

109 110 113 117

Introduction

Not much more than half a century ago, Mr. A. W. Howard, a farmer in South Australia, wrote to the Adelaide press stating that he had found a plant which, he believed, would go far to solving the problem of introducing nitrogen into the soil (Hill, 1936). This seemingly extravagant claim was apparently received with scepticism, but Howard, undaunted by indifference, continued to observe his discovery and to press for its recognition as a plant of great agricultural potential. His shrewd observations had not led him astray. The plant referred to by Howard, Trifolium subterraneum L. (hereinafter called sub. clover), was recorded as naturalized in Victoria in 1887 (Audas, 1921). It was observed on a South Australian farm in the Mount Barker district by Howard in 1889. In what now seems a classic understatement, Maiden (1896), the Government Botanist in New South Wales, wrote “it is not an introduction which need render us uncomfortable.” Gardner and Dunne (1933) reported that the species was naturalized in Western Australia even prior to 1893, probably having been accidentally introduced in ryegrass (Lolium spp.) seed from Europe. In 1909 Howard stated that this clover was regarded as sari insignificant weed growing in poor soils” and was “unknown to any seedsman in Europe or Australia as a fodder plant.” The first requirement for the evaluation of a plant, the production of sufficient seed for trials on an adequate scale, was met by Howard, who wrote in his diary “30 lbs of clean clover seed to Messrs E. and W. Hackett and Company on 18th January, 1906, at 2s.6d. [then about 50 cents] per pound.” By the mid-thirties 200 or more tons of seed was being produced annually; today the average production is over 3000 tons of certified seed annually. Sub. clover is established on 20 to 30 millions of acres, with a potential of the order of 100 million acres ( Davies, 1952). This article attempts to trace the development of this plant from a roadside weed to its present role as one of the most valuable plants in Australia, and to describe its cultivation and usage, its taxonomy, physiology, and techniques of improvement. It is not intended to include an exhaustive list of references. Nutritional aspects will be largely omitted because recent reviews by Anderson (1956) and Stephens and Donald (1958) have covered these. The general background, the climate, soils,

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and grassland agronomy of Australia have been described by Trumble (1952) and will not be considered in detail here. Davies (1951) stated that “there is no evidence that our agricultural scientists played any part in this development until the plant was very well on its way.” Sub. clover was obviously recognized as a plant of great potential, and it had became widely established, at least in South Australia and Victoria, by the early 1920’s. Perhaps this was largely a result of Howard’s propaganda in the early part of the century, for its use was not officially advocated until Mullett (1922) discussed its value on various Victorian farms judged in a farm competition, and also suggested methods of management. He referred to a number of tests which had been conducted by the Agricultural Department of Victoria. The first sowings were being made about this time on the southern tablelands of New South Wales. Results were disappointing until the sowing of clover was accompanied by topdressing with superphosphate According to Elliott et al. (1948) there were only 25,000 acres under sown pastures in Western Australia in 1923, a figure which sets the upper limit for clover acreage in that State. There was thus a considerable delay between the first recognition of the possibilities of the plant by Howard and its wide adoption by farmers as the basis of improved pastures. But it is by no means clear that Davies is justified in singling out for criticism agricultural scientists any more than other sections of the community (e.g., banks, finance and pastoral companies). Indeed it seems he could scarcely have expected faster progress in that period. There are probably several reasons for this lag in adoption. The role of nutrition (especially phosphorus) was not fully appreciated. Until most soils in the areas climatically suitable for sub. clover were topdressed with superphosphate, the potential of sub. clover could not be visualized. World War I, in which Australia was involved from 1914 to 1918, and various economic recessions in the 1920’s and 1930s, diverted the capital, expended the means, and destroyed the incentives for pasture improvement. Seed was not available in large quantities, and techniques for harvesting on a commercial scale were yet undeveloped. Establishment methods required investigation, and genetic variation was neither known nor appreciated in relation to the range of unexploited environments. In the years following World War I1 the crest of postwar expansion of plantings was not reached until the mid-fifties, even though the financial and social climate favored the establishment of improved pastures and technical knowledge was usually adequate. Transport, labor, equipment, and fertilizer were all in short supply and imposed limits on expansion for years after hostilities ceased.

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At present certified seed production is of the order of 3500 to 4000 tons annually. Assuming that 1 acre of pasture is established for each 3 to 4 pounds of certified seed produced, the current rate of expansion would be of the order of two million acres per annum. At this rate it will be many years before the area on which sub. clover might profitably be grown is fully planted. This slow relative increase reflects not so much a failure of agricultural science, but rather social and financial barriers to rapid expansion. The picture in New Zealand is similar to that in Australia, but on a reduced scale. Hamblyn (1937) stated that the species had been known in that country since the 1880'~~ but it was not exploited until the 1930's. He estimated that 15 million acres of the North Island alone could benefit from its establishment. Saxby (1956)) distinguished four stages of development. Between 1906 and 1925 sub. clover was regarded as a weed of little value relative to white clover. From then until 1934 it was being tried out in many localities, but it was not sown on a large scale. During this period the importance of phosphate was slowly appreciated. The use of sub. clover was restricted early in the period by lack of knowledge, later by lack of finance. From 1934 to 1947, sowings were expanded on the topographically more favored sites, but during and following World War I1 expansion was curtailed by lack of fertilizer and seed. Since 1947 expansion has been rapid, especially following the advent of aerial seeding and topdressing. The probable level of use in 1956 was about 200 tons of seed per annum. The utilization of sub. clover in other countries has not advanced to the same level as that in Australia and New Zealand. Several forms are cultivated in the subtropical zones of U.S.S.R., most of these being local ecotypes (Ivanova, 1938). In South Africa it appears to be promising in certain areas (Smit, 1943; Vorster, 1943; Penzhorn, 1944). In view of the extent of areas with a Mediterranean-type climate, it is to be expected that sub. clover will play an increasingly important role in South Africa. Sub. clover was introduced into North America about 1920 (Mestre, 1948) but has only recently been cultivated there in Oregon (Rampton, 1952), Texas (Crouch and Jones, 1945), and California (Bentley, 1946). In Texas it was judged inferior to white clover (Trifolium repens) and other species. In California, Love (1952) and Love and Jones (1952) stated that rose clover ( TrifoZium hirtum) is more widely preferred. In Israel sub. clover has become commercialized. According to Katznelson (private communication), 200 acres have been artificially established as a cover crop in deciduous orchards. Seed supply is at present the main limitation to expansion. In 1958 there were only two small nurseries of sub. clover, but this species was present on fairly extensive

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areas of range land. Perhaps improved understanding of its nutrition and management will enable this species to enhance production over extensive areas of range in the Mediterranean region. There have been reports from many countries of trials that include sub. clover, but most of these are of a preliminary nature only. The possible role of sub. clover in these regions will be evident only after further experiments, so that most of these reports will not be discussed here. II. Taxonomy

A. DISTRIBUTION

Trifoliurn subterruneurn is largely but not wholly Mediterranean in its distribution, as is shown in Fig. 1. This map was compiled by Mr. J. Katznelson, Agricultural Experiment Station, Neveh Yaar, Israel, from collections, about 30 floras from Mediterranean and Middle Eastern countries; monographs from Spain, Russia, and Italy; herbarium specimens in Jerusalem and Paris; and including some from the Food and Agriculture Organization of the United Nations. The boundaries of its area of distribution are necessarily inexact, but they clearly include a wide range of climates and presumably other environmental variables.

B. MORPHOLOGY Many descriptions have been published, for example by Gardner and Dunne ( 1933) and Aitken and Drake (1941). Some plant parts are illustrated in Fig. 2. The main features of interest are: 1. Annual, self-fertilizing, winter-growing, prostrate habit. 2. Inflorescence of 3 to 7, usually 4, perfect papillionate flowers, usually white, which form the outer part of the inflorescence. The inner part is developed after fertilization as a cluster of barbed processes, which are the calyxes of many abortive flowers. These processes form a bur enveloping the seeds. 3. Following fertilization peduncles elongate toward the ground, burying a variable proportion of burs in the soil (Yates, 1957, 1958). 4. The ovary contains two ovules, but usually only one develops. In some varieties both frequently develop, forming “twin,”hemispherical seeds. 5. Seed weight varies between and within strains. Average seed weight may be from less than 5 mg to more than 8 mg. Seed color is usually black, but white and amber forms occur. 6. A wide range exists of leaf markings, anthocyanin marks on leaves, stipules, and calyxes, and pigmentation of stems and petioles.

FIG.1. Distribution of sub. clover in original habitat (inset is the Canary Islands). Broken shading indicates uncertain whether present. (Compiled by J. Katznelson, Agricultural Experiment Station, Neveh Yaar, Israel, as part of a research project supported by the Palestine Endowment Fund of New York.)

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FIG.2. Morphology of sub. clover. A. Flowering runner from a sterile F, plant. x 0.25. B. Vegetative runner. x 0.25. C. Leaf. x 1. D. Flower. x 5. E-E3. Inflorescence development to formation of bur; approximately weekly intervals between stages. F. Pod. G. Seed.

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Varieties are distinguished by characteristic combinations of leaf marks, calyx colors, seed colors, and flowering times, which are of great value for seed certification. Some hundreds of lines, discovered in Australia or collected in the original habitat, have been assembled in Canberra. Of these Aitken and Drake ( 1941) have described fifty, mostly Australian isolates, found either by chance or in a systematic search for variants. According to C. M. Donald (quoted by Frankel, 1954) no lines introduced from the Mediterranean were identical with any from elsewhere, or with any Australian lines. The number of genotypes must be legion, but few have been evaluated from the agronomic viewpoint. C. CYTOCENETICS Wexelsen (1928) reported 2n = 16, but Yates and Brittan (1952) found chromosome counts of both 2 n = 16 and 2n= 12, the latter number being characteristic of a race from Israel, the former of six Australian varieties. Attempts to cross the two groups have so far been unsuccessful. Brock ( 1953) obtained counts of collections from Australia, England, Malta, Morocco, and Portugal, all of which were 2n = 16 whereas Israel material was 2n = 12. Zohary and Katznelson (1957, 1958) studied the distribution and morphology of the two chromosomal races in Israel, where they are sympatric in some localities. Since the races are clearly distinguishable by flower and fruit morphology, they considered that, both on morphological and genetical grounds, the Israel race should be regarded as a distinct species. Accordingly they named it T . israeliticum and published a description. They suggested that this new species, because of its earliness of flowering and seed development and its adaptation to relatively dry places (even on calcareous bedrock), could be a useful legume where drought escape was especially necessary for survival. Morley et al. (1956) found pollen abnormalities and high levels of sterility in several F1 and F2 plants (see Fig. 1 ) . In some Fa plants vegetative abnormalities were also present. Gross chromosomal rearrangements accounted for less than 40 per cent of observed pollen sterility, Morley ( 1 9 5 8 ~ )reported that, on the basis of F1 fertility, 20 Australian strains could be divided into three groups, one with 16 strains the others with 2 strains each. The largest Australian group, which includes TALLAROOK, MT. BARKER, BACCHUS MARSH, and DWALGANUP, produced highly or fully fertile (judged by pollen examination) F1 plants in crosses with 15 of 33 strains from Portugal, Morocco, Algeria, Turkey, and Greece, One Moroccan strain was fully fertile with another group (CLARE, WENIGUP),

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but no strain fertile with the third group (YARLOOP, YABBA NORTH) has yet been found, although many F1plants have been examined. Since then the wide distribution of the TALLAROOK group has been confirmed. In addition a Western Mediterranean group of several strains from Portugal, Morocco, Algeria, and the Canary Islands has been shown to constitute a fourth group, one from Portugal a fifth, and one from Israel a probable sixth. It seems likely that the TALLAROOK group, being distributed very widely in the known habitat, is the oldest of the “subspecies,” others having arisen from time to time, these eventually becoming more or less widely distributed, depending on their adaptability and perhaps on the rate of speed dispersal. The Western Mediterranean group is presumably quite ancient, but less so than the TALLAROOK group. Other explanations are, however, possible, since area of distribution is not necessarily wholly determined by age. It is perhaps of significance that, to the present, no representative of the Western Mediterranean group has been found in Australia and that a far higher proportion of the Australian varieties belongs to the TALLAROOK group than is found in the collections from any locality in the Mediterranean area. This suggests either that the TALLAROOK group is composed of much more invasive strains than other groups or that the Australian strains came from a relatively restricted area, probably not from the Western Mediterranean region. The extreme variation within this group makes it unlikely that more than a small proportion of Australian strains has arisen de nouo. Further examination of collections is proceeding. From the plant improvement viewpoint this subspeciation is a nuisance. The recombination of desirable characters is made more difficult because many hybrids are highly infertile. Admittedly fertile segregants may be obtained in later generations, but this involves growing larger populations and selection for additional characters.

D. THEBREEDING SYSTEM Because of the cleistogamous flowers and the stability of lines from generation to generation (despite absence of measures to prevent crossing), sub. clover has usually been regarded as wholly self-fertilized. Symon (1954) suggested that the frequency of seedlings with chlorophyll aberrations found by him, approximately 1 and 1500, indicated either a total mutation rate of the order of 1 in 700 to 800 for all genes capable of such mutation, or the presence of heritable variation (presumably as a result of occasional hybridization). This is not necessarily so for, even assuming that the majority of the aberrants were genetically determined, and were completely lethal, a frequency of the order observed would be expected if approximately 150 loci with mutation rates

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about were involved-a figure that is by no means excessive. Further, it is evident that the frequency of aberrants of this type must depend on the mutation rate, not on the breeding system, if the population is maintained in an equilibrium between mutation and elimination. Also the proportion of heterozygosity for recessive lethals is kept relatively low by even moderate (20 per cent) levels of selfhg (Wright, 1931), so that the imposition of increased levels of selfing would be unlikely to markedly raise the frequency of aberrants. Therefore Symon’s results seem to indicate no more than that a moderately large number of mutational blocks can affect chlorophyll content. This is supported by Carpenter’s (1958) results on irradiation of sub. clover and is consistent with numerous investigations on chlorophyll defects in other species. Chlorophyll defects comprise a large fraction of either natural or induced mutations. Frankel and Williams (1958) used the dominant marker, redleaf, to demonstrate the occurrence of natural crossing in the field. They emphasized that since the recessive parents were segregates of a cross ( YARLOOP BURNERANG) in which many plants were highly infertile, their results might not represent those to be expected from normally fertile plants growing together in the field. I have observed honeybees working sub. clover swards and noted that they made brief visits to many flowers, but these flowers were at a stage at which fertilization would already have taken place. Nevertheless the possibility certainly exists that occasional hybridization occurs. Indeed, as discussed elsewhere (Morley, 1960), a certain amount of out-crossing has probably been of great significance to the evolution of this species. However, I have no evidence that cross fertilization is other than very rare in Australia, Some pollinating agent may be present in the natural habitat, but no evidence for this has been reported. However, Katmelson (private communication) observed in Israel that some flowers were damaged by a beetle before fertilization, and this may have increased the chances of cross pollination.

x

111.

Physiological Variation

Sub. clover is present in a wide range of natural habitats and therefore might be expected to possess a wide range of physiological adaptations to these habitats. In this section present knowledge of the physiology of this species is summarized. A. EMBRYO DORMANCY Germination may be inhibited by hardseededness, which prevents absorption of water. This must be distinguished from embryo dormancy,

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which may be defined as the inhibition of germination in viable, fully imbibed seeds. Woodforde (1935), Toole and Hollowell ( 1939), and Loftus Hills (1942b, 1944b, c, d, e ) showed that high temperatures inhibit germination, as is found in many winter growing species. Large differences between varieties were observed. Ballard ( 1958) showed that carbon dioxide, even at less than 1per cent by volume, relieved dormancy in sub. clover and in several other annual legumes in which germination is inhibited by high temperatures (Grant Lipp and Ballard, 1959). Morley (1958b) found differences between strains and between plants of the same strain, the latter presumed to be environmentally determined. He showed that the genotype of the embryo was implicated and that there was a tendency for high dormancy to be inherited as a dominant. Dormancy has been shown in these publications to be relieved by low temperatures, carbon dioxide, activated carbon, hydrogen peroxide, and removal of the testa. Per cent germination is also higher if seeds are almost wholly immersed in water, rather than being placed on moist paper. Dormancy is high immediately after harvest but decreases with time at rates differing with variety. I have found that differences in dormancy between and within varieties may persist for five or more years but may not be evident unless tests are made at sufficiently high temperatures (e.g., 25" to 28°C.). Although pH within the range normally encountered in soil does not appreciably affect germination ( Aitken and Davidson, 1954), I have found that exposure for an hour or two to HC1, H2SO4,or a wide range of organic acids at a pH in the region of 2.5 to 3.0 stimulates extrusion of the radicle within 2 to 4 hours. This is usually, but not invariably, followed by germination unless toxic effects have supervened. The extrusion of the radicle may be inhibited by calcium, but not by magnesium. The present evidence is consistent with the theory that germination may be prevented by the absence of one or more promoters, or the presence of inhibitors. Inhibitors may be prevented from diffusing away by the testa, or may be destroyed by acids or other chemicals. But gas exchange affecting the synthesis of promoters must also be considered and seems at present equally probable. Embryo dormancy is clearly not a simple phenomenon. Nor should it be expected to be simple since germination involves a number of steps that may be blocked by different mechanisms. Thus, I have been able to separate initial cell elongation as shown by radicle protrusion from subsequent steps that require cell division. The clarification of the nature of genetic differences and of the influence of environment may depend very largely on a separation of the component processes. Genetic and biochemical tools may both be required.

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B. PHYSIOLOGY OF FLOWEFUNG The period from germination to flowering may conveniently be divided into two intervals: from germination to flower initiation, and from initiation to the appearance of the flower. The first of these intervals is subject to a variety of physiological processes, each influenced by environmental stimuli. Sub. clover is a long-day plant (Aitken, 1955a); that is, flower initiation is promoted by increased day length. It shares with many other long-day plants a requirement for low temperatures during the vegetative phase (vernalization). This low temperature requirement is greatly modified by day length. Evans (1959) showed that, in continuous light, vernalization was not required by any of the varieties tested. Both he and Aitken (1955a) also showed that plants of some varieties, when exposed continuously to low temperatures, initiated flowers even in continuous darkness. In the normal range of growing conditions a genetically variable amount of vernalization was necessary for initiation. Flowering time in the field seemed to depend on the interaction of vernalization with day length. As the vernalization process continues, the requirement for long days decreases until the critical day length corresponds with the prevailing day length. Initiation then takes place. Unfortunately, the facts were not found to be in full accord with this theory. Morley and Davern (1956) described large genotype-environment interactions in the field. Attempts to explain these in terms of differing day length and vernalization requirements were not wholly convincing. The demonstration in the laboratory (Evans, 1959) and in the field (Morley and Evans, 1959) of a third process, one that is necessary for initiation and that is promoted by high temperatures, offered a more satisfying explanation of varietal behavior. In cold climates, such as in Canberra, flower initiation may be prevented by the retarding effect of low temperatures on a promotive process which follows vernalization. This process is evidently also of significance in the flowering of lettuce and wheat. Other processes may be operative, but the three described seem sufficient to explain varietal behavior in different environments. Earliness in all environments demands small vernalization and long-day requirements and the capacity to initiate at low temperatures. Lateness may be determined by requirements for extended vernalization, for long days, or for high temperatures for initiation, or some combination of these, The time of flowering is thus a function of the particular physiological limitations and of the environment in which tests are conducted.

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Flowering time is polygenically determined, approximately 90% of the total variance being additive genotypic (Davem et al., 1957). Requirements for different stimuli (e.g., vernalization) appear to be continuously distributed and are therefore probably determined by many genes with small effects. They are not “all-or-none,” discontinuously distributed, as if determined by few genes with large effects.

C. GROWTH RATE The iduence of temperature and light energy on the growth rate of sub. clover was examined in two adjoining papers by Black (1955a) and Ozanne (1955) using very different methods and leading to widely different conclusions. Black made 52 consecutive weekly measurements of the growth rate of widely spaced plants of the BACCHUS MARSH variety. These were grown in the open with ample moisture and nutriment, in the relatively mild climate of Adelaide, South Australia. Growth w& determined, on a series of plantings, between two points; the first when plants reached the second trifoliate leaf stage, the second 1 week later. Light energy received was measured on an integrating recorder. Mean temperatures were obtained from thermograph charts in a Stephenson screen. Black then estimated partial regression coefficients, using various measures of growth rate as the dependent variable; light energy, mean temperature, maximum and minimum temperatures were included as independent variables. He found positive associations of light energy and maximum temperature, and a negative association of minimum temperature, with net assimilation rate. Mean temperature was not significantly correlated with net assimilation rate. Relative growth rate was significantly correlated with light energy received, but not with temperature. Black constructed an equation relating plant weight to light energy received. He continues to maintain that light energy was the principal factor affecting seasonal growth (Blackman and Black, 1959) and that temperature is of minor significance, except perhaps in extreme conditions. Ozanne (1955),in a study of the effect of day length and light intensity on the response to zinc, grew plants for 6 to 8 weeks in natural daylight in a greenhouse, subjecting them to different light intensities and day lengths by screening. His results showed a fairly consistent effect of day length and light intensity. The growth rate at 1400 foot-candles light intensity and 11.75 hours daylight, was about equal to that at 8000 foot-candles light intensity and the same day length. If one substituted the appropriate values for the independent variable of this experiment into Black‘s prediction equation, one would predict a negative growth rate. Ozanne’s findings are thus very much at variance with Black‘s con-

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clusions. However, it is not valid to conclude from these results that the optimum light intensity for growth of sub. clover lies near 1400 footcandles, for leaf temperature may have been above optimum in the highintensity treatment. Millikan ( 1957) demonstrated the effect of light intensity (below 1220 foot-candles ), day length, temperature, and variety on the growth rate of plants grown in the Earhart Laboratory. Mitchell (1956a, b ) showed that MT. BARKER sub. clover has, at rather low light intensities, a relatively low optimum temperature for growth compared to T . repens or Lotus major. Morley (1958a) compared growth rates of five varieties at three temperatures in temperature-controlled greenhouses, at relatively high light intensities. Temperature and strain effects and strain by temperature interactions were significant for relative growth rate and net assimilation rate (production per unit leaf area), especially for the former. Black (1955a) and Blackman and Wilson (1951) have both emphasized the limitations imposed by light energy, and Black discounts the effect of temperature. In contrast, the other authors have found maximum or high growth rate in spaced plants at relatively low light energy values and have demonstrated clear temperature responses. How can these conflicting conclusions be reconciled? The statistical technique used by Black, multiple regression, might readily give false conclusions, with relatively few degrees of freedom, if two or more of the independent variables were strongly correlated and if responses were nonlinear. Light energy is strongly correlated with temperature, even more so with day length (which Black did not consider). The response of growth to temperature is clearly nonlinear. The environment prior to his first harvest could also have influenced his results. Black's conclusions must therefore be rejected. They are subject to several criticisms and give results incompatible with those obtained by other workers with techniques less open to question. The suggestion by Blackman and Wilson (1951) that sub. clover is ill adapted to shading because adjustment of the leaf area:plant weight ratio to shade is slight may nevertheless be entirely valid. Seasonal variations in sward productivity may be influenced by light energy, temperature, and day length, to a degree depending on the density of plants. Shortly after germination, growth rate will depend largely on the variety, temperature, and day length. As a leaf canopy forms, mutual shading will cause light intensity to become increasingly limiting for the lower leaves in the canopy. Light energy must then become the main factor determining sward producton. But, at this advanced stage, livestock production is unlikely to be greatly influenced by growth

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rate, since the production of the pasture is far greater than can be consumed. In southern Australia the value of the direct contribution of sub. clover to livestock nutrition will depend on the rapidity with which the photosynthetic factory is built. Contrary to the opinion expressed by Donald and Black (1958), light is unlikely to be so much the “operative limiting factor to the level of production” as are the reaction of varieties to low temperatures and short days and the capacity for producing and conserving sufficient seed to assure high density (Davidson and Donald, 1958). As Davidson and Philip’s ( 1956) calculations show, the greater the leaf area index, the more sensitive is total stand photosynthesis to changes in illumination. At small leaf area indexes, when the amount of fodder available to grazing animals limits animal production, increases in light intensity are unlikely to increase growth rate to an important extent. On the other hand the production of swards destined for harvesting for fodder conservation will be profoundly affected by light intensity whereas, at this time, temperature is likely to be nearly optimum and days long. This subject is discussed further in Section V, D. I have perhaps overemphasized the differences between viewpoints of different workers, but this will serve to draw attention to the necessity for further investigation of the role of temperature, day length, light energy, and light intensity in determining the contribution of sub. clover to animal production. Techniques for selection for growth in critical environments may impart logic and speed to plant improvement. But it is necessary first to be certain what factors of the environment are limiting. Such knowledge may also lead to improvements in chemical treatment to raise production at certain seasons. The increase in winter production following use of gibberellic acid reported by Scurfield and Biddiscombe (1959) may be an initial step in this direction.

D. CHEMICAL COMPOSITION The chemical composition of the whole plant, and of its parts, is expected to vary greatly according to degree of maturity. Hardwick (1954) found that the content of free and reducing sugars was high in the young plant, but lowest in the mature plant. May and Davidson (1958) found that nonstructural carbohydrates decreased after defoliation but that a period of recovery followed this decrease. They postulated that this decrease results largely from the use of these materials as respiratory substrates. Beck (1952) found that the crude fiber content rose from 10 to 12 per cent in early growth, to 30 to 35 per cent in late spring. The proportion of lignin and the ratio of lignin to cellulose increased during

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the later stages of the life cycle. Shapter (1935) found 23 per cent of fiber at flowering time. The main interest in chemical composition has become focused on estrogens, which are discussed in Section VIII, and on protein content. Shapter (1935) and Beck (1952) found 20 to 29 per cent of protein in the leaves petioles of DWALGANW, but this diminished to 10 to 12 per cent in the late spring. The composition of seeds is of special interest because of their role as a fodder reserve. Shapter (1935) and Franklin and Powning (1942) reported about 40 per cent protein and 6 and 15 per cent fat in seed, but Beck (1952) reported only 28 per cent protein. Rossiter (private communication, 1960) found values similar to those of Shapter in seed of DWALGANUP, YARLOOP, and BACCHUS MARSH. The fats are mostly unsaturated, with more than 10 carbon atoms per molecule of fatty acid (McArthur, 1942). The amino acid composition suggests the presence of cysteine, and tryptophan (Lugg deficiencies in methionine, cystine and Clowes, 1945) and also isoleucine and threonine (Holmes, 1953), but the arginine content is apparently ample (Holmes, 1951). Williams and Moir (1951) fed Merino lambs a basal diet of chopped oaten hay and wheat grain supplemented with various sources of nitrogen. They estimated the biological value of the protein in sub. clover seed to be about the same as that of linseed. However, since 60 per cent of the nitrogen in the diet came from the basal ration (probably quite high in methionine), this result is scarcely critical. It is by no means clear that sub. clover plant or seeds are deficient in any amino acid or other chemical component required for ruminant nutrition. Since sub. clover would seldom be the only item in the diet, any deficiency may not be serious if it is made good by the nonclover portion of the ration. Differences between genotypes in amino acids or other chemical compounds remain to be explored.

+

+

IV. Nodulation and Nitrogen Fixation

A. NITROGEN ECONOMY UNDER SUB.CLOVER The most important contribution of sub. clover to pastures is the fixation of nitrogen, through symbiosis with Rhizobium trifolii, in root nodules. This nitrogen may not become available to associated grasses until the clover plant has virtually completed its life cycle (Trumble and Strong, 1937; Trumble and Shapter, 1937a,b) except as nitrogen returned to the pasture after passage through the grazing animal. However, having been fixed, it accumulates in quantities sufficient to main-

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tain a highly productive grass-clover sward in nitrogen equilibriumprobably requiring a turnover of 50 to 100 pounds of nitrogen per acre per annum. As a large proportion of this nitrogen would be returned to the soil in plant debris, or after animal passage, the actual amount fixed each year is diEcult to estimate. It seems unlikely to approach figures such as 400 pounds per acre per annum quoted by Russell (1950)), or found by Sears (1951) in New Zealand pastures, but it may be much greater than 100 pounds. The nitrogen economy of Australian soils has been inextricably bound with their phosphate status, and without phosphate, nitrogen is seldom fixed in appreciable amounts in the higher-rainfall districts. Cook (1939) found at Kybybolite, South Australia, an increase in soil nitrogen of the order of 1000 pounds per acre in the top 6 inches after 15 years under sub. clover which was topdressed annually with 90 pounds of superphosphate per acre. Neglecting the small loss of nitrogen from sale of livestock products and losses from leaching, erosion, and denitrification, this figure represents the fixation of about three-fourths of a pound of nitrogen for each pound of superphosphate applied. Donald and Williams (1954) estimated that 0.76 pounds of nitrogen were fixed per pound of superphosphate applied to sub. clover pastures in the Crookwell district on the southern tablelands of New South Wales. The agreement between these estimates is probably fortuitous since neither figure is likely to be near the potential. If, as Jensen (1947), Greaves and Jones (1950), and Gibson and Nutman (1960) have suggested, the presence of nitrogen inhibits nodulation, and perhaps nitrogen fixation, it seems that fixation would be below maximum, especially in the later years. There is no indication of a decline in the Crookwell results, and both estimates were obtained from predominantly sub. clover pastures in which the nitrogen level would be high for most of the period under consideration. It is nevertheless possible, since yields of clover were increasing with time (and phosphate status), that soil nitrogen would have inhibited, to some extent, the fixation of nitrogen per unit of pasture grown, Further, the rate of build up of soil organic matter, and thus nitrogen, was probably limited at Crookwell by sulfur deficiency (Williams and Donald, 1957). It is relevant that most of the extra nitrogen observed by Cook lay in the top 4 inches of soil; leaching may not have been an important cause of nitrogen loss in this instance.

B. THE SYMBIOTIC RELATIONSHIP Certain rhizobia that were effective on sub. clover were shown by Strong (1937) to be ineffective on T. pratense and T . repens, and vice versa. Since then strains of clover rhizobia have been further investigated

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by Purchase and Vincent ( 1949), Vincent (1954a, b, 1956), and others. Isolates from the coast or wetter localities were generally effective with T. repens or T. pratense, but not with sub. clover. Isolates from the drier inland were mostly effective on sub. clover and T. glomerutum. Vincent (1954b) placed sub. clover with T. incarnatum, T. glorneratum, and T . alexandrinum in a group with broadly similar nodulation reactions. Nutman (1956) proposed the inclusion of T. ornithopodoides in this group. He suggested that the species within groups tended to be related ecologically rather than taxonomically, possibly reflecting cross inoculation in native habitats. Nutman (1956) and Vincent (1954b, 1956) have reviewed all but the most recent literature on root nodule symbiosis in sub. clover, so that much earlier material will not be referred to again here. However, the factors that limit nitrogen fixation by sub. clover will be considered briefly. These may be classified by their effect on the host plant, on the rhizobia, or on the processes of nodulation and nitrogen fixation which involve both host and rhizobia. The amount of nitrogen fixed may be limited by poor growth of the host plant. The most obvious and important limitation in Australia is inadequate water. It is illogical to expect appreciable nitrogen fixation in regions where plant growth is restricted by lack of moisture; and a sward which is sparse, whether because of lack of water or some other deficiency, is unlikely to contribute useful amounts of nitrogen to soil, pastures, or crops. For this reason sub. clover may not always benefit non-irrigated pastures in semiarid regions, nor contribute to livestock production in such areas. In such places water is more likely to limit livestock production than is nitrogen, although soils in such regions may be nitrogen deficient. Nutritional deficiencies, especially of phosphorus (Anderson and McLachlan, 1951; and many others), sulfur (Anderson and Spencer, 1950), and trace elements have seriously limited nitrogen fixation by a direct effect on the host plant. Nodulation is dependent on the presence of a healthy and dense population of rhizobia in the rhizosphere. Purchase and Nutman (1957) found that the nodulating capacity of a clover root is saturated by a rhizosphere population of lo4 bacteria per milliliter, such densities being attained by rapid multiplication in the rhizosphere under suitable conditions. Soil acidity may be more critical for the growth of rhizobia than for that of sub. clover: indeed sub. clover flourished at pH4.5 although rhizobia failed to grow (Loneragan, 1958, 1960; Loneragan and Dowling, 1958). Anderson and Moye (1952) showed that harmful effects of low

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pH on sub. clover nodulation and pasture establishment could be overcome by relatively light dressings (224 pounds per acre) of lime, applied so as to be in close contact with the seed. This approach was taken further by Loneragan et al. (1955a, b ) , who obtained satisfactory nodulation and establishment of sub. clover from 3 pounds of seed pelleted with 5 pounds of lime per acre, results being quite comparable to those obtained from drilling 224 pounds of lime per acre with the seed. In practice this development has often, but not always, given satisfactory results (Paton, 1960; Cass Smith and Holland, 1958; Fitzpatrick, 1958). Some soils may be so deficient that they are unsuitable for renewed nodulation and nitrogen fixation in following years unless heavy dessings are applied. The viability of rhizobia on seed seems to be somewhat precarious (Vincent, 1958), these organisms being particularly sensitive to contact with superphosphate. The addition of equal parts of lime to the fertilizer reduced the decline in viable organisms fivefold. Lime pelleting of seed may be beneficial by aiding both the survival of rhizobia and their multiplication. The population density of rhizobia in soil apparently depends largely on characteristics of the soil in relation to type of organism. Survival of rhizobia effective on sub. clover is no problem in some soils, but requires some soil amendment (e.g., adjustment of pH) in others which support different types of rhizobia (Vincent, 1954a). In still other soils there may be competition due to rhizobia that are ineffective (Harris, 1953, 1954) or to unidentified soil microorganisms that may be antagonistic or competitive (Hely et al., 1957; Cass Smith and Holland, 1958). In such soils effective nodulation has been obtained by 100- to 1000-fold normal doses of inocula or by soil fumigation or sterilization; these measures would be impractical on a commercial scale, although calcium cyanamide treatment, as proposed by Cass Smith and Holland, might be a practical proposition. Bergersen et a,!. (1958) proposed pelleting seed with a nutrient material to enable an increase in inoculation level around the seed during germination. A bentonite-dried blood pelleting greatly improved nodulation at three localities, Trace elements and insecticides may also decrease rhizobial populations (Vincent, 1958; Braithwaite et al., 1958). The factors involved in gradual improvement in nodulation without lime (Anderson and Moye, 1952) are not clear. In view of Loneragan’s (1958) findings, this probably does not result from the supply of calcium to rhizobia, through superphosphate, but phosphorus may be involved. It is possible, although evidence is yet inadequate, that selection may occur of rhizobia capable of surviving and nodulating effectively under the generally unfavorable conditions (Vincent, 1956). If this were so

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these could be valuable for use in similar problem areas. Possibly a gradual increase in numbers of effective organisms may 'determine the change in nitrogen status (Paton, 1960). The process of nodulation may fail because specific nutritional requirements are not met (Loneragan, 1960) or because of the inability of strains of rhizobia to complete certain stages of infection or nodule formation (Bergersen, 1957). Combined nitrogen may inhibit nodule formation, as discussed earlier, but it is not clear whether such inhibition is important in old, nitrogen-rich, pastures. The processes of nitrogen fixation may be restricted by nutritional deficiencies (review by Nutman, 1956). The calcium requirement for nitrogen fixation seems to be higher than that for growth of the host plant (Loneragan, 1958), moderate calcium deficiency resulting in extensive nodule degeneration. The special role of molybdenum in nitrogen fixation has been reviewed by Anderson (1956). Small amounts are required for nitrate reduction in the host plant. Larger amounts are needed for symbiotic nitrogen fixation: plants with moderate molybdenum deficiency may thus be nitrogen deficient. An increase in soil pH may correct molybdenum deficiency by rendering the element available to clover. Other trace elements may play a critical role (Hallsworth et al., 1960). High temperatures may also inhibit nitrogen fixation in sub. clover (Meyer and Anderson, 1959). A. Gibson (private communication, 1960) has found that different strains of rhizobia have different temperature optima and digerent ranges of adaptation to temperature. Whether temperature tolerance of rhizobia is a factor limiting nitrogen fixation in the field is yet to be determined. The presence of variety-strain interactions has been demonstrated in number of nodules formed in the field (Vincent, 1956; Waters, 1956). Nutman (1961) discounted the existence, in sub. clover, of strain-variety interaction in effectiveness, but recently A. Gibson (private communication, 1960) has demonstrated such interactions in tube cultures. The investigation of interactions in the field is difficult, but the use of a genetic indicator may be a useful aid to such studies. The redleaf gene may be used as an indicator of effectiveness of nodulation in media deficient in nitrogen (Brockwell, 1956). Backcrossing of this gene into different genetic backgrounds may prove of value for rapid determination of field response, but this technique has not yet been exploited. There are thus many factors that may inhibit the survival of rhizobia, nodulation, and effective nitrogen fixation. Many environments are unfavorable to a satisfactory sub. clover-rhizobium symbiosis (Vincent, 195413, 1956). Therefore inoculation of seed with strains of known effectiveness, in addition to precautions against nutritional deficiency or

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unfavorable pH, are to be recommended generally unless local experience has clearly shown these steps to be unnecessary. V. Ecology

Sub. clover, as discussed earlier, is naturally distributed around the Mediterranean region in areas having predominantly wet winters and dry summers. Within this area of distribution it may be found in habitats from sea level to over 1500 meters, and with annual precipitation from 350 to over 1100 mm. Therefore it may be expected to exhibit considerable variation in those attributes that are of major significance in adaptation. Unfortunately the physiological and morphological bases of adaptation in this, or any other, species still largely elude us. An attempt is being made to examine this experimentally (Morley and Frankel, 1959; Morley, 1960), but results must await the action of natural selection and the subsequent assessment of changes in different characteristics.

A. MATURITYOR FLOWERING TIME Flowering date, or maturity grade, has attracted the interest of most workers associated with sub. clover, probably because of its constancy within any variety at any location and its obvious significance in adaptation. Thus Donald and Smith (1937) emphasized the effect of maturity grading on seed yield of spaced plants, and Rossiter (1959) in spaced plants and in swards. These workers, and results from our spaced plants in Canberra, showed that the seed yield of spaced plants increased with lateness of flowering. A curvilinear association was noted by Donald and Smith, apparently caused by lack of moisture late in the season. TALLAROOK appeared to be an exception among late varieties in possessing the ability to produce seed under unfavorable conditions. In swards Rossiter (1959) observed a decline in seed production with maturity grading (lateness), the steepness of decline varying with location and year. In swards at Canberra the trend has been, if anything, in the reverse direction. As Rossiter suggested, the seed production potential in his swards was presumably reduced by insufficient moisture in late spring, a limitation of much less significance in the irrigated plots at Canberra. In general one would expect early flowering to be important, as a mechanism of drought escape, in the arid parts of the area of distribution. But the advantage of late flowering in the moister parts of the area of distribution is by no means obvious. Morley et al. (1961) proposed an explanation based on the higher proportion of leaf in later strains relative to early strains, as demonstrated by Lawson and Rossiter (1958) and others, The early varieties would tend to be deprived of light

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by the leafier, later varieties and would thus tend to be eliminated from the mixed sward. Examination of strains collected by C. A. Neal-Smith over a range of habitats in the Mediterranean region disclosed a rather loose relationship between flowering time at Canberra and moistness of habitat. The results set forth in the tabulation were obtained. Maturity Habitat Dry Moist

Late

Very early

Early

Midseason

8

7 11

10

1

7

15

-

Number of strains 26 33

However, such data are not especially critical. The flowering time in a cool locality such as Canberra may not reflect the flowering time in the natural habitat. A variety might be late flowering at Canberra because of limitations on a process requiring high temperatures (Evans, 1959; Morley and Evans, 1959) whereas in a warm environment, where such limitations are not imposed, it may be early. Examples were given by Morley and Davern (1956). In a study discussed by Frankel (1954), a wide range of maturity was often found in plants from the same locality. Although this does not necessarily reflect a wide range in flowering time in the natural habitat, the relationship between maturity and aridity is by no means clear cut, yet the general pattern is unmistakable. Perhaps heterogeneity within localities results from microclimatic variation as well as from variation from season to season. My own observations of natural populations show that such heterogeneity is not confined to flowering time. Since the correlation is imperfect in the natural habitat, it is scarcely surprising that neither Donald and Smith (1937), nor Aitken and Drake (1941), were able to demonstrate a clear relationship, such as that proposed by Trumble (1937), between flowering time and the distribution of strains in various climatic regions of Australia. With prolonged natural selection, a relatively clear pattern can be expected to emerge only in areas with a regular and distinctly Mediterranean pattern of rainfall, and observations by Rossiter (private communication, 1960) appear to conform to this expectation. Otherwise, irregularity of rainfall pattern from season to season, coupled with the conservation of hard seeds over several seasons, and the influence of other traits such as rate of seed maturation (Millington, 1956), might be expected to maintain a degree of heterogeneity which may itself be an adaptation to a fluctuating environment ( Morley, 1960). This compromise situation suggests that in

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all but extreme situations a fairly wide range of maturity types should be tested, and perhaps included in pasture mixtures.

B. SEEDCONSERVATION Conservation of seed may be achieved by two mechanisms: hardseededness and embryo dormancy. Aitken (1939) showed that hardseededness depends on the continuity of an impermeable suberized thickening. This may be broken by physical or chemical agencies. Under natural conditions, softening of seed is probably a consequence of fluctuating temperatures. We have found that over 90 per cent of mature seeds of several varieties are impermeable to water if they have been handled very carefully. Threshing will reduce the proportion of hard seed to levels that apparently depend largely on the size of seeds and severity of treatment. Embryo dormancy may be defined as failure of fully imbibed and viable seed to germinate. Loftus Hills (1942b, 1944b, c, d, e, f ) examined both dormancy and hardseededness in seed samples from different varieties grown in different sites. He showed that dormancy, which was maximized by high temperatures, differed strongly among varieties, in contrast with hardseededness, in which varietal differences could not be demonstrated. The physiology of embryo dormancy was discussed in detail in Section 111, A. Donald (1959) showed that, in Canberra, 92 per cent of the seed crop which germinated did so in the year following its production, 6.3 per cent in the second year, values falling to 0.07 per cent in the fifth year, after which germination was nil or negligible. There was a regular seasonal pattern, the peak of germination occurring in the late summer and autumn both in the year following seed formation and in subsequent years. The relative contributions of hardseededness and dormancy to delay in germination could not be assessed from Donald's results. The fact that a large proportion of the total germination occurred shortly after seed formation, at a time when dormancy would still be maximum, suggests that neither mechanism was very effective. Nevertheless imbibed, but highly dormant, seeds may not germinate in soil at high temperatures, although moisture is ample. They will remain viable despite several repeated cycles of wetting and drying (Morley, 1958b). Further, I have found that seed five years old may still be highly dormant at 30" C. The fact that the most dormant strains are to be found in the coolest parts of the area of natural distribution suggests that dormancy is probably an important ecological adaptation serving to diminish seed losses. The mortality in the young seedlings which emerge after summer

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rains, and which frequently encounter conditions unfavorable for survival, would thus be avoided. Adams (1943) showed that hard seeds of the variety DWALGANUP survived with unimpaired germinating capacity from 1929 until 1942. Meadley (1947) found no decline in viability in seed stored in closed, but not sealed, containers after nine years. I have found no indications of loss of viability in samples stored in envelopes in laboratory drawers for six years, and approximately 40 per cent viable seeds from samples stored in cans for sixteen years, compared with over 95 per cent viability in fresh samples. Although maintenance of viability under such conditions may not be indicative of the viability to be expected of seed in soil, these results do suggest that the virtual absence of germination observed by Donald, after five years, was not caused by loss of viability but indicated that practically all seeds had germinated. His results are consistent with a germination of about 90 per cent of seeds of BACCHUS MARSH, MT. BARKER, or TALLAROOK and 80 per cent of DWALGANUP for each year in the ground. Since the seed yields obtained by him were unusually light, the observed numbers of plants germinating per unit area in the fifth and subsequent years might be appreciably less than that to be expected under normal stands. However, it is possible that a large proportion of seeds never germinated, since Rossiter (private communication, 1960) found that only 40 per cent of the original seeds could be accounted for after five or six years in the ground. The various mechanisms for maintenance of viability thus provide insurance for survival over a number of consecutive seasons. They are also the means of maintaining genotypic diversity-in itself an adjustment to the ecological range encountered within any locality.

c. SEED SIZE Seed size and weight changes with germination have been the subject of several papers by Black (1955b, 1956, 1957a, b, 1958) and Lawson and Rossiter (1958) and of a review by Black ( 1959). Seed size largely determines the growth rate of the plant (but not the relative growth rate) and thus influences the time at which competition becomes operative in swards of given density. Black (1956) concluded that seed size was important also in limiting the maximum hypocotyl elongation, and hence emergence at any depth of sowing (or natural burial). Once cotyledons had emerged seed reserves were of no further significance, the growth rate being determined by cotyledon area, which depended on seed size, not on depth of planting. However, it is possible that growth rate could equally depend on meristem size, which is perhaps a function of seed size, so Black's contention that growth rate depends on cotyledon area

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is not yet wholly proved. The competitive advantage of seed size in dense swards was revealed by Black (1958), who found that the proportion of the total leaf area contributed by plants from small seeds rapidly diminished. There would thus be natural selection for large seed size, especially in dense communities. The ecological significance of the marked strain variations in seed size found in sub. clover remains obscure. Two opposing influences may be visualized. As Salisbury (1942) emphasized, large fruit size is advantageous in closed communities, whereas the production of large numbers of small seeds is preferable in open communities. In sub. clover there appears to be a tendency for large seed size to be more frequent in the warmer parts of the area of distribution, especially Morocco and Israel, than in the cooler parts. This may reflect intensity of competition, but it seems more likely to result from selection for rate of root growth, this being sufficient to keep pace with drying out of surface layers of the soil in seedlings from large seeds, perhaps inadequate in those from small seeds, In our natural-selection plots, large seed size does not seem to have conferred any overwhelming advantage. There are considerable varietal differences, almost independent of seed size, in the proportions of testa, cotyledons, and hypocotyl of seeds (Morley and Rogers, unpublished, 1957). Apparently these differences in proportions have little or no influence on seedling growth rate, as judged by results from five rather contrasting varieties (Morley, 1958a). D. COMPETITION Intraspecific competition in sub. clover has been examined by a number of workers in relation to yield of swards of different density (Donald, 1951, 1954; Lawson and Rossiter, 1958), the effects of seed size (Black, 1958), and petiole length (Black, 1960). The essential theory of these studies was examined by Davidson and Philip (1956). In the presence of adequate moisture, and in normal growing temperatures, competition between plants for light begins at a stage of growth that is dependent on density of planting and increases as the ratio of leaf area to ground area (leaf area index = LAI) increases. The interception of light by leaves is almost complete at an LA1 of approximately 5 (depending somewhat on light intensity). At a higher LA1 many leaves receive insufficient light to maintain essential functions, and these must degenerate. According to Davidson and Philip (1956), at LA1 values above optimum, sward production will decrease because increasing numbers of leaves in the lower layers become parasitic. However, as Saeki (1960) suggested, the evidence from translocation studies indicates that, since such leaves do not receive assimilates from other leaves, they do not become para-

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sitic. Nevertheless, since their losses from respiration exceed gains from photosynthesis, these leaves would lose weight. The rate of increase of plant material in the sward would thus decline as LA1 increases above optimum, although the rate of decline might be appreciably less than that proposed by Davidson and Philip. At high LA1 values genotypes at a disadvantage in competing for light may be rapidly eliminated. Thus Black (1960) showed that the variety YARLOOP, which has very long petioles, rapidly suppressed varieties with shorter petioles when grown in mked swards at high density. Competitive ability is, however, a function of many characteristics. In our natural selection experiments the most predominant variety, MT. BARKER, in those plots where plant density has been consistently high, is not blessed with large seed size or with long petioles. Presumably it has some compensating advantages, but what these are is yet obscure. Interspecific competition follows the same general principles as intraspecsc competition, but of course competing species may differ more than do competing varieties. Sub. clover has competed successfully with endemic species and with many introduced species over many millions of acres. This is no accident. Davies et al. (1934) and Trumble and Donald (1938a) demonstrated the effect of sub. clover on yield, nitrogen status, and the proportion of grass to clover in native and sown pastures. Anderson and McLachlan (1951) and Donald and Williams (1954) have described the characteristic disappearance of endemic species under the impact of sub. clover and superphosphate, the result being initially a sward composed predominantly of sub. clover with some introduced grasses if these had been sown. But after several years of superphosphate application to pure sub. clover swards, grasses and other nonleguminous species, such as thistles, invade the swards and may even at times dominate them. The appearance of thistles and other nonlegumes in old sub. clover pastures is generally ascribed to improved soil fertility, and especially to increased nitrogen content, which provides an environment more favorable for such species (Fig. 3). Control of the invaders, or at least their replacement by more useful species, may often be achieved by sowing grasses, especially Phalaris tuberosa and Lolium rigidurn, or by cropping with cereals ( Meadley, 1946). The mechanism of control has been assumed to be related to a decrease in the nitrogen levels in the soil. Willoughby (1954) suggested that two conditions must be met in order fully to exploit nitrogen fixation. First, an associate grass should be grown to acquire and store nitrogen, thereby lowering the level of nitrogen in the soil and thus reducing the inhibition of further nitrogen fixa-

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tion. Second, the nitrogen status under the sward should be periodically reduced either by a harvest of hay or silage or by a cropping phase. Williams and Donald (1957) and Cook (1939) reported that the nitrogen contents in the top few inches of soils under old sub. clover

FIG.3. A sward of sub. clover established without a companion grass becomes dominated by thistles and volunteer annuals (here Hordeum Zeporinum) after a few years.

pastures were about 2000 pounds per acre. The former authors showed that the carbon:nitrogen ratio was relatively constant, and they therefore concluded that almost all the nitrogen was combined with carbon in soil organic matter. Therefore, it would not be immediately available to

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plants, but would become available at a rate that would depend on the rate of decomposition of organic matter. A grass growing in a sub. clover pasture might acquire nitrogen as fast as it becomes available and would store a certain amount in roots and other organs. The amount so stored, which might be 50 pounds per acre, would simply be transferred from one kind of store to another and would scarcely affect the total nitrogen content of the soil. The grass could thus affect the level of available nitrogen, but not the amount stored. The evidence for a significant inhibition of nitrogen fixation by high levels of soil nitrogen is by no means convincing. Results of Donald and Williams (1954) show increases in soil nitrogen linear with time or phosphate, unaffected by nitrogen level. These results, as they pointed out, do show some decline in quantity of nitrogen fixed per unit weight of clover grown. But if inhibition did occur, it was far from complete. Any increase in the rate of nitrogen fixation per unit weight of clover, which might result from the removal by grass of available soil nitrogen, would surely be more than counteracted by the decrease in weight of clover in the sward which results from competition by the grass. As Willoughby showed, at the highest level of superphosphate application the proportion of clover in the sward is reduced to about 20 per cent. The amount of nitrogen removed in animal products would be approximately 8 per cent of the weight of greasy wool, 3 per cent of the weight of undressed carcasses. Even at high levels of animal production the loss of nitrogen would therefore be little more than 10 pounds per acre per year, although dairy farming might remove more than twice this figure. Since nitrogen fixation in highly productive stands of pure clover may approach 100 pounds per acre per annum, the nitrogen balance of grazed swards may be maintained by a relatively small clover component, although other elements (e.g,, potassium) may become limiting, especially if hay crops are removed (Williams and Lipsett, 1960). A crop such as wheat might remove up to 50 pounds of nitrogen per acre (Purchase et d.,1949),a crop of hay a similar, or perhaps a slightly larger, amount. The amount of nitrogen stored under old sub. clover pastures is thus unlikely to be reduced greatly by an occasional harvest of hay or cereals, for a single year of nitrogen fixation by a good sub. clover pasture would provide ample nitrogen for a crop, and a margin for losses. Hence rotations that include a year or two of sub. clover for every two crops should maintain the nitrogen content of the soil at high levels. Grasses in old sub. clover-grass pastures may show symptoms resembling those of nitrogen deficiency, despite the presence of large amounts of organic nitrogen in the soil. These symptoms seem more

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common in early spring and tend to disappear with the arrival of warmer weather. They probably indicate that the rate of uptake required for current maximum growth by the grass exceeds the rate at which nitrogen is becoming available through decomposition of organic matter in the soil. This process presumably proceeds relatively slowly at low temperatures, such as in winter or early spring. Suppression of thistles and other invaders might therefore seem to be achieved by competition for available nitrogen, even though the total amount of nitrogen in the soil may be far greater than that required for plant growth. But this conclusion appears to be refuted by the fact that thistles flourish in many old sub. clover pastures, despite the presence of a dense cover of volunteer annuals such as Hordeum Zeporinum. Thus the effect of perennial grasses in closing the sward, and in competing for light and moisture, seems to offer a better explanation for the control of weeds in established pastures. From the practical viewpoint the result is satisfactory, no matter what the mechanism happens to be. If phosphate is adequate, the proportions of grass and clover in a sward might be expected to reach an equilibrium. Improved nitrogen status would result in grass dominance, hence reduced fixation of nitrogen, leading in turn to decreased growth of grass and temporary increase in the proportion of clover. If phosphate is inadequate the growth of grass appears to be limited more than that of clover, and clover dominance results. This has been a major cause of animal infertility, through high intake of estrogen (see Section VIII), Grazing may influence grassclover relationships through preferential consumption, by the removal or deposition of nutrients, and by the differential effects of defoliation on competition for light. Rossiter (private communication, 1960) has sought, but not found, evidence for preferential grazing by sheep, but further studies of intake are required. Heavy grazing, accompanied by deposition of excreta, may mobilize fertility and increase the available nutrients in the soil. But if excreta are not deposited uniformly one would expect the heavily manured areas to become grass dominant. Heavily grazed areas, especially those on which excreta are not deposited, might well become clover dominant. Species with relatively erect growth habits, such as most grasses, probably have considerable advantages over clover in competition for light, Therefore lightly grazed pastures are likely to become grass dominant (Fig. 4).If there were any tendency for the more erect species to be preferred by livestock, heavily grazed pastures might well become clover dominant. Further, Blackman and Wilson (1951) suggested that sub. clover is ill adapted to shading, so that it would be prone to succumb in competition for light in lightly grazed swards.

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F. H. W. MORLEY

Competition for light may well explain the results of Rossiter and Pack (1956),who found that a heavily grazed area became dominated by sub. clover and Cryptostemma calendula (cape weed). In an adjacent enclosure the percentage of clover dropped below 10, and the sward became grass dominant.

FIG.4. A grass-dominated sward of sub. clover and Phaluris tuberosa several years after establishment.

The effect of rainfall on competition with sub. clover depends on the time of the year and the competing species. Sub. clover is stated to make more rapid growth in early autumn than Lolium rigidum (Willoughby, 1954). Early autumn rains therefore tend to be followed by aclover years.” On the other hand, late “opening” rains tend to be “grass years.”

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This may not be true if sub. clover is growing with a vigorous perennial, such as Phalaris tuberosa. Personal observations suggest that the same general tendency is present but is less marked than with annual grasses. Whether this seasonal variation in grass:clover ratio reflects nutritional influences, or arises from differential responses to temperature and day length, is not clear at present. Presumably decomposition of soil organic matter would release not only nitrogen, but phosphorus, sulfur, and other nutrients. The differential effects on grass and clover of the amounts and balance of nutrients made available might account for some of Willoughby’s observations. Where sub. clover and white clover ( T . repens) are growing in association the effect of moisture on the balance between the two is very marked. Bartels et aZ. (1932) found that sub. clover dominated infrequently watered pastures, whereas white clover dominated pastures in which natural rainfall was supplemented by more than four watering of 3 or more inches. In the northern tablelands of New South Wales the balance obviously changes with rainfall; improved pastures resemble pure stands of white clover in wet years, but sub. clover is dominant in years when the rainfall is less, or when rain falls mostly in the winter.

E. WEEDCONTROL One of the outstanding contributions of sub. clover to Australian agriculture lies in the control of weeds of pasture and cultivation. Stening (1933) in New South Wales, and Clarke (1934) and Orchard (1951) in South Australia, discussed control of H ypericurn perforatum by competition with sub. clover. Moore and Cashmore (1942) demonstrated almost complete control of this weed by sub. clover, especially if combined with PhaZaris tuberosa. It has also been recommended for control of ConvoZvuZus arvensis (Pearson, 1946), Senecio jacobea (Hexter, 1950), NUsella trichotoma (Green, 1956), and Chondrilla juncea (Cameron, 1952; Tindale, 1955). The replacement of weeds of wheat fallows by sub. clover and Wimmera ryegrass, is now commonplace. The ecological control of weeds by sub. clover pastures is probably dependent on the fact that most weeds of pastures and fallows are able to compete, under conditions of depleted fertility, more aggressively than species adapted to high levels of fertility. Since most agricultural plants fall in the latter category the raising of fertility favors them at the expense of weeds. Not all weeds are controlled by sub. clover, for example, Erodium botrys and Cryptostemma calendula ( Meadly, 1946; Rossiter and Pack, 1956) and Wimmera ryegrass, which is in some respects a weed in certain wheat-growing areas. It seems that plants which respond markedly to high fertility are unlikely to be dominated by sub. clover,

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but they may be dominated by other aggressive species, such as Phalaris tuberosa, which are highly responsive to increased fertility.

F. LIMITSTO DISTRI~UTION IN AUSTRALIA The biological limits to the persistence of sub. clover are far from precisely understood. Donald ( 1960b) described three climatic boundaries of the area of distribution, these being determined by aridity, heat, and cold. In the markedly Mediterranean climates of the southern comer of Western Australia and portion of South Australia the persistence of sub. clover is chiefly a matter of drought escape, largely by early flowering (Donald and Smith, 1937; Trumble, 1937). Donald suggested that 12.8 million acres of Western Australia are climatically suitable to the late midseason variety MT. BARKER, 38.4 million to the early variety DWALGANUP.

In the southeastern States the field evidence mentioned by Donald is far from convincing. It is difEcult, if not impossible, to separate the influence of aridity from edaphic and temperature effects. In recent years a succession of good seasons has enabled large seed reserves to accumulate in some marginal areas, so that we now find midseason varieties persisting in areas formerly considered doubtful for even the earliest varieties (see results from Deniliquin in Morley et al., 1961). There seems to be no powerful reason why sub. clover might not extend as far as species such as Medicago hispicla, M . minima, and M . laciniata, well beyond the recognized sub. clover zone. Perhaps edaphic factors, strongly correlated with rainfall in this region, are critical. The observations of Yates (1957, 1958) on seed burial are probably also relevant. He found that seed setting was considerably better when development occurred at or below the soil surface or when burs were enclosed in sponge rubber. In high temperatures and low humidity, such as are commonly experienced in early spring in the drier inland areas, seed-set above ground was poor relative to that in buried burs. In general, varieties with a strong tendency to bury burs tended to have poor seed setting above ground, this presumbaly being a reflection of past selection. In addition, Hutton and Peak (1954a) found that germination of pollen grains was adversely affected by high temperature, and especially by low humidity. Both pollen germination and seed-set may be limited by the microenvironment of the inflorescence, which may be of greater significance than general aridity in the eastern region. The warm boundary of the area of distribution will depend in part on vernalization requirement, as was first suggested by Aitken (1955b). Donald considered that the boundary for MT. BARKER would correspond with the 55" F. mean July isotherm. The warm boundary for varieties

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which require little vernalization-DwALGmp, YARLOOP, cLm-is probably determined by rainfall distribution rather than temperature per se. The warm areas to the north usually have winters that are too dry for sub. clover. Further, wet summers, which are usual in these areas, favor the growth of competing species at critical times, such as germination and seed setting, in addition to providing conditions unfavorable for survival of seed during summer. Jenkins (1957) showed that in subtropical regions such as at Lismore, New South Wales (see Donald, 1960b, for climatic data) the late varieties set relatively little seed and over 90 per cent of this was destroyed or germinated (equally fatal) during summer. The survival picture was little better for early varieties, but seed production was at a much higher level. The seed production of the variety BACCHUS MARSH, which requires a moderate amount of vernalization (Morley and Evans, 1959), was as high as that of early varieties. The difference in vernalization requirement between this variety and MT. BARKER, although small, nevertheless seems to be critical. On the basis of these results MT. BARKER and TALLAROOK could not be recommended for the Lismore region (Arthur and Jenkins, 1955; Jenkins, 1957). The presence of a cold boundary of the area of distribution was first proposed by Donald (1959) on the basis of meteorological data and differences among seed yields of certain varieties in four years. He interpreted his results as indicating that “frosts during flowering were apparently the principal factor governing seed production at Canberra in the four seasons under study.” The seed production observed was unusually low, and other climatic factors could have operated, so that one may not unreservedly accept this interpretation. But there can be little doubt of the validity of his conclusion that lateness of flowering is an adaptation to cold climates, as is the vernalization requirement ( Morley and Davern, 1956). These mechanisms ensure that flowering does not take place before winter, thereby avoiding frost damage to the highly vulnerable floral primordia. Whether this adaption affects the distribution in Australia is open to doubt. Some varieties, especially CLARE and some from Morocco, suffer severe frost damage even when vegetative (personal observations). At high elevations in southern Australia, precipitation is sufficient to promote competition to sub. clover, by spring- and summer-growing species, at the critical period for seed setting. Germination of seed which did set would be encouraged by the high moisture and low temperatures of elevated localities, at a time when competition would be most severe. At altitudes over 4500 feet snow cover would probably severely prejudice the survival of a winter annual. But whatever the mechanism or combination of mechanisms that de-

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F. H . W. MORLEY

termine the precise cold boundary of the distribution, in fact the extent of the area beyond this boundary is small relative to the actual or potential total. Thus, pushing back this boundary is unlikely substantially to increase the total area suitable for sub. clover. In any case, white clover is likely to be more productive in such localities. Finally, the edaphic limits of the distribution should be considered. In the natural habitat the species is usually restricted to soils on the acid side of neutrality, and in Australia it has been most successful on acid soils. Whether this distribution depends on nutritional or other properties of soils is yet uncertain. Trumble and Donald (1938b) proposed that symptoms shown by sub. clover on calcareous soils indicated a nutrient deficiency. This suggestion was confirmed by the discovery by Higgs (1958) that CLARE, the variety best adapted to alkaline soils, differed from BACCHUS MARSH in responses to zinc and manganese. Thus, the supply and balance of trace elements may be important. Extensive investigations of the role of nutrition in adaptation seem to be called for. If edaphic limitations can be overcome by supplying trace elements, attemps to select for edaphic adaptation would seem unjustified. VI. Agrotechnology

A. ESTABLISHMENT Several techniques of pasture establishment are used, with results that depend greatly on the particular soil and season and especially on the incidence of rain after sowing (Toms, 1958). The technique favored is a compromise between economy and probability of establishment. It may depend very much on topography and the state of development of the land as regards clearing and cultivation. Probably 30 to 50 per cent of the sown areas have been established by broadcasting from the air or ground (e.g., Parish and Dillon, 1956). This is cheap but unreliable (Hudson, 1935; Harrison, 1936; Hardy, 1949) because good results are unlikely unless prolonged moist weather, and other conditions favoring germination and establishment, follow the initial sowing. Broadcasting remains the best, often the only practicable, method of establishment on steep, rocky, or partly cleared country. It is worth gambling the cost of seed and fertilizer if land can be obtained cheaply or is of little value without pasture improvement, provided it is known to be of reasonable potential productivity. The chances of successful establishment are increased if sub. clover is sown into prepared seed beds (Fig. 5 ) , but the amount of preparation is often slight compared to that usual in crop husbandry. Frequently the seed is only scratched into an unimproved pasture with a drill or similar

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machine (Figs. 6 and 7 ) . This is inexpensive and usually adequate. Some pastures are established under cover crops such as oats (Donald and Smith, 1938). Very seldom is a seed bed prepared thoroughly, and it is doubtful if thorough preparation is justified except for the benefit of the cover crop According to Davies (1953) careful preparation is usually

FIG.5. A lightly grazed sward of sub. clover with Phalaris tuberosa and volunteer annuals 18 months after sowing in a prepared seedbed.

a waste of time and money. However, as Tiver (1957) indicated, the extra cost may be worth while if seed of the most expensive varieties, such as CLARE, is being used. The extra expense may also be offset in part by better grazing during the first year. Successful establishment has been obtained in stony and tussocky country by sowing the clover, with fertilizer, in shallow grooves scratched by chisel points (Fig. 8 ) . Sub. clover spreads naturally over extensive areas, probably mainly

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by animal passage. Hardy (1949) suggested that animal passage might be used to establish the clover on new areas by transferring livestock from a well-seeded pasture. This is likely to result only in very patchy establishment. Moreover topdressing with fertilizer is almost invariably necessary, so that this method is unlikely to eliminate broadcasting operations. It is not sufficiently reliable to be generally recommended, al-

FIG.6. Sub. clover and a grass mixture were scratched into this degenerate, tussocky pasture 6 months previously. The furrows were 1.5 feet apart.

though “naturally pelleted seed may start with a slight advantage over broadcast seed. Nevertheless, seed has become widespread over extensive areas by animal passage, and good establishment may eventually follow with no more treatment than the application of fertilizer. As Davies (1953) stressed, the main limitation to establishment over large areas is nutritional, and it is of little advantage to disseminate seed if nutritional deficiencies are not corrected. Seeding rates vary greatly depending on variety, seedbed preparation, cost of seed, and the value of land. Trumble (1935b) recommended

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5 pounds per acre; Dunne ( 1936) and Tiver (1957), 4 pounds per acre for prepared seedbeds, sown to % inch deep, or on the surface if covered lightly by harrows. One or 2 pounds are probably sufficient, under a cover crop, to set sufficient seed to establish a useful stand the following season. Two to 4 pounds is usual when broadcasting, especially with later varieties. If conditions are satisfactory for establishment

FIG.7. Sub. clover pasture similar to that shown in Fig. 6, but 12 months later. The sown species are competing with the tussocks.

this will be sufficient. Heavy seed rates will not compensate for unsatisfactory climatic conditions following seeding. It is important to realize that the fertility and texture of soils in native pastures seldom favor establishment, and repeated failures to establish do not mean that a dense sward will not regenerate itself. Sub. clover plant debris (Williams et d.,1954) and more favorable rhizobia, soil nitrogen, and soil texture all favor regeneration in old pastures. The problems of establishment of clover into the relatively dense pastures of high-rainfall areas have called for a novel method of seeding.

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Breakwell and Jenkins (1953) developed a machine for overseeding paspalum pastures which are too dense to be readily penetrated by seed drills. This machine, called a sod-seeder, places seed and fertilizer in shallow furrows cut into the sod. Before sod-seeding, the pasture must be closely grazed or mown, thus diminishing competition encountered by the newly emerged seedlings. This procedure avoids the cost of preparing land, which would be heavy in very dense swards, and it scarcely interrupts the normal utilization of pastures.

FIG.8. The pasture on the right was sown with sub. clover 18 months previously. The pasture on the left has not been sown. Note the tussocks remaining close to the fence line, where the clover could not be sown.

The best time of sowing is relatively simple to determine in regions with a strictly Mediterranean rainfall pattern. Otherwise it is a gamble between higher yield from early sowings if the rainfall is favorable, and greater certainty of establishment but lower average yield from later sowings. Later sowings are more likely to obtain sufficient moisture for establishment, but growth will be inhibited by cold to such an extent that seed yield may be unsatisfactory in the first year. Management of the freshly established pasture should be directed to obtain maximum yield of seed. In general, grazing should be avoided, but it may at times be useful in aiding control of weeds and grasses.

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Stands established in early autumn, if they have formed a nearly continuous ground cover, may be grazed heavily in the first season. But such results are unusual. Grazing by cattle is preferable to that by sheep during the first season, or until a good stand is established. The companion grass, if any, may be sown at the same time as the clover or, as shown by Fricke (1939), may be readily sown into a prepared seed bed on land which has been brought to a reasonably high level of fertility by some years of clover. In general, sowing the grass with clover is likely to be more economical, although growth of grasses may be unsatisfactory until appreciable amounts of nitrogen become available. If the soil is moderately fertile, seedling losses of grasses during the first year may be avoided by sowing grasses a year or two before sowing clovers (D. F. Smith, private communication, 1960). The variety, species, or mixture of companion grasses cannot be generally recommended. Probably the most common grass is Wimmera ryegrass (Lolium rigidurn), which has often been introduced accidentally. Phalaris tuberosa is outstanding in many environments, and perennial ryegrass (Lolium perenne) is generally favored in higher rainfall areas. There is no evidence favoring any particular grass, although Andrew and Neal-Smith (1956) proposed that the inclusion of an additional perennial grass ( such as Agropyrum obtusiusculum, Bromus coloratus, Bromus inermis, Dactylis glomerata, or Festuca arundinacea) might be advantageous in a Phalaris-sub. clover pasture. B. MANAGEMENT OF SUB. CLOVER PASTURES The outstanding problems of management of an established sub. clover pasture are, first, the amount and kind of fertilizer to apply and, second, the intensity and management of stocking. As the first lies in the realm of plant nutrition it will be discussed only in outline, without detailed references. Australian soils are rich only in the diversity and intensity of their deficiencies. Correction of these deficiencies is fundamental to the exploitation of sub. clover or other exotic species. Many deficiencies can be induced in artificial culture, and these have been described and illustrated by Millikan ( 1953). In the field calcium, nitrogen, phosphorus, potassium, sulfur, molybdenum, manganese, zinc, and copper deficiencies have been found alone or in various combinations. The most important of these, Ca, P, S, and N, can be rectified by the combination of superphosphate with rhizobial nitrogen fixation. Other deficiencies require application of suitable quantities of chemicals, alone or in fortified superphosphate. As has been repeatedly emphasized, phosphorus is deficient on all but

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a few soils, and sub. clover will seldom flourish unless phosphorus is applied. Ten pounds of phosphorus per acre (as superphosphate) give handsome dividends, but on most soils responses to increasing amounts are almost linear up to several hundreds of pounds of superphosphate per acre. If finance permits, heavy initial applications (400 to 500 pounds per acre) may be advisable except on some soils in which phosphorus is rendered unavailable. If finance limits the amount that can be applied, the best policy may be to sow seed over a wide area with 100 pounds superphosphate per acre, thus establishing a pasture that will respond rapidly to heavy applications in the following years. Since the costs of clearing and preparation of the land, the purchase of seed, and the closing of the area during establishment, are likely to be far greater than that of the fertilizer, it is probably false economy to restrict superphosphate. The maximum stocking rate of a pasture used for wool production depends on the number of animals the pasture will sustain during the season in which the fodder available is minimal. Supplementary feeding is seldom considered economical except as a last resort to keep animals alive during a severe drought. Two periods of scarcity, late winter and early autumn, are usual. The former is almost invariably critical because it is the season of minimum fodder availability, and at this time late pregnancy and lambing are demanding a rising or high plane of nutrition. The term “sustain” is used advisedly for reasons initially advanced by Moore et al. (1946) and Davies (1946) and recently given increased emphasis by Willoughby (1959a). The period during which the amount of pasture available is adequate for high production is short, so that the production of animal matter per acre is largely determined by the number of animals that can be placed on the pasture at this time. The growth of the pasture during this period is far in excess of the demands at any practicable intensity of stocking. The number of animals that a sub. clover-based pasture will sustain is probably minimal after rains in winter, following a dry autumn. Any dry fodder will have been greatly reduced in value by the rains, and the amount of new pasture available is insufficient to provide even a minimal maintenance ration. Note, it is not necessarily the growth rate of the pasture that is critical at this time, but the amount of fodder available to the sheep. Willoughby (1959b) emphasized the point that, if the amount available were lower than 1400 pounds of dry matter (as green material) per acre, animal production would be below maximum because of the inability of sheep to gather a full ration. Nevertheless Willoughby’s (1959a, Fig. 3) results show that sheep may maintain body weight, at 8 sheep per acre, on the production of somewhat less than 200 pounds green material, plus a substantial amount

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of dry residues. Presumably the high protein content of the young pasture would have aided the digestion of the low quality roughage. The relative growth rate of a sub. clover-Phalaris pasture in winter conditions similar to those in Canberra may be determined approximately by using Willoughby’s figures obtained from a pasture which was left unstocked in July, the coldest month. The figure obtained, 0.05 g./g./day, is comparable with a determination of my own on several sub. clover varieties. This was 0.04 g./g./day at 8°C. day, 3°C. night, 10% hours day length, in growth cabinets. If 1% pounds dry matter per day is sufficient to maintain a sheep, 300 pounds dry material per acre, as green plant tops, could produce sufficient for 10 sheep per acre, if they were able to harvest this quantity, Willoughby’s results indicate that they might not be able to do so, but the exact points of equilibrium between consumption, production, and animal requirements must be very difficult to determine. If these calculations are approximately correct, a pasture presenting sufficient material (probably less than 500 pounds per acre) to enable a sheep to harvest its needs will produce considerably more than sufficient for the demands of any practical stocking intensity, even during winter months, provided water and nutrition are not limiting. If the amount of available material is less than this, the ability of the sheep to harvest would limit intake to submaintenance levels at any stocking intensity. High stocking intensity will prolong the submaintenance period. It may be possible to increase stocking intensity for the whole year by removal of stock from the pasture until a sufficient photosynthetic factory is built, which may occupy 4 to 6 weeks after opening rains. The cost of conserving, storing, and using fodder to do this may not be offset by the gains from increased stocking, but the practicality and economics of such practices deserve examination. The effects of stocking rate on reproductive performance, growth rate, and longevity, and the bionomics of alternate forms of utilization, await further investigation. The food required by a sheep on a maximum ration for 4 months, and on little above maintenance for the rest of the year, is approximately 750 pounds dry matter per annum. The amount of dry matter that might be produced by a good sub. clover-based pasture is not known, but Donald (1951) suggested 4 tons per acre as a maximum harvest. For reasons discussed by Brougham ( 1956) and Davidson and Philip (1956), this figure may well be far short of maximum production under continuous harvest, even during the sub. clover growing season. Production, especially by perennials, at other seasons could add substantially to this figure. Stocking rates of twelve sheep per acre, with careful management, may not be entirely impossible, but they may not be the most economic.

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MORLEY

Willoughby’s experiments have demonstrated ( C.S.I.R.O., 1956), and other evidence is confirming, the possibility of stocking rates of up to eight sheep per acre on unirrigated sub. clover based pastures. The significant idea emerging from this discussion is that the usual stocking rate of two to four sheep per acre on sub. clover-based pastures is well below the maximum possible in the higher rainfall areas. Maximum production per acre is more likely to result from a high intensity of stocking at a relatively low plane of nutrition for most of the year than from a low intensity of stocking at levels of nutrition perhaps only slightly higher. But removal of stress from one season may impose greater stress in another. The problem of the nutritive value of dried residues of sub. clover pastures, discussed by Donald and Allden (1958) and Allden (1958), may be as crucial to productivity in a Mediterranean environment as is winter production on the tablelands of New South Wales. As these authors and Fels et al. (1958) showed, the dry residues may be as deficient in quality as are the winter pastures in quantity. But in either case a decrease in stocking rate may not offer a real solution to the deficiency. The observation by Ferguson et al. (1949), that wool production is more efficient at lower levels of intake, is highly relevant to the problem of stocking rate. One of the most valuable attributes of sub. clover is that it lays down large quantities of seed. After a good spring, as much as a ton of seed per acre may be lying on the surface or in the top inch of soil. This represents a substantial drought reserve, sufficient to keep eight sheep alive for 6 months or more, even if they are able to harvest only 80 per cent of this material. However, if a pasture consists of only 20 per cent clover the seed yield, and hence fodder reserves, will be greatly diminished. Therefore light stocking, which will result in grass dominance, may not be the best method of avoiding losses in drought. Indeed it may result in inadequate fodder reserves at a time when the need is most urgent. Clover dominance must be avoided because of the dangers of estrogens, and probably low winter productivity. Grass dominance must be avoided because it may result in inadequate reserves of seed. A compromise is probably best achieved by ample phosphate, perennial grasses, and intense stocking. Obviously such arguments may not apply to dairy farming, and perhaps only in part to beef cattle production or even sheep operations mainly intended for meat production. In these a high plane of nutrition must be maintained for sufficient time to fatten the animal for marketing. Such forms of utilization may make demands on the pasture very difEerent from those made by wool production. It is perhaps necessary to consider the use of rotational grazing. The

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results of Moore et al. (1946), and the discussion of these by Davies ( 1946), seem to have disposed of arguments favoring rotational grazing. The concepts of leaf area and its relationship to pasture production, considered by Davidson and Philip (1956) and Davidson and Donald (1958), indicate that there is no known theoretical basis, or reported result, supporting the expectation that rotational grazing will increase production. Rossiter’s (1952) and Willoughby’s (1959%b) results have provided further experimental confirmation, if such were needed, of the lack of value of rotational grazing. It is to be hoped that future investigations on pasture management will be directed to more profitable topics. “Autumn saving,” or conserving growth at one period for use at another period when livestock demands are more acute, may have an important place in increasing productivity. This form of management should not be confused with rotational grazing, and its value should not be taken as an indication of the value of rotational grazing. In the few decades that sub. clover has been an important contributor to pastures a considerable body of information on such topics as the nutrition, the distribution, the physiology, and the ecology of this species has been accumulated. Knowledge of the utilization of the extra fodder, the nitrogen, and the energy produced through sub. clover has not kept pace. There are reasonable grounds for believing that the value of many existing pastures based on this species could be doubled by the integration of plant and animal sciences. C. THE ROLEOF SUB. CLOVER IN LEY FARMING Since writers such as Cook (1935 and other papers) and Hayman (1945) referred to the value of sub. clover for mixed cereal and fat lamb farming in southern Australia, the urgency of problems of declining soil fertility, increased erosion, and decreased yields of cereals have been emphasized by Forster ( 1939), Cornish ( 1949), Sims and Jardine ( 1949), Kaleski (1956), and others. The reports of experiments in Victoria on the effect of sub. clover leys have been summarized in a series of papers delivered in 1949 to a specialist conference on “Plant and Animal Nutrition in Relation to Soil and Climatic Factors” (H.M.S.O., London). As this publication is not widely available, the reader is referred to Morrow et a2. (1948), Bath (1951), and Cameron (1952). The evidence indicates that, despite the use of superphosphate and the advent of better varieties, yields were declining over wide areas of the wheat belt in South Australia and in Victoria. Deterioration in soil structure and loss of soil nitrogen, the inevitable consequence of wheatfallow-wheat rotations, seem to have been the cause of this decline. Sub. clover leys, associated with increased emphasis on livestock, offer a

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solution to the problems of declining fertility and have been credited with increases of 8 to 10 bushels per acre in wheat yields (Bath, 1951; Gruen, 1956) as well as substantial increases in livestock production on wheat farms. There are some arguments as to the rate of improvement in fertility status which probably arise from variations in seasons, phosphate status, soils, and the degree of deterioration at the time of commencement, but the general picture is not questioned. It seems worth while to emphasize that rapid improvement can scarcely be expected unless a heavy sward has been grown. Trumble (1935a) suggested that approximately 43 pounds of nitrogen is fixed for every ton of dry clover herbage. This is sufficient to supply the needs of a crop, but it will not restore high fertility to a soil impoverished by years of abuse. The advent of sub. clover in wheat rotations has not been a wholly unmixed blessing. High fertility levels have, especially in wet seasons, created a severe problem because of lodging, so that resistance to lodging may need increased attention in breeding programs of the future. The encouragement of Wimmera ryegrass has resulted in severe competition to the young wheat. Finally, there are indications (Butler, 1959) that some fungous diseases may flourish in sub. clover leys. These are problems that can and will be met by modification of techniques and the development of new varieties. Such difficulties scarcely constitute an argument against the use of sub. clover for conserving soil and fertility. A development of special interest has been the use of sub. clover in orchards as a cover crop and source of nitrogen (Davey, 1942; Gayford, 1947; Botting, 1957) and as a “sod culture” for control of soil loss (Pascoe, 1946; Nicholas, 1952). The essential feature of sod culture is the encouragement of a dense sward, in deciduous orchards, during autumn and winter, and the frequent mowing of this in spring. The advantage of sub. clover is that it sets seed, dies, and forms a surface mulch during summer and early autumn, and thus does not compete with the trees for moisture, while it provides in spring a substantial supply of nitrogen for use by the trees. D. SUB. CLOVER AND SOILCONSERVATION Cameron (1959a, b) recently reviewed the role of sub. clover in soil conservation. The ability to establish a sward as dense as permitted by moisture supply, to raise soil fertility to a level where nonlegumes can flourish, and to improve soil structure so that water intake is increased, and runoff and gully erosion minimized, make sub. clover invaluable in control of erosion. On steep or recently cleared land, sub. clover can be established even though preparation of a seedbed would be impossible (McCaffrey, 1952). On badly eroded land it can be established in gullies (Fig. 9) as well as on adjacent areas (Kaleski, 1956), and on arable land

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it will help prevent erosion and also aid reclamation through improved soil fertility and structure ( Lightfoot, 1948). Cameron ( 1959b) very reasonably does not give unqualified approval of sub. clover, pointing out that, during the summer, land on which sub. clover is dominant may be vulnerable to erosion unless care is taken to conserve some surface trash. He strongly recommends the establishment of perennial grasses with sub. clover to minimize this erosion hazard.

FIG. 9. An eroded gully filling in and becoming grassed in a heavily stocked sub. clover pasture.

Sub. clover is not the panacea for all problems of erosion in southern Australia (and similar areas) but, where the climate is suitable, it seems to provide an economic and practical basis for erosion control, The key to erosion control lies in soil fertility and surface cover rather than in mechanical earthworks, and for much of southern Australia sub. clover has demonstrated its value as builder of the fertility necessary for production of good surface cover.

E. SEEDPRODUCTION Sub. clover seed production, as first described by Howard in 1906 (Hill, 1936), has undergone many changes in technique. The harvesting equipment discussed by Duggan (1940), and that described by Meadley

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(1945), seems crude in comparison with the recently developed machinery now used for large-scale harvesting operations ( Ballard, 1956). The essential features of seed production consist in removal of top growth by rakes (this may be threshed if it contains much seed), loosening of the surface soil by some kind of cultivator, collection of the burs by rakes and by road sweepers or sheepskin rollers, threshing the burs, decoating the seeds, and cleaning the seed by grading, winnowing, and gravity separation. Modem equipment includes machines specially developed for each process, rather than machines designed for other purposes and adapted to sub. clover seed harvesting. The crude sheepskin roller, drawn by a horse, has been replaced by gangs of rollers or sweepers, with hydraulic unloading devices, drawn by tractors or self-propelled. If a large proportion of the seed is not buried, as may frequently be the case with later varieties, direct harvesting by a header fitted with pea lifters may be more economical. Wilkie (1946) stated that a suitably adjusted machine would harvest and thresh at one operation two bags of field-dressed seed per acre. Thaine (1949) using this method obtained 750 pounds machine-dressed seed per acre the first year after sowing, and 1260 pounds in the second year. It is doubtful whether this method could be used on all varieties, but it would often seem to be far more economical to “skim the cream” than to attempt to harvest much of the buried seed. The yield of seed per acre varies greatly with variety, season, and year. Average yields of clean seed are less than 500 pounds (Duggan, 1940) although more than 1000 pounds per acre may frequently be obtained. Most commercial operations probably leave some hundreds of pounds of seed in the ground. In small experimental tests we have obtained seed yields of 1200 to 1800 pounds per acre in the first year after sowing, in both early and late varieties. The relationship between seed yield, maturity grade, and climate has been discussed in Section V, A. The production of certified seed in 1956 to 1959 in the main seedproducing Australian States is shown in Table I. These figures, kindly supplied by my colleagues in State Departments of Agriculture and Dr. R. C. Rossiter, probably represent over 80 per cent of the total seed crop. The value of this crop would be approximately J?,A1,000,000 ($2.24 million) at 1960 prices. The actual cost of production is difficult to estimate, but probably 2 shilling (22 cents) per pound is near the irreducible minimum. The present levels are probably fairly stable, the output having declined from a peak in the middle 1950’s when production was almost twice that shown in Table I. High wool prices and taxation in the early

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1950's provided the capital and the incentive for expansion of pasture improvement. Superphosphate and equipment were readily available, in contrast to the 1940's, and seasons were favorable for seed production TABLE I Certified Production of Subterranean Clover Seed Averaged over 1956 to 1959 (tons/annum) Variety State Western Australia South Australia Victoria New South Wales

DWALGANUP

894

343 1247

MT.

TALLA-

BACCHUS

BARKER

ROOK

MARSH

207 319 61 834 1421

1

7 56

64

CLARE YARLOOP

6 26 260 167 459

1 5 34 8 48

191 1

192

Total

1299 352 362 1418 3431

and pasture establishment. But this boom seems to have passed, perhaps because of lower wool prices-certainly not because all land suitable for improvement is already improved. VII. Diseases and Pests

A. FUNGOUS DISEASES 1. Rust Infection of sub. clover by rust (Uromyces trifolii) was observed in Tasmania by Radel (1935), who noted that the variety WENIGUP was resistant but MT. BARKER was susceptible. Henrick (1935) stated that in a warm, moist spring MT. BARKER was heavily attacked, the foliage being completely lost within a week. Levy and Gorman (1937) noted varietal differences in resistance. These observations were extended by Loftus Hills (1942a, 1944a), who also presented Fz results suggesting that susceptibility was dominant. Latter (1953) found three races of rust, with differential patterns of host varietal resistance. Crofts et al. (1955) reported that Misses B. Carne and H. Philpotts found a number of additional races in collections from five States of Australia. Only three varieties, WENIGUP, BAULKAMAUGH NORTH, and an introduction from Spain, were found resistant to all. The importance of rust to productivity of sub. clover is difficult to evaluate, and I have yet to be convinced that resistance should be a major objective in a breeding program. Although defoliation is severe and spectacular in certain years, these are generally years of plenty. However, the effect on fodder conservation could be critical.

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2. Anthracnose Walker (1956a, b ), described outbreaks of "scorch" caused by Kabatiella caulivora, resulting in loss of yield and reduced seed setting. He also found differences between strains in resistance. The disease has apparently been unimportant in more recent years but could cause losses in the future. Other fungous diseases have been recorded, such as those caused by Rhixoctonia solani and Corticum praticola, reported by Ludbrook et al. (1953), but these appear to be only of minor or negligible importance.

B. VIRUSDISEASES 1. Phaseolus Virus 2 Pierce (Bean Mosaic) Aitken and Grieve (1943) and Watson (1949) described a mosaic disease which was identified by Hutton and Peak (1 9 5 4 ~)as the Phaseolus virus 2 Pierce. They found lethal necrotic reactions in some varieties, but mild mosaic reactions in others. Lethal reactors were resistant in the field, and the hypersensitive reaction was inherited as a dominant in most crosses. The disease may be transmitted mechanically or by aphid vectors. Although it may not be very important in the field, the disease is a major nuisance in spaced plants. No attempt is being made to incorporate resistance into commercial varieties. 2. Sub. Clover Stunt This disease, first noticed by various agronomists in the early 1950's, was described by Grylls and Butler (1956, 1959), who demonstrated transmission by aphids. These insects are persistent carriers once they have become infected from a diseased plant. OLoughlin (1958) found a high degree of resistance in the variety TALLAROOK, which was confirmed by Grylls and Peak (1960), who also located other resistant genotypes. Resistance of Fz plants was found to lie between that of the parents. The disease may affect growth in late autumn and winter, and thus could adversely influence production. It is also the cause of serious losses in experimental spaced plantings. Breeding programs to incorporate resistance in commercial lines of sub. clover are proceeding ( Grylls and Peak, 1960) . C. PESTS Ludbrook et al. (1953) reported damage due to nematodes (Meloidogyne spp.) in South Australia, but whether damage is frequent or widely distributed is not known. Andrewartha (1945) and Came (1948, 1951) reported serious damage from the cockchafer scarab, Aphodius

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howitti. This may constitute an erosion hazard (Cameron, 1959b). Swan (1934) and Norris (1944) reported damage by the red-legged earth mite Halotydeus destructor. Sub. clover seems to be especially attractive to this pest. Control of both cockchafers and mites may be achieved by mixing 3 to 4 ounces of lindane per acre with superphosphate applied in autumn. Disease is not yet a major problem with sub. clover, but this happy state may not continue indefinitely. The existence, year after year, of large populations of similar genotypes offers ideal conditions for the evolution of parasites. Perhaps it is not a coincidence that the most widespread variety, MT. BARKER, appears to be highly susceptible to all diseases of any importance. In view of the probable increase in importance of diseases, our own breeding program includes the production and maintenance of many agronomically acceptable lines derived from a wide variety of crosses. If new diseases should have drastic effects on productivity, any disease resistance in the species stands a fair chance of being already present in some useful material. VIII. The Estrogen Problem

In the early 1940’s reproductive disorders and infertility in ewes and abnormal development of the mammary and accessory sex glands of wethers, when grazing sub. clover-dominant pastures, were reported (Bennetts, 1944, 1946; Bennetts et al., 1946). The symptoms, which appeared after a few weeks of grazing on the potent pasture, were suggestive of the action of an estrogen or estrogenlike substance, since they resembled lesions produced by prolonged administration of stilbestrol (Bennetts, 1946,1947). Curnow et al. (1948) found that crude ether extracts from artificially dried DWALGANUP sub. clover produced symptoms similar to those following injection of estradiol. East et al. (1949) and East (1950) seemed to clinch the matter neatly by demonstrating that male sex hormones prevented symptoms of estrogenic action of sub. clover from appearing in castrated male guinea pigs or sheep. This also offered an explanation for the nonappearance of symptoms in rams. Here was a serious economic problem. Lambing percentages sometimes fell to as low as 10 per cent; neonatal deaths of lambs and difficulties associated with parturition caused severe losses. Infertility was shown to be permanent ( Schinckel, 1948; Underwood and Shier, 1951). Further, experiments by Underwood and Shier (1952) and Underwood et al. (1953, 1959) showed that, even at low levels (0.01 mg. stilbestrol per day) of estrogen administration, reproductive performance was adversely affected, although not in any obvious way. At somewhat higher rates of administration, reproduction was severely affected, although the

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estrous cycle did not appear to be abnormal. This suggests that important losses could be sustained at relatively low intakes of estrogen. Unless special investigations were made, these might not be diagnosed. A technique of bioassay of estrogenic activity of sub. clover, by uterine weights of spayed guinea pigs, was developed by Alexander and Watson (1951). They found that, although dehydration of clover reduced its estrogenic activity, little change took place in stored material. However, Bennetts and Underwood (1951) found that wilted or dry clover did not maintain uterine weight in spayed ewes. Rossiter (private communication, 1960) states that there is ample evidence that such material has a low estrogen content. On the other hand, Bickoff et al. (1960) reported estrogenic activity in sun-cured forages, including sub. clover from Australia, and many other dehydrated legume meals. So it seems that at least some estrogens are reasonably stable and persistent in forage. The estrogenic potency of sub. clover was not affected by various minerals added to superphosphate, but clover from plots to which superphosphate had not been recently applied was significantly more estrogenic than clover from recently fertilized plots (Alexander and Rossiter, 1952). Bradbury and White (1951) identified an isoflavin derivative, genistein, with estrogenic activity one-fifth that of estrone, in potent sub. clover. Biggers and Curnow (1954) found that genistein, which is of relatively low estrogenic activity, is a proestrogen, a compound which, when absorbed into the body, gives rise to true estrogenic substances in the course of metabolism. Curnow (1954) described techniques for isolation and quantitative estimation of genistein, and suggested that the quantities found in sub. clover are sufficient to account for the symptoms observed. He found similar and high levels of genistein in four varieties of sub. clover, but no such substances were found in T . repens, T . fragiferum, T . pratense, or Melilotus alba. Curnow and Rossiter (1955) found appreciable amounts of genistein in all of 123 strains of sub. clover, and in T. israeliticum and T. globosum. Both T. pratense and T . campestre gave suspicious reactions, while T. repens and thirteen other species were negative. Production of genistein-free strains by breeding therefore seems improbable. In view of the problems of assay, the use of radiation or other mutagenic agents is unlikely to solve these difficulties. It seems that genistein is not the only estrogen in pasture legumes. Others, including biochanin A (Pope and Wright, 1954), formononetin ( Bradbury and White, 1951), and coumestrol (Lyman et al., 1959), have been found, the last in several pasture legumes. The significance of these in reproduction of sheep is not known; nor is it known whether estrogenic compounds in sub. clover affect grazing animals other than sheep. This would seem possible, even probable.

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Symptoms have been reported from widely separated lo