JEAN BAPTISTE JOSEPH DlEUDONNE BOUSSiNCAULT 1802 - I8S-7 The Pounder of Modern Agricultural Chemistry, .SOIL CONDITIONS AND PLANT GROWTH* BY EDWARD J. RUSSELL, D.Sc. (LoND.), F.R.S. DIRECTOR OF THE ROTHAMSTED EXPERIMENTAL STATION, HARPENDEN WITH ILLUSTRATIONS FOURTH EDITION LONGMANS, GREEN AND CO. 39 PATERNOSTER ROW, LONDON FOURTH AVENUE & 30TH STREET, NEW YORK BOMBAY, CALCUTTA, AND MADRAS 1921 BIBLIOGRAPHICAL NOTE. First Edition (in Monographs on Biochemistry) June, 1912 New Impression ...... April, 1913 New Edition February, 1915 Third Edition June, 1917 Fourth Edition (in Rothamsted Monographs on Agricultural Science) July, 1921 PREFACE TO THE FOURTH EDITION. WHEN the revision of the present edition was begun it became obvious that some fundamental change would be necessary in the treatment. Since the first edition was published in June, 1912, the subject has grown enormously : it has now completely run away from the 1 66 pages that proved a sufficient allowance in those days. Several courses were open ; the one chosen was to retain the general characteristics of the older editions, dealing broadly with the whole subject, emphasising the outlines and endeavouring to maintain a dispassionate balance between the various parts. Detailed treatment has been avoided, partly because it is now hopeless for any one individual to attempt it, and partly because I did not wish to overload the book and make it into a huge " Handbuch " where the outlines are all obscured and the perspective lost. At the same time, however, the student must have access to the more detailed treatment, and it would have helped him little had I simply published a card index and left him to find out the rest for himself. The problem was solved by enlisting the sympathy of the Heads of the Departments at Rothamsted, who each undertook to write a monograph dealing with his or her special branch, and which Messrs. Longmans will publish uniformly with this volume. Each one has discussed with me the corresponding sections of this 460725 vi SOIL CONDITIONS AND PLANT GRO WTH book, with the results that the book is much improved, and a definite continuity is assured between this general monograph and the detailed monographs to which it will serve as a link. The whole series is to be called "The Rothamsted Monographs on Agricultural Science". It is hoped that this new arrangement will prove satisfactory to students. It has the advantage that the book still covers the whole ground, while remaining of a manageable size, and that it is closely linked up with a series of similar books, each dealing with separate sections, in which more complete treatment and more fully informed criticism are given than I could myself undertake. On the other hand, the necessity for freedom of treatment, and the difficulty of placing such subjects as >Soil Physics or Soil Protozoa under the heading of Biochemistry, compelled the withdrawal of the book from the important " Biochemical Series," of which it has hitherto been a member. And, although Drs. Plimmer and Hopkins willingly consented, this course was not taken without serious consideration and regret : it in- volves the separation of the book from a very useful series of volumes. Perhaps the most striking feature of the past ten years' developments has been the increasing recognition of the complexity of soil phenomena. The American investigations have shown how complex are the physico- chemical relations of the soil, the soil solution and the plant. English work has shown that the soil population is numerous and very varied. It is not sufficiently recognised, however, that this complexity necessitates a difference in method of investigation from that usually adopted in scientific laboratories, and this I have tried to bring out in the course of the book. A chemist PREFACE TO THE FOURTH EDITION vii dealing with a single substance can treat it in a certain way, and be reasonably certain that the result obtained is the direct consequence of the treatment. The soil investigator has no such certainty : definite treatment of a soil may be followed by a definite result, but there may be no direct relationship with the factor under in- vestigation— the result may be merely a reflex of some far-reaching change produced in some other factor which is entirely overlooked. Further, if observations are attempted in the field it is impossible to ensure such simple variation as the recorded data seem to suggest. It follows that the ordinary laboratory method in which factors are varied only one at the time requires con- siderable modification when used for soil work. The methods in use at Rothamsted fall into two groups : — 1. Observations are made in natural conditions as ac- curately as is feasible, and repeated sufficiently frequently to allow of treatment by modern statistical methods. These enable the investigator to study the variations, and hence to make deductions as to the numbers and properties of the factors involved. The factors can then be studied in the laboratory as single factors, using more precise methods and more rigid controls than are possible in the field. 2. Experiments are made on the soil, and from the results deductions are drawn as to' the probable nature of some new factor. Direct experiment is then made to test the operation of the factor in the field, and precise laboratory experiments are also undertaken. Further, just as the ordinary methods of investigation are insufficient, so also the customary divisions of science cannot be rigidly maintained in soil work. The chemist is constantly confronted with physical and biological viii SOIL CONDITIONS AND PLANT GROWTH problems : the biologist constantly needs the help of the statistician, the physiologist and chemist : most of the work is essentially " team work," requiring close co-operation between experts in different branches ot science. The recognition of these considerations, and the fact that the Ministry of Agriculture has now, through the Development Fund, enabled Rothamsted to interest some of the more promising of the younger scientific workers in agricultural problems, justifies the hope that the future will see even greater advances than the past. In preparing this edition thanks are due to my colleagues at Rothamsted who have read sections or chapters of the book and made useful comments, es- pecially to Miss Aslin, Dr. B. Muriel Bristol, Miss Jewson, Messrs. W. B. Brierley, D. W. Cutler, E. M. Crowther, B. A. Keen, and H. J. Page ; to Professor V. H. Blackman, Mr. G. W. Robinson, of University College, Bangor, and Dr. E. J. Salisbury ; to M. Georges Matisse, of Paris, who is translating this edition into French ; and in America to Dr. H. L. Walster for a valuable critical review and list of refer- ences, and others for useful suggestions. The book has been helped by so many that I hope it will still find friends in spite of its larger size. PREFACE TO THE THIRD EDITION. CONSIDERABLE alterations have been made in the text and a new chapter has been added discussing the col- loidal properties of the soil. It is abundantly clear that the soil investigator of the future will have to be thoroughly familiar with the ways of colloids, and I fully expect that much of the older work will require careful re-examination in the light of what has been done in this direction by chemists and physicists. Although the volume has necessarily expanded I have tried to keep it as a monograph : I have not attempted to turn it into an extended card index by re- ferring to every paper published on the subject since the first edition came out. Many papers have been omitted ; the guiding principle has been to include only those that brought in some new idea or profoundly modified an old one. Some of the papers omitted from the last edition have been included in this because they now fall into their place, while before they did not. Doubtless this will happen again. Continued progress is being made. Since the book was first begun two Journals have sprung up devoted entirely to soil : Soil Science, under the editorship of the indefatigable J. G. Lipman, and the International Mitteilungen fur Bodenkunde. Another Journal, the Journal of Ecology, has also arisen and is vigorously developing another aspect of the same subject, while the older agricultural journals are finding more and more of their space taken up by soil papers. The subject now only lacks a name, and though many have been pro- posed— pedology, agrogeology, edaphology, etc. — I have not felt drawn to any of them. CONTENTS. CHAPTER PAGE I. HISTORICAL AND INTRODUCTORY i II. SOIL CONDITIONS AFFECTING PLANT GROWTH - 30 III. THE COMPOSITION OF THE SOIL - 92 IV. THE COLLOIDAL .PROPERTIES OF SOIL - 148 V. THE CARBON AND NITROGEN CYCLES IN THE SOIL - 175 VI. THE BIOLOGICAL CONDITIONS IN THE SOIL - 218 VII. THE MICRO-ORGANIC POPULATION OF THE SOIL AND ITS RELATION TO THE GROWTH OF PLANTS - 250 VIII. THE SOIL IN RELATION TO PLANT GROWTH - - 301 IX. SOIL ANALYSIS AND ITS INTERPRETATION - - 322 APPENDICES. I. THE METHODS OF SOIL ANALYSIS - - 347 II. AMOUNTS OF VARIOUS SUBSTANCES ABSORBED FROM THE SOIL BY THE COMMON AGRICULTURAL CROPS OF ENGLAND - -357 III. A SELECTED BIBLIOGRAPHY OF PAPERS ON SOIL CON- DITIONS AND PLANT GROWTH "- - 359 AUTHOR INDEX - 387 SUBJECT INDEX 395 LIST OF PLATES. PORTRAIT OF J. B. J. DIEUDONNE" BOUSSINGAULT (1802- 1887) Frontispiece TO FACE PAGE FIG. 9. — INFLUENCE OF MEDIUM ON ROOT DEVELOPMENT OF LUPINES - 53 FIG. ii. — SWEDES FROM AGDELL FIELD, BROADBALK : UN- MANURED; SUPERPHOSPHATES AND POTASSIC FERTILISERS; AND COMPLETE FERTILISER - 65 FIG. 30A. — POOR GRASS PASTURE, UNTREATED PLOT, ^ HORNDON-ON-THE-HILL, ESSEX (LONDON CLAY) - between FIG. 306. — ADJOINING PLOT, BUT TREATED WITH |3I2&3I3 GAFSA PHOSPHATE, FEB. 27™, 1918 CHAPTER I. HISTORICAL AND INTRODUCTORY. IN all ages the growth of plants has interested thoughtful men. The mystery of the change of an apparently lifeless seed to a vigorous growing plant never loses its freshness, and consti- tutes, indeed, no small part of the charm of gardening. The economic problems are of vital importance, and become more and more urgent as time goes on and populations increase and their needs become more complex. There was an extensive literature on agriculture in Roman times which maintained a pre-eminent position until compara- tively recently. In this we find collected many of the facts which it has subsequently been the business of agricultural chemists to classify and explain. The Roman literature was collected and condensed into one volume about the year 1 240 by a senator of Bologna, Petrus Crescentius, whose book l was one of the most popular treatises on agriculture of any time, being frequently copied, and in the early days of printing, passing through many editions — some of them very handsome, and ultimately giving rise to the large standard European treatises of the sixteenth and seventeenth centuries. Many other agricultural books appeared in the fifteenth and early sixteenth centuries, notably in Italy, and later in France. In some of these are found certain ingenious speculations that have been justified by later work. Such, for instance, is Palissy's remarkable statement in i<6j (222) 2: "You will admit that when you bring dung into the field it is to return to the soil something that has been taken away. . . . When a lDe agricultura vulgare, Augsburg, 1471, and many subsequent editions. 2 The numbers in brackets refer to the Bibliography at the end of the book. I 2 SOIL CONDITIONS AND PLANT GROWTH plant is burned it is reduced to a salty ash called alcaly by apothecaries and philosophers. . . . Every sort of plant with- out exception contains some kind of salt. Have you not seen certain labourers when sowing a field with wheat for the second year in succession, burn the unused wheat straw which had been taken from the field ? In the ashes will be found the salt that the straw took out of the soil ; if this is put back the soil is improved. * Being burnt on the ground it serves as manure because it returns to the soil those substances that had been taken away." But for every speculation that has been confirmed will be found many that have not, and the begin- nings of agricultural chemistry must be sought later, when men had learnt the necessity for carrying on experiments. The Search for the " Principle " of Vegetation, 1630-1750. The earlier investigators sought for a " principle " of vegetation to account for the phenomena of soil fertility and plant growth. The great Lord Bacon (8) believed that water formed the " principal nourishment " of plants, the purpose of the soil being to keep them upright and protect them from excessive cold or heat, but he also considered that each plant drew a " particular juyce " from the soil for its sustenance, thereby impoverishing the soil for that particular plant and similar ones, but not necessarily for other plants. Van Hel- mont regarded water as the sole nutrient for plants, and his son thus records his famous Brussels experiment (131) : "I took an earthen vessel in which I put 200 pounds of soil dried in an oven, then I moistened with rain water and pressed hard into it a shoot of willow weighing 5 pounds. After exactly five years the tree that had grown up weighed 169 pounds and about three ounces. But the vessel had never received anything but rain water or distilled water to moisten the soil when this was necessary, and it remained full of soil, which was still tightly packed, and, lest any dust from outside should get into the soil, it was covered with a shjet of iron coated with tin but perforated with many holes. I did not take the weight of the leaves that fell in the autumn. In the end I dried the soil HISTORICAL AND INTRODUCTORY 3 once more and got the same 200 pounds that I started with, less about two ounces. Therefore the 164 pounds of wood, bark, and root, arose from the water alone." The experiment is simple and convincing, and satisfied Boyle (50), who repeated it with " squash, a kind of Indian pompion " and obtained similar results. Boyle further dis- tilled the plants and concluded, quite justifiably from his prem- ises, that the products obtained, " salt, spirit, earth, and even oil (though that be thought of all bodies the most opposite to water), may be produced out of water ". Nevertheless, the conclusion is incorrect, because two factors had escaped Van Helmont's notice — the parts played by the air and by the missing two ounces of soil. But the history of this experiment is thoroughly typical of experiments in agricultural chemistry generally : in no other subject is it so easy to overlook a vital factor and draw from good experiments a conclusion that appears to be absolutely sound, but is in reality entirely wrong. Some years later — about 1650 — Glauber (107) set up the hypothesis that saltpetre is the "principle" of vegetation. Having obtained saltpetre from the earth cleared out from cattle sheds, he argued that it must have come from the urine or droppings of the animals, and must, therefore, be contained in the animal's food, i.e. in plants. He also found that addi- tions of saltpetre to the soil produced enormous increases in crop. He connected these two observations and supposed that saltpetre is the essential principle of vegetation. The fertility of the soil and the value of manures (he mentions dung, feathers, hair, horn, bones, cloth cuttings) are entirely due to saltpetre. This view was supported by Mayow's experiments (195). He estimated the amounts of nitre in the soil at different times of the year, and showed that it occurs in greatest quantity in spring when plants are just beginning to grow, but is not to be found " in soil on which plants grow abundantly, the reason being that all the nitre of the soil is sucked out by the plants ". Kiilbel (quoted in 293), on the other hand, regarded a magma i * SOIL CONDITIONS AND PLANT GROWTH unguinosum obtainable from humus as the " principle" sought for. The most accurate work in this period was published by John Woodward, in a remarkable paper in 1.699 (321). Set- ting out from the experiments of Van Helmont and of Boyle, but apparently knowing nothing of the work of Glauber and of Mayow, he grew spearmint in water obtained from various sources with the following results among others : — Source of Water. Weight of Plants. Gained in 77 days. Expense of Water (i.e. Transpira- tion). Proportion of Increase of Plant to Expense of Water. When put in. When taken out. Rain water River Thames Grains. 28J 28 Grains. 45i 54 Grains. *7i 26 Grains. 3004 2493 I to lyiff i to 95ff Hyde Park conduit ,» + i£ ozs. garden mould 110 92 249 376 139 284 13140 14950 i to 94lz& I tO 52iff Now all these plants had abundance of water, therefore all should have made equal growth had nothing more been needed. The amount of growth, however, increased with the impurity of the water. " Vegetables," he concludes, " are not formed of water, but of a certain peculiar terrestrial matter. It has been shown that there is a considerable quantity of this matter contained in rain, spring, and river water, that the greatest part of the fluid mass that ascends up into plants, does not settle there but passes through their pores and exhales up into the atmosphere : that a great part of the terrestrial matter, mixed with the water, passes up into the plant along with it, and that the plant is more or less augmented in proportion as the water contains a greater or less quantity of that matter ; from all of which we may reasonably infer, that earth, and not water, is the matter that constitutes vegetables." He discusses the use of manures and the fertility of the soil from this point of view, attributing the well-known falling off in crop yield when plants are grown for successive years on HISTORICAL AND INTRODUCTORY 5 unmanured land to the circumstance that " the vegetable matter that it at first abounded in being extracted from it by those successive crops, is most of it borne off. . . . The land may be brought to produce another series of the same vege- tables, but not until it is supplied with a new fund of matter, of like sort with that it at first contained ; which supply is made several ways, either by the ground's being fallow some time, until the rain has poured down a fresh stock upon it ; or by the tiller's care in manuring it." The best manures, he continues, are parts either of vegetables or of animals, which ultimately are derived from vegetables. In his celebrated textbook of chemistry Boerhaave (41) taught that plants absorb the juices of the earth and then work them up into food. The raw material, the " prime radical juice of vegetables, is a compound from all the three kingdoms, viz. fossil bodies and putrified parts of animals and vege- tables ". This " we look upon as the chyle of the plant ; being chiefly found in the first order of vessels, viz., in the roots and the body of the plant, which answers to the stomach and intes- tines of an animal ". For many years no such outstanding work as that of Glauber and Woodward was published, if we except Hales' Vegetable Staticks (i 19), the interest of which is physiological rather than agricultural.1 Advances were, however, being made in agricultural practice. One of the most important was the introduction of the drill and the horse hoe by Jethro Tull (286)-, an Oxford man of a strongly practical turn of mind, who insisted on the vital importance of getting the soil into a fine crumbly state for plant growth, Tull was more than an inventor ; he discussed in most picturesque language the sources of fertility in the soil. In his view it was not the juices of the earth, but the very minute particles of soil loosened by the action of moisture, that constituted the " proper pabulum " of plants. The pressure caused by the swelling of the growing roots forced these particles into the " lacteal mouths of the roots," where they entered the circula- tory system. All plants lived on these particles, i.e. on the 1 He shows, however, that air is " wrought into the composition " of plants. 6 SOIL CONDITIONS AND PLANT GROWTH same kind of food ; it was incorrect to assert, as some had done, that different kinds of plants fed as differently as horses and dogs, each taking its appropriate food and no other. Plants will take in anything that comes their way, good or bad. A rotation of crops is not a necessity, but only a convenience. Conversely, any soil will nourish any plant if the temperature and water supply are properly regulated. Hoeing increased the surface of the soil or the " pasture of the plant," and also enabled the soil better to absorb the nutritious vapours condensed from the air. Dung acted in the same way, but was more costly and less efficient. So much were lull's writings esteemed, Cobbett tells us, that they were " plundered by English writers not a few and by Scotch in whole bandittis ". The position at the end of this period cannot better be summed up than in Tull's own words : " It is agreed that all the following materials contribute in some manner to the in- crease of plants, but it is disputed which of them is that very increase or food : (i) nitre, (2) water, (3) air, (4) fire, (5) earth ". The Search for Plant Nutrients. I. The Phlogistic Period, 1750-1800. Great interest was taken in agriculture in this country dur- ing the latter half of the eighteenth century. " The farming tribe," writes Arthur Young during this period, "is now made up of all ranks, from a duke to an apprentice." Many experi- ments were conducted, facts were accumulated, books written, and societies formed for promoting agriculture. The Edin- burgh Society, established in 1755 for the improvement of arts and manufactures, induced Francis Home (137) " to try how far chymistry will go in settling the principles of agriculture ". The whole art of agriculture, he says, centres in one point : the nourishing of plants. Investigation of fertile soils showed that they contain oil, which is therefore a food of plants. But when a soil has been exhausted by cropping, it recovers its HISTORICAL AND INTRODUCTORY 7 fertility on exposure to air,1 which therefore supplies another food. Home made pot experiments to ascertain the effect of various substances on plant growth. " The more they (i.e. farmers) know of the effects of different bodies on plants, the greater chance they have to discover the nourishment of plants, at least this is the only road." Saltpetre, Epsom salt, vitriol- ated tartar (i.e. potassium sulphate) all lead to increased plant growth, yet they are three distinct salts. Olive oil was also- useful. It is thus clear that plant food is not one thing onlyr but several ; he enumerates six : air, water, earth, salts of different kinds, oil, and fire in a fixed state. As further proof he shows that " all vegetables and vegetable juices afford those very principles, and no other, by all the chymical experi- ments which have yef been made on them with or without fire ". The book is a great advance on anything that had gone before it, not only because it recognises that plant nutrition depends on several factors, but because it indicates so clearly the two methods to be followed in studying the problem — pot cultures and plant analysis. Subsequent investigators, Wal- lerius (293), the Earl of Dundonald (90), and Kirwan (149) added new details but no new principles. The problem indeed was carried as far as was possible until further advances were made in plant physiology and in chemistry. The writers just mentioned are, however, too important to be passed over com- pletely. Wallerius, in 1761, professor of chemistry at Upsala, after analysing plants to discover the materials on which they live, and arguing that Nutritio non fieri pot est a rebus hetero- geneis, sed homogeneis^ concludes that humus, being homogeneis^ is the source of their food — the nutritiva — while the other soil constituents are instrumentalia^ making the proper food mix- ture, dissolving and attenuating it, till it can enter the plant root. Thus chalk and probably salts help in dissolving the "fatness " of the humus. Clay helps to retain the " fatness " and prevent it being washed away by rain : sand keeps the soil open and pervious to air. The Earl of Dundonald, in 1 Recorded by most early writers, e.g. Evelyn (Terra, 1674) (96). 8 SOIL CONDITIONS AND PLANT GROWTH 1795, adds alkaline phosphates to the list of nutritive salts, but he attaches chief importance to humus as plant food. The " oxygenation " process going on in the soil makes the organic matter insoluble and therefore useless for the plant ; lime, "alkalis and other saline substances" dissolve it and change it to plant food ; hence these substances should be used alternately with dung as manure. Manures were thus divided, as by Wallerius, into two classes : those that afford plant food, and those that have some indirect effect. Throughout this period it was believed that plants could generate alkalis. "Alkalies," wrote Kirwan in 1796, " seem to be the product of the vegetable process, for either none, or scarce any, is found in the soils, or in rain water." In like manner Lampadius thought he had proved that plants could generate silica. The theory that plants agreed in all essentials with animals was still accepted by many men of science ; some interesting developments were made by Erasmus Darwin in 1803 (77> Between 1770 and 1800 work was done on the effects of vegetation on air that was destined to revolutionise the ideas of the function of plants in the economy of Nature, but its agricultural significance was not recognised until later. In 1771 Priestley (230), knowing that the atmosphere becomes vitiated by animal respiration, combustion, putrefaction, etc., and realising that some natural purification must go on, or life would not longer be possible, was led to try the effect of sprigs of living mint on vitiated air. He found that the mint made the air purer, and concludes " that plants, instead of affecting the air in the same manner with animal respiration, reverse the effects of breathing, and tend to keep the atmosphere pure and wholesome, when it is become noxious in consequence of animals either living, or breathing, or dying, and putrefying in it ". But he had not yet discovered oxygen, and so could not give precision to his discovery : and when, later on, he did discover oxygen and learn how to estimate it, he unfortunately failed to confirm his earlier results because he overlooked a vital factor, the necessity of light. He was therefore unable to HISTORICAL AND INTRODUCTORY 9 answer Scheele, who had insisted that plants, like animals, vitiate the air. It was Ingen-Housz (142) who reconciled both views and showed that purification goes on in light only, whilst vitiation takes place in the darkness. Jean Senebier at Geneva had also arrived at the same result. He also studied the con- verse problem — the effect of air on the plant, and in 1782 argued (259) that the increased weight of the tree in Van Helmont's experiment (p. 2) came from the fixed air. " Si done 1'air fixe, dissous dans 1'eau de I'atmosphere, se combine dans la parenchyme avec la lumiere et tous les autres elemens de la plante ; si le phlogistique de cet air fixe est surement precipite dans les organes de la plante, si ce precipite reste, comme on le voit, puisque cet air fixe sort des plantes sous la forme d'air dephlogistique, il est clair que Tair fixe, combine dans la plante avec la lumiere, y laisse une matiere qui n'y seroit pas, et mes experiences sur I'etiolement suffisent pour le demontrer." Later on Senebier translated his work into the modern terms of Lavoisier's system. 2. The Modern Period^ 1800-1860. (a) The Foundation of Plant Physiology. — We have seen that Home in 1756 pushed his inquiries as far as the methods in vogue would permit, and in consequence no marked advance was made for forty years. A new method was wanted before further progress could be made, or . before the new idea intro- duced by Senebier could be developed. Fortunately, this was soon forthcoming. To Theoglorje-d^-Saussure, in 1804 (244), son of the well-known de Saussure of Geneva, is due the quan- titative experimental method which more than anything else has made modern agricultural chemistry possible : which formed the basis of subsequent work by Boussingault, Liebig, Lawes and Gilbert, and indeed still remains our safest method of in- vestigation. Senebier tells us that the elder de Saussure was well acquainted with his work, and it is therefore not surpris- ing that the son attacked two problems that Senebier had also studied — the effect of air on plants and the nature and origin io SOIL CONDITIONS AND PLANT GROWTH of salts in plants. De Saussure grew plants in air or in known mixtures of air and carbon dioxide, and measured the gas changes by eudiometric analysis and the changes in the plant by " carbonisation ". He was thus able to demonstrate the central fact of plant respiration — the absorption of oxygen and the evolution of carbon dioxide, and further to show the de- composition of carbon dioxide and evolution of oxygen in light. Carbon dioxide in small quantities was a vital neces- sity for plants, and they perished if it was artificially removed from the air. It furnished them not only with carbon, but also with some oxygen. Water is also decomposed and fixed by plants. On comparing the amount of dry matter gained from these sources with the amount of material that can enter through the roots even under the most favourable conditions, he concludes that the soil furnished only a very small part of the plant food. Small as it is, however, this part is indispens- able : it supplies nitrogen — une partie essentielle des vegetaux — which, as he had shown, was not assimilated direct from the air ; and also ash constituents, qui peuvent contribuer a former, comme dans les animaux, leur parties solides ou osseuses. Fur- ther, he shows that the root is not a mere filter allowing any and every liquid to enter the plant ; it has a special action and takes in water more readily than dissolved matter, thus effect- ing a concentration of the solution surrounding it ; different salts, also, are absorbed to a different extent. Passing next to the composition of the. plant ash, he shows that it is not constant, but varies with the nature of the soil and the age of the plant ; it consists mainly, however, of alkalis and phos- phates. All the constituents of the ash occur in humus. If a plant is grown from seed in water there is no gain in ash : the amount found at the end of the plant's growth is the same as was present in the seed excepting for a relatively small amount falling on the plant as dust. Thus he disposes finally of the idea that the plant generated potash. After the somewhat lengthy and often wearisome works of the earlier writers it is very refreshing to turn to de Saussure's concise and logical arguments and the ample verification he HISTORICAL AND INTRODUCTORY n gives at every stage. But for years his teachings were not accepted, nor were his methods followed. The two great books on agricultural chemistry then current still belonged to the old period. Thaer and Davy, while much in advance of Wallerius, the textbook writer of 1761, nevertheless did not realise the fundamental change introduced by de Saussure ; it has always been the fate of agricultural science to lag behind pure science. Thaer pub- lished his Grundsdtze der rationellen Landwirtschaft in 1809- 1812: it had a great success on the Continent as a good, practical handbook, and was translated into English as late as 1844 by Cuthbert Johnson. Davy's book (79) grew out of the lectures which he gave annually at the Royal Institu- tion on agricultural chemistry between 1802 and 1812; it was published in 1813, and forms the last textbook of the older period. Whilst no great advance was made by Davy himself (indeed his views are distinctly behind those of de Saussure) he carefully sifted the facts and hypotheses of previous writers, and gives us an account, which, however de- fective in places, represents the best accepted knowledge of the time, set out in the new chemical language. He does not accept de Saussure's conclusion that plants obtain their carbon chiefly from the carbonic acid of the air : some plants, he says, appear to be supplied with carbon chiefly from this source but in general he supposes the carbon to be taken in through the roots. Oils are good manures because of the carbon and hydrogen they contain; soot is valuable, because its carbon is " in a state in which it is capable of being rendered soluble by the action of oxygen and water". Lime is useful because it dissolves hard vegetable matter. Once the organic matter has dissolved there is no advantage in letting it decompose further: putrid urine is less useful as manure than fresh urine, whilst it is quite wrong to cause farmyard manure to ferment before it is applied to the land. All these ideas have long been given up, and indeed there never was any sound .experimental evidence to support them. It is even arguable that they would not have persisted so long as they 12 SOIL CONDITIONS AND PLANT GROWTH • did had it not been for Davy's high reputation. His insistence on the importance of the physical properties of soils — their relationship to heat and to water — was more fortunate and marks the beginning of soil physics, afterwards developed con- siderably by Schubler (254). On the Continent, to an even greater extent than in England, it was held that plants drew their carbon from the soil and lived on humus, a view sup- ported by the very high authority of Berzelius.1 (ti) The Foundation of Agricultural Science. — Hitherto ex- periments had been conducted either in the laboratory or in small pots: about 1834, however, Boussingault, who was already known as an adventurous traveller in South America, began a series of field experiments on his farm at Bechelbronn in Alsace. These were the first of their kind : to Boussin- gault, therefore, belongs the honour of having introduced the method by which the new agricultural science was to be de- veloped. He reintroduced the quantitative methods of de Saussure, weighed and analysed the manures used and the crops obtained, and at the end of the rotation drew up a bal- ance sheet, showing how far the manure had satisfied the needs of the crop and how far other sources of supply — air, rain, and soil — had been drawn upon. The results of one experiment are given in Table I. on the opposite page. At the end of the period the soil had returned to its original state of productive- ness, hence the dry matter, carbon, hydrogen, and oxygen not accounted for by the manure must have been supplied by the air and rain, and not by the soil. On the other hand, the manure afforded more mineral matter than the crop took off, the balance remaining in the soil. Other things being equal, he argued that the best rotation is one which yields the great- est amount of organic matter over and above what is present in the manure. No fewer than five rotations were studied, but it will suffice to set out only the nitrogen statistics (Table II. on the opposite page), which show a marked gain of nitrogen 1 J. J. Berzelius, Lehrbuch d. chemie, ubersetz. v. F. Wohler, 3 Aufl., 1839, Bd. 8. HISTORICAL AND INTRODUCTORY TABLE I. — STATISTICS OF A ROTATION. BOUSSINGAULT (46). Weight in kilograms per hectare of Dry Matter. Carbon. Hydrogen. Oxygen. Nitrogen. Mineral Matter. i. Beets . 3172 13577 184-0 1376-7 53'9 199-8 2. Wheat . 5 V \ ; 3006 1431-6 164-4 1214-9 31-3 163-8 3. Clover hay . , 4029 1909-7 201-5 1523-0 84-6 310-2 4. Wheat. 4208 2004*2 230*0 1700-7 43'8 229-3 Turnips (catch crop) 716 307-2 39*3 302-9 I2'2 54*4 5. Oats . . * 2347 1182-3 137*3 890-9 28-4 108-0 Total during rotation 17478 81927 956-5 7009-0 254*2 1065-5 Added in manure IOl6l 3637-6 426-8 2621-5 203-2 3271-9 Difference not ac- + 7317 + 4555' i + 5297 + 4387-5 + 51*0 - 2206-4 counted for, taken from air, rain, or soil. 1000 kilograms per hectare = 16 cwt. per acre. TABLE II. — NITROGEN STATISIICS OF VARIOUS ROTATIONS. BOUSSINGAULT (46). Kilograms per hectare. Rotation. Nitrogen in Nitrogen Excess in Crop over that supplied in Manure. Per Rotation. Per Annum. (i) Potatoes, (2) wheat, (3) clover, (4) wheat, turnips,1 (5) oats 203-2 2507 47*5 9*5 (i) Beets, (2) wheat, (3) clover, (4) wheat, turnips,1 (5) oats 203-2 254-2 51-0 10-2 (i) Potatoes, (2) wheat, (3) clover, (4) wheat, turnips,1 (5) peas, (6) rye 243-8 353*6 109-8 18-3 Jerusalem artichokes, two years 188-2 274-2 860 43-0 2 (i) Dunged fallow, (2) wheat, (3) wheat .... , 82-8 87-4 4-6 i'5 Lucerne, five years 224-0 1078-0 854 170-8 1 Catch crop, i.e. taken in autumn after the wheat. 2 This crop does not belong to the leguminosae, but it is possible that the nitrogen came from the soil, and that impoverishment was going on. 14 SOIL CONDITIONS AND PLANT GROWTH when the newer rotations are adopted, but not where wheat only is grown. Now the rotation has not impoverished the soil, hence he concludes that u 1'azote peut entrer directement dans 1'organ- isme des plantes, si leur parties vertes sont aptes a le fixer ". Boussingault's work covers the whole range of agriculture and deals with the composition of crops at different stages of their growth, with soils, and with problems in animal nutrition Unfortunately the classic farm of Bechelbronn did not remain a centre of agricultural research and the experiments came to an end. Some of the work was summarised by Dumas in a very striking essay (88, see also 47) that has been curiously overlooked by agricultural chemists. During this period (1830 to 1840) Carl Sprengel was studying the ash constituents of plants, which he considered were probably essential to nutrition (270). Schiibler was working at soil physics (254), and a good deal of other work was quietly being done. No particularly important discoveries were being made, no controversies were going on, and no great amount of interest was taken in the subject. But all this was changed in 1840 when Ljebig's famous report to the British Association upon the state of organic chemistry, afterwards published as Chemistry in its Application to Agriculture and Physiology (1740), came like a thunderbolt upon the world of science. With polished invective and a fine sarcasm he holds up to scorn the plant physiologists of his day for their continued adhesion, in spite of accumulated evidence, to the view that plants derive their carbon from the soil and not from the carbonic acid of the air. " All explanations of chemists must remain without fruit, and useless, because, even to the great leaders in physiology, carbonic acid, ammonia, acids, and bases, are sounds without meaning, words without sense, terms of an unknown language, which awake no thoughts and no associations." The experiments quoted by the physi- ologists in support of their view are all "valueless for the de- cision of any question ". " These experiments are considered by them as convincing proofs, whilst they are fitted only to HISTORICAL AND INTRODUCTORY 15 awake pity." Liebig's ridicule did what neither de Saussure's nor Boussingault's logic had done : it finally killed the humus theory. Only the boldest would have ventured after this to assert that plants derive their carbon from any source other than carbon dioxide, although it must be admitted that we have no proof that plants really do obtain all their carbon in this way. Thirty years later, in fact, Grandeau (112) adduced evidence that humus may, after all, contribute something to the carbon supply, and his view still finds acceptance in France ; l for this also, however, convincing proof is lacking. But for the time carbon dioxide was considered to be the sole source of the carbon of plants. Hydrogen and oxygen came from water, and nitrogen from ammonia. Certain mineral substances were essential : alkalis were needed for neutrali- sation of the acids made1 by plants in the course of their vital processes, phosphates were necessary for seed formation, and potassium silicates for the development of grasses and cereals. The evidence lay in the composition of the ash : plants might absorb anything soluble from the soil, but they excreted from their roots whatever was non-essential. The fact of a sub- stance being present was therefore sufficient proof of its necessity. Plants, Liebig argued, have an inexhaustible supply of carbonic acid in the air. But time is saved in the early stages of plant growth if carbonic acid is being generated in the soil, for it enters the plant root and affords extra nutriment over and above what the small leaves are taking in. Hence a supply of humus, which continuously yields carbonic acid, is advantageous. Further, the carbonic acid attacks and dissolves some of the alkali compounds of the soil and thus increases the mineral food supply. The true function of humus is to evolve carbonic acid The alkali compounds of the soil are not all equally soluble. A weathering process has to go on, which is facilitated by liming and cultivation, whereby the comparatively insoluble 1 See e.g. L. Cailletet (64), Jules Lefevre (169), and J. Laurent, Rev. gen, hot., 1904, 16, 14. 1 6 SOIL CONDITIONS AND PLANT GROWTH compounds are broken down to a more soluble state. The final solution is effected by acetic acid excreted by the plant root, and the dissolved material now enters the root. The nitrogen is taken up as ammonia, which may come from the soil, from added manure, or from the air. In order that a soil may remain fertile it is necessary and sufficient to return in the form of manure the mineral constituents and the nitrogen that have been taken away. When sufficient crop analyses have been made it will be possible to draw up tables showing the farmer precisely what he must add in any par- ticular case. An artificial manure known as Liebig's patent manure was made up on these lines and placed on the market. Liebig's book was meant to attract attention to the subject, and it did ; it rapidly went through several editions, and as time went on Liebig developed his thesis, and gave it a quan- titative form : " The crops on a field diminish or increase in exact proportion to the diminution or increase of the mineral substances conveyed to it in manure ". He further adds what afterwards became known as the Law of the Minimum,1 "by the -deficiency or absence of one necessary constituent, all the others being present, the soil is rendered barren for all those crops to the life of which that one constituent is indispensable ". These and other amplifications in the third edition, 1843, gave rise to much controversy. So much did Liebig insist, and quite rightly, on the necessity for alkalis and phosphates, and so impressed was he by the gain of nitrogen in meadow land supplied with alkalis and phosphates alone, and by the con- tinued fertility of some of the fields of Virginia and Hungary and the meadows of Holland, that he began more and more to regard the atmosphere as the source of nitrogen for plants. Some of the passages of the first and second editions urging the necessity of ammoniacal manures were deleted from the third and later editions. "If the soil be suitable, if it con- 1 The underlying principle was not discovered by Liebig, having already been enunciated by political economists of the Malthus School. He was, how- ever, the first to apply it to plant nutrition. HISTORICAL AND INTRODUCTORY 17 tains a sufficient quantity of alkalis, phosphates, and sulphates, nothing will be wanting. The plants will derive their ammonia from the atmosphere as they do carbonic acid," he writes in the Farmer s Magazine}- Ash analysis led him to consider the turnip as one of the plants " which contain the least amount of phosphates and therefore require the smallest quantity for their development ". These and other practical deductions were seized upon and shown to be erroneous by Lawes (161-162) who had for some years been conducting vegetation experiments. Lawes does not discuss the theory as such, but tests the deductions Liebig himself draws and finds them wrong. Further trouble was in store for Liebig ; his patent manure when tried in practice had failed. This was unfortunate, and the impression in England at any rate was, in Philip Pusey's words : " The mineral theory, too hastily adopted by Liebig, namely, that crops rise and fall in direct proportion to the quantity of mineral substances present in the soil, or to the addition or* abstraction of these sub- stances which are added in the manure, has received its. death-blow from the experiments of Mr Lawes ". And yet the failure of the patent manure was not entirely the fault of the theory, but only affords further proof of the, numerous pitfalls of the subject. The manure was sound in that it contained potassium compounds and phosphates (it ought, of course, to have contained nitrogen compounds), but it was unfortunately rendered insoluble by fusion with lime and calcium phosphate so that it should not too readily wash out in the drainage water. Not till Way had shown in 1850 that soil precipitates soluble salts of ammonium , potassium and phosphates was the futility of the fusion process discovered, and Liebig saw the error he had made (\J^b). Meanwhile the great field experiments at Rothamsted had been started by Lawes and Gilbert in 1 843. These experiments were conducted on the same general lines as those begun 1 Farmer's Magazine, 1847, vol. xvi., p. 511. A good summary of Liebig's position is given in his Letters on Chemistry, 34th letter, 3rd edition, p. 5195, 1851. 2 1 8 SOIL CONDITIONS AND PLANT GROWTH earlier by Boussingault, but they have the advantage that they are still going on, having been continued year after year on the same ground without alteration, except in occasional details, since 1852. The mass of data now accumulated is consider- able and it is being treated by modern statistical methods. Certain conclusions are so obvious, however, that they can be drawn on mere inspection of the data. By 1855 the fol- lowing points were definitely settled (i66r) : — (i) Crops require phosphates and salts of the alkalis, but the composition of the ash does not afford reliable information as to the amounts of each constituent needed, e.g. turnips re- quire large amounts of phosphates, although only little is present in their ash. Some of the results are : — Composition of ash, per cent. (1860 crop) — K00 . 44-8 P205 ... 7-9 Yield of turnips, tons per acre (1843)- Unmanured . . . 4-5 Superphosphate . . . 12-8 ,, +potassic salts irg (2) Non-leguminous crops require a supply of some nitro- genous compounds, nitrates and ammonium salts being almost equally good. Without an adequate supply no increases of growth are obtained, even when ash constituents are added. The amount of ammonia obtainable from the atmosphere is insufficient for the needs of crops. Leguminous crops behaved abnormally. (3) Soil fertility may be maintained for some years at least by means of artificial manures. (4) The beneficial effect of fallowing lies in the increase brought about in the available nitrogen compounds in the soil. Although many of Liebig's statements were shown to be wrong, the main outline of his theory as first enunciated stands. It is no detraction that de Saussure had earlier published a somewhat similar, but less definite view of nutrition : Liebig had brought matters to a head and made men look at their cherished, but unexamined, convictions. The effect of the stimulus he gave can hardly be over- HISTORICAL AND INTRODUCTORY 19 estimated, and before he had finished, the essential facts of plant nutrition were settled and the lines were laid down along which scientific manuring was to be developed. The water cultures of Knop and other plant physiologists showed conclusively that potassium, magnesium, calcium, iron, phos- phorus, along with sulphur, carbon, nitrogen, hydrogen, and oxygen are all necessary for plant life. The list differs from Liebig's only in the addition of iron and the withdrawal of silica; but even silica, although not strictly essential, is ad- vantageous to cereals. In two directions, however, the controversies went on for many years. Farmers were slow to believe that "chemical manures" could ever do more than stimulate the crop, and declared they must ultimately exhaust the ground. The Rothamsted plots falsified this prediction ; manured year after year with the same substances and sown always with the same crops, they even now, after sixty years of chemical manuring, continue to produce good crops, although secondary effects have sometimes set in. In France the great missionary was Georges Ville, whose lectures were given at the experimental farm at Vmcennes during 1867 and 1874-5 (288). He went even further than Lawes and Gilbert, and maintained that artificial manures were not only more remunerative than dung, but were the only way of keeping up fertility. In recom- mending mixtures of salts for manure he was not guided by ash analysis but by field trials. For each crop one of the four constituents, nitrogen compounds, phosphates, lime, and potassium compounds (he did not consider it necessary to add any others to his manures) was found by trial to be more wanted than the others and was therefore called the " dominant " constituent. Thus for wheat he obtained the following results, and therefore concluded that on his soil wheat required a good supply of nitrogen, less phosphate, and still less potassium : — 20 SOIL CONDITIONS AND PLANT GROWTH Crop per acre. Bushels. Normal manure . .* , . "/.. •-.,.. . . . . 43 Manure without lime . ; . ... . . .41 ,, ,, potash . . . . . . .31 „ ,, phosphate . ... . . . 26^ „ ,, nitrogen . 14 Soil without manure ........ 12 Other experiments of the same kind showed that nitrogen was the dominant for all cereals and beetroot, potassium for potatoes and vines, phosphates for the sugar cane. An excess of the dominant constituent was always added to the crop manure. The composition of the soil had to be taken into account, but soil analysis was no good for the purpose. In- stead he drew up a simple scheme of plot trials to enable farmers to determine for themselves just what nutrient was lacking in their soil. His method was thus essentially empirical, but it still remains the best we have ; his view that chemical manures are always better and cheaper than dung is, however, too narrow and has not survived. The second controversy dealt with the source of nitrogen in plants. Priestley had stated that a plant of Epilobium hirsutum place'orln a small vessel absorbed during the course of the month seven-eighths of the air present. De Saussure, however, denied that plants assimilated gaseous nitrogen. Boussingault's pot-experiments showed that peas and clover could get nitrogen from the air while wheat could not (45), and his rotation experiments emphasised this distinction. He himself did not make as much of this discovery as he might have done, but Dumas (88) fully realised its importance. Liebig, as we have seen, maintained that ammonia, but not gaseous nitrogen, was taken up by plants, a view con- firmed by Lawes, Gilbert, and Pugh (164) in the most rigid demonstration that had yet been attempted. Plants of several natural orders, including the leguminosae, were grown in surroundings free from ammonia or any other nitrogen com- pound. The soil was burnt to remove all trace of nitrogen compounds while the plants were kept throughout the ex- HISTORICAL AND INTRODUCTORY 21 periment under glass shades, but supplied with washed and purified air and with pure water. In spite of the ample supply of mineral food the plants languished and died : the conclusion seemed irresistible that plants could not utilise gaseous nitrogen. For all non-leguminous crops this con- clusion agreed with the results of field trials. But there re- mained the very troublesome fact that leguminous crops required no nitrogenous manure and yet they contained large quantities of nitrogen, and also enriched the soil considerably in this element. Where had the nitrogen come from? The amount of combined nitrogen brought down by the rain was found to be far too small to account for the result. For years experiments were carried on, but the problem remained un- solved. Looking back over the papers l one can see how very close some of the older investigators were to the discovery of the cause of the mystery: in particular Lachmann (158) in 1858 and Bretschneider (54) in 1861. Lachmann showed that the nodules invariably present on the roots contained " vibrionenartige " organisms, while Bretschneider showed that the nitrogen fixation which occurred in normal soil did not take place in ignited soil. But these papers were both published in obscure journals and attracted little attention, and once again an investigation in agricultural chemistry had been brought to a standstill for want of new methods of attack. The Beginnings of Soil Bacteriology. It had been a maxim with the older agricultural chemists that " corruption is the mother of vegetation ". Animal and vegetable matter had long been known to decompose with formation of nitrates : indeed nitre beds made up from such decaying matter were the recognised source of nitrates for the manufacture of gunpowder during the European Wars of the seventeenth and eighteenth centuries.2 No satisfactory 1 A good summary of the voluminous literature is contained in Lohnis Handbuch der Landw. Bakteriologie, pp. 646 et seq. 2 " Instructions sur 1'etablissement des nitrieres, public par les . Re"gisseurs ge"neraux des Poudres et Salpe"tre. Paris, 1777. 22 SOIL CONDITIONS AND PLANT GROWTH explanation of the process had been offered, although the discussion of rival hypotheses continued up till 1860, but the conditions under which it worked were known and on the whole fairly accurately described. No connection was at first observed between nitrate formation and soil productiveness. Liebig rather diverted attention from the possibility of tracing what now seems an obvious relationship by regarding ammonia as the essential nitrogenous plant nutrient, though he admitted the possible suitability of nitrates (I74 401-418. 28 SOIL CONDITIONS AND PLANT GROWTH for new factors : synthesis is hardly attempted as yet. As the factors are discovered attempts are made to give them mathematical expression. Thus Liebig's Law of the Minimum and F. F. Blackman's Limiting Factors are expressed mathe- matically by Mitscherlich (p. 32) : V. H. Blackman l ex- presses plant growth by the " compound interest law " : Miyake (pp. 185 and i88)bringsammonification and nitrification within the equation for autocatalytic actions ; and the modern agricul- tural chemist is acquiring a taste for mathematical formulae and constants unknown to the older generation of workers. This attempt to find mathematical expressions has been resisted on two grounds : some suppose that phenomena associated with life cannot in any case be expressed mathe- matically and that nothing but a hollow appearance of agree- ment can be obtained ; others consider that the mathematical formula, if it is to hold at all, must be expressed in such general terms as to become meaningless, e.g. many of the actions going on in Nature can be expressed by exponential equations if the terms are chosen with sufficient ingenuity. The soil investigator, however, will be wise to secure all the assistance he can, as the subject is complex, and it cuts across the conventional divisions of science. In modern Experimental Stations the tendency is towards team work. As an instance chosen because it is best known to the writer : at the Rothamsted Experimental Station, instead of a number of isolated individuals, there is a body of workers investigating the subject, each from his own special point of view, but each fully cognisant of the work of the others, and periodically submitting his results to discussion by them. Separate workers investigate respectively the bacteria, protozoa, fungi, algae, helminths, and insects of the soil ; in addition physical and organic chemists are studying the soil conditions, while others are concerned in the study of the growing plant. A body of workers by harmonious co- operation is able to make advances that would be impossible for any single individual, however brilliant. 1 Annals of Botany, 1919, 33, 353. HISTORICAL AND INTRODUCTORY 29 The nature of the subject necessitates a further departure from the usual procedure. In purely laboratory investigations it is customary to adopt the Baconian method in which factors are studied one at a time, all others being kept constant except the particular one under investigation. In dealing with soils in natural conditions, however, it is impossible to proceed in this way : climatic factors will not be kept constant, and however careful the effort to ensure equality of conditions there is always the probability, and sometimes the certainty, that the variable factor under investigation is interacting with climatic factors and exerting indirect effects which modify or even obscure the direct effects it is desired to study. Of recent years statisticians have devised methods for dealing with cases where several factors are varying simul- taneously. The data obtained by the various workers at Rothamsted are therefore examined by a statistician who endeavours to disentangle the effects of various factors and to state a number of probable relationships which can then be investigated in the laboratory by the ordinary single factor method. The modern methods as applied at Rothamsted include three distinct processes : — 1. Observations or experiments in the field by a group of specialists working independently, but with full cognisance of each other's results. 2. Examination of the data by modern statistical methods so as to ascertain the probable effects of the known factors and to indicate where known factors are insufficient to account for the results, and where, therefore, new factors must be sought. 3. Laboratory studies by the specialist staff of the relation- ships indicated by the statistical examination, these being reduced to single factor problems. CHAPTER II. SOIL CONDITIONS AFFECTING PLANT GROWTH. IN order to put limits to our discussion, it is necessary at the outset to state what soil factors influence plant growth and how, in general, the influence is manifested. We shall, there- fore, in the present chapter bring together certain of the results obtained by plant physiologists which are indispensable for the proper study of the subject. It has been shown that the five following soil factors pro- foundly affect the growth of plants : — 1. Water supply. 2. Air supply. 3. Temperature. 4. Supply of plant nutrients. 5. Various injurious factors. The plant may be affected in two general ways — in the amount of growth (i.e. the total amount of dry matter formed), or in the habit or other characteristic of growth. The former is susceptible of quantitative investigation, the latter is not, or only with difficulty ; it has, therefore, proved less attractive to investigators. The Effect on the Amount of Growth. I. The Study of Single Factors. Of the five factors concerned plant nutrients are, on the whole, the easiest to investigate quantitatively. The general relationship between the supply of a given plant nutrient and the amount of dry matter formed was demonstrated by Hell- riegel at Dhame in the eighties of the last century. Barley 30 SOIL CONDITIONS AFFECTING PLANT GROWTH 31 was grown in pots of sand, all necessary factors were amply provided, excepting only potassium salts, the amount of which varied in the different pots. The weights of dry matter formed are shown in Table IV. TABLE IV. — EFFECT OF POTASSIUM SALTS ON GROWTH OF BARLEY HELLRIEGEL (130^). Mgms. of K2O per pot . o 23-5 47 70'5 9* 1 88 282 Drv matter formed when KC1 was given 2*271 5 '4 H 9-024 9-963 I5-322 21-246 24-417 K2SO4 2'549 5^40 5-283 13-363 14-768 21-593 23-774 KN03 4-552 6-621 9*949 14-576 21-499 24-206 KH2P04 „ K2HPO4 „ — 4-687 6-346 6-684 9-93I I2'377 11-736 17-171 20-255 - Average 2-410 4-948 6-791 10*801 I3-755 20-357 24-132 These results are plotted in Fig. i. 25 o lL.5 10 0 o J88 28? 25-5 47 70-5 94 KCl. used (MgmsKaO.) FIG. i. — Relation between potash supply and growth of barley (Hellriegel). Similar curves are obtained when the varying nutrient is nitrogen or phosphorus. The smoothness of the curve suggests that it can be ex- pressed by a mathematical equation, and E. A. Mitscherlich has attempted to do this, proceeding in the following manner : If all the conditions were ideal, a certain maximum yield would be obtained, but in so far as any essential factor is deficient 32 SOIL CONDITIONS AND PLANT GROWTH there is a corresponding shortage in the yield. The yield rises if some of the lacking factor is added, and goes up all the further the lower it had previously fallen. Mitscherlich put this as follows : the increase of crop produced by unit incre- ment of the lacking factor is proportional to the decrement 100 90 Lo.g ,9615- 80 .1 co 60 20 0 10 25 50 100 C&lcium Monophosphate. FIG. 2. — Mitscherlich's curve showing relation between yield of oats and amount of phosphate supplied. from the maximum. The advantages of this form is that it can be expressed mathematically : — where y is the yield obtained when x = the amount of the SOIL CONDITIONS AFFECTING PLANT GROWTH 33 factor present and A is the maximum yield obtainable if the factor were present in excess, this being calculated from the equation. Mitscherlich's own experiments were made with oats grown in sand cultures supplied with excess of all nutrients except- ing phosphate. This constituted the variable x : the yields actually attained when monocalcium phosphate was used and those calculated from the equation shown in Table V. (Fig. 2). Experiments were also made with di- and tri-calcic phos- phates and constants were calculated corresponding to k. The ratio of these constants £2(di-calcic phosphate) /^(mono-calcic phosphate) is a measure of the relative nutrient efficiency of the two salts ; k is therefore called the efficiency value (Wirkungswert). There are some very attractive possibilities about this method of treatment, since it gives a constant independent of the yield and having a definite mathematical meaning.1 TABLE V. — YIELD OF OATS WITH DIFFERENT DRESSINGS OF PHOSPHATES. MlTSCHERLICH (2Ol6). P2O5 in Manure. Dry Matter Produced. Crop Calculated from Formula. Difference. Difference x Probable Error. Grams. Grams. Grams. O'OO 9-8 + 0'50 9-80 - 0-39 - 0-8 0-05 19-3 + 0-52 18-91 - 0-56 - 0-3 O'lO 27'2 + 2'00 26-64 - 2*37 - 2-8 O'20 41-0 ± 0-85 38-63 + 3'22 + 2'9 0*30 43-9 ± I-I2 47-12 + 2-49 + 0-7 0-50 54-9 + 3-66 57'39 + 6*64 + 3*o 2'OO 61-0 ± 2-24 67-64 _ The method has given rise to considerable controversy in 1 For an interesting application see Zur Frage der Wurzelausscheidungen der Pflanze (Landw. Versuchs-Stat., 1913, Ixxxi., 467-474), in which Mitscherlich argues that the root excretions of clover cannot differ from those of oats. 3 34 SOIL CONDITIONS AND PLANT GRO WTH Germany. Pfeiffer * insists that the curves must be profoundly modified by other limiting factors, while Frohlich takes excep- tion to the method of calculation.2 Mitscherlich apparently recognises the force of these criticisms, for he now admits that Liebig's Law of the Minimum is not a correct expression of the facts: the yield is determined not by the one factor which is lacking, but by all the factors. Baule (9) has deve- loped an equation on these lines which, he claims, agrees satisfactorily with the experimental results. Curves of similar type have been obtained for variations in water supply and in temperature, but owing to greater experi- mental difficulties the data are less reliable, and, therefore, not susceptible to mathematical treatment. The temperature re- sults are of interest, in that they bring out an important differ- 1 Th. Pfeiffer, E. Blanck, and M. Fliigel, Wasser und Licht als Vegetations- factoren undihre Beziehungen zum Gesetze von Minimum (Landw. Versuchs-Stat., 1912, Ixxvi., 169-236. See also 2240). 2 The method of calculation is as follows : Obtain two equations by sub- stituting two of the numerical values of x and y obtained experimentally. Calling these numbers xlt xv etc., the equations are log, (A - yj = c - kxi ..... (i) loge (A - y2) = c - kxz ..... (2) Then by subtraction log (A - yj - log (A - yz) = k (#2 - xj . . . (3) Obtain another equation like (3) but select the numerical values so that X$ — X% — #2 ~~ % loge (A - y.,} - loge (A - y3) = k (x3 - *2) . . . (4) By subtracting (4) from (3) loge (A - yj + loge (A - y3) = 2 loge (A - j2)» Since yvy^ and jy3 are all known, the value of A is easily calculated. The value of k is then found from equation (3) k = log4A - jyj - loge(A - yj^ xz - xl As all the quantities on the right-hand side are known the value of k is readily obtained. A difficulty is that different values for A are, in fact, obtained when other equations like (3) are worked out. If there were a very large number of points, a probable value for A could be obtained : with a small number, such as almost necessarily are obtained in practice, some selection apparently has to be made, which is objectionable. This was done by Mitscherlich in the case quoted in Table V. SOIL CONDITIONS AFFECTING PLANT GROWTH 35 ence between sustained growth and the individual processes of assimilation, etc. In the classical experiments of F. F. Black- man and of Miss Matthaei (now A. G. L. Howard) (193) the effect of temperature on assimilation, all other factors being eliminated, was precisely that obtaining in an ordinary chemi- cal reaction ; so also for respiration. Miss Matthaei found that the amounts of carbon dioxide assimilated by a cherry laurel leaf per 30 sq. cms. (about 8 sq. ins.) per hour at various temperatures were : — Temperature, deg. C. ~ 6° + 8-8° 11-4° 15° 237° 30-5° 37-5° 40-5° 43° ' Weight of CO2 as- similated, grams . •OOO2 •0038 •0048 •0070 •OI02 •0157 •0238 •0149 •OIO2 By interpolation, the values at o°, 10°, 20°, etc., can be found, and the rate of assimilation is thus seen approximately to double for every increase of 10°, the usual order of increase in chemical reactions : — 2 Temperature . . . . ) . 0° 10° 20° 30° 37° Amount of CO2 assimilated per hour 17*5 42 89 157 238 Increased rate for 10° C. . ) . 2-4 2'I 1-8 1-8 But on the other hand, the effect of temperature on the rate of growth of a plant as a whole is in no wise like its effect in accelerating chemical change. Bialoblocki's (37) results with barley were as follows : — Temperature .... Dry matter formed, grams (o) (nil) 10° 7-64 20° 8-22 30° 3-85 40° 0-93 The two curves are shown in Fig. 3 ; the difference between JThe rates were maintained only for a short time at the higher tempera- tures. 2 A list of the papers dealing wijth the temperature co-efficient for cell growth is given in Science, 6th November, 1908. See also Zaleski, W., Ber. Deut. Bot. Ges., 1909, 27, 56-62. 3* 36 SOIL CONDITIONS AND PLANT GROWTH them is of fundamental importance to our subject and must be discussed in some detail. Each of the separate processes — assimilation, respiration, i/coc/ £40 #00 /20 80 40 n / A / ' \ / \ 2 \ x / X .x""1 — *• Temperature FIG. 3. — Relation between temperature and assimilation. +50" (Miss Matthaei.) etc., gives, so far as is known, curves like Fig. 3, continuous over the whole range of temperature nearly up to the death point. At higher temperatures it is necessary to work for a ^ u Q ? Grams of Dry Matter foi W 1 o t« * o* oo c ^ / \ 7 \ / \ L/ \ s. 0° 0° +10° +20° 4-30° +40° +50° — »~ Temperature ation between temperature and plant growth. (Bialoblocki.) short period only, so as to reduce the injurious effects then produced, but there is no break in the curve. For the growth of the plant, however, it is necessary that all the processes should work harmoniously together, and that SOIL CONDITIONS AFFECTING PLANT GROWTH 37 the protoplasm should remain healthy and vigorous. Now the temperature range over which protoplasm lives and the somewhat delicate adjustment of the processes holds together is very restricted ; beyond a certain point, which varies with different plants, further temperature increases do not cause more growth, but throw the adjustment out of gear. Thus the curve begins to bend over. The student will observe a close similarity between this Qu A TEMPERATURE FIG. 4. — Influence of temperature on enzyme action, showing fall in quantity but increase in activity as temperature rises. (Duclaux.) curve and that obtained by Duclaux 1 for the relation between enzyme action and temperature. In Fig. 4 AB shows the relation between enzyme action and temperature so long as the activity remains unimpaired ; CD shows the relation between temperature and quantity of enzyme, the enzyme being destroyed as the temperature rises ; AOE is the resul- tant curve showing the relation between temperature and the activity of a given initial quantity of enzyme. 1 E. Duclaux, T-raite de Micro biologie, Tome 2, Paris, 1899. 38 SOIL CONDITIONS AND PLANT GRO WTH In this case the falling off in activity at higher tempera- tures (OE) is due to the destruction of the enzyme ; in the case of the plant it is attributable to two factors, disadjust- ment of processes and injury to protoplasm. (2) Variation of Two or more Factors. The simple case presented by single factor variation is unusual in natural conditions : more generally two or more factors are present in quantities insufficient for perfect growth. This case has been discussed by F. F. Blackman who has in- troduced the very happy phrase " limiting factor " to express a conception previously used by H. T. Brown under the name "throttle valve". Generally speaking the effect of each separate factor is expressible by the single factor curve up to the point where a second factor begins to be insufficient, and then the curve alters considerably : instead of going on con- tinuously the increase of growth falls off considerably or even is brought to an end. A factor that thus proves insufficient and stops or greatly retards what ought to be a continuous process is called a " limiting factor ". Growth is once more resumed when the amount of the limiting factor is increased until again this factor proves insufficient, or some new factor comes into play. These phenomena are illustrated by von Seelhorst's inves- tigations on the effect of water supply on plant growth. • In one of their investigations Tucker and von Seelhorst (256) put up three series of soil pots in which the water was kept at a definite amount ; one was just moist, another was moister, and a third still moister. These were then each subdivided into three others, one receiving no nitrogen compounds, another one dose, and the third two doses. Oats were sown in all nine sets with results that are given in Table VI. When only little water is present the added 0-5 grm. of nitrogen is without effect, the supply in the soil being suf- ficient for the crop needs : the water and not the nitrogen is the limiting factor. When more water is added the plant can SOIL CONDITIONS AFFECTING PLANT GROWTH 39 make more growth, and can therefore utilise more nitrogen : the added O'5 grm. now raises the crop by 10 grms. Again, however, the water supply sets a limit, and the second o* 5 grm. of nitrogen is without effect. When a liberal supply of water is added the first O'5 grm. of nitrogen gives 20 grms. of crop, double the previous increment ; but even this does not repre- sent the whole possibility, for the second 0*5 grm. of nitrogen gives a still further increase of 1 5 -5 grms. TABU VI. — INFLUENCE OF WATER SUPPLY ON THE EFFECTIVENESS OF MANURES. VON SEELHORST AND TUCKER (256). Dry Weight of Oat Crop. Nitrogen Series. Increased Crop for Manuring. KP. KPN. KP2N. First Increment of Nitrogen. Second Increment of Nitrogen. I. Moist soil l 67'5 68-5 68-5 1-0 0 II. Moister soil 83-6 93 '4 94-0 9-8 •6 III. Wettest soil 99*5 H9'5 i35'o 20 '0 *5'5 K = i gram of K2O as K2CO3 per pot ; P = i gram of P2O5 as Ca(H2PO4)2 per pot ; N = -5 gram of N as NaNO3 per pot. Phosphate Series. Increase for Manuring. First Incre- ment of P. Second Incre- ment of P. Complete Manure. None. KN. KNP. KN2P. KNP. I. Moist soil l 4!'5 38-5 68-5 79*2 30-0 107 27 II. Moister soil 47*2 40*0 93*4 108-0 53*4 I4-6 46'2 III. Wettest soil 68-5 63-5 iiQ'5 127-5 56*0 8 51 The results of the phosphate series are somewhat different in detail, but not in principle. The first dose of P2O5 in the dry soil gives an increased crop, and so does the second, the first not having been large enough ; in the wetter soil, however, the increase is much larger. There is a still further increase in the wettest soil, but less than before, some other limiting 1 The moist soil contained 14*35 per cent, of water (41 '6 per cent, of satura- tion), the moister soil 15-41 per cent, at the beginning, increasing to 18-43 (51-7 per cent, of saturation) as the experiment proceeded, and the wettest soil, 16-44 per cent, at the beginning, increasing to 22-59 (63*7 per cent, of saturation). SOIL CONDITIONS AND PLANT GROWTH factor now coming in. These relations are shown in the curves of Fig. 5. A further illustration is afforded by experiments made by the writer on the growth of tomatoes in pots of sand in which supplies both of water and of nitrate were varied. The results are shown in the curves of Fig. 6. The series of curves is expressible by a surface, which is the proper way of represent- ing the effect of two varying factors on plant growth. No account was taken of temperature variations : to do this would i 90 65 MastWai:er.(.22-6°/0) More Vf 0-41-0-45 0-49 Effect of Soil Conditions on Consumption of Water by the Plant. In the experimental work just described the water content of the soil is kept artificially constant, thus eliminating the effect of the rate of consumption by the plant. In natural conditions, however, the supply is not maintained constant and the rate of consumption therefore becomes an important factor. Many determinations have been made of the weight of the water transpired per gram of dry matter formed, the figure being called the transpiration coefficient. This mode of 1 o-i normal NaCl has an osmotic pressure of approximately 4 atmospheres. SOIL CONDITIONS AFFECTING PLANT GROWTH 47 expression is convenient and it gives useful information to the practical grower in irrigated districts, but from the strictly scientific point of view it suffers from the disadvantage that it implies a causal relationship between transpiration and as- similation when in reality there is none. The transpiration coefficient is not constant but varies with the plant and the conditions, increasing with the tempera- ture and to some extent with the water supply, but decreasing as the food supply increases. TABLE IX. — TRANSPIRATION COEFFICIENTS, i.e. AMOUNT OF WATER TRANS- PIRED DURING THE PRODUCTION OF ONE PART OF DRY MATTER. BRIGGS AND SHANTZ (550). Crop. Extreme Values for Different Varieties. Mean Value for Genus. Proso Millet Sorghum . . . . .•„ Maize Wheat Barley Oats Flax 268 to 341 261 to 444 285 to 467 315 to 413 473 to 559 502 to 556 559 to 622 293 310 322 368 513 534 597 905 Sugar beet Potato — 397 636 Cow pea Clover 780 to 80 5 57i 7Q7 Lucerne Grasses Various native plants (i.e. weeds) 651 to 963 277 to 1076 831 861 With the plant variations we are not concerned. It is demonstrated, however, that a difference exists between different varieties of the same crop and that there are con- siderable prospects for breeding or selecting varieties specially suited for dry conditions. This work is already in hand with good results in Australia, the Western States and else- where. Unfortunately no correlation has been traced between water requirements and plant structure, so that the breeder has no guide in his selection except actual and tedious trials. The effect of soil conditions has been studied at the Be- sentchuk Agricultural Experiment Station, Samara, Russia, situated in a district which suffers from prolonged summer drought and excessive variations in crops. The transpiration SOIL CONDITIONS AND PLANT GROWTH coefficient is found to vary from year to year with the external meteorological conditions, being greatest in dry years and lowest in wet years ; it is also higher in wet soils than in dry ones. Toulaikoff considers that it is these conditions, rather than the biological character of the plant, that determines the magnitude of the coefficient (284). Some of his results are : — 1911. 1912. 1913- 1914. Wheat, var. Polltawka 628-4 444"5 338-6 387-6 „ „ Bieloturka 756-3 475*9 316-5 397-1 Oats „ Giant 655-1 5io-3 347*4 369-9 Barley „ Moravian 617-9 461-6 230-3 413*3 The year 1911 was excessively dry, 1913 was very rainy: 1912 was an average year and 1914 was rather dry. The effect of variations in water and food supply on the water requirements of plants was studied by Hellriegel, and subsequently by von Seelhorst at Gottingen (256-258), who has worked extensively at the various water relationships of plants. His results with oats are given in Table X. TABLE X. — EFFECT OF VARYING WATER SUPPLY * AND FOOD SUPPLY ON THE WATER REQUIREMENTS OF OATS. VON SEELHORST (257*1). Dry Matter Produced, Grams. Total Water Transpired, Grams. Water Transpired per Gram of Dry Matter. Soil Moist. Soil Moister. Soil still Moister. Soil Moist. Soil Moister. Soil still Moister. Soil Moist. Soil Moister. Soil still Moister. No manure 39-6 48-8 52-6 10-215 I5-245 16-290 259*9 312-9 307-1 Complete manure . 49'9 86-7 95-1 II-I70 20-490 23-030 225-1 236-8 231*6 1 The variations in water supply were : — 5 to 12 May. 12 May to i June. i June to i July. Soil moist Soil moister . Soil still moister . 54-4 59-2 64-0 54*4 64-0 73'9 44-8 59-2 73-6 Where 100 = saturation of the soil. SOIL CONDITIONS AFFECTING PLANT GROWTH 4$ Similar results have been obtained by Wilfarth (Table XL), (309*2) with sugar beets grown in pots of soil containing known but varying amounts of nitrate. TABLE XI. — EFFECT OF VARYING FOOD SUPPLY ON THE WATER REQUIRE- MENTS OF SUGAR BEET. WILFARTH. Nitrogen supplied, grams . •42 1-26 2'IO 2'94 3*36 378 Weight of dry matter pro- duced, grams . 23-0 73'9 96'5 132-4 167-6 188-8 Water transpired, grams 13,100 34,570 39,420 55,190 62,600 72,280 Stated as inches of rain 3-6 9'4 107 15*8 I7-0 19-6 Water used per gram of dry matter formed » 569 468 409 417 374 383 15 10 20 Water applied. 30 40 FIG. 8. — Effect of water supply on the effectiveness of farmyard manure. Yield of maize (stover and grain) in tons per acre for o to 40 inches irrigation water. (Harris and Butt, Utah, 123.) Two deductions may be drawn: (i) water is economised by increasing the food supply ; (2) the total amount of water required during the growing season may be greater than is supplied by the rain, in which case the balance must be otherwise provided, or the food cannot be utilised. 4 SOIL CONDITIONS AND PLANT GROWTH Over large areas of the world the rainfall is insufficient, and recourse is had to irrigation. In endeavouring to ascer- tain the best way of irrigating crops two considerations have to be kept in view: (i) excessive watering has secondary injurious effects on the soil, such as the deterioration of the physical condition, the accumulation of alkali salts, or the formation of toxic reduction products ; (2) the requirements of the plant are not always the same, more water being needed during the period of active growth than during germination or ripening. Some of the results obtained in Utah are set out in Table XII. and in Fig. 8 :— TABLE XII. — AVERAGE YIELD OF DRY MATTER AND NITROGEN FROM THE EXPERIMENTAL PLOTS ON GREENVILLE FARM, UTAH. GREAVES, STEWART, AND HIRST (1136). (Expressed as Ibs. per Acre.) Lucerne. Potatoes. Hay. Nitrogen. Tubers. Nitrogen. ins. 37'5 10,464 282*5 1,464 20-4 25-0 9,963 265-0 1,54° 24-8 15-0 9,779 259'I i,759 33*2 None 6,808 170-1 1,075 19-1 Oats. Maize. Grain. Straw. Nitrogen. Grain. Stover. Nitrogen. ins. 37'5 2,273 2,989 Sg-S 2,o8o 3,3I6 66-3 25-0 2,093 2,581 83'9 1,995 3,332 69-6 15-0 l,885 1,821 717 2,179 3,605 76-6 None 1,560 1,928 64-0 1, 600 3,280 62-3 Field experiments like those conducted by the Punjab Irrigation Department l have shown that the cultivator every- 1 These and similar experiments are discussed by A. and G. L. C. Howard in Wheat in India: Its Production, Varieties, and Improvement (Imperial Depart- ment of Agriculture, India, 1909). German experience is recorded in Erfahrung bei der Ackerbewasserung (Jahrb. Deutsch. Landw. Gesell., 1913, 28, 76). SOIL CONDITIONS AFFECTING PLANT GROWTH 51 where tends to take too much water, with loss not only to others on the same irrigation system, but also to himself. Air Supply. It is well known among farmers and gardeners that soil aeration is essential to fertility but exact measurements are difficult to obtain. The phenomena are more complex than appears at first sight, involving two wholly distinct factors : — 1. The necessity of a supply of oxygen to the plant root. 2. The toxic effect of the carbon dioxide which invariably accumulates in a non-aerated soil or other medium. Moreover, plants vary considerably in their sensitiveness to these factors. The simplest case is seen in water cultures where aeration produces marked effects. In the experiments of Hall, Brenchley, and Underwood (izic) the amounts of dry matter produced were as follows : — Barley, Grams per Plant. Lupins, Grams per Plant. Non-aerated .... Continuously aerated 1*314 2-122 0-83 i'53 .- The more recent experiments of E. E. Free (looa) show that plants do not all stand in equal need of oxygen. Buck- wheat was grown in water cultures through one set of which was blown air, and through others nitrogen, oxygen, and carbon dioxide respectively. No difference was observable between the plants supplied with nitrogen and those supplied with air or oxygen : they all grew normally to maturity. In this case, therefore, the root apparently can dispense with gaseous oxygen. When, however, carbon dioxide was given the plants sickened and wilted within a few hours, and died in a few days. Stiles and Jorgensen (272) have confirmed this difference between barley and buckwheat. Soil experiments are more difficult to carry out. B. E. SOIL CONDITIONS AND PLANT GRO WTH Livingstone and E. E. Free (178^) grew plants in soil so sealed that its atmosphere could be controlled. Different plants varied in their susceptibility to the exclusion of oxygen ; Coleus blumei and Heliotropium peruvianum were the most sensitive, the intake of water in their roots ceasing within 12 to 24 hours owing to death of the roots and the entire plants ultimately died when oxygen was replaced by nitrogen. Salix nigra, on the other hand, successfully en- dured the exclusion of oxygen. Application of these general results to field conditions have been made in India by A. and G. L. C. Howard (139), who have shown that increased soil aeration resulting from lessened irrigation or addition of potsherds to the soil leads to increased plant growth. Some of their results are given in Table XIII. :- TABLE XIII. — THE EFFECT OF DILUTING THE PUSA SOIL WITH POTSHERDS OR SAND. A. AND G. L. C. HOWARD (139). Crop. Yield per Acre of Control Plot. Yield per Acre with i in. of Potsherds. Increase per Acre. Percentage Increase. lb. lb. lb. Oats . 1954 2312 358 18 Wheat 1316 1580 264 20 Tobacco . *. 1680 1846 1 66 10 EFFECT OF REDUCING IRRIGATION AND THEREBY INCREASING AIR SUPPLY. WHEAT. Three Waterings. Two Waterings. One Watering. Yield of wheat : Grain lb. 1222 1302 788 Straw, etc. lb. 1764 2004 1714 Hall, Brenchley, and Underwood (i2i and W. A. Cannon and E. E. Free, Science, 1917, 45, 178-180, 54 SOIL CONDITIONS AND PLANT GROWTH time throughout. It varies even for different varieties of the same crop ; plant breeders are continually trying to evolve strains suited to particular ranges, e.g. wheats have been bred at Ottawa to ripen in the northern parts of Canada. 2. It affects the lengths of the periods of vegetative growth and of maturation and therefore causes certain modi- fications in the plant itself. A long ripening period gives wheat a plump kernel with a low percentage of protein, while a short ripening period gives an increased protein content (Lawes and Gilbert in 1857, 1664 Turnips in the south of England not only make less growth than in the north, but have a somewhat different composition. Oat straw in Scotland differs in composition from that in England, the translocation of material to the grain being apparently less complete. 3. The temperature at the time of ripening profoundly affects the germination capacity of the seed.1 Light. Although light is not a soil factor it nevertheless indirectly affects the soil by modifying the flora which, as we shall 1 For a discussion of the physiological effects produced by temperature see — Action de la Chaleur et dufroid sur Vactivite des etres vivants : par Georges Matisse. Paris (Larose), 1919. Bull. Internal. Inst. Ag. Rome, 1917, 8, 340. Darwin, Francis, On the growth of the fruit of " Cucurbita " (Annals of Botany, 1893, 28, 459-487). Leitch, I., Some experiments on the influence of the rate of growth of " Pisum sativum " (Ann. Botany, 1916, 30, 25-46). Lehenbauer, P. A., Growth of maize seedlings in relation to temperature (Physiol. Researches, I, 247-288, 1914). Lepeschkin, W. W., Zur Erkenntnis der Einwirkung supramaximaler Temperaturen auf die Pflanze (Ber. Deutsch. Bot. Gesell., 30, 703-714, 1912). Maximow N. A., Chemische Schutzmittel der Pflanzen gegen Erfrieren (Ber. Deutsch. Bot. Gesell. , 30, 52-65, 293-305, 504-916, 1912). Groves, J. F., Temperature and life duration of seeds (Bot. Gazette, 63, 169-189, 1917). Livingstone, B. E., and Livingston, Grace J., Temperature Coefficients in Plant Geography and Climatology (Bot. Gazette, 1913, 56, 349-375. Abs. in Journ. Ecol., 1914, 2, 179). SOIL CONDITIONS AFFECTING PLANT GROWTH 55 subsequently see, largely determines the nature and amount of the organic matter of the soil. Tall vegetation keeps off light from the lower growing plants and more or less sup- presses them. Thus on the Rothamsted grass plots clover is seriously reduced in amount by nitrate of soda which causes tall growing grasses to flourish : Lathy rus, on the other hand, in consequence of its tall growing habit is not adversely affected but grows vigorously. Numerous other instances are recorded in the Journal of Ecology}- The extreme case is seen in wood-land where there is very little undergrowth and where, therefore, organic matter has not accumulated in the soil. Adjacent pieces of land at Rothamsted, both untreated and differing only in the flora, showed the following differences in composition : — Long Established. Established 40 Years. Open Land. Flora of Grass and Clover. Wood-land. No Green Plants. Open Land. Flora of Grass and Clover. Wood-land. No Green Plants. Organic matter — 0-9 ins. 9 - 18 „ 8-5 4'8 67 4-8 7*9 67 8-1 5 '2 Nitrogen — 0-9 ins. 9 - 18 „ 0*256 0-097 0*185 0-093 % 0-182 0-084 0-173 0-081 One of the most effective ways of suppressing weeds is to grow a heavy crop which, in the farmer's language " smothers " them by excluding light and by exerting certain root effects. Food. The nutrition of plants is complicated by the fact that plants synthesise their own food from various substances taken out of the air and the soil. It is common in farmers' lectures to speak of these as the actual foods, but the student must be perfectly clear in his own mind that they are only the raw materials out of which the food is made. It is 1See e.g. E. Farrow, Journ, Ecology, 1916, 4. 56 SOIL CONDITIONS AND PLANT GRO WTH convenient to make a distinction between the elements neces- sary in large quantities, and those of which mere traces suffice : the effect of the former can readily be demonstrated in water and sand cultures ; the latter are more difficult to study, as traces are always present in the seed, and often also in the nutritive medium, or the vessel in which the plant is grown. The substances needed in quantity are carbon dioxide, water, oxygen, and suitable compounds of nitrogen, phos- phorus, sulphur, potassium, calcium, magnesium, and iron. Of these nitrogen, phosphorus, and potassium compounds are re- quired in such large amounts that they usually have to be added to soils as artificial fertilisers in order to obtain maxi- mum yields in agricultural practice. The other nutrient ele- ments are generally present in air, in soils or in rain, in sufficient amounts to exert their full effect. In addition to the above substances, small amounts of manganese and silicon are known to be beneficial : recent evidence suggests that others may be also. The list of ele- ments of which traces only are needed has been much extended in an important investigation by Maze (197), who includes boron, fluorine, iodine, chlorine, aluminium, and zinc. Evidence is steadily accumulating that these substances, in suitable traces, are beneficial to plant growth, as will be shown below. Between nutritive effects and toxicity the margin appears to be narrow, and almost all the elements essential to plant nutrition are capable of producing toxic effects under other conditions. In the case of the major nutritive elements toxi- city occurs only when a sufficiently large excess is present to alter considerably the osmotic relationships or the proper physiological balance of the nutrient medium : the transition from beneficial to harmful effects is very gradual and proceeds through the long inert stage shown in Fig. 7. In the case of elements of which only small amounts are needed the transition is much sharper : the limits are easily overstepped and toxic effects set in when the minute beneficial amount is exceeded. Carbon. — It is generally assumed that plants derive all their carbon from the air, but the French investigators have SOIL CONDITIONS AFFECTING PLANT GROWTH 57 persistently held the view that some may come from the soil (p. 1 5). The physiological work has usually been done in water cultures. Knudson l finds that saccharose, glucose, maltose, and fructose are directly absorbed and utilised by green maize, Canada field pea, timothy, radish, vetch, etc., giving rise to a characteristic branched root system. While these substances do not occur in the soil other soluble carbon compounds are present, especially in glasshouse soils, and may exert important effects. There is considerable evidence, however, that by far the larger portion of the carbon of the plant is taken up from the atmosphere and not from the soil, but the phenomena are not wholly independent of the soil. The amount of carbon dioxide in the atmosphere is subject to slight variations which may arise from variations in biochemical activity in the soil, and may be a factor of importance in crop production. Brown and Escombe (59*2) found that the amount varied at Kew from 2*43 to 3*60 2 volumes per 10,000 volumes of air, the average being 2-94. Taking the month of July as an example, the average values were : — 1898. 1899. 1900. 1901. CO2 in 10,000 volumes of air . 2-83 2*88 2*86 3'ii It is probable that the plant as a whole would respond to variations of this order, making greater or less growth as the amount of carbon dioxide rises or falls.3 Nitrogen. — Of all the nitrogen compounds yet investigated nitrates are the best, and, in natural conditions, probably the only nitrogenous food for non-leguminous plants. The seed- ling, still drawing its sustenance from the seed, lives on other compounds : H. T. Brown (59^) found that asparagine was the most effective nutrient for the detached embryo of barley, followed by other relatively simple substances like nitrates, glutamic and aspartic acids, ammonium sulphate, etc., the more complex substances being less useful. The experimental ^•Cornell Repts. (Ithaca, N.Y.), 1917, 747-813 (Memoir 9 of 1916). 2 Only on one occasion was so high a number obtained. 3 See E. Demoussy (83) and Otto Warburg, Biochem. Zeitsch.> 1919, 100, 230, 58 SOIL CONDITIONS AND PLANT GROWTH study of the nitrogen nutrition of adult plants is complicated by the difficulty of growing plants under sterile conditions in which the decompositions effected by bacteria are obviated ; much of the earlier work is vitiated by this circumstance. Later work has satisfactorily shown that ammonia is readily assimilated from solutions of ammonium sulphate if the con- centration is not too high ; but even cri per cent, was found injurious by Maze (196). Kriiger(i56£) concludes that am- monium sulphate is less beneficial than sodium nitrate for mangolds ; both compounds are equally useful for oats, barley, and mustard, while ammonium sulphate is better for potatoes. Brigham l maintains that maize also thrives better on am- monium sulphate than on sodium nitrate. Hutchinson and Miller (1400) found that peas assimilate nitrates and ammon- ium salts equally well, while wheat showed a decided preference for nitrates. Soderbaum 2 found that the effect varied with the salt : ammonium phosphate was more beneficial than the sulphate, while the chloride was harmful. An interesting attempt has been made by Prianichnikow (229^) to elucidate the phenomena of ammonia assimilation by plants. He supposes that the ammonia taken up by the roots is transformed in the plant into as^paragine, which is then con- verted into protein. Some plants, e.g. barley, maize, pumpkin, etc., readily take up ammonia and effect the conversion ; others, such as peas and vetches, do so only in presence of calcium carbonate ; whilst others, such as lupin, will not nor- mally take up ammonia at all. He suggests that this difference is due to the different quantities of carbohydrates at the dis- posal of the plant ; by increasing or diminishing the amount of carbohydrate it is possible to pass from one type of assimi- lation to another. In a state of inanition the power of forming asparagine is lost ; with plentiful supply of carbohydrate, on the other hand, even plants of the lupin type could absorb ammonia and convert it into asparagine. Asparagine can accumulate in the plant without detriment, and can be built 1 Soil Sci., 1917, 3, 155. ^Kungl. Landt. Handlingar, 1917, 56, 537-561. SOIL CONDITIONS AFFECTING PLANT GRO WTH 59 up into protein when sufficient carbohydrate is present This accumulation would account for the fact on which all experi- menters agree, that plants fed on ammonium salts contain a higher percentage of nitrogen than those fed on nitrates (Table XIV.). TABLE XIV. — PERCENTAGE OF NITROGEN IN DRY MATTER OF PLANTS. Fed on Nitrates. Fed on Ammonium Salts. Observer. Maize . 3*17 3*43 Maze (196) Mustard 2-87 3H8 Kruger (1566) Oats . I -80 2-05 » » Wheat . I -QI 2-17 Hutchinson and Miller (1400) Nitrites are also assimilated so long as the solution is not too concentrated or too acid.1 In spite of a considerable amount of work it is not known whether nitrogen compounds other than nitrates and am- monia are assimilated by plants. That many other com- pounds serve as nitrogen nutrients, even without the inter- vention of bacteria, seems to be certain (140^), but it has never been shown whether assimilation of the compound as a whole takes place, or whether there is decomposition at the surface of the root. Many of the supposed assimilated compounds are as a matter of fact more or less easily hydrolys- able, or otherwise decomposable, with formation of ammonia, and the decomposition will obviously proceed as fast as the ammonia is removed by the plant. Two factors that determine how far a given compound serves as a nitrogen nutrient are : (i) the ease with which it splits off ammonia, (2) the effect on the plant of the other decomposition products : if these happen to be toxic the whole process stops as soon as they have suf- ficiently accumulated. The normal nitrogenous food of plants is, however, a nitrate, and there is a close connection between the amount supplied and the amount of plant growth, which is well shown in Hellriegel and Wilfarth's (130*) experiments (Table XV.). 1 See Perciabosco and Rosso, Staz. Speriment. Agrar. ital., 1909, xlii., 5. 6o SOIL CONDITIONS AND PLANT GRO WTH TABLE XV. — EFFECT OF NITROGENOUS FOOD SUPPLY ON THE GROWTH OF BARLEY IN SAND CULTURES. HELLRIEGEL. Milligrams of nitrogen supplied o 56 112 168 280 420 Dry matter in crop, grams 742 4-856 10*803 17-528 21-289 28727 Increased yield for each extra 56 mgms. nitrogen — 4-114 5*947 6725 i -880 2'975 Grain, per cent, of dry matter in crop . . ... 11-9 37'9 38 42-6 38-6 43 '4 Weight of one grain, mgms. . *9'5 30 33 32 21 3° The figures are plotted in Fig 10. Similar results are ob- tained on the field plots at Rothamsted (Table XVL). Dry Matter produced , in Grams. V4 & 2 Gb 0> -k > ^ >^ BX x5 4 / r Z O 112 224 336 ' 44-8 M.gms. N supplied as Ca (NOj)g FIG. 10. — Effect of nitrogenous food supply on the growth of barley. (Hellriegel.) TABLE XVI. — BROADBALK WHEATFIELD, AVERAGE YIELDS, FIFTY-SIX YEARS, 1852-1907. Plot 5. Plot 6. Plot 7. Plot 3. Nitrogen supplied in manure, Ib. per acre . o 43 86 129 Total produce (straw and grain), Ib. per acre 2315 3948 5833 7005 Increase for each 43 Ib. nitrogen 1633 1885 1172 The increasing effects produced up to a certain point by successive increments of nitrogen may be due to the circum- stance that the additional nitrate not only increases the con- SOIL CONDITIONS AFFECTING PLANT GROWTH 61 centration of nitrogenous food in the soil, but also increases the amount of root, i.e. of absorbing surface, and of leaf, i.e. assimilating surface. The process thus resembles autocatalysis, where one of the products of the reaction acts as a catalyser and hastens the reaction. The increase does not go on in- definitely because some limiting factor steps in. The effect of nitrogen supply on the grain is very marked. In Table XV. it is seen that the grain formed, when nitro- genous food is wholly withheld, is only two-thirds of the normal weight per individual. The first addition of nitrate causes a marked rise in the weight per grain and the proportion of grain to total produce, but successive additions cause no further rise. Indeed other experiments prove that excess of nitrogen- ous food causes the proportion of grain to fall off somewhat. The leaf and the general character of growth are affected to a much greater extent. Nitrogen starvation causes yellowing of the leaf, especially in cold spring weather, absence of growth, and a poor starved appearance generally : a moderate supply of nitrogen leads to more rapid growth, very useful in cold weather or in case of attacks by insect pests. Abundance of nitrogen, on the other hand, leads to the development of large dark green leaves which are often crinkled, and usually soft, sappy, and liable to insect and fungoid pests (apparently because of the thinning of the walls and some change in com- position of the sap) and to retarded ripening: the effects resemble those produced by abundant water supply. A series of plants receiving varying amounts of 'nitrate are thus at somewhat different stages of their development at any given time, even though they were all sown on the same day, those supplied with large quantities of nitrate being less advanced than the rest. If they could all be kept under constant con- ditions till they had ripened this difference might finally disappear, but in crop production it is not possible much to delay the harvest owing to the fear of damage by autumn frosts, so that the retardation is of great practical importance. Seed crops like barley that are cut dead ripe are not supplied with much nitrate, but oats, which are cut before being quite 62 SOIL CONDITIONS AND PLANT GRO WTff ripe, can receive larger quantities. All cereal crops, however, produce too much straw if the nitrate supply is excessive, and the straw does not commonly stand up well, but is beaten down or " lodged " by wind and rain. Swede and potato crops also produce more leaf, but not proportionately more root or tuber, as the nitrogen supply increases ; no doubt the increased root would follow, but the whole process is sooner or later stopped by the advancing season — the increased root does, in fact, follow in the case of the late-growing mangold. Tomatoes, again, produce too much leaf and too little fruit if they receive excess of nitrate. At the Cheshunt Experiment Station l the omission of nitrogen compounds from the fertiliser mixture has caused the yield of fruit to increase 1 1 per cent. With the variety Comet the following quantities of fruit have been obtained : — Lb. per plant. Tons per acre. Relative Weights, 1916. 1917. 1918. 1919. 1916. 1917- 1918. 1919. 1916-1919. Complete artificials 4'9 5'n 3 '32 5'57 387 35-8 25-8 42-2 IOO No nitrogen . 57 5-60 3-62 5-98 45 'o 39'2 28-2 47'4 III On the other hand, crops grown solely for the sake of their leaves are wholly improved by increased nitrate supply : growers of cabbages have learned that they can not only improve the size of their crops by judicious applications of nitrates, but they can also impart the tenderness and bright green colour desired by purchasers. Unfortunately the softness of the tissues prevents the cabbage standing the rough handling of the market. These qualitative differences are of great import- ance in agriculture and horticulture. Three cases are illustrated in Table XVII. ; as the nitro- gen supply is increased wheat shows increases in straw greater than those in grain ; white turnips show increases in leaf greater than those in root, but mangolds show substantially the same increase both in leaf and root, because their growing 1 Annual Reports for 1917 et seq. SOIL CONDITIONS AFFECTING PLANT GROWTH 63 period is so much longer than that of the other crops, con- tinuing until the end of October. TABLE XVII. — EFFECT OF VARYING SUPPLY OF NITROGENOUS MANURE ON THE GROWTH OF CROPS. ROTHAMSTED. Wheat 1000 Ib. White Turnips, Mangolds, 1000 Ib. Nitrogen in Manure, Ib. per acre (1852-1864). Nitrogen in Manure, Ib. looo Ib. per acre (1845-1848). Nitrogen in Manure, Ib. per acre. (1906-1910). Grain. Straw. Roots. Leaves. Roots. Leaves. none I -06 1-86 none l8'37 6-05 none 11-84 2*55 43 1-68 3-03 47 22-18 9-63 86 40*12 8-51 86 2-18 4-28 137 22-96 1378 134 65-67 13-88 129 2-27 4-78 — — 172 2-29 5*22 ~ ~ ~ ~ _ The actual increase of growth brought about by successive increments of nitrogenous food depends on the amount of water and other nutrients, on the temperature, and so on ; any of these may act as limiting factors. Table XVIII. shows the crops obtained on some of the Rothamsted mangold plots ; in one case the supply of potassium is so small that it becomes the limiting factor, in the other sufficient potassium is supplied. TABLE XVIII. — INFLUENCE OF POTASSIUM SALTS ON THE ACTION OF NITROGENOUS MANURES. ROTHAMSTED. Average Weights, Mangolds, 1906-1910. Roots, looo Ib. per acre. Leaves, 1000 Ib. per acre. Insufficient potassium (Series 5) Sufficient potassium (Series 4) . 11-97 11-84 14-68 40-12 18-62 65-67 2'59 2'55 7-25 8-51 775' 13-88 Nitrogen supplied in manure, Ib. per acre .... — 861 184* — 861 i842 The effect of varying water supply is more conveniently studied in pot experiments than in the field, since any com- parison between yields in wet and dry seasons is complicated 1 From 400 Ib. ammonium salts. 2 From 400 Ib. ammonium salts and 200 Ib. rape cake. 64 SOIL CONDITIONS AND PLANT GROWTH by the great differences in temperature conditions. Tucker and von Seelhorst's experiments have already been described (pp. 38 and 44). From the practical point of view the important result is that a given increase in the food supply may produce no in- creased growth, small increase, or a larger increase, according to the extent of the water supply. Phosphorus. — Phosphates are by far the most efficient phosphorus foods known for plants. The relationship between phosphorus supply and growth has been measured by E. A. Mitscherlich (p. 33) in a series of experiments on oats grown in sand with each of the three calcium phosphates. For equal weights of the three salts the relative efficiencies corresponded with the basicity ; for equal weights of P2O5, however, the values were 2 '66 : 2*31 : 1-65. This was in sand cultures; in soils different efficiencies were found : thus for the mono- phosphate the values were : — Sand. Soil i. Soil 2. Soil 3. 2*66 1*80 i'74 2*40 The effect of a phosphate on the crop is twofold. In the early stages of growth it promotes root formation in a re- markable way. So long ago as 1847 Lawes (161) wrote: " Whether or not superphosphate of lime owes much of its effect to its chemical actions in the soil, it is certainly true that it causes a much enhanced development of the under- ground collective apparatus of the plant, especially of 'lateral and fibrous root, distributing a complete network to a con- siderable distance around the plant, and throwing innumerable mouths to the surface ". Dressings of phosphates are par- ticularly effective wherever greater root development is re- quired than the soil conditions normally bring about. They are invaluable on clay soils, where roots do not naturally form well, but, on the other hand, they are less needed on sands, because great root growth takes place on these soils in any case. They are used for all root crops like swedes, turnips, potatoes, and mangolds ; in their absence swedes and turnip roots will not swell but remain permanently dwarfed like '": rt 1.1 11 S S 2 £ J5 5 J^,Q II jjd 3 SOIL CONDITIONS AFFECTING PLANT GROWTH 65 radishes (Fig. 1 1 shows one of Lawes and Gilbert's photographs) : the introduction of superphosphate as a fertiliser revolutionised agriculture on some of the heavier soils by allowing better growth of these crops. Phosphates are needed also for shallow-rooted crops with a short period of growth, like barley. Further, they are beneficial wherever drought is likely to set in, because they induce the young roots to grow rapidly into the moister layers of soil below the surface ; probably, as Hall has suggested, this explains the marked effect of superphosphate on wheat in the dry regions of Australia. Later on in the life of the plant phosphates hasten the ripening processes, thus producing the same effect as a de- ficiency of water, but to a less extent ; for this reason they are applied to the wheat crop in some of the northern districts of England, and the oat crop in the west, to bring on the harvest a few days earlier and obviate risk of loss by bad weather. The northern limit of growth of several crops may in like manner be extended. This ripening effect is well shown on the barley plots at Rothamsted ; crops receiving phosphates are golden yellow in colour while the others are still green. But these effects, important as they are, are nothing like as striking as those shown by nitrogen compounds. . There is no obvious change in the appearance of the plant announcing deficiency or excess of phosphate 1 like those changes showing nitrogen starvation or excess ; the hastening of maturity is seen only when there is a control plot unsupplied with phos- phates : it leads to no increase in the proportion of grain borne by the plant. On the Rothamsted plots supplied with nitrogen and potassium compounds, but no phosphate, the grain formed 44*9 per cent, of the total produce during the first ten years of the experiment (1852-1861), and almost exactly the same proportion (447 per cent.) during the fifth ten years (1892-1901) when phosphate starvation was very 1 Barley grown in water cultures without phosphorus compounds acquires a red colour in the stem, but this is not commonly seen in the field. 66 SOIL CONDITIONS AND PLANT GROWTH pronounced; it fell a little to 41-3 per cent, in the sixth ten years (1902-1911), but rose to 46*8 per cent, in the period 1913-1919. Even in sand cultures the difference is not very marked: Hellriegel (130^) grew barley with varying supplies of phosphate with results given in Table XIX. In absence of phosphate no grain was formed ; when a little was added grain formation proceeded normally, and the resulting grain was nearly full weight per individual ; as the phosphate supply increased the percentage of grain increased, but soon reached a maximum beyond which it would not go. TABLE XIX. — EFFECT OF VARYING PHOSPHATE SUPPLY ON THE GROWTH OF BARLEY IN SAND CULTURES. HELLRIEGEL (130^). Weight of P2O5 supplied, mgms. per pot Weight of dry o 14-2 28-4 56-8 85-2 113-6 142 213 284 matter in crop, grams per pot . 1-856 8-254 I2'6l3 I9'505 I9'549 20-195 18-667 17785 31-306 Grain per cent, of dry matter — 22'4 SI'S 38-4 41'6 43-8 4^3 40-1 43 '4 Weight of one grain, mgms. . 27 29 38 34 4i 38 30 34 It is in the total growth of straw and of grain that the effect of phosphate is manifested as shown in Table XX. The Rothamsted results are plotted in Fig. 12. The effect of phosphate starvation shows itself in depressing the yield of straw and of grain, the straw being the first to suffer. Potash starvation takes longer to set in, not because potassium is less necessary but because the soil contains a larger quan- tity ; it also affects the straw first. Nitrogen starvation sets in at once, rapidly bringing both grain and straw down to a very low level. It is difficult to get behind these effects and ascertain their causes. The function of phosphoric acid in the cell is not easy to discover ; even when the problem is reduced to its simplest state by experimenting with Spirogyra in culture solutions little more has been ascertained than that phosphates are essential for mitotic cell division, doubtless because phos- SOIL CONDITIONS AFFECTING PLANT GRO WTH 67 phorus is a constituent of the nucleus, and also for the normal transformations of starch. Loew (i8o£) found that fat and albumin accumulated in absence of phosphates, but the colour was yellow and there was no cell division ; as soon as a trace of potassium phosphate was added, however, energetic cell division took place. Reed (236) showed that starch was formed in absence of phosphorus, but did not change to sugars ; erythrodextrin was formed instead and also cellulose. TABLE XX. — RESULTS OF. WITHHOLDING PHOSPHATES, POTASSIUM COMPOUNDS, AND NITROGEN COMPOUNDS FROM BARLEY. Hoos FIELD EXPERIMENTS, ROTHAMSTED. Yield of Grain, 1000 lb. per acre. Plot. 5 yea", 1852-56. 5 years, 1857-61. 10 years, 1862-71. 10 years, 1872-81. i o years, 1882-91. 10 years, 1892-1901. 10 years, 1902-11. 7 years, 1913-19. 7 Dung . 2-31 2-78 3-00 2-88 2-66 2-56 2-50 2'35 A4 Complete manure (salts of NH4 K and P) 2-47 271 2-67 2'34 2-24 2'O2 2-25 2*06 A3 No phosphates 2-27 I-7I I -99 1-68 i-38 1-26 1-23 1-38 A 2 No potassium 2-42 2*70 276 2*29 2'OI I-63 1-81 1-88 04 No nitrogen 1-86 i*57 i'39 •98 •92 •74 '94 1-31 Yield of Straw, 1000 lb. per acre. Plot. 5 years, 1852-56. 5 years, 1857-61. 10 years, 1862-71. 10 years, 1872-81. 10 years, 1882-91. 10 years, 1892-1901. 10 years, 1902-11. 7 years. 1913-19, Dung . 2-82 3-15 3*35 3*37 3-28 3*35 3*54 273 A4 Complete manure (salts of NH4 K and P) 3*29 3*I7 3'14 2-63 2'6l 2-36 2-83 2-17 A3 No phosphates 2-86 2-03 2'2O 1-64 1-56 1-57 A2 No potassium 3*21 3'°3 3 '07 2-30 2'2O 1-90 2'l6 178 04 No nitrogen 2-03 i-58 I'42 •95 *94 •90 1-39 I-46 The effects of phosphates in raising the quality and feeding value of the crop are very great. The most nutritious pastures in England and the best dairy pastures in France are those richest in phosphates. Soils deficient in phosphates are nearly alway unsatisfactory. Paturel l has also shown that the best 1 Bull. Soc. Nat. Agric., 1911, p. 977. 68 SOIL CONDITIONS AND PLANT GRO WTH wines contain most P2O5 (about 0*3 grm. per litre), the second and lower qualities containing successively less. Further, when the vintages for different years were arranged in order of their P2O5 content a list was obtained almost identical with the order assigned by the wine merchants. Davis (780) has emphasised the importance of phosphate supply for the indigo crop. 55 52'5657'6I 627l 72'8J 82'9I 92'0l O2'll I3'I9 FIG. I2A. — Effect on yield of grain of withholding various nutrients from barley. (Hoos field, Rothamsted.) The close connection between cell division and phosphate supply may account for the large amount of phosphorus com- pounds stored up in the seed for the use of the young plant, and also the relatively large amounts of phosphate taken from the soil during the early life of the plant. Potassium. — Hellriegel has shown (Table IV., p. 31) that SOIL CONDITIONS AFFECTING PLANT GRO WTH 69 equivalent amounts of the soluble compounds of potassium have practically all the same nutritive value. The effect of potassium compounds is more localised than that of phosphates, so that potash starvation can be more readily detected. The colour of the leaf becomes abnormal ; the potash-starved grass plots at Rothamsted have a poor, dull colour, as also have the mangold plots ; the leaves also 5256 5 '61 6271 72'8I 82'9I 92'0l O2'll I3'I9 FIG. I2B. — Effect on yield of straw of withholding various nutrients from barley. (Hoos field, Rothamsted.) tend to die early at the tips. The stem is weaker so that the plant does not stand up well ; this is apparently a tur- gidity effect, although anatomical differences were observed by Miss O. N. Purvis.1 The most striking effect, however, is the loss of efficiency in making starch, pointed out long ago by Nobbe (2150) ; either photosynthesis or translocation — it is not yet clear which — is so dependent on potassium salts that 1 Journ. Agric. Sci., 1919, 9, 360. SOIL CONDITIONS AND PLANT GROWTH the whole process comes abruptly to an end without them. Mangolds, sugar beets, potatoes, and other sugar- and starch- forming crops reduce their production of sugar with decreasing potassium supply even before the leaf area has been dimin- ished. Thus, in the mangold experiments of Table XVIII. (P- 63), 7255 Ib. of leaf give rise to 14,684 Ib. of root where potash food is deficient, while very little more leaf, 8508 Ib., give rise to nearly three times as much root, 40,128 Ib., where more potassium salts are supplied. The harmful effect of potash starvation on carbohydrate production does not seem to be the result of a pathological condition of the chloro- plastids. Reed found that they remained normal for two months and even increased in numbers in potash-starved algae. A second effect is on the formation of grain ; unlike phos- phates and nitrates, potassium compounds have a very marked effect on the weight of the individual grains, as may be seen by comparing Table XXI. with the corresponding Tables XV- (p. 60) and XIX. (p. 66) ; indeed, to withhold potash is the surest way of producing stunted grain. At Rothamsted the average weights per bushel of wheat for the ten years 1910- 1919 were: — No Manure. Plot 3. Farmyard Manure. Plot 2. Complete Artificials. Plot 7. Artificials without Nitrogen. Plot 5. Artificials without Potash. Plot ii. 61-5 62-3 62-2 61-8 607 TABLE XXI. — EFFECT OF POTASSIUM SALTS ON THE DEVELOPMENT OF BARLEY. HELLRIEGEL (130^). K2O supplied, mgs. o 23%5 47 70'5 94 188 282 Dry matter in crop, grams . 2*271 5'4i4 9-024 11-636 15-302 20*946 29-766 Grain, per cent of dry matter — 4-8 21-5 27-2 30-1 38-5 427 Weight of one grain, mgs. . 5 9'5 13 17 26 34 Lastly, the vigour and healthiness of the plant are very dependent on the potassium supply ; potash-starved plants are the first to suffer in a bad season, or to succumb to SOIL CONDITIONS AFFECTING PLANT' GROWTH 71 disease. The Broadbalk wheat plots receiving potassium salts give conspicuously better results than the others when- ever the year is unfavourable to plant growth ; taking the yield on the unmanured plot as an index of the character of the season, we obtain the following results for a series of good and of bad years respectively : — TABLE XXII. — YIELD OF WHEAT IN THOUSAND POUNDS PER ACRE. ROTHAMSTED. Plot No. In Nine Bad Seasons.! In Nine Good Seasons.1 Grain. Straw. Grain. Straw. Unmanured .... Insufficient potash Sufficient potash 4 ii 13 •55 i -06 170 •87 1-86 3*02 •88 1*5* r98 ro8 2*20 3'i6 Percentage increase due to potash — 60-3 62-3 31-1 43-6 In the bad years the average rainfall was 32*55 inches (harvest years, September-August), while in the good years it was 27-10 inches ; the badness of the season may be connected with the high rainfall and corresponding low tem- perature. Similar results are obtained, however, if other un- favourable conditions set in. The improvement in healthiness is well exemplified by the power of resisting disease. At Rothamsted the potash-starved wheat and mangolds are liable to be attacked by disease, especially where there is excess of nitrogen, while the sur- rounding plots, equally liable to infection, remain healthy. FlaXjgrowers in the north of Ireland have found that potassic fertilisers increase the resistance of the plant to the attacks of the wilt organism. At the Cheshunt Experimental Station liberal treatment with potassic fertilisers makes the tomato plant more resistant 1 The bad years were 1867, '71, '72, '75, '76, 77, '79, '86, '88; the good years were 1868, '69, '70, '81, '83, '85, '87, '89, '91. 72 SOIL CONDITIONS AND PLANT GRO WTH to the bacterial stripe disease : the numbers of plants affected out of a total of 1 20 in each plot were : — l Complete Fertiliser. No Potassic Fertiliser. Var. Comet 4.O 78 Var. Kondine Red .... •13 33 Potassic fertilisers often afford the simplest method of dealing with fungoid diseases and they are usually more effective than other fertilisers under glass. Next to the sugar-producing plants, the leguminosae seem to stand most in need of potassium salts. The potash-starved grass plots at Rothamsted contain notably less clover than those fully manured, the actual depression fluctuating accord- ing to the season. Some of the weeds, especially the sorrel, require a good supply of potash. There is some controversy as to whether potassium plays any important part in protein synthesis in plants.2 In absence of potassium salts mitotic cell division does not go to completion ; Reed observed that the cell and nucleus both elongate, but actual division does not occur (236). It is not at present possible to say whether all these phenomena are different manifestations of one and the same specific action of potassium in the plant, or whether there are several different causes at work. Zwaardemaker 3 puts forward the interesting suggestion that the potassium ion (which is somewhat radioactive) may be replaced by any other radioactive element, light or heavy, or by free radio- active radiation, provided the doses are equi-radioactive. Sodium does not appear to be essential even to salt marsh plants, although salicornia grew better in presence of salt than in its absence.4 It can partially, but not completely, 1S. G. Paine and W. F. Bewley, Annals of Applied Biology, 1919, 6, 185. 2 J. Stoklasa, Biochem. Zeitsch., 1917, 82, 310-323 ; T. Weevers, ibid., 1917, 78, 354- 3y. Physiol., 1920, 53, 273. 4 A. C. Halket, Annals of Botany , 1915, 29, 143-154. SOIL CONDITIONS AFFECTING PLANT GROWTH 73 replace potassium as a plant nutrient ; it thus delays the setting in of potash starvation, but will not keep it off alto- gether. Hellriegel (130^) found that sodium salts always gave increases in crop even when potassium salts were present in quantity. TABLE XXIII. — EFFECT OF SODIUM SALTS WITH SMALL AND WITH LARGE AMOUNTS OF POTASSIUM SALTS ON THE GROWTH OF BARLEY. HELLRIEGEL (130^). K2O supplied, mgs. ; . 0 94 188 282 376 Dry matter produced when sodium salts added . . Dry matter produced when no sodium salts added 4'925 2-658 23-019 15-638 32-278 29724 36-535 34-897 38-270 36-281 Difference due to sodium salts . 2-267 7-38I 2'554 1-638 1-989 Breazeale (5 1 ft) has more recently obtained similar results in water cultures. It is well ascertained in farming practice that sodium salts can be used with great effect as manures wherever there is any deficiency of potash in the soil.1 J. A. Voelcker (290) has made the interesting observation that sodium hydrate and sodium carbonate, unlike most other salts, cause an increase in the percentage of nitrogen in the wheat grain, besides increasing the yield of crop. The sulphate and the chloride increased the crop, but beyond a relatively low concentration limit further increases in amount of sodium chloride proved toxic. Lithium salts, on the other hand, have a toxic action on plants. Gaunersdorper's older experiments 2 have been con- firmed by J. A. Voelcker (290, 1912), who found that amounts of the chloride, sulphate, or nitrate, corresponding to -003 per cent, of the metal were distinctly injurious to wheat ; JFor fuller details see Kruger, Zeitschr. Ver. Deut. Zuckerindus., 1914, 694- 702; B. Schulze, Beitrag zur Frage der Dungung mit Natronsalzen (Landw- Versuchs-Stat., 1913,79-80,431, and 1915, 86, 323-330); E. J. Russell, Journ. Ed. Agric., 1915, 22, 393-406. 2 Landw. Versuchs-Stat., 1887, 34, 171-206. 74 SOIL CONDITIONS AND PLANT GROWTH smaller amounts, however, appeared to cause an increased growth. Ccesium salts are less harmful (290). Calcium is an essential plant food, the function of which was first carefully studied by von Raumer (234), but has not yet been satisfactorily cleared up. Little has been inferred from the fact that, like potassium, it occurs more in the leaf than in the seed. It certainly gives tone and vigour to the plant ; gypsum is used in alkali regions to counteract the harmful effects of excessive amounts of saline matter in the soil. It also appears to stimulate root production : if calcium is withheld from water cultures the size of the root is much reduced. Maquenne and Demoussy * show that the amount present in the seed of the pea is insufficient for root develop- ment, which therefore ceased on the third or fourth day of germination in pure water. Addition even of traces of calcium sulphate was followed by further root growth: O'Oi mg. of calcium sulphate per seed, representing an addition of calcium equal to 1/40,000 the weight of the dry seed, led to formation of root hairs and a 40 per cent, increase in root length. The close relationship between calcium and nitrogen content suggests that the calcium may be associated with protein metabolism, perhaps combining with the acids to which such metabolism gives rise. Plants fall into two groups so far as calcium is concerned : — (a) Those (including calcifuges) with low content of calcium and low calcium-nitrogen ratio ; (<£) those (including calcicolous plants) in which these quantities are high.2 Barium and strontium cannot replace calcium in the nutrition of plants. McHague (187) has" shown that the carbonates are toxic, though in the presence of calcium car- bonate they cause an increase in plant growth, strontium being more effective than barium. In Voelcker's experiments (290) the addition to the soil of even OT per cent of strontium sulphate, hydrate, or carbonate was without effect, but the 1 Compt. Rend., 1917, 164, 979-985 ; and 165, 45-51. 2 F. W. Parker and E. Truog (Soil Sci., 1920, 10, 49). SOIL CONDITIONS AFFP:CTING PLANT GROWTH 75 chloride was distinctly toxic. In Loew's experiments on algae (i8Of) strontium salts injuriously affected the chlorophyll bodies, causing loss of starch-making power and finally death. Magnesium, like phosphorus, finally moves to the seed, and is thus in contrast with calcium and potassium, which re- main behind in the leaf or the straw. Willstatter has shown (310) chlorophyll to be a magnesium compound, an observa- tion that accounts for the unhealthy condition of the chloro- phyll bodies, and the final etiolation of magnesium-starved plants. Further, magnesium seems to be necessary for the formation of oil, the globules being absent from algae growing in solutions free from magnesium salts ; oil seeds are richer in magnesium than starch seeds. An excess of magnesium salts produces harmful effects which, as we shall see (p. 78), can be lessened by addition of calcium salts ; Loew indeed considers CaO (l8o#) that plants require a definite ratio in their food, but neither Gossel x nor Lemmermann 2 could obtain evidence of any such necessity. In J. A. Voelcker's experiments3 magnesium oxide, car- bonate, and chloride had, like sodium hydroxide, the unusual effect of causing an increase in the nitrogen content of the wheat grain. The sulphate did not act in this way, although in suitable small amounts it caused increases in yield of grain and of straw. The chloride proved toxic at higher concentra- tions. Aluminium compounds have been, found beneficial by Stoklasa.4 Iron. — For some reason difficult to explain the formation of chlorophyll is absolutely dependent on the presence of a trace of some ferric salt, although iron does not enter into the composition of chlorophyll. So little is wanted that iron salts never need be used as manures, excepting for water or sand cultures. 1 Bied. Zentr., 1904, xxxiii., 226. *Landw. Jahrbuch, 1911, xl., 175 and 255. *y. Roy. Agric. Soc., 1915, 76, 354 ; 1916, 77. 260. *Biochem. Zeitsch., 1918, 91, 137. 76 SOIL CONDITIONS AND PLANT GRO WTH Manganese is considered by Bertrand to be a constituent of oxidases, and, therefore, necessary to the plant ; minute traces only are required, larger quantities being harmful. Dr. Brenchley's water cultures (54^) show that barley benefits by small doses of manganese salts, and a number of field ex- periments in Japan and in Italy l have indicated some manurial value. Bertrand regards manganese salts as " engrais com- plementaires " (35). Field trials at Rothamsted, however, gave negative results. Chlorine does not appear to be necessary to the plant in large quantity ; indeed, Knop grew even the halophytes without it. Maze" finds that small amounts are necessary, which, how- ever, would not need to be added in manure, as rainwater in- variably contains chlorides : at Rothamsted the amount of chlorine brought down per acre averages 16 Ib. per annum, the annual fluctuations varying with the rainfall between 10*3 and 24/4 Ib.2 Voelcker finds that, on the whole, chlorides are more toxic than sulphates at equivalent concentrations (290). In small quantities both fluorine and iodine appear to in- crease plant growth : this was first shown in Japan by Loew (i8o<:) and Suzuki (277*2): it is also accepted in France by Maze" (197). Gautier and Clausmann (102) go even further and claim that a dressing of 5 kgms. of amorphous calcium fluoride per acre was followed by increases in cereal crops of 5 to 1 8 per cent, and sometimes considerably more in the case of root crops. Sulphur is an essential food constituent, and occurs in plants, especially in cabbages and swedes, to a greater extent than is usually recognised, the older analytical methods giving low results (Hart and Peterson (127), Peterson (223)). Sul- phates are present in rain and in soil, but further additions in 1 The Japanese experiments are recorded in the Bull. Coll. Agric., Tokyo, 1906 et seq. (210), and the Italian experiments in the Studi e Ricerche di Chimica Agraria, Pisa, 1906-8 ; pot experiments have also been made by J. A. Voelcker at the Woburn Experiment Station (Journ. Roy. Ag. Soc., 1903, 64, 348-359). See also E. P. Deatrick, Cornell Mem., 1919, 19, 371. 2E. J. Russell and E. H. Richards, Journ. Ag. Sci., 1919, 9, 309. SOIL CONDITIONS AFFECTING PLANT GROWTH 77 manure were found by Bogdanow to be helpful.1 Dymond (92) showed that sulphates increased the yield of heavy crop's rich in protein, although they were not needed for cereals or per- manent pastures. A number of recent investigations in the United States by Pitz,2 H. G. Miller, C. B. Lipman and W. F. Gericke (175^) and others have confirmed and extended these observations : the last-named authors found that sulphate of ammonia was superior to nitrate of soda for barley on certain Californian soils, though it was no better than a mix- ture of nitrate and sulphate of soda. Silicon does not seem to be essential in any quantity, but it occurs to so large an extent in some plants that it is not likely to be wholly useless. Wolff and Kreutzhage (315) found that soluble silicates increased the yield of oats in water cultures and also the proportion of grain, behaving, in their opinion, much like phosphates. On some of the phosphate- starved plots at Rothamsted marked crop increases are ob- tained by addition of sodium silicate (Table XXIV.). Hall and Morison (120^) conclude that silicates act by causing an TABLE XXIV. — EFFECT OF SILICATES ON THE GROWTH OF BARLEY, 1864-1904. ROTHAMSTED. Yield of Dressed Yield of Slfaw, . Total Grain Grain, bushels. cwts. "Straw Without With Without With Without With Silicate Silicate. Silicate. Silicate. Silicate. Silicate. Nitrate only 27-3 33-8 16-2 I9'8 85-I 86-6 Nitrate + phosphate . 42'2 43*5 24-6 25-8 87-2 85-8 Nitrate + potassium salts 28-6 36'4 17-9 217 8o'6 85*0 Nitrate + phosphate + potas- sium salts 4I-2 44-5 25-3 27-6 827 82-1 increased assimilation of phosphoric acid by the plant, the seat of action being in the plant and not in the soil. Bene- ficial results were likewise obtained by Jennings.8 iExpl. Stat, Record, 1900, n, 723, and 1903, 15, 565. 2 Pitz, Journ. Ag. Research, 1916, 5, 771-780; H. G. Miller, Journ. Ag. Research, 1919, 17, 87-102. 3 Soil Sci., 1919, 7, 201. SOIL CONDITIONS AND PLANT GROWTH Boron, given as boric acid, was found by Dr. Brenchley (53^) to increase the growth of peas, but not of barley, in water cultures at concentrations of about 1/100,000; above this point harmful effects were produced. In Voelcker's pot ex- periments (290) even I part in 200,000 of soil proved toxic for barley, but at lower concentrations there was a slight stimulating effect. More recent experiments are recorded by Cook and Wilson.1 Physiological Balance. It is not only necessary to supply all the essential nutrient substances to the plant, in addition there must be maintained some kind of proportion between the various salts ; this is spoken of as the physiological balance. Plant physiologists have long recognised that single salts of potassium, magnesium, sodium, etc., are toxic to plants, while a mixture of salts is not. Calcium salts are by much the most powerful reducers "of this toxic effect. Thus Kearney and Cameron (1450) found that a root of Lupinus albus was just killed when immersed in •00125 N magnesium sulphate solution (7 parts per 100,000), but the effect was modified by added salts, as shown in Table XXV. TABLE XXV. — EFFECT ON VARIOUS SALTS IN REDUCING THE TOXICITY OF MgSO4. KEARNEY AND CAMERON (i45a). Alone. + MgCla (•0025 N). + Na2C03 (•0025 N). + Na«SO4 (•or N). + NaCl (•015 N). + CaCl3 (-2 N). + CaS04 (Saturated). Strength of MgSO4 that just kills the root . . •00125 N •000625 N •00125 N •00375 N •0075 N •2 N •6 N Hansteen found that the toxic effect of potassium salts used singly was overcome even when so little lime was Osterhout found (2200) added that the ratio K2O 840' that Vaucheria sessilis lived for three weeks in distilled water, 1 F. C. Cook and J. B. Wilson, Journ. Ag. Research, 1918, 13, 451. SOIL CONDITIONS AFFECTING PLANT GROWTH 79 but was killed in a few minutes by — N NaCl, and in a few 32 days by *oooi N NaCl ; yet the toxic effect even of the stronger solution disappeared on adding one gram-molecule of CaCl2 for every loo gram-molecules of NaCl. It does not appear that calcium prevents the entrance of the sodium or other ion into the plant : apparently it gives greater vigour to the plant1 Magnesium chloride and sulphate, potassium chloride and calcium chloride were also toxic when used singly, but in admixture they formed a nutrient medium in which the plant grew normally and developed fruit even when — N NaCl was also present. This action is called 32 antagonism of ions.2 Osterhout shows (220^) that the phen- omena hold generally both for land and water plants. Other facts are less easy to explain, such as Grafe and Portheim's observation that the toxic effects of a single salt fail to appear, or are much delayed, when sugar is supplied.3 These and many other experiments all indicate that a complex equilibrium normally exists in the cell between colloids and electrolytes which can be maintained only when the external medium has an appropriate composition. Even when all the nutrient salts are present and the total osmotic concentration is maintained constant at a suitable level it is still possible to produce the most diverse effects on the growing plant, from violent injury to excellent growth, by varying the proportions in which the salts occur. This problem has been much investigated by Tottingham (283) and by Shive (262^). Tottingham studied the effects of eighty-four mixtures of KH2PO4, KNO3, MgSO4 and Ca(NO3)2, plus a trace of an iron salt, all of which had the same total osmotic con- centration. Shive simplified the investigation by reducing the salts to three, KH2PO4, Ca(NO3)2 and MgSO4, and using 1 J. A. Le Clerc and J. F. Breazeale, jfourn. Ag. Research, 1920, 18, 347 ; L. Maquenne and E. Demoussy, Compt. Rend., 1920, 170, 420. a For sand culture experiments see Wolkoff, Soil Sci., 1918, 5, 123-150. 9 Bied. Zentr., 1908, xxxvii., 571. 8o SOIL CONDITIONS AND PLANT GRO WTH thirty-six different combinations of equal total concentra- tion. The extent of the injurious effect depends on the concen- tration : at O'l atmosphere osmotic pressure none of the solutions proved injurious to seedlings ; at a higher concen- tration seven caused severe and four slight injury ; fifteen caused various degrees of productiveness without injury. The specific injury caused in these circumstances by mono- potassic phosphate and by sulphate of ammonia to soy beans was studied by Shive (262*:) and by Wolkoff (316) respectively. The amount of growth produced by well-balanced solutions also depends on the total concentration, increasing up to a certain point. The optimum ratio of nutrients for a given stage of plant development alters with the concentration ; it is not the same at O'l, 175, and 4 atmospheres. But it is not affected by the nature of the medium ; it is the same in sand as in water culture. So also it is independent of variations in the moisture content of the sand, being the same for degrees of moistness varying from 40, 60, to 80 per cent, of the water- retaining capacity of the sand. But it is not constant for the whole range of growth of the plant, being different in seedling and ripening stages and different for the growth of " top " and of roots. It must not be supposed, however, that the physiological balance is a rigid ratio: Hoagland and Sharp (136*:) obtained satisfactory growth with a wide range of mixtures so long as the total supply and concentration of essential elements was adequate. Maze has adduced evidence that physiological balance is an important factor in soils and is affected by calcium carbonate and sometimes by humus (196). Absence of Injurious Substances. We have seen that many salts have a toxic effect if given alone to the plant, but for our purpose we need consider only those causing injury in presence of other compounds. Two cases arise in practice : some substances are injurious even in small quantities, others only in excess. SOIL CONDITIONS AFFECTING PLANT GROWTH 81 Substances Injurious in Small Quantities: Acids and Al- kalis.— H and OH ions. It has long been known that plants do not grow well on acid soils, and the conclusion has been drawn that the acidity is harmful to plants. As a general statement this is true : additions of a strong acid, such as HC1 or H2SO4 to a culture solution or to soil soon kills the plant. But the change in reaction thus induced is vastly greater than is found in soils, and from our point of view it is necessary to know the effect of changes of the same magnitude as occur in nature. The proper basis of comparison is the hydrogen-ion concen- tration (see p. 113). • Hoagland has shown (1360) that an acid condition up to 07 x io~5 H ion (PH = 5'i6) is favourable to the growth of barley seedlings, while stronger acidity is harmful. Alkaline solutions stronger than I -8 x io~ 6 OH ion (PH = 8-26) caused injury, and those stronger than 2 -5 x icr 5 (PH = 9'4°) were extremely toxic. Salter and Mcllvaine (243) obtained the best growth in slightly acid conditions ; for wheat, soy beans, and lucerne PH = 5*94, and for maize PH = 5-16; 2-96 was harmful, and 2-16 fatal. Alkalinity was decidedly more harmful than acidity1 (Fig. 13). It will be shown later that the PH value of soils varies between 37 and 97, values beyond 4-5 and 8-5 being, how- ever, unusual. The acidity is often less than that of cell sap, the PH value of which varies between 4-0 and 6'O.2 Thus, 1 See also J. S. Joffe, Soil Sci., 1920, 10, 301. 3 Truog and Meacham, Soil Sci., 1918, 5, 177. For further data showing variation with stage of growth and conditions see A. R. C. Haas, Soil Sci., 1920, 9, 341. For acidity of root sap (which is generally less than that, of the aerial portion), H. Kappen (Landw. Versuchs-Stat. 1918, 91, 1-40) obtained the follow- ing PH values : — Wheat . \ . 7-2 Barley .'.... 6-85 Oats '.,... 6-7-6-8 Rye . . . . . . . , . . 6-6 Mustard • . . ' - 6-2 Horse beans 6-0 Lupins > v, . . . 5'6-5'8 Buckwheat . . . . . . . . . 5'o-5'3 For other determinations see C. B. Clevenger, Soil Sci., 1919, 8, 217 (values 5-8-6). 6 82 SOIL CONDITIONS AND PLANT GROWTH direct injury to the plant caused by true acidity of the soil is probably not very frequent in nature : harmful effects of alkalinity, however, do occur. The unsuitability of the atmosphere of industrial towns has been traced in part to the presence of acids, which affect the leaves as well as the roots. Wieler l found that assimila- tion of carbon dioxide was profoundly modified by sulphur o 100 90 70 60 50 20 10 Maize. A/ fa/ fa — Reaction as PH FIG. 13. — Relation between hydrogen-ion concentration (Pa value) and growth of crops. (Salter and Mcllvaine, 243.) dioxide, most injury being done in moist weather when the stomata were more widely opened and the gas could readily enter the leaf tissues. Crowther and Ruston (71) obtained the following yields from pots of Timothy, showing that acid water gradually kills the plant : — 1 Bied. Zentr., 1908, xxxvii., 572. SOIL CONDITIONS AFFECTING PLANT GRO WTH 83 TABLE XXVI. — EFFECT OF ACID RAIN-WATER ON THE GROWTH OF TIMOTHY GRASS. CROWTHER AND RUSTON (71). Weight of dry matter obtained when plants were regularly watered with : — Solution of Sulphuric Acid, Parts per Country Rain Leeds Rain 100,000 of Water. (ACIQ). i 2 4 8 16 32 ist crop, igo8 . 28*0 gms. 23-8 gms. 30-5 287 28-8 24-8 23-8 14-1 2nd crop, igog . 24*9 „ I7*5 » 18-2 I7-8 IO'O 8-2 1-8 0 3rd crop, igio . 147 „ 6-6 „ I2*O 8-0 3 '9 37 0 0 Metallic Salts. — Complaints are sometimes made by farmers in mining districts that their crops suffer damage from the waste products — generally metallic salts — turned into the streams from the works, especially where the water is wanted for irrigation, or where, as in Japan, rice is grown in the marshes. The damage done to pastures by the lead mines of Cardiganshire has been investigated by J. J. Griffith (115) at Aberystwyth. Clover is particularly susceptible. A heavy dressing of lime proved a useful remedy. Zinc also causes injury in parts of Wales.1 Traces of zinc are regarded as essential by Maze (197)- Working with larger quantities Dr. Brenchley (53#) was un- able to find definite indications of stimulating action in water culture, although Javillier (143) claimed to obtain increases in soil. Ehrenberg (930) concludes that zinc salts are always toxic when the action is simply on the plant, but they may lead to increased growth through some Indirect action on the soil itself (see p. 281). A vast number of experiments have shown that copper salts are extraordinarily toxic in water cultures or where they actually come into contact with the plant, even the minute trace sometimes present in distilled water being harmful. This property finds useful application in removing algae from water and in killing weeds. For example, a 3 per cent, solu- tion of copper sulphate is sprayed over cornfields in early 1 The soil of an Anglesey garden examined at Rothamsted contained 078 per cent, of zinc. It proved, as might be expected, highly infertile. 84 SOIL CONDITIONS AND PLANT GRO WTH spring at the rate of fifty gallons per acre to destroy charlock (Brassica sinapis\ one of the most troublesome weeds on light soils. The solution adheres to the rough horizontal leaves of the charlock and kills the plant, but runs off the smooth vertical leaves of the wheat without doing much damage. Even the insoluble complex copper salt present in Bordeaux mixture and sprayed on to fruit trees to kill fungoid pests, was found by Amos1 to retard assimilation of carbon dioxide by the leaf. Copper salts do not appear to be anything like so toxic in the soil as in water culture. It is often asserted that any toxic substance must, at proper dilutions, act as a stimulant to plants ; with copper sulphate, however, Dr. Brenchley (53L. ljourn. Ag. Science, 1907, ii., 257-266. 3 yourn. Ag. Research, 1916, 6, 389-416. 3H. H. Green, 5th and 6th Report, Veterinary Research, South Africa, 1918, 593-624. SOIL CONDITIONS AFFECTING PLANT GROWTH 85 few metals required for nutrition. No unexceptionable evid- ence of a stimulating effect on the plant has yet been obtained, although certain effects may be produced in the soil leading to increased growth (see p. 280). The literature of the sub- ject is summarised by Dr. Brenchley in her monograph (53^). Whenever infertility is traced to any of these metallic salts a good dressing of lime is usually found to be an effective antidote. Various Other Substances — Sulphuretted hydrogen is ex- tremely toxic, so also is ammonium sulpho-cyanide which, in the early days, used to cause trouble as an impurity in ammonium sulphate made from gas liquor. It is rarely, if ever, found now. Toxic nitrogen compounds include nitrites, which have to be removed from synthetical calcium nitrate used for manure, the dicyano-diamide associated with com- mercial cyanamide, and ammonium salts at too high a con- centration. None of these, however, are for long harmful in the soil, since all are ultimately converted into nitrates. Perchlorates are harmful and used sometimes to occur in sodium nitrate, but they are now carefully removed. Substances Injurious in Large Quantities : Carbon Dioxide. — In an interesting series of investigations Kidd (147) has shown that CO2 exerts a marked inhibiting effect on the germination of seeds, even though all other conditions are favourable. The seed is not permanently affected, and it will germinate freely, though not always immediately, when the CO2 is removed. He suggests that this is the cause of the remarkable dormancy of certain seeds, especially weed seeds, buried below a certain depth in the soil ; some of these will survive for years, and will produce a copious and vigorous crop of weeds when brought to the surface by deeper ploughing or breaking up of grass land.1 Soluble Salts. — In many arid districts the soil contains such large quantities of sodium and potassium salts that the soil water is too concentrated to permit of plant growth. Sodium carbonate not infrequently occurs and directly poisons 1 W. E. Brenchley, Journ. Ag. Science, 1918, 9, 1-31. 86 SOIL CONDITIONS AND PLANT GRO WTH the plant. Such soils are called alkali soils : they may be treated with gypsum, or, still better, carefully washed with irrigation water, adequate provision being simultaneously made for drainage. Calcium Carbonate is sometimes considered harmful be- cause plants are liable to chlorosis on chalky soils. It is equally probable, however, that the general soil conditions are responsible for the disease (see p. 304). Magnesium Salts. — The toxicity of magnesium salts was discovered by Tennant in the eighteenth century in studying the alleged harmful effects of certain limestones found near Doncaster (280). Modern investigations1 on magnesian limestone, however, have failed to show any harmful effect ; indeed, in the Woburn experiments (290) Voelcker has ob- tained an actual benefit both on wheat and on mangolds by using magnesia (MgO). But the soluble salts, the sulphate and especially the chloride, are harmful. Cases are reported by Loew where excess of magnesia in the soil has caused infertility ; none, however, have fallen under the writer's observation in this country. The soil of the Greenville Ex- perimental Farm, Utah, is rich in magnesia — containing over 6 per cent of MgO — and is remarkably fertile. It also con- tains, however, 17 per cent, of CaO and 20 per cent CO2.2 As already stated, any injurious effect can be overcome by treatment with lime. Effects of Salts on Germination. — Salts generally cause a retardation in the rate of germination ; some of Guthrie and Helms' (117) results are given in Table XXVII. Sigmund has studied the effects of a very large number of substances (267). The technical interest in the work lies in the fact that seeds are sometimes treated with antiseptics before sowing in order to kill any spores of disease organisms, and, moreover, certain soluble salts — artificial manures — are often put into the soil about the same time as the seeds are sown. ^ee, e.g., New Jersey Bull., 267, 1914, and on the other side Durham Coll. Bull., 12, 1915. 2 J. E. Greaves, R. Stewart, and C. T. Hirst, Journ. Ag. Research, 1917, 9, SOIL CONDITIONS AFFECTING PLANT GRO WTH 87 TABLE XXVII. — EFFECT OF SOLUBLE SALTS ON GERMINATION. GUTHRIE AND HELMS (117). Sodium Chloride, per cent. Sodium Carbonate, per cent. Sodium Chlorate, per cent. Arsenic Trioxide, per cent. Barley. Rye. Barley. Rye. Barley. Rye. Barley. Rye. Germination affected . O'lO O'lO 0-25 0-25 0*005 0-004 0-6 O*2O ,, prevented 0-25 0-40 0-60 0*50 0-007 0-OO6 — 0-4 Growth affected O'lO 0-15 0-15 0-25 0-003 0-002 0*05 0-15 „ prevented 0-2O 0'20 0-40 0-40 0*006 0-004 O'lO 0-30 When a solution comes in contact with a seed it does not necessarily enter as a whole. Adrian Brown (58) has shown that the barley seed is surrounded by a membrane which has the remarkable property of keeping out many dissolved sub- stances and allowing the water only to pass in, so that the solution loses water and becomes more concentrated. A number of substances can, however, pass through the mem- brane, and to these H. E. and E. F. Armstrong (5) have applied the term Hormones,. In general they have no great affinity for water ; in the Armstrongs' nomenclature they are anhydrophilic : they pass into the cell and there disturb the normal course of events. Ammonia, toluene, ether, chloro- form, are all highly effective hormones readily entering the cells of seeds, leaves, etc., and hastening the normal sequence of processes. Supposed Stimulation of Plants by 'Electricity, Heat, and Radium. The Electric Discharge. — It has often been stated that an electric discharge increases the rate of growth of plants either by direct action on the plant, or by indirect action in the soil. As far back as 1783 the Abbe Bertholon (34) constructed his electro-vegetometre, a kind of lightning conductor that collected atmospheric electricity and then discharged it from a series of points over the plant. The view that atmospheric electricity is an important factor in crop growth has always found 88 SOIL CONDITIONS AND PLANT GROWTH supporters in France. Grandeau (112^) stated that plants protected from atmospheric electricity by a wire cage made less growth than control plants outside. Lesage 1 confirmed this observation, but found that silk thread caused as much retardation as wire, so that the effect is not necessarily electrical': in point of fact the rate of evaporation was con- siderably less under the cage than in the open. Instead of relying on atmospheric electricity Lemstrom (171) generated electricity on a large scale and discharged it from a series of points fixed on wires over the plant. This method has been used at Bitton, near Bristol, and studied on the electrical side by Sir Oliver Lodge, on the botanical side by J. H. Priestley (231), and on the practical side by J. E. Newman, J. H. Priestley, I. Jorgensen, and Miss Dudgeon. Numerous field experiments are recorded, but there is usually some uncertainty about the check plots. The Bromberg experiments (lo^d) gave negative results. Further studies are in hand by V. H. Black man who has put the whole subject on a sound basis for investigation. Various Rays. — Experiments by Miss Dudgeon suggest that the rays of the Cooper-Hewitt mercury vapour lamp2 may have a stimulating effect, accelerating germination and in- creasing growth. Priestley found that the rays from a quartz mercury vapour lamp were harmful at close range, whilst farther off they stimulated growth. There is much scope for work in this direction ; the problem is of great economic importance, because of the enhanced market value of early crops. Effect of Heat. — Molisch3 has shown that perennial plants steeped in hot water towards the close of their deepest period of rest come at once into activity. His hypothesis is that the " rest " required by plants is of two kinds, the freiwillig rest due to external conditions and therefore cap- able of being shortened, and the unfreiwillig rest inherent in 1 Compt. Rend., 1913, 157, 784. 2 A glass envelope was used. 3 Das Warmbad ah Mittel zum Treiben der Pflanzen, 1909, Prague. SOIL CONDITIONS AFFECTING PLANT GRO WTH 89 the nature of the plant. Parkinson l has tested the method with satisfactory results; spirea, rhubarb, seakale, etc., steeped for twelve hours in water at 95°, at the end of November, or early in December, made rapid growth when subsequently forced. Effect of Radium. — R. J. Strutt has shown 2 that the typical soil deposits contain measurable amounts of radio- active substance equivalent to 0^25 (but more usually i) to 5-8 x icr12 grms. of radium per gram. Zircon and apatite are much richer, the figures being 75 to 865 x io~12 and n to 30 x io~12 respectively. Joly3 finds that the amount of radium emanation in soil air is many thousand times greater than in the atmosphere ; J. Satterly 4 at Cambridge gives a lower estimate, viz. 2, x icr12 curie per litre, or 2000 times the usual amount in the atmosphere ; while J. C. Sanderson 5 in America estimates the amount in I c.c. of soil air as the quantity in equilibrium with 2-4 x io~13 grms. of radium. Among the many remarkable properties of radium it was perhaps natural to expect that it might have some definite effect on plants or micro-organisms. The suggestion has even been made that radium emanations might, under suitable conditions, cause sufficient increase in the amount of growth to justify its use in horticulture and agriculture. The early observations of Dixon and Wigham 6 at Dublin, however, did not seem very promising ; I oo seeds of cress (Lepidium sativum) were uniformly distributed on an even surface of moist quartz sand, and after germination .had taken place, a sealed tube containing 5 mgms. of radium bromide was set i cm. above the central seed. The seedlings grew up, but without any curvature indicating positive or negative ' * radio- tropism," and the only noticeable effect was a slight depression 1 Journ. South-Eastern Agric. Coll., 1909, 19, 245-257. 2 Proc. Roy. Soc., 1906, 77a, 472, and 1907, ySa, 150. 3Sci. Proc. Roy. Soc., Dublin, 1911, 13, 148. 4 Proc. Camb. Phil. Soc., 1912, 16, 514. The soil air was taken at a depth of 100-150 cms. For his estimates of the amount in the atmosphere see Phil. Mag., 1910 (vi.), 2O, i. 5 Amer. J. Sci., 1911 (iv.), 32, 169. 6 Proc. Roy. Soc., Dublin, 1904, 10, 178-192. 90 SOIL CONDITIONS AND PLANT GRO WTH of growth in those within I cm. radius of the tube. As stronger preparations of radium became available more definite retardations and inhibitions were observed ; thus Gager, in an elaborate report,1 noted a more or less complete inhibition in cell activities in younger and especially embryonic tissues, with a few exceptions. The action of radium through the soil, however, was different ; germination and growth were both accelerated, and the plants farthest away were stimulated most. Acqua2 found that different plants, and even different organs of the same plant, were differently affected, the root system in general responding more markedly than the aerial parts, and in his experiments being arrested in their development. The intensity of the radiation, however, is important, and G. Fabre,3 using Linum catharticum as a test plant, was able to obtain increased development and germination of seedlings by working with emanations up to I -5 microcuries per 2 litres of air, and to retard development by using emanations of 40 microcuries per litre of air. H. Molisch 4 obtained a like result ; young plants of vetches, beans, sunflower, etc., were stimulated in growth by weak emanations, but checked, or entirely stopped, by stronger ones. He further claimed that the " rest period " could be broken by the radium emanation, and he forced lilac into bloom in November by attaching pipettes containing small quantities of radium chloride to the terminal buds.5 In his earlier experiments he, like Dixon and Wigham, failed to detect any radiotropism, but later on he found indications in the case of certain heliotropically-sensitive plants, e.g. oats and vetches.6 These, and similar results, naturally suggested that the residues left after the extraction of radium, but still containing radio-active material, might have definite manurial value, and it was not long before definite statements were forthcoming. Baker 7 claimed that increased yields of wheat and radishes 1 Mem. New York Bot. Gard., 1908. zAnn. Bot. (Rome), 1910, 8, 223-238. 8 Compt. Rend. Soc. Biol. (Paris), 1911, 70, 187-188. 4 Umschau, 1913, 17, 95-98. *Oesterr. Gart. Ztg., 1912, 7, 197-202. 8 Sitzber. K. Akad. Wiss. (Vienna), 1911, 120, 305-318. 7 Journ. Roy. Soc. Arts, 1913, 62, 70-78. SOIL CONDITIONS AFFECTING PLANT GROWTH 91 had been obtained by mixing I part of radio-active material (2 mgs. Ra per ton) with 10 of soil. It isitrue that Stoklasa's1 results were negative (although in his other experiments radium emanations increased growth to a marked extent), but this did not prevent the introduction of radio-active fertilisers, and the enterprising syndicates and companies concerned were by no means loth to push their wares. These Vere investigated by Martin H. F. Sutton,2 the experiments being made with radishes, tomatoes, potatoes, onions, carrots, and marrows, some grown in pots, others in plots out of doors. Eight different radium residues were used, in addition to pure radium bromide ; the dressings were so arranged that equivalent quantities of radium were given in each case (rj^ grm. radium bromide to I 5 Ib. of soil : 2-j- times this amount per sq. yard to the plots). In no case was there any clear evidence of increased growth, even the pure radium bromide seemed to be without action. We are therefore left with the apparent discrepancy already observed on p. 35. The work of the physiologists, assuming it to be sound, indicates that radium emanation is capable of stimulating certain cell activities. Button's results show that such stimulus, if it exists, does not affect the final growth of the plant. This discrepancy is periodically confronting the agricultural investigator. Thus, Dr. Winifred Brenchley at Rothamsted has failed to obtain increases in growth by supply- ing plants with inorganic poisons which .have been supposed to stimulate certain cell functions in suitable dilutions. The result opens up the prospect of an interesting discussion, but it also shows the danger of arguing from a simple physiological observation to a complex phenomenon like the growth of a plant in soil. Zwaardemaker 3 has obtained some interesting results in the case of animal organs which deserve close study by plant physiologists. 1 Chem. Ztg., 1914, 38, 841-844. a Messrs. Button's Bull., No. 6, 1916. d jfourn. Physiology , 1920, 53, 273, discussed by V. H. Blackman in Annals of Bo tany, 1920, 34, 299. • m CHAPTER III. THE COMPOSITION OF THE SOIL. IT is well known that only the top six or eight inches of the soil is suited to plant life, and that the lower part, or subsoil, plays only an indirect part in plant nutrition. We shall, therefore, confine our attention almost exclusively to the surface layer. The soil was in the first instance derived from rocks, partly by disintegration and partly by decomposition. In most cases the fragments split off were sooner or later carried away by water and deposited at the bottom of a river or sea. There they mingled with residues of living organisms which have subsequently played an important part in the history of the soil as its chief source of calcium carbonate and calcium phos- phate. In course of time the material accumulated to con- siderable depths ; then, as the result of some earth change, the water retreated leaving the deposited material as dry land or rock. No sooner was this exposed to the air than it began once again to undergo disintegration and erosion. Air, water, and frost all played a part in the disintegration process ; water and sometimes ice have acted as transporting agents. For immense ages the particles have been subjected to these actions, and the fact that they have survived shows them to be very resistant and practically unalterable during any period of time that interests us. Reference to Table LXXXVII., p. 332, shows that the particles in the surface soil which have been exposed to weathering ever since the soil was laid down, and in some cases, to cultivation for some hundreds of years, are almost indistinguishable in size from those in the subsoil which have been protected from all these changes. 92 THE COMPOSITION OF THE SOIL 93 However, the soil particles are not wholly unalterable. The rain water and its dissolved carbonic acid exert a slight solvent action, and the soil water always contains small amounts of calcium and magnesium compounds, silica, and other substances in solution. Each individual particle only loses a very minute amount of substance to the soil water, and its life is extraordinarily long ; nevertheless dissolution is perpetually taking place. The surface soil contains less of the smallest, and, therefore, most easily attacked, particles than the subsoil. In any region where the rainfall and temperature conditions are favourable, soil rapidly covers itself with vegetation ; even a bare rock surface is not without its flora. The first vegeta- tion must obviously have obtained its mineral food from the dissolved material of the soil particles, but when it died and decayed all the substances taken up were returned to the soil, so that subsequent vegetation has food from two sources : from the substances dissolved direct out of the soil particles during the life of the plant, and from those dissolved out at earlier times and taken up by previous races of plants. Thus in the natural state, and where the vegetation is not removed, the mineral plant food can be used over and over again and indeed tends to accumulate as fast as it is extracted from the soil particles by the rain water. The plant, however, returns to the soil more than it takes away ; during its life it has been synthesising starch, cellulose, protein, and other complex unstable and endothermic material, much of which falls back on the soil when it is dead. This added organic matter introduces a fundamental change because it contains stored-up energy ; the difference between the soil as it now stands and the original heap of mineral matter is that the soil contains sources of energy while the mineral matter does not. Hence it soon becomes the abode of a varied set of organisms, drawing their sustenance and their energy from the accumuluted residues, and bringing about certain changes to be studied later ; some, as we shall see, are capable of fixing gaseous nitrogen and so increasing the supply 94 SOIL CONDITIONS AND PLANT GRO WTH of protein-like compounds, whilst others can assimilate carbon dioxide. Thus the complex that we speak of as the soil consists of four parts :— 1. The mineral matter derived from rock material, which constitutes the frame-work of the soil and is in the main unalterable, but it contains some reactive decomposition products. 2. The calcium carbonate and phosphate (the latter being usually in much smaller amount), and organic matter derived from marine or other organisms deposited simultaneously with the soil. 3. The soil water, a dilute solution of carbonic acid con- taining small quantities of any soluble soil constituent. 4. The residues of plants that have grown since the soil occupied its present position, consisting of the mineral plant food taken up from the soil water and of part of the complex organic matter synthesised during their life period. As the source of energy for the soil population this may be regarded as the distinguishing characteristic of soils. These four constituents are invariably present, but not in the same proportion ; their relative abundance affords the basis on which soils are classified From the agricultural point of view we thus have : (a) mineral soils consisting mainly of rock material, subdivided into sands, loams, and clays ; (£) calcareous soils containing notable amounts of chalk or limestone ; (<:) alkali soils rich in soluble, saline matter ; (d} acid humus or peat soils where much organic matter has ac- cumulated in absence of calcium carbonate ; (e) neutral humus soils where much organic matter has also accumulated, but in presence of sufficient calcium carbonate to prevent acidity. By far the greater proportion of agricultural soils belong to the first group. The Mineral Portion of the Soil. It is usual to divide the soil into a number of fractions by a sedimentation process, and this method has been used for a THE COMPOSITION OF THE SOIL 95 large number of analyses, which have given information of considerable value to the student and the cultivator. The method is fundamentally defective in that the grouping is quite arbitrary, sharp lines being drawn where none exist in Nature, and the soil is represented as a mixture of five or six different substances when in point of fact the number of components is indefinitely large. It is very difficult to use the resulting figures for further investigations : they cannot be plotted on curves or reduced to simple factors. They can, it is true, be set out in columns which are very convenient for lecture purposes. Some limited success has followed the attempts to correlate the figures with other soil properties, as will be shown later ; but in the main the actual figures obtained have not proved very fruitful to investigators, though the broad general results have been useful. Very much better results might be expected if a distribu- tion curve could be obtained, showing how the particles are distributed according to size. A certain amount of artificiality seems inevitable, and one must still keep to the conventional " effective radii " and deal with the soil particles, not as they are, but as they would be if they were perfect spheres. A distribution curve on these lines, while not perfectly expressing the soil conditions, would be a great advance on anything that we have at present. A serious effort to obtain such a curve has been made by Sven Oden, whose elegant method of mechanical analysis deserves serious attention from soil investigators. Instead of ascertaining the weights of the fractions of soil falling between certain limits of size, Oden proceeds in the reverse order, and ascertains the time taken for small successive equal weights to fall through a column of water. A suspension of soil in water is poured into a cylinder near the bottom of which is a large flat plate attached to one arm of a balance, the other arm being counterpoised and containing in its pan a small weight. As soon as the weight of the soil particles settling on the plate begins to exceed the weight in the pan the balance moves : this makes an electric connection which registers the time on 96 SOIL CONDITIONS AND PLANT GROWTH a chronograph and causes a second weight to fall into the pan, thereby restoring the balance to its original position (Fig. 14). The process is repeated when the weight of sediment again just exceeds the weight on the pan. The second step consists in calculating from the known Jfc B n IP M FIG. 14. — Sven Odin's apparatus for mechanical analysis of soil diagrammatically represented (2i8c). laws of motion of solids in fluids the mass or number of particles of any given radius present in the soil : but as the actual particles are very irregular in shape it is necessary to adopt an "equivalent" or "effective" radius, i.e. the radius of a sphere which would fall with the same velocity as the particle. THE COMPOSITION OF THE SOIL 97 Oden gives a formula for effecting the calculation, but its use requires some mathematical training.1 The type of curve finally obtained is shown in Fig. 1 5. This is a mass distribution curve : not a frequency curve. On the horizontal axis are plotted successive values of the radii. The axis of y represents a complex function which gives the percentage weight of particles comprised between successive integral values of //,. Thus the percentage weight of particles between 1/j, and 2fj, in diameter is the area bounded by ordinates drawn at points I and 2 on the axis of x : without i6 O LJi2 £ LU LJ O QC LJ Q- 30 35 0 5 10 15 20 25 EQUIVALENT RADII /x FIG. 15. — Mass distribution curve obtained by Odin's method of soil analysis. sensible error it is represented by the ordihate drawn from the point i -5. It is improbable that this could ever become a working analytical method ; but as a method for investigation it sur- passes any other at present available, because for the first' time it affords the possibility of representing the soil fractions by a distribution curve of the type familiar to physicists and mathematicians. 1 A full description of the method and the mode of calculating the results will be given in B. A. Keen's monograph in this series. A simpler method is described by O. Wiegner, Landw. Versuchs-Stat., 1918, 91, 41-80. 7 98 CONDITIONS AND PLANT GROWTH 1 8 lila Us6 N N i I N N *£) g S^§? 88? 9§| 99| b o "3 ^> -II M « g Jg* rt b ^ b § g g s Ih ^te "cT 0 5,5 -s ^ _. ^ — ' ^± H c< "p g *-w p ? in ci *"< J° 0 w •" J> j.2<2 cd 0 13 •2-S w !1 "§ • ' ' •S (3 c rt "o £ 1 S 1 S | | | 4> "2 2 V s •§ « s Ills co O & -o '£ s § ^ I ^ c •§ 2 H S ^ v u s i 0 13 £ «- c > 1 u CO 5 •§ S s w •SS8 J? 2 0' 0^ 1 ^ li1 m ^ b N H 9 9 9 § m m o T "° b b g b _o *<£ 0 o JB 1*8 If 1 ^ > rt 1 -8 1 P £ o c ii II 2 w ? S 1 1 i ! I i fib 0 fe U s > | I'l 6 H O t P i b 99 ^ M >W O'OO2 N 00 jj 4 Qfe o ob XI n "S 'c -0 *o g rt ^ T^U »^ s! &) 4J 03 .^ ^ 03 a rt .§ 1 . < O Oi hy S K £ z s w o 5 O B P< O H S Q Id w ° H J ° 5 Sw ^ ^ as ^ w w o S£P^ a ! ^ oo *2 0 o M N ro TJ- » _n o 0 0 0 0 0 9, <3 £ o| 3 !? SCO 0 M ^- ^- t^ Sf o 0 0 0 O O O O a £ in oo r^ in in H 0 1 H H H H M H d 'o'c 0 oo m ro 0* 0> Ti- S| 0 0 H M ro >n ri- ro § ^ Tj- 0 H IQ VO Q 1 0 0 H H M H M < S . 5 oo >n O m 01 rj- O 0 W"Sb 0 0 0 0 0 O H M § 00 0 H •^- iO O rt 1 b H ro N H 0 O . rt Tj- Tj- Tj- 00 M o? 10 in SI 0 0 0 0 M N H H § H H 01 oo oo oo « H H TJ- m tx. W o IN 2 «< 01 •8| M N 01 in 0 0 N H W'S) 01 H M H OJ t^ ro fO M M c jj T3 0 ro m o .» « 1 00 °^ ro •*• H H oo r^ H N o < < 'o'c 0 0 H OJ ro N 00 W *tU} ro H N in {H. 9 M b< "W I P 0. CTl H OJ H in ro ro 0 ts« ^~ u 00 00 t"N. t>» VO ^~ o < C/3 T3 o £ «*• o^ 0 Tj- H ro M O si S ro CTi c^ oo •^- ^ ro O\ oo ^o in Tt- 'S > rt T3 C rt T3 c 1 , ' , Sb s S ' 'S S fa og 1) •S 5 fa C/3 • w >> C rt tS 5 THE COMPOSITION OF THE SOIL 103 approximately equal to 2. Other soils of volcanic origin from Java gave up larger amounts of base relative to the silica, but in no case were the ratios constant whole numbers ; the alkaline bases showed the same lack of constancy in the ratios to A12O3. TABLE XXX.— RATIO MOLECULES OF SiO2 MOLECULES OF ALO, EXTRACTED FROM VARIOUS SOILS. VAN BEMMELEN (22). Solvent. Temperature and Time of Extraction. Alluvial Soils, Holland. Volcanic Soils, Java. Laterite Soils, Surinam. HC1 of sp. gr. 1*03 15 mins. at 55° V7 5*o '9 2*1 I'l HC1 of sp. gr. 1*2 i hour boiling temp. 3'4 4-6 2*2 27 1-6 H2SO4 cone. . — 2'0 2-4 3*2 2*O 1-6 Alkaline bases extracted from a heavy clay, Surinam. Mols. of Bases Extracted Solvent. Tempera- ture of A1203 Dissolved, Ratio Mols. Si02 for i Mol. of A12O3. Extraction. per cent. Mols. A13O3 CaO. MgO. K20. Na20. HC1 of sp. gr. '03 55° 1*2 1*3 '33 •83 •10 HClofsp.gr. -i. 100° 3'4 27 •05 •32 •08 HClofsp.gr. -2. boiling 4-6 27 •03 '!4 •OQ •01 HC1 of sp. gr. -2 . ii 2'5 27 •03 TO •10 HClofsp.gr. -2. Cone. H2S04 — 1-9 8-8 27 2-0 •03 •005 •08 •06 •12 •17 02 Different soils gave up different proportions of alkaline bases, but again without showing definite simple ratios one to another. Detailed studies of clay revealed the presence of chemically unchanged crystals of the original silicates and also of easily soluble substances including a fusible group with molecules of SiO2 ratio — \ 1 f A1 ^ varying from 3 to 6, and a silicate molecules of A12O3 J resembling kaolin with ratios varying between 2 and 3. The* easily soluble material represents the products of weathering since it does not occur in rocks. If it were a definite chemical compound the ratio of its constituents should be constant whole numbers, but this is not the case. It behaves, however, 104 SOIL CONDITIONS AND PLANT GROWTH precisely like a solid solution and is therefore regarded by van Bemmelen as an "absorption compound," SiO2, mA!2O3, nFe2O3 . . . pH2O, in which the constituents are not chemic- ally united but are held by the feebler forces manifested by colloids in their attractions one for the other.1 The Physical Properties of the Various Fractions. — Serious studies of the soil by competent physicists have scarcely been attempted as yet, and the work hitherto done can only be regarded as preliminary. The fundamental difficulty in applying the ordinary physical methods is to synthesise the soil ; numerous studies have been made of the physical properties of sand, silt, clay, etc., considered as separate entities, but no one has worked out the resultant when all the varying grades of sand, silt, and clay are inti- mately mingled, or drawn up a scheme or formula to express the properties of the soil in terms of the mechanical analysis. More useful results are obtained by the method of correlation ; soils of known properties are analysed and the results are correlated so far as is possible with the properties ; even this method, however, can only be used very crudely, because the physical properties of the soil as a whole cannot at present be expressed by definite numbers. Only a very general summary will therefore be attempted. The Clay Fraction. — The word "clay" is unfortunately used in many senses ; the soil worker and the ceramic chemist in particular attach widely different meanings to it. In soil investigations clay is the material of less than -002 (or in the U.S.A. '005) mm. diameter : in ceramic work it is the material of cri mm. diameter downwards. Clay in the soil sense may be regarded as a plastic colloid, but its special properties are seen only when a certain amount of water is present.2 If it is well rubbed with water it becomes very sticky and absolutely impervious to air or water ; it is also Gedroiz' method of estimating the zeolitic silicic acids see Bull. Internal. Instit. (Rome), 1917, 8, 1190. 2 Older work on the constitution of clay is summarised by Rohland in Abegg's Handbuch der Anorganischen Chemie, 1906, 3, 97-119. THE COMPOSITION OF THE SOIL 105 highly plastic, and can be moulded into shapes which remain permanent on drying and baking. It shrinks very much on drying and absorbs heat ; on moistening again, however, there is a considerable swelling and evolution of heat. The re- versibility of the process has been studied by van Bemmelen (20, 25), who has also shown that the rate at which water is lost on drying over sulphuric acid is not essentially different from the rate at which evaporation takes place from a pure water surface under the same conditions. The separate particles of clay are so small that, when placed in water, they assume a state of Brownian movement and sink only very slowly in spite of their high specific gravity. Traces of electrolytes have a profound effect on these properties ; small quantities of acids or salts cause the temporary loss of plasticity, impermeability, and the property of remaining long suspended in water without settling ; the clay is now said to be flocculated. The change can be watched if a small quantity of any flocculating substance is added to the turbid liquid obtained by shaking clay with water; the minute particles are then seen to unite to larger aggregates which settle, leaving the liquid clear. There is, however, no permanent change, deflocculation takes place and the original properties return as soon as the flocculating agent is washed away. Alkalis (caustic soda, caustic potash, ammonia and their car- bonates) produce the reverse effect : they deflocculate clay, intensifying its stickiness and impermeability and causing it to remain suspended in water for long periods1 (see p. 161). Clay is thus an electro-negative colloid, its reaction prob- ably being conditioned by a trace of potash liberated by hydrolysis. Further, it appears to act as a semipermeable membrane in relation to the movement of water : 2 this property might 1 Leoncini and Masoni (172) were unable to find that the modification in permeability in soil caused by saline solutions had any relation to their powers of flocculating clay. 2Lynde, J. Phys. Chem., 1912, 16, 759-778, and jf. Amer. Soc. Agron., 1913, 5, 102-106. io6 SOIL CONDITIONS AND PLANT GROWTH cause some peculiarities of behaviour on sandy soils, such as the Bagshot beds, where there are thin partings of clay. A remarkable change sets in when clay is sufficiently heated, and it permanently loses all its special properties. Several theories have been put forward to account for the special properties of clay, and in particular its plasticity. It has often been assumed that these properties are the necessary result of the smallness of the particles, which brings into prominence the surface forces. Rohland l attributes plasticity to hydrated colloidal substances forming a gelatin- ous film round the clay particles, and Le Chatelier2 to minute flake-like particles. At present the evidence is insufficient to allow of discrimination.3 These clay properties are of great importance to the fertility of the soil, and no constituent is more necessary in proper proportions, or more harmful in excess. Clay impedes the movement of water in the soil and keeps it in the surface layers within reach of the plant roots, thus making the soil retentive of water. Excess of clay, however, interferes too much with the water movements, making the soil water- logged in wet weather and parched in dry seasons even though the permanent water level is near the surface ; it also impedes the movement of air to the roots and lowers the temperature of the soil. The adhesive properties of clay cause the soil particles to bind together into those aggregates on which "tilth" depends; soil without clay would be very like a sand heap. Here also, however, excess of clay does harm and makes the soil so adhesive that it sticks to the tillage implements and retards their movements ; it also tends to form large clods unfavourable to vegetation. These effects are intensified in wet weather ; the soil becomes sticky or " poached" and must not be worked or the tilth is injured for a long time. Another effect of a large amount of clay is to make the soil shrink very much on drying, so that large cracks appear in the fields in summer time. These harmful 1 P. Rohland, Die Tone (Vienna), 1910. 2 La silice et les silicates. Paris, 1914. 3For discussion see A. B. Searle, British Clays, Shales, and Sands, 1912 THE COMPOSITION OF THE SOIL 107 effects are reduced by flocculation effected by dressings of lime or chalk (which become converted into calcium bicarbon- ate in the soil) and by organic matter ; on the other hand,, they are intensified by the deflocculation resulting from the use of alkaline manures like liquid manure, or by sodium nitrate, which leaves a residue of sodium carbonate in 'the soil. Further, as pointed out above, clay " fixes " and retains the ammonia and potash supplied as manure. In general 8 to 1 6 per cent is a satisfactory proportion of clay in a soil where the rainfall is 20 to 30 inches per annum. Fine silt (croi to croo2 mm. in diameter) has also great water-holding power, and in excessive amounts (above 10 to i 5 per cent.) it increases the difficulty of working the soil, especially if much clay is present. It does not possess the marked plastic and colloidal properties of clay, it behaves differently towards electrolytes (see p. 162) and is less altered by lime ; indeed no method is known for making it tractable. It is usually less in amount than the clay ; certain peculiarities in cultivation are in some cases manifested where the reverse obtains, e.g. in the Lower Wealden strata, the Upper Green- sand and the Lincolnshire warp lands ; in North Wales, how- ever, many soils possess more silt than clay without any apparent disadvantage. The coarser grade of silt (0^04 to CTOI mm. in diameter) appears to be very valuable, and constitutes 30 to 40 per cent of many of the loams most famous in the south-east of England for carrying their crops well and not drying out The fertile loess soils of the United States are also rich in silt, containing 55 or more per cent of material of -05 to •005 mm. diameter.1 Light, sandy loams, on the other hand, may contain only 10 to 20 per cent ; some of these are highly fertile, but as a rule they require large dressings of dung, or a situation favourable for water-supply. Probably silt plays a very important part in maintaining the even con- ditions of moisture so desirable for plant growth. t It is fine 1]. G. Mosier and A. F. Gustafson, Soil Physics and Management, 1917 p. 64. io8 SOIL CONDITIONS AND PLANT GROWTH enough to retard, but not to prevent, percolation, and it facilitates capillary movement of water. Fine sand (cr2 to 0*04 mm. in diameter) forms a consider- able fraction — usually 10 to 30 per cent, or more — of nearly all soils. Although its dimensions are relatively large, it still possesses cohesiveness and a tendency to cake together ; it has not, however, so great an effect as silt in maintaining a good moist condition. Soils containing 40 per cent, or more of fine sand tend to form, after rain, a hard crust on the surface, through which young plants can only make their way with difficulty until it has been broken by a roller. But they have no great water-holding capacity or retentive power, and are not infrequently described by their cultivators as hungry soils that cannot stand drought. The notoriously infertile Bagshot sands and the barren Hythe beds in West Surrey are largely composed of this fraction, as much as 70 per cent, being sometimes present. In all these cases, however, clay is deficient and the situation is dry ; better results are obtained when the clay exceeds 8 or 9 per cent., or when the water table is near the surface, especially if the amounts of coarse sand and gravel are not too high. Coarse sand (i to O'2 mm. in diameter) is perhaps the most variable of all soil constituents in amount, and, as its pro- perties are in many ways the reverse of those of clay, it exercises a very great effect in determining fertility. Through its lack of cohesion it keeps the soil open and friable ; in moderate amounts it facilitates working, but in excess it increases drainage and evaporation so much as to interfere seriously with the water-holding capacity of the soil. Many good loams contain less than 4 per cent, and in general strong or tenacious soils contain less coarse sand than one-half the quantity of clay present. When the coarse sand exceeds the clay in amount the soil becomes light, unless of course the clay is above 20 per cent, when the soil must always remain heavy. Not being a colloid, it possesses no power of absorbing water or soluble salts. Soils containing 40 per cent, or more of coarse sand and less than 3 per cent, of clay THE COMPOSITION OF THE SOIL 109 are cultivated only where large quantities of dung are avail- able, or where the water-supply is exceptionally good. As the amount of coarse sand increases, the soils become less and less suited to cultivation, till finally the sand dune con- dition may be reached, though in adequately moist conditions cultivation may still continue even when 90 per cent, of coarse sand and no clay are present (e.g. part of Anglesea). Fine gravel is not usually present to any great extent, and is of importance only when the coarse sand is already danger- ously high. Stones cannot be determined quantitatively by any method of sampling in use, and their effect must be judged by a visit to the field. If they are uniformly scattered through a stiff soil, as in the Clay-with-Flints, they are on the whole beneficial, because they facilitate tillage. Where they form a bed underlying the soil they may do harm by causing over-drainage. Some typical examples are discussed in Chapter VII. Reactive Inorganic Constituents. It was formerly supposed that zeolites occurred in the soil and accounted for many of the soil reactions and absorp- tions. The direct evidence is so slight \that this view is now generally given up. It seems necessary, however, to assume that some of the inorganic constituents of the soil are very reactive, since certain chemical changes are brought about which can hardly be attributed to micro-organisms. Thus, calcium cyanamide is decomposed by soil with formation of urea, and the change proceeds whether the soil be fresh or ignited ; this change can also be effected by certain minerals, e.g. prehnite. So phenol reacts instantly with soil to form some insoluble compound not decomposable by steam. These obscure reactions are being studied at Rothamsted (p. 215). Soil Acidity. It has long been known that many soils are acid to litmus paper but become neutral on addition of lime or calcium no SOIL CONDITIONS AND PLANT GROWTH carbonate. Many cultivated plants, notably clover and its allies, fail to grow well on such soils, and they succeed only after lime has been added : Azotobacter and other organisms are also adversely affected (p. 239). The older chemists took the simple and obvious view that these soils contained an acid or acids, and, as "high moor" peat showed the same property, they concluded that the acid was of the same general nature in both cases. It was assumed that plant residues at a certain stage in their decomposition formed some acid substance which accumulated in circum- stances where decomposition became very slow, e.g. in badly drained and badly aerated soils. But instances were observed of acid soils, well drained arid therefore not suffering from slowness of decomposition, containing so little organic matter that it was difficult to attribute acidity to organic compounds. It was therefore necessary to assume the presence of acid mineral substances in the soil, and a number of investigations were made showing that kaolin and similar silicates, which might be expected to occur in soil, become more and more acid to litmus paper on treatment with CO2 solution.1 Chemical hypotheses, satisfactory as they might otherwise have been, suffered from the serious drawback that no one was ever able to isolate an undeniable acid from soils in any significant quantity. A wholly different hypothesis was therefore put forward by Cameron. Setting out from van Bemmelen's demonstration that humus is a colloid he showed that all the phenomena of soil acidity could be explained as simple colloidal manifestations and did not require the assumption of soil acids at all. It was only necessary to suppose that the soil colloids absorbed the base more readily than the acid from blue litmus and the whole phenomena are explained. In support of this view Cameron showed that cotton and other absorbents behaved exactly like " acid " soils, slowly turning blue litmus red ; the phenomenon was therefore a general property of a class of absorbents. 1 See Gans (101) for a review of the literature of this problem. THE COMPOSITION OF THE SOIL 1 1 1 Baumann and Gully (io£) applied this idea to the case of peat and showed that it fully explained all the facts then known. In the first instance they pointed out that it was not necessary to assume that the " acid " was a decomposition product because the original sphagnum was almost as " acid " as the peat. Secondly, the acid if it exists must be insoluble because the water extract of the peat is practically neutral to litmus. It must, however, be very potent, because solutions of neutral salts such as calcium chloride, sodium nitrate, etc., are decom- posed with liberation of free hydrochloric and nitric acids when treated with peat or sphagnum. Baumann and Gully argue that no acid of this character is known to chemists, and it involves less strain to conceive of a physical absorption of the base from the dissolved salt with liberation of the acid than to imagine an insoluble organic acid capable of decomposing simple salts in solution. Further reasons for supposing that the phenomena are due to absorption and not to chemical action are : — 1. The amounts of acid liberated from equivalent quantities of different salts of the same base are not equal as they should be in a chemical action. 2. The amounts of base absorbed are not equivalent, e.g. potassium is absorbed to a greater extent than sodium. 3. The amount of action varies with the concentration of the solution and the mass of the sphagnum, but not in the way that would be expected of a chemical change. 4. The electrical conductivity of peat is very low, much less than that of an acid having the same solvent action on tricalcic phosphate. The view that acidity of the mineral acid soils is due to preferential absorption of the base was developed by Harris (124) in an investigation of Michigan soils. The phenomena are substantially the same as for peat : the soil turns blue litmus red : an aqueous extract is neutral : but an extract ii2 SOIL CONDITIONS AND PLANT GROWTH made with a solution of a salt, e.g. calcium nitrate, is acid. We must therefore assume either an insoluble but very potent mineral acid, or a preferential absorption of the base over the acid. The latter is indicated because, as in the case of peat, the amount of acid liberated from equivalent quantities of different salts is not the same, as it should be in a chemical reaction. Daikuhara (74$) has applied this view to the case of the acid mineral soils of Japan and Korea, but he has modified the explanation and made it more easily intelligible to the chemist, who finds it difficult to understand why an unparal- leled physical decomposition of a simple salt should be ac- cepted, and the assumption of a difficultly soluble but potent acid rejected. Daikuhara shows that the development of acidity in the salt solution is due to an exchange of bases and not to simple absorption of the base from the salt. If the acid solution is analysed it is found to be really a solution of an aluminium salt : aluminium being given up from the soil in amount approximately equivalent to the base absorbed. Aluminium salts, as is well known, turn blue litmus red and therefore are indicated as acids. The phenomenon is still essentially an absorption, but the seat of the reaction is located. This view is supported by Rice's experiments (238) which have demonstrated the substantial identity in hydrogen ion concentration of a solution of aluminium nitrate and the solution obtained by treating an "acid" soil with potassium nitrate solution. Hartwell and Pember (1280) also support this view by an ingenious line of argument. True acids added to nutrient solutions affect barley and rye in water culture similarly, while extracts of acid soils affect them differently. Therefore acid soil extracts contain something not present in acid solutions ; on testing they were found to contain aluminium. The effect of aluminium salts on plant growth was examined l 1J. B. Abbott, S. D. Conner, and H. R. Smalley had previously attributed the infertility of an Indiana soil to the presence of aluminium salts (Ind. Expt. Sto. BuL, 170, 1913). THE COMPOSITION OF THE SOIL 113 and found to resemble that of the acid soil extract. Mirasol l has confirmed and extended these observations. Ramann also adopts this physical hypothesis and gives up the expression "acid soils," using instead " absorptiv ungesattigte Boden". Kappen 2 confirms the observations without entirely accepting the explanation. Recent work shows that both views have a foundation of truth ; there are at least two causes at work : lack of bases in the soil brings about the absorption phenomena discussed above, but there are also true acids present in soil under certain conditions. Rindall of Helsingfors (239), Sven Od6n of Upsala (2i8#), Tacke (278), and Ehrenberg and Bahr (93^) have each argued in favour of definite humic acids in peat (see p. 139). Truog (285) finds, in the case of mineral soils, that equivalent amounts of different bases are required to neutralise the acid properties of the soil — which if generally true would be easier to explain by assuming an acid than an adsorption. The view that soil acidity is caused by actual acids has gained support from recent work 3 on the hydrogen ion con- centration in soils. Chemists study acids in two ways : — 1. By measuring the hydrogen ion concentration, a value based on the assumption that an acid on solution in water dissociates into two parts, called ions — one being hydrogen, and the other the rest of the molecule. 2. By determining the titration value, i.e. the number of c.c. of standard alkali solution which a given volume of the acid solution will neutralise. In the language of the dis- sociation hypothesis this value measures the total quantity of hydrogen ions producible under the conditions of the experiment, supposing them to be neutralised or linked up with - OH ions as quickly as they are liberated. lSoil Sci., 1920, 10, 153. 2 H. Kappen, Studicn an saurem Mineralboden aus der Ndhe von Jena (Landw. Versuchs-Stat., 1916, 88, 13-104). 3 For a critical summary of recent work see E. A. Fisher, J. Ag. Sci., 1920, xx, 19. 8 H4 SOIL CONDITIONS AND PLANT GROWTH The titration value measures the quantity of the acid but it makes no distinction between a strong acid, such as sulphuric acid, and a weak acid, like acetic acid : the great difference in action on plant life exhibited by the two acids is missed altogether. Moreover, the titration value is not an absolute constant ; polybasic acids, which are by far the most numer- ous, have several titration values, according to whether one, two or more of the hydrogen atoms are affected. Different indicators, therefore, give different numerical results. So long as the constitution of the acid is known this does not matter, but it causes complications in dealing with a mixture of un- known acids. Many of the methods suggested for measuring soil acidity, including all the " lime requirement " methods, afford a more or less rough measure of the titration value 1 : the differences in results are partly due to absorptions and partly to differences in the number of hydrogen atoms concerned. The determination of the hydrogen ion concentration, i.e. of the ions actually present, as distinct from those that would finally be liberated on neutralisation, is an attempt to measure the intensity of the acid as distinct from its quantity. The principle of the method is simple in dealing with solutions of pure acids : it is based on the ordinary dissociation law : = K where H* and X' are the ions produced on the dissociation of the acid HX, and K is a constant. The number or the concentration of the ions is measured by the electrical conductivity of the solution, but once this has been determined for a given acid it can become a standard against which the hydrogen ion concentrations of other acids can be rapidly compared by means of a set of indicators.2 The problem becomes more complex when salts are present, because the additional ions thus introduced affect the 1 Probably all the results are too high, as they include the absorptions. See H. R. Christensen (676), also 105^. 2 A useful account of the measurement of the hydrogen ion concentration is given by J. F. McClendon in Physical Chemistry of Vital Phenomena (1917), which was written for biological students. THE COMPOSITION OF THE SOIL 115 amount of dissociation : in particular they make it difficult for the hydrogen ion concentration to increase when another acid is added. Colloidal substances act in the same way. This effect, known as " buffer action," is of great importance in vital phenomena and is well marked in the case of soils. Thus if an acid is added to soil the increase in the hydrogen ion concentration is not nearly so great as if the same quantity of acid were added to water. The numerical values of the hydrogen ion concentrations are rather unmanageable and a convention has therefore been adopted in dealing with them. In pure water or in a neutral solution H* = OH' = I x io~7'07 gram-ions per litre. Acidity means a hydrogen ion concentration in excess of that of water, i.e. the index instead of being -7-0 is greater: as the quantity is negative this means that the number itself is less : it may fall to zero. Simple trial will show that it is very difficult to plot on the same curve numbers ranging from i to i x io~7. A further complication began to arise but fortunately was checked : investigators expressed their' results in terms not of i, but of some other number, multiplied by io to some negative power, e.g. one solution might be repre- sented by the value 4-4 x io"6, and another by 6 -8 x io"7, and it was rather difficult to make a comparison. To overcome the first of these difficulties Sorensen l adopted the familiar device of plotting the logarithms of the numbers instead of the numbers themselves ; and to overcome the second he reduced all to terms of I x some power of io : thus the two cases just quoted become I x io"5'36 and i x icr6'17 respectively. He calls the numbers 5-36 and 6*17 the PH values of the solutions: in reality they are the values for the - log (H'). On this notation the PH value for neutrality is 7*07 ; that for acidity is anything less — down to nothing — that for alkalinity is anything greater — up to I4'I4- 1S. P. L. Sorensen, Biochem. Zeitsch., 1909, 21, 130, and 22, 352; see also Ergebnisse d. Physiologic (Asher and Spiro), 1912, 12, 393. 8* ii6 SOIL CONDITIONS AND PLANT GROWTH In consequence of the buffer action already mentioned these values are not liable to disturbance by small additions of substances to the soil. A large number of determinations of the PH or - log (H') values for American soils have been made by Gillespie (1050) and G. Sharp and D. R. Hoagland (260), while for British soils a beginning has been made in the Rothamsted laboratories. Some of the values are as follows : — American Soils. Sharp and Hoagland. British Soils. Acid soils, extreme value .... Fertile soils Alkaline soils, extreme value 3'7 7-04-7-52 97 4-6 The acidity of root sap is of the order of 5 '5 -6 '8 (p. 81). The relatively small range of variation in comparison with the large variation in amount of titratable acidity is explained by the buffer action described above. In addition to and probably distinct from this natural acidity it is possible to induce acidity in soils deficient in calcium carbonate by the long-continued application of am- monium sulphate. This was first observed by Wheeler in Rhode Island (303) and it is demonstrated in a remarkable manner at the Royal Agricultural Society's Experiment Station at Woburn. Apparently this is not the same as " acidity " liberated by solutions of neutral salts in acids, because am- monium sulphate appears to act specifically, no other fertiliser behaving in this manner at Woburn.1 Hall supposes, with considerable probability, that the ammonia is taken up by the plant, leaving the sulphuric acid in the soil, and this view seems justified by the fact that nitrate of soda behaves altogether differently, leaving an alkaline residue in the soil. 1 There are a few cases on record where potassium salts are said to have reduced yields, and here it is possible that acid substances have been liberated by the dissolved salt. THE COMPOSITION OF THE SOIL 117 TABLE XXXI. — EFFECT OF CALCIUM CARBONATE ON THE TEXTURE OF SOILS. Hamsey Green. Rothamsted. Arable Soil. Too Sticky for Arable. Arable Soil, Barnfield. Too Sticky for Arable, Geescroft. Fine gravel . • . Coarse sand . .. 17 5'3 1-6 9'5 2*4 5*5 1-8 4*9 Fine sand . „ 287 22'3 20'3 27-8 Silt . 26-3 25*4 24-4 25*4 Fine Silt IO'2 9-9 127 io'6 Clay . 16-4 i6'O 22*0 ig'O Loss on ignition . 4-8 5-2 4-7 5'i Calcium carbonate I'02 •48 3-0 •16 The present position may be summarised as follows : — The existence of a hydrogen ion concentration greater than that of water is satisfactory evidence of the presence of true acids in soils. The measurements at present throw no light on the nature or quantity of the acids. The acids may be— 0) Organic, (b) Siliceous, (<:) Formed by hydrolysis of iron or aluminium salts. They may adversely affect plants or micro-organisms by reason of their strength or their quantity. On the other hand, the acids themselves may be without serious action (see p. 81). The special properties of an " acid soil " may result from a lack of basicity, whereby — 1. Special absorption relationships appear; 2. Lack of calcium may affect plants or micro-organisms ; 3. Certain toxic substances, e.g. metallic salts, may remain effective, which in presence of lime would be thrown out of action ; • 4. The clay may become deflocculated and therefore as- sume a sticky condition unfavourable to plant growth. Table XXXI. shows pairs of soils similar in constitution and in general external conditions, temperature, water-supply, n8 SOIL CONDITIONS AND PLANT GROWTH etc., but very different in agricultural value because of their different content of calcium carbonate, one being readily cultivated while the other is wet and sticky, and suitable only for pasture land. "Sourness" of Soil: Its Relation to Acidity and Calcium Carbonate. Soils which are infertile from lack of calcium carbonate are called "sour" by farmers. The older chemists substituted the word "acid," but, as shown above, the effect is not necessarily in all cases attributed to acids, and therefore it seems desirable to retain the farmers' term as the broad one and distinguish between " sourness " due1 to acidity and that due to lack of basicity.1 Whatever its cause in a given case sourness can be over- come by addition of lime or calcium carbonate ; soils containing calcium carbonate are never sour, and they are generally fertile. It by no means follows, however, that soils devoid of calcium carbonate are infertile. Hendrick and Ogg at Aberdeen (132) and Robinson in N. Wales (240) have both described soils free from calcium carbonate but fertile and indeed neutral in reaction : Hoagland observes 2 that certain Cali- fornian soils with low PH value, i.e. considerable intensity of acidity, and large " lime requirements," are nevertheless able to produce excellent crops of many types. In the present state of our knowledge it is hardly possible to say from chemical examination of the soil alone whether it is "sour" or not, i.e. whether it is or is not infertile through lack of calcium carbonate. A combination of chemical analysis and field observation, however, enables something to be done : soils known by vegetation observations to be comparable can 1 This seems preferable to the distinction made by some of the investigators between "positive acidity" caused by actual acids, and "negative acidity" caused by adsorption of base (cf. Lyon, Soils and Fertilisers, p. 112. See also H. R. Christensen, Soil Sci., 1917, 4, 115, and C. J. Schollenberger, Soil Sci., > 279). 2 Private communication to the author. THE COMPOSITION OF THE SOIL 119 be set out in order of their calcium carbonate content by the usual analytical methods, or of their lack of calcium carbonate by one of the lime requirement methods.1 Before any indi- cation can be given of the amount of lime required for cultiva- tion it is necessary to make field trials (p. 335). In general, sandy soils require only sufficient calcium carbonate to prevent sourness, while clay soils need in addi- tion enough to keep the texture good. Sands well supplied with calcareous water and under ordinary arable cultivation may get along with cri per cent, or even less calcium carbon- ate. Many light soils that are intensively farmed respond to dressings of chalk or of ground limestone, even though 0-2 or 0*3 per cent is already present. 0*5 per cent, of calcium carbonate commonly proves insufficient for clay soils, and even ro per cent, may not be enough in highly-farmed districts, especially where cattle are fed on the land and tread the soil into a somewhat sticky state. Further increases in calcium carbonate over and above the proper amount are not known to have any effect except to provide a margin of safety. Considerable work has been done on the effect of lime on peat soils, which seems to be more than a neutralisation of acidity. Oden (218^) suggests that the calcium humate produced may be directly beneficial to plants. Calcium carbonate is not a permanent constituent of the soil, but changes into the soluble bicarbonate and washes out into the drainage water ; the average loss per acre per annum throughout England and Wales has been estimated at 500 lb., and at Rothamsted on the arable land at 800 to 1000 lb. (i2O<$). The rate of loss is influenced by the treatment, being increased by the use of ammonium sulphate and decreased by dung and by a growing crop ; it is much less on pasture than on arable land. Repeated additions of calcium carbonate to 1 Such as Veitch's (287) or Hutchinson and McLennan's (140^). L. J. Wild, y. Ag. Sci., 1917, 8, 154, shows that the latter when modified by a correction factor of 0*1 per cent, proves helpful in the study of New Zealand soils. For a critical account of the various methods see Ames and Schollenberger (4) and E. A. Fisher (99). It is hardly necessary to point out that the " lime require- ment" measures something entirely distinct from- the PH value (p. 115). 120 SOIL CONDITIONS AND PLANT GROWTH the soil are, therefore, necessary : indeed, chalk and lime are among the oldest of manures. Soils lying immediately above chalk and limestone are no exceptions, and in wet regions they may become thoroughly decalcified. On chalk soils the percentage of calcium carbonate may rise very high, and then a wholly new set of properties comes in. It is impossible to draw any exact line showing where these properties begin to appear, but they entirely mask the effects of the silica and silicate particles and obliterate the distinctions between sands, loams, clays. Chalk soils, there- fore, form a class by themselves to which the ordinary laboratory methods of analysis and investigation do not apply : unfortunately, appropriate methods have not yet been worked out. The Phosphorus, Potassium, and Calcium Compounds of the Soil. Part of the phosphorus in the soil is probably present in organic combinations derived from plant or animal resi- dues : this has been investigated by Vincent,1 by Potter and Snyder 2 and by Schollenberger ; 3 none of whom, however, were able to effect identification of the compounds. Part, however, is in inorganic combination and Bassett has shown * that it most probably occurs as hydroxyapatite — (Ca3P208)3Ca(OH)2, this being the solid phase stable over a range extending from faintly acid to alkaline conditions : any phosphate such as superphosphate or basic slag added to the soil as fertiliser would tend to be converted into this substance. The potassium compounds in the soil are probably silicates. Their distribution has been studied by Dumont 5 who found that they may occur in the finer or coarser portions of the 1 Compt. Rend., 1917, 164, 409. 2 Soil Sci., 1918, 6, 321-332. 3 Ibid., 365-395 ; 1920, 10, 127. In this case about one-third of the phos- phorus occurred in organic combination. 4 Trans. Chem. Soc., 1917, in, 620-642. 5 Compt. Rend., 1904, 138, 215-217. THE COMPOSITION OF THE SOIL 121 soil with very different effects in either case. He instances two soils of nearly equal potash content, one of which from la Creuse responds to potassic fertilisers, while the other from Grignon does not : in the former case the potash is present mainly in the coarser material, in the latter mainly in the fine (Table XXXIL). TABLE XXXII. — DISTRIBUTION OF POTASH AMONG SOIL PARTICLES (DUMONT). Percentage Distribution Per cent, of KcO in Soil. Response to Potassic Fertilisers. Per cent, of " Argile". Per cent. K2Oin Argile of Potash in Soil. Sable Grossier. Sable Fin. Argile. Grignon . 0-85 Nil 16-8 0-94 16-6 65-8 17-7 La Creuse 0-89 Good 4'5 0-51 70-9 .'26-4 27 The calcium compounds of a number of soils have been studied by E. C. Shorey, Fry and Hazen (263). The follow- ing were detected by petrographic methods ; — Frequently — Hornblende, chiefly Ca(MgFe)3Si4O12 with Na2 Al2Si4O12 and (MgFe)2(AlFe)4Si2012. Plagioclase, isomorphous mixtures of CaAlSi2O8 and NaAlSi3O8. Epidote, Ca2H(AlFe)8Si3O18. Occasionally — Calcite, CaCO3. Titanite, CaTiSiO5. Garnet (Ca, Mg, Fe, Mn)3(Al, Fe, Cr, Ti)2(SiO4)8. Rarely — Dolomite (Ca, Mg)CO3. Augite, CaMgSi^A with (Mg, Fe)(Al, Fe)2SiO6. Gypsum, CaSO4 + 2H2O. Chemical analysis showed that pairs of soils containing the same total CaO might differ considerably in their content of the various calcium compounds (Table XXXIII.): — 122 SOIL CONDITIONS AND PLANT GROWTH TABLE XXXIII.— CALCIUM COMPOUNDS IN SOIL (SHOREY, FRY & HAZEN (263)). Soil No. Total CaO. CaCOs. CaO as :— Easily Decom- posable Silicates. Difficultly De- composable Silicates. Humus Combination. 58 61 537 4-64 2'5o O'o6 O'I2 0-49 0'54 O'll 2'33 4*34 Nil 0-13 24 36 1-98 I '92 O'02 0'34 1-84 0-98 Nil O'll The Soil Water or Soil Solution. The soil retains by absorption and surface attractions some I o to 20 per cent, of its weight of water, distributed as films over its particles. This water is of obvious importance as the medium through which plants and micro-organisms derive their food ; indeed, it may be regarded as the culture solution for the plant. Its relationship to the mineral matter is dis- cussed by Cameron (65^, d, and e) and the displacements of equilibrium under the influence of climate, plant roots, etc., by Nolte (216). Several methods have been used for extracting the solution in order to determine its composition. As often happens the pioneer work was done in France : Schloesing in 1866 (245*) devised a method which is still often used — displacement of the soil solution by means of water. Displacement Methods. — Schloesing placed 30-35 kgms. of freshly taken soil in a large inverted tubulated bell jar and poured on it water coloured with carmine, this being done to simulate the action of rain. The added water at once dis- placed the soil water and caused it to descend so that it could be collected : a sharp horizontal line of demarcation between the added and the original water persisted throughout the ex- periment even when 8 days were occupied in the descent. A typical analysis of the displaced liquid in milligrams per litre was : — THE COMPOSITION OF THE SOIL 123 SiO2. Nitric Acid. Carbonic Acid. CaO. MgO. K» » • 8-40 4-08 2 1 '60 38-40 1-48 •72 V8o 6-75 ,, poor 975 4-80 8-50 IQ-25 2'45 1*21 2-12 5-10 The method, however, is difficult in application and has not come into general use. Pressure methods have been adopted by several investi- gators. In Ramann's (233^) laboratory 3 kilos of soil were subjected to a pressure of 300 kilos per sq. cm. The amounts of calcium and of potassium were found to vary considerably in different extracts obtained from the surface soil, the pro- portions relative to the other constituents increasing as the solution became concentrated by dry weather and falling as the solution became diluted by rain. In extracts prepared 1 The Bureau of Soils prefers to express the compositions in terms of dry soil rather than of solution. THE COMPOSITION OF THE SOIL 125 from the subsoil, on the other hand, the amount of calcium showed less variation, except only for a rise at midsummer. There was evidence of a transportation of calcium and potassium from the subsoil to the surface during a prolonged period of drought. No indication was obtained, however, that soil adsorption exercised any regulating effect on the concentration of the soil solution ; an exchange of bases took place only when the proportions between the dissolved sub- stances were altered. In van Zyl's experiments (323) the concentration of the solution obtained also varied according to manurial treatment and season of the year, but the percentage composition of the ignited dry matter of the extract remained constant ; this being the reverse of what Ramann obtained. A steel plunger was used by C. B. Lipman.1 An absorption method used by Briggs and McCall consists in driving a Pasteur Chamberland filter into the soil and con- necting it with an exhausted 2-litre bottle.2 A totally different method is adopted by Bouyoucos who studies the changes in the soil solution in situ by determining the changes in the freezing-point (49^). He finds the osmotic pressure varies between 0*2 to I atmospheres in moist soils, and 4'5 to 16*5 at low percentages of moisture. The con- centration of the extracted solution was 0*04 to 0*18 per cent, in moist soils and O'Q to 3 -o per cent, in drier soils. Some of his results are given in Table XXXVI. Hoagland and Sharp (136^) have applied the method to- Californian soils with interesting results.3 They find no evi- dence that the soil water is a saturated solution : on the con- trary, it appears to be dilute, probably containing less total dissolved matter than the soil extract, and it varies in con- centration with the conditions : in their experiments the range 1 Univ. California Pubn. Ag. Sci., 1918, 3, 131. 2 L. J. Briggs and J. R. McCall, " An Artificial Root," Science, 1904, 20, 566-569; Bull. 31, Bureau of Soils, 1901. 3 For further results see J. C. Martin and A. W. Christie, Journ. Ag. Res.,. 1919, 18, 139. 126 SOIL CONDITIONS AND PLANT GROWTH jj O Is M P O CO c E •5 g o o 8 « H P Tl- co 8 ^ 1" k H CO H YOUCOS, 4< If fl 0 S, s a 0 £ J3 o-J 01 Q O^ O PQ M 1 *% P z gcfl oo s* bi o CO CM ^ s~*. 00 P •^« rf- j c fj ™ U") 0 III CO 00 o |13 vo 00 8 8 C/J »^ w ^ CO vp 00 CO . c/5 M vo" ob 0 H H (£ ^ CO en 00 H CO c c^ t* P o en w Its' CM o o c/5 H 00 H o cf CO H s CO O I 0 M 0 m O S* ^ .S >, CTi M a .2 CO ^" oU o H n » 8 00 VO CM^ 00 CM « H 3 C/3 ob H M C? CM M 55 & O 55 0 • O M M M o> CM 00^ o> CO O O P cT M 1. 11 0 CO 0 a ^, ^ C/3 ^ co JO X V in K X w X u «3 bo o 6 ' H 3 s »O rt g 0) to i- || « rt . 1 " w C "o c . 'e'S §/§ ' CJ ^JD '?• c &D . CL. ** o S ^ § ll sS \3 D 0 H2 J3 JO O« a U w rt oo en PU fe CL, THE COMPOSITION OF THE SOIL 127 was generally from cri to I atmosphere. The seasonal varia- tions are shown in Fig. 16. • These various methods naturally give different results. The general conclusion, however, seems to be that the solution in a normal agricultural soil contains mainly calcium nitrate and bicarbonate ; with some organic matter, sodium, mag- nesium, silica, chlorine, sulphuric acid, less potassium, a trace of ammonia, and only little phosphate. In ordinarily moist 24W*s 32WKS. 40WKS. JuUO.Jui24.Au62J. Ocr23 Dtc.18. F«.I7 MAY I Time from planting. FIG. 16. — Variations in concentration of soil solution with crop and season, show- ing that the barley crop much lowers the concentration even after its removal in August. On the vertical axis one-quarter of each freezing-point depression is plotted :* an approximate estimate of the corresponding osmotic pressure is obtained by multiplying the plotted value by 50 (Hoagland and Sharp, 1366). soil it has a concentration of the order of OT to I'O per cent., or some O'5 to 5 atmospheres osmotic pressure ; this varies however, with the rainfall and the manuring. The propor- tions of the components also varies, again changing with the season and the manuring. The nitrates are perhaps the most variable constituents, but the calcium and potassium also vary both relatively and absolutely — excepting in van Zyl's ex- periments. The numerous analyses of land drainage water that have 128 SOIL CONDITIONS AND PLANT GRO WTH been made in this country and on the Continent throw some light on the composition of the soil solution. As might be expected from the known absorptive properties of clay and of humus, drainage water contains mere traces of NH4 and PO4, and only little K ; it contains chiefly carbonic acid, SiO4, Cl, SO4, NO3, Ca with some Fe, Mg, and Na. Typical analyses. are given in Table XXXVII. TABLE XXXVII. — ANALYSIS OF DRAINAGE WATERS FROM CULTIVATED FIELDS r PARTS PER MILLION OF SOLUTION. Rothamsted: Broadbalk Field.i Field at Gottingen.2 No Manure. Dung. Complete Artificials. Highest Result. Lowest Result. Plots 3 and 4. Plot 2. Plot 6. CaO 98'l 147-4 J43'9 184 157 MgO 5'1 4'9 7*9 46-4 31*3 K,0 17 5*4 4*4 37 1-7 Na2O 6-0 137 10-7 Fe203 57 2-6 27 — — Cl . 10-7 20-7 20*7 — — S03 247 1 106-1 73*3 59*2 43 '5 P205 •6 — i'54 — — Si02 10*9 357 24-7 — — N as NH3 'J4 •20 •24 — — N as Nitrate 15-0 62*0 32-9 8-2 I'D Organic matter, CO2, etc 677 77'3 84-6 — — Total solids . 246-4 476-0 407-6 — — It will be observed that the total concentration of the Rothamsted drainage water varies from -02 to "05 per cent. Organic Matter. The distinguishing characteristic of soil is that it contains part of the complex material synthesised by plants. This material affords energy to numerous micro-organisms, and is gradually converted by them into simple substances appro- 1 A. Voelcker's analyses of five samples collected between 1866 and 1869 (289^); 2 Von Seelhorst's analyses of samples collected weekly, or fortnightly, from a field between August, 1899, and August, 1900 (258) THE COMPOSITION OF THE SOIL 129 priate for plant nutrition. We may look upon its constituents as taking part in a perpetual cycle : in one stage nourishing the growing plant and storing up the energy of sunlight, in the other stage nourishing micro-organisms and liberating energy. In addition, it has important physical effects on the soil. Unfortunately, not much is known of the highly com- plex components of the plant and even less is known about the important organic substances of the soil. The difficulty of working with insoluble, unstable bodies mingled with twenty times or more their weight of sand, silt, and clay has hitherto proved almost insuperable. The ideas current in the textbooks go back' to the time before organic chemistry arose, and have come down direct from C. Sprengel (270^), Mulder (205) and Detmer (84). We can thus only speak in the most general terms about what is admittedly the characteristic component of soil. Two great groups are to be carefully distinguished : one furnished by recent generations of plants ; the other deposited with the soil during its forrricition, and therefore as old as the soil itself. Unfortunately, no actual method of separation is known, but some idea of the amount and properties of the original organic matter can be obtained from a study of the subsoil at depths below the root-range of plants. Ten feet or more below the surface, sandy subsoils usually contain less than -oi per cent of nitrogen and clays less than -05 per cent., but shales con- tain more than O'l per cent. The percentage of carbon fluctuates, but is usually five to ten times that of nitrogen (2000). Now these values are about one-tenth to one-fifth of those obtained in the surface soil, so that at the very outside, and assuming there has been no decomposition, not more than I o to 20 per cent, of the surface organic matter is original. The organic matter furnished by recent vegetation may roughly be classified as: (i) material that has not yet had time to decompose and still retains its definite cell structure ; (2) partially decomposed and still decomposing material ; (3) simple soluble decomposition products ; (4) plant or animali constituents not decomposable in the soil. 9 130 SOIL CONDITIONS AND PLANT GROWTH The undecomposed material is important as the reserve supply for the entire chain of reactions to be considered later. It also has a certain mechanical effect in opening up the soil and facilitating aeration and drainage, an effect useful on clays but often harmful on sands where these processes already tend to go too far. The partially decomposed material forms a particularly vague and indefinite group containing all the non-volatile products of bacterial, fungal, enzymic and other actions on the plant residues. It shades off in one direction into the simple soluble decomposition products, and in the other into undecomposed plant fragments, so that it cannot be sharply defined or accurately estimated. A detailed study of the group being thus out of the question, we must ascertain in the first instance what part it plays in determining those relation- ships between the soil and the living plant that it is our business to study, and then, when we know what to look for, try to discover what constituents are important from our point of view and fix attention on them. For the preliminary inquiries recourse is had to the indirect method of correlation already used in ascertaining the properties of the mineral fractions of the soil. Numerous studies on these lines have proved that this group (or some component) possesses at least six properties not usually shown by the undecomposed plant residues. 1. It gives a dark brown or black colour to the soil. 2. It can withdraw various ions — NH4, K, PO4 — from their solutions. The experiments of van Bemmelen (19, 21) indicate a complete parallelism with clay in this respect. Baumann and Gully (10) and Oden (218^) show that un- decomposed sphagnum can absorb ions from solutions, but the phenomena differ in detail from those shown by humus. 3. It causes the soil to puff up, or in the expressive phrase of the German farmer, to "ferment" (Bodengarung], and so leads to an increase in the pore space (see p. 220). From this results a marked improvement in the tilth and general mechanical condition. The Rothamsted mangold plots re- THE COMPOSITION OF THE SOIL 131 ceiving no organic manure, and therefore poor in partially decomposed organic matter, get into so sticky and "un- kindly" a state that the young plants have some difficulty in surviving however much food is supplied, and may fail altogether if bad weather intervenes in the spring (as in 1908 and 1911); the dunged plots which are rich in this group are much more favourable to the plant and never fail to give 9* 132 SOIL CONDITIONS AND PLANT GROWTH a crop. But the puffing up or " lightening " may go too far, and sometimes causes much trouble in old gardens that have long been heavily dunged. 4. It increases the water-holding capacity of the soil. The amounts of moisture present in adjacent plots at Rothamsted are shown in Fig. 17, from which it appears that the plot annually receiving farmyard manure contains normally 3 or 4 per cent, more water than the adjoining plot receiving no organic manure. The variations in water content follow very closely the variation in the amount of organic matter present. So marked are these physical effects that if 1 5 or 20 per cent, of organic matter is present in a soil the operation of other factors ceases to count for much, and the distinctions between sands, loams, and clays are obliterated. Thus, much of the famous Red River prairie soil of Manitoba is identical in mineral composi- tion with certain poor infertile Wealden soils, but the presence of 26 per cent, of organic matter completely masks the harm- ful effect of the clay and fine silt. A similar pair of soils, owing their difference in agricultural properties to their different organic matter content, have been analysed by C. T. Giming- ham (106) (Table XXXVIII.) TABLE XXXVIII. — EFFECT OF ORGANIC MATTER l ON THE TEXTURE OF SOILS. Good Texture. Poor Texture. Good Texture. Poor Texture. Manitoban Prairies. Weald Clay. (Reported by C. T. Gimingham.) Fine gravel Coarse sand Fine sand Silt Fine silt Clay . 1-6 3'8 17-1 28-2 23'3 •5 I tO 2 10 „ 12 20 „ 30 25 „ 30 2O ,, 25 •6 4'3 n«2 287 23-8 8-4 13-8 26-5 25-0 Loss on ignition . 26-3 5 „ 8 19-8 I4-5 1 Measured by the loss on ignition. THE COMPOSITION OF THE SOIL 133 5. It swells when wetted1 6. Although the group is essentially transitional it has a certain degree of permanency and only slowly disappears from the soil. It disappears more rapidly from chalky and sandy soils than from loams and clays. These properties greatly enhance the fertility of the soil, and in most schemes of husbandry definite arrangements are made to keep up or even increase the supply of organic matter, while in forests the removal of leaves and other decomposable material has led to such bad effects that in all state forests of France, Belgium, Germany, etc., the practice is absolutely forbidden. The group of substances possessing these properties is generally called " humus," and so long as the word is used in a collective sense as a convenient label it may be retained. But the practice has been responsible for a good deal of loose thinking, because it obscures the fact that the group is an in- definite and complex mixture, and gives instead the impression that it is a single definite substance. From these half-dozen general properties we may infer that humus is a brown, slowly oxidisable colloid. Careful ex- amination of a number of soils in their vegetation relationships has shown that there must be several distinct types of humus but the laboratory methods are not yet as sensitive as the growing plant and fail to explain some of the differences ob- served by ecologists. We have to look to field observations for the facts on which to base a scheme of classification. Peats. — Most of the recorded investigations are on peats 1 Peat shows this phenomenon in a marked degree ; indeed, after heavy rain- fall inadequately-drained peat bogs may swell so much as to overflow into valleys with disastrous results. After drainage, however, drying and shrinkage set in, followed by a slow but steady erosion as air penetrates into the newly-formed spaces and starts the oxidation processes. When Whittlesey Mere was drained in 1851 a pillar was driven through the peat into the underlying gault, and the top of the pillar was made flush with the surface of the soil. So great has been the subsequent shrinkage that over 10 feet of the pillar is now out of the ground and the process has not yet reached its limit, for a perceptible shrinkage took place during the dry season of 1911. 134 SOIL CONDITIONS AND PLANT GROWTH of various kinds, and in this country the work has been done largely by Dr. Moss and other members of the British Vege- tation Committee (279). At least three great classes and another two that may be transition forms were recognised : — I. Dry peat (the German Trockentorf) found on heaths in relatively dry regions and on poor sandy soils. It is often only a fraction of an inch in thickness, and is largely formed by lichens and mosses (e.g. Cladonia rangiferina, Polytrichum piliferum, and others). The dominant plant is Calluna. Much of the organic matter of heath soils, however, often consists of undecomposed vegetation, e.g. bracken fronds, etc. la. In wetter districts the layer of peat becomes thicker, and no doubt changes in composition, but it still carries essentially " heath " vegetation, although it shows resemblances to 2. 2. Wet peat (the German Hochmoor) formed in wet tracts or regions of high rainfall, and accumulating to so great a depth that it entirely determines the character of the vegetation whatever the underlying rock. It receives no supplies of spring or underground water, and, therefore, no dissolved salts ; the drainage water is acid and poor in soluble mineral matter. Two great divisions are recognised : lowland moors or mosses, formed in low-lying wet places largely from sphag- num, cotton grass (Eriophorum\ and Calluna; and upland moors > formed mainly from Eriophorum spp. and Scirpus ccespitosus in elevated districts of high rainfall. 3. Fen (Niedermoor in German, see (299)) formed from a calcicolous vegetation (Phragmites, Cladium, Scirpus, Carex, etc.), in presence of calcium carbonate and soluble mineral salts, showing no acid properties and giving alkaline drainage waters. 3#. Carr, genetically related to the fen, containing much decaying tree residues, and formed in what at one time was a marshy wood. Between fen and peat several transition forms have been described by Weber (299) and also recognised in England. Some of our moors are built up on older fens. THE COMPOSITION OF THE SOIL 135 Forbes l recognises three classes of peat in Ireland : — 1. Mountain peat, corresponding with the wet peat low- land moss of the British Committee, which originates wherever the conditions are too sterile or the subsoil too impervious or water-logged to allow deep-rooted vegetation to flourish, and where, therefore, shallow rooted plants come in and, on dying, form a layer of organic matter on which sphagnum, cotton grass, etc., begin to develop. This occurs above the 800 feet level in most parts of Ireland, but in the west it often covers the entire surface down to the sea-level. 2. Marsh peat, corresponding with the British fen, which arises from reeds, sedges, rushes, etc., and which, so long as the water contains lime and nutrient salts, is as favourable a medium for plant growth as ordinary soil, though it affords no root-hold for trees, so that they are liable to be overturned in strong gales. This kind of peat forms the basis of all the low- land bogs in Ireland and of many of the small bogs in mountain districts. 3. This marsh peat finally becomes so consolidated with time and pressure that it loses connection with the water table, and a surface swamp forms on which a sphagnum bog of the "mountain type" arises. This, therefore, becomes similar in character to the first group : it differs, however, in its uniform- ity of growth, being higher in the centre than at the margins where soil water can get in and where, therefore, decomposition is more rapid. The Scotch peats have been described by Lewis 2 and the Yorkshire moors by Elgee.3 Within each of the great classes described above several subdivisions are recognised, but how far they arise from differ- ences in the organic matter, or from other differences, cannot yet be ascertained. 1 A. C. Forbes, Clare Island Survey, 1914, 9 (Proc. Roy. Irish Acad., , 31)- 2 Trans. Roy. Soc., Edinburgh, 1905, 41, 699-724; 1906, 45, 335-360; 1907, 3-70 ; 1911, 48, 793-833- 3 F. Elgee, The Moorland of North-Eastern Yorkshire. London, 1912. 1 36 SOIL CONDITIONS AND PLANT GROWTH Numerous chemical analyses of the peats have been made by Tacke at Bremen (278), Gully (i 16), Michelet and Sebelien (199) and others.1 As a general rule, though with many exceptions, the per- centage of nitrogen varies with that of the lime, and the high moor contains least of these, the low moor a larger quantity, and the fens a still larger quantity (Table XXXIX.). TABLE XXXIX. — NITROGEN AND CALCIUM CONTENT OF VARIOUS PEATS. Nitrogen CaO per cent, in per cent, in Observer. Dry Matter. Dry Matter. I High moor Bremen I-I4 °*44 Tacke Lancashire 0-85 O'll Russell and Prescott Low moor Bremen 1*62 1-24 Tacke Cheshire O'QI 0*16 Russell and Prescott Fen Norfolk 2-85 7'5 » >i The investigations on the cause of the acid properties of peat are dealt with on p. ill. Humus of Forest Soils. — An admirable series of studies has been made by P. E. Miiller (206) of the types of humus oc- curring in the Danish forests.2 In beech forests he found two types, which he called mull and torf, our nearest equivalents being mould and peat. On mull the characteristic plants were Asperula odorata with its associated Mercurialis perennis, Mi- lium effusum, Melica uniflora, Stellaria nemorum, and others, moss being absent. The mull itself was only a few inches thick, and was underlain by I to 5 feet of loose soil, lighter in colour than mull, but almost equally rich in organic matter ; still lower came a compact but porous layer of soil. The surface of the soil was covered by a layer of leaves, twigs, etc. Earthworms were numerous throughout ; their potent influence in the soil had recently been shown by Darwin (76). Detailed chemical examination was not made : it was shown, however, that mull was free from acid and contained about 5 to I o per 1 See Jahresber. Agrik. Chem., 1878, p. 29 ; 1904, pp. 87, 88, etc. 2 Other investigations on forest humus are dealt with by Ramann, Forstliche Bodenkunde u. Standortslehre, 1893. THE COMPOSITION OF THE SOIL 137 cent, of organic matter completely disintegrated and most intimately mingled with the mineral matter. Torf differed completely. The characteristic plant was Trientalis europaa with the associated Aira flexuosa and moss, but surface vegetation was not very common. The loose layer of leaves was absent, and the torf itself was so tough and compact that rain water could not readily penetrate. Below it was a layer of loose, greyish sand (bleisand), and lower still a layer of reddish soil (roterde), or else a pan (prtsteiri). Prac- tically no earthworms were found in the torf, but there were numerous moulds and fungi, Cladosporium humifaciens Rostrup and Sorocybe Resince Fr. being perhaps the commonest. Torf was acid, contained about 30 per cent, of organic matter not completely disintegrated, nor well mixed with the mineral matter. It was not very favourable to the growth of young trees, and the forest tended to become an open heath as the old trees died. The distribution of mull and torf did not seem to be deter- mined by the nature of the soil, or by the amounts of soluble alkali salts or calcium carbonate present, but rather by the nature of the living organisms in the soil. Animals, especially earthworms, gave rise to mull, fungi produced torf. If the conditions were favourable to earthworms mull was therefore found, if not, torf was produced. The nature of the vegetation was also a factor : oak only rarely formed torf but commonly gave rise to mull, at least two varieties of which were observed ; pine, like beech, could form either torf or mull, while Calluna vulgaris and V actinium myrtillus generally produced torf. Humus of Field Soils. — It is commonly assumed that the humus of field soils is of the same nature as that of peat, fen, or forest. There are undoubtedly certain properties in common. Alway and Neller (3^) find that the differences in moisture content of adjacent plots of varying organic matter content can be explained on the assumption that soil organic matter has the same water-holding capacity as the most absorbent peats, i.e. three to four times its own weight. The difference in organic matter was 1-37 per cent. ; the difference in moisture 138 SOIL CONDITIONS AND PLANT GRO WTH was up to 4 per cent. A close study jointly by a botanist and a chemist would carry the problem much nearer to a solu- tion. The observations indicate that the mixture we have agreed to call humus does not vary erratically from field to field, but produces much the same effects over any tract where similar soil and climatic conditions prevail. The mixture changes when a new set of conditions occurs, but its general character persists over a certain range and then it merges into another type. Two great divisions of the types can already be recog- nised : a neutral group commonly spoken of as neutral humus or " mild humus," and a group reacting like an acid and called " sour humus," acid humus, or by German writers Rohhumus. TABLE XL. — ANALYSES OF THE ORGANIC MATERIAL EXTRACTED BY ALKALIS FROM SOIL (OFTEN CALLED HUMUS, SOLUBLE HUMUS, ACTIVE HUMUS, MATIERE NOIRE, ETC.). Source. Carbon. Hydrogen. Oxygen. Nitrogen. Ash. Observer. Arable land 56-3 4'4 36'0 3'3 -0 u Mulder (205) Garden soil 56-8 4 '9 34*8 3*5 ~ "& Pasture land 56-1 5*3 6-1 rt w , Peat 59'o 47 327 3*6 °3| M ,, 57'4 4'6 — F. Fuchs1 Rich prairie soil Soil never cultivated 44' I 37 6-0 28-6 35*2 10-4 8-1 12*2 6-6 Snyder (268) Cultivated subsoil (a) 48-2 5 '4 33*2 9'i 4'2 f| " Humic acid " from 50-1 4-8 337 6-5 4'9 Berthelot and sugar . " Humic acid " from 66-4 4'6 29-0 — — Andre (30-32) Berthelot and compost " Humic acid " from 53'3 5-6 37 '5 3-6 — Andr£ (30-32) sugar through K Robertson, Irvine salt . 6470 4'55 3071 — — and Dobson 2 "Humic acid" from sugar through NH4 Robertson, Irvine salt 64*74 4*69 29-81 0-76 — and Dobson 2 " Humic acid " from Robertson, Irvine soil through K salt 56-67 5'i6 35*68 2-49 — and Dobson 2 " Humic acid " from soil through NH4 Robertson, Irvine salt 54*29 4*94 38-16 2'6l — and Dobson 2 1 Chem. Zeit., 1920, 44, 551. The molecular weight was about 680. 2 Biochemical Joum., 1907, 2,458. For other studies see Mary Cunning- ham and Chas. Doree, Trans. Chem. Soc., 1917, III, 589-608. THE COMPOSITION OF THE SOIL 139 In order to understand much of the chemical work that has been done, it is necessary to remember that the older chemists regarded humus as a single definite substance, or as a mixture of two or three definite substances. The favourite view was to consider neutral humus as the calcium salt of t( humic acid," which could be extracted from the soil by dilute alkalis after preliminary treatment with hydrochloric acid. On acidifying this alkaline extract the " humic acid " came down as a brown colloidal precipitate. Acid humus was the actual humic acid itself. It was further supposed that humic acid could be synthesised by boiling sugar with hydrochloric acid, on the singularly inadequate ground that the product thus obtained is also a brown colloid, soluble in alkalis and pre- cipitated by acids. Numerous analyses have been made both of the natural and the synthetic " humic acid," some of which are given in Table XL.1 Robertson, Irvine, and Dobson give the formula C39H32OU to humus from sugar and C24H24OnN to the soil humus ob- tained through the potassium salt ; the latter may be related to the acid prepared from Dopplerite by Mayer,2 containing 1 Many partial analyses have been made. Cameron and Breazeale (650) in nineteen samples obtained percentages of carbon varying from 33*3 to 50*1^ whilst Hilgard (1336) found the nitrogen content to be : — Percentage of Humic Acid in the Soil. Percentage of Nitrogen in the Humic Acid. Soils of the arid regions (decom- position rapid) Soil of the sub-irrigated arid regions Soil of the humid regions (de- composition slow) . 0*20 to 3*0 0-36 „ 2-0 I'O ,, lO'O 87 to 22-0 (average 15-2) 5-4 „ 10-8 ( „ 8-4) 17 ,. 7*o ( ,, 4*2) C. B. Lipman, however, is convinced that there is an error here : he has not found more than 7 or 8 per cent, of N in the humus from arid soils : this is no more than occurs in the humus from humid soils (Soil Science, 1916, i, 285-290). He also finds no difference in general nitrifying power (Joum. Ag. Research, 1916, 7, 47). Westermann (302) has analysed humus from the Danish moors, and Gully (116) has studied humus from South Bavarian moors. Many of the older analyses have been collected by Wollny (318). ^Landw. Versuchs, 1883, 29, 313. 140 SOIL CONDITIONS AND PLANT GROWTH 58-2 per cent, carbon and 5*0 per cent, hydrogen. In view, however, of Hoppe-Seyler's discovery, confirmed by Oden,1 that soil humus is partly soluble and partly in- soluble in hot alcohol, it is obvious that at least two sub- stances must have been present in the material analysed in the above cases and the validity of any formula becomes doubtful.2 The purification of humus for investigation in the labora- tory is rendered extraordinarily difficult by its colloidal nature, which was demonstrated by van Bemmelen in a remarkable paper in 1888 (19). Baumann's researches (10) have carried the subject a good deal farther, and it is now known that " humus," freshly precipitated by acids from an alkaline ex- tract of soil, compost, etc., possesses the following colloidal properties : — (1) Very high capacity for retaining water. (2) Extraordinary shrinkage on drying. (3) Is coagulated by acids and salts, the electric current and frost. (4) Reversibility, i.e. the freshly precipitated material re- dissolves when the precipitant is washed away. (5) Decomposes salts — calcium carbonate, calcium phos- phate, etc. (6) Forms difficultly soluble and easily decomposable col- loidal mixtures with other colloids. (7) Masks certain ion reactions (e.g. Fe cannot be detected by potassium ferrocyanide, etc.). (8) Forms absorption compounds. The colloidal nature of the humus accounts for much of the failure of the earlier chemical work, and it adds to the difficulties of those modern chemists who have had the courage to tackle the problem. In spite of these difficulties several good investigations have been made. lBer., 1912, 45, 651. 2 Miklauz (Zeit. f. Moorkultur u. Torfverwertung, 1908, 285) and Mayer both showed that variations in the time or the method of extraction cause differ- ences in the results even from the same soil. Sostegni (Landw. Versuchs-Stat., 1886, 32, 9) had shown that humus is readily fractionated. THE COMPOSITION OF THE SOIL 141 Schreiner and Shorey (250) have attempted a resolution of the " humic acid " and of the " crenic acid " (the part not precipitated by HC1) and have obtained the following sub- stances from the alkaline or alcoholic extract : — Substances Precipitated by Acids (the so-called Humic and Ulmic Acids). Substances not Precipitated by Acids (the so- called Crenic and Apocrenic Acids). Resin acids. Resin esters. Glycerides. Paraffinic acid, C24H48O2, m.pt. 45°-48°, probably identical with the acid formed on treating paraffin with fum- ing nitric acid. Lignocericacid, C24H48O2, m.pt. 8o°-8i°, isomeric with above. Agroceric acid, C21H42O3, m.pt. 72°-^°, a hydroxy fatty acid. Agrosterol, C26H44O, m.pt. 237°. Phytosterol, C^H^O . H2O, m.pt. 135°. Both of the cholesterol group. Dihydroxystearic acid, C18H36O4,.m.pt. g80-g9°, identical with the acid formed on oxidising elaidic acid. a-Picoline y-carboxylic acid, C7H7O2N, m.pt. 239°, identical with the acid formed on heating uvitonic acid to 274°. Xanthine, C5H4O2N4. Hypoxanthine, C5H4ON4. Cytosine, C4H5ON3 . H2O. Histidine, C6H9O2N3. . Arginine, C6H14O2N4. A pentosan. None of these, however, is the black substance which is the real characteristic of " humus," and until recently this had eluded investigation and was vaguely described as a " melanoid body ". It can be split up by solvents into the following sub- stances which are no doubt further resolvable : — Insoluble in NH4HO I Soluble in acids {Mulder's apocrenic acid) Pptd. by acids Humus Humin Insoluble in alcohol • Soluble in alcohol Humic acid Hymatomelanic acid1 (see Schreiner and Shorey) Insoluble in pyridine Soluble in pyridine The substances are probably related : at any rate they occur together. There is good evidence that soil humus is formed mainly from cellulose : no great amount is produced from protein. Addition to the soil of wool, silk waste, flour, and lucerne meal causes no gain, but on the contrary loss of 1 For Hoppe-Seyler's hymatomelanic and fulvic acids see Od£n, 2190. 142 SOIL CONDITIONS AND PLANT GROWTH humus.1 This formation from cellulose suggests an interest- ing possibility as to its origin. It has already been stated that a substance resembling humus is formed on heating sugar with hydrochloric acid (e.g. 250 grms. dextrose with I litre 24 per cent. HC1 for 12 hours). Maillard has shown (189^) that the reaction is general and not confined to strong acids. He was led by his investigations on the function of alcohol in the synthesis of albuminoids to study the action of a typical amino acid, glycocoll, on d-glucose. When I part of glycocoll and 4 parts of glucose were added to 3 to 4 parts of water and heated on the water-bath to facilitate dissolution the re- action mixture rapidly changed to yellow and finally to dark brown, CO2 being given off. The action was a true condensa- tion and not an oxidation, for practically no oxygen was absorbed. The black material had the properties of soil humus : it was insoluble in water, dissolved in alkalis, but was reprecipitated on adding acids, and contained 4-4 to 6 per cent, nitrogen : this question is discussed at length in the later paper (189^). The reaction turned out to be general, and was given by all the amino acids tried (glycocoll, sarcosine, alanine, valine, leucine, tyrosine, glutamic acid and also the polypeptides) and all the sugars. Xylose and arabinose reacted especially quickly, fructose, galactose, glucose, and mannose less quickly, and lactose and maltose still less, while saccharose acted only slowly. The reaction proceeds at ordinary tem- peratures, but more slowly. Now it is certain that both amino acids and pentose com- pounds are formed during the decompositions in the soil, the former from the proteins and the latter from the celluloses, and it is highly probable that the black substance is formed by their interaction. A detailed study of the reaction was there- fore made at Rothamsted by V. A. Beckley (12) ; setting out from an observation by Fenton, he showed that sugars, on treat- ment with acids, give rise to hydroxymethylfurfuraldehyde, which readily condenses to form a substance closely re- 1 Gortner, Soil Sci., 1917, 3, 1-8. THE COMPOSITION OF THE SOIL 143 sembling humus. He also found indications of hydroxy- methylfurfuraldehyde in a dunged soil and in rotting straw in which humus was being produced. He suggests, therefore, that the formation of humus in the soil proceeds in two stages : — Carbohydrate (Cellulose, etc.) ( + Aminoacid)-> Hydroxymethylfurfuraldehyde. Hydroxymethylfurfuraldehyde + amino acid -> humus + furfural + CO2 by condensation. Eller and Koch,1 on the other hand, suppose that humus is formed by the oxidation of quinones which arise by the elimination of water from hexoses : — C6HI206 = 4H20 + C,H4Of Beijerinck (17) has already found quinone among the products of certain soil organisms working in culture solution. As to the constitution of humus little is known. Sven Oden of Upsala (218*2) has adduced evidence from the con- ductivity of the solution in ammonia that the alcohol insoluble portion is a true acid, as was supposed by the older chemists and by Tacke (278). Ehrenberg and Bahr (93» .... NH4OH 0-147 0*461 Brustlein »> .... NH4OH 0-054 0-386 We still, however, have to account for the fact that the absorbed bases displace an equivalent amount of some other bases from the soil — a procedure which would be wholly un- necessary if nothing but adsorption were involved. This is done by supposing that only the hydroxide is absorbed : the acid radicle in general is not : it therefore dissolves out some of the bases from the soil. As this is a purely chemical re- THE COLLOIDAL PROPERTIES OF SOIL 157 action the amount of base brought out is equivalent to the acid set free, i.e. to the amount of base adsorbed by the soil. Thus the modern position is essentially that of Knop, but the idea has been expanded, and, above all, the phenomena have been connected up with a wide range of others. Miyake's experiments (2020) indicate that the rate of ab- sorption of the ammonium ion by soil follows the ordinary time rate for the diffusion of liquids into absorbing substances, viz. x = ~K.tm where x = the amount absorbed, t = time and K and m are constants. The rate, however, is affected by the presence of other ions. The absorption of potassium by soil has been studied by Schreiner and Failyer,1 by Patten and Waggamann 2 and re- cently by McCall, Hildebrandt and Johnston.8 The Action of Dilute Acids on Soils. It will be shown later that the reaction between dilute acids and soil is of very great importance to the soil chemist in enabling him to form some estimate of the amounts of the mineral plant nutrients present. The reaction ought in principle to be simple, but numerous investigations by Hall and Amos (i2O 7> 529- Summaries of other results are given by A. W. Blair and H. C. McLean, Soil Sci., 1917, 4, 283-293. I 82 SOIL CONDITIONS AND PLANT GROWTH TABLE LI. — LOSSES OF NITROGEN FROM CULTIVATED SOILS, BROADBALK WHEAT FIELD, ROTHAMSTED, FORTY-SEVEN YEARS, 1865-1912. Rich Soil, Plot 2, Ib. per Acre. Poor Soil, Plot 3, Ib. per Acre. Nitrogen in soil in 1865 . Nitrogen added in manure, rain (5 Ib. per annum) and seed (2 Ib. per annum) •175 per cent. = 4,340 9,730 •105 per cent. = 2,720 330 Nitrogen expected in 1912 Nitrogen found in 1912 . Loss from soil . Nitrogen accounted for in crops Balance, being dead loss . Annual dead loss 14,070 •245 per cent. = 5,730 3,050 •103 per cent. = 2,510 8,340 2,55o 540 750 5,790 123 -2IO -5 Experiments of this kind have led to the conclusion that some gaseous product is formed in addition to nitrates, and, as no sufficient amount of ammonia can be detected, it is supposed that gaseous nitrogen is given off. The conditions for this decomposition appear to be copious aeration, such as is produced by cultivation, and the presence of large quantities of easily decomposable organic matter. Now these are precisely the conditions of intense farming in old countries and of pioneer farming in new lands, and the result is that the reserves of soil and manurial nitrogen are everywhere being depleted at an appalling rate. Fortunately there are recupera- tive actions, but one of the most pressing problems at the present time is to learn how to suppress this gaseous de- composition and to direct the process wholly into the nitrate channel. It is evident that there must be some recuperative agency or the stock of soil nitrogen, which is never very great, would long ago have disappeared in old countries. Experiment has shown that soil gains nitrogen when it is allowed to remain undisturbed and covered with unharvested vegetation as in natural conditions. On the Broadbalk field a third plot adjacent to the two already mentioned was in 1882 allowed to go out of cultivation and has not been touched since ; it CARBON AND NITROGEN CYCLES IN THE SOIL 183 soon covered itself with vegetation, the leaves and stems of which go to enrich the soil in organic matter. The gain in nitrogen is very marked, as shown in Table LI I. The gain is much influenced by the amount of calcium carbonate in the soil, and is considerably less on another plot in Geescroft field TABLE LII. — GAINS IN NITROGEN IN SOILS PERMANENTLY COVERED WITH VEGETATION — ROTHAMSTED SOILS LEFT TO RUN WILD FOR 22-24 YEARS. HALL (i2o/). Broadbalk : CaCO3, 3*32 per Cent. Geescroft: CaCO3, 0-16 per Cent. Carbon, per Cent. Nitrogen, per Cent. Carbon, per Cent. Nitrogen, per Cent. 1881. 1904. 1881. 1904. 1883. 1904. 1883. 1904. ist 9 inches 2nd 9 inches 3rd 9 inches I-I4 •62 •46 1-23 •70 "55 •108 •070 •058 •145 •095 •084 Pit •60 *45 I-49 '63 "44 •108 •074 •060 •131 •083 •065 'Approximate gain in nitrogen, Ib. per acre Lb. per acre per annum . , V .. 2 20O 917 - . 1400 60 LAND LAID DOWN TO GRASS IN 1856 AND MOWN ANNUALLY (DR. GILBERT'S MEADOW, ROTHAMSTED). Per cent, of N in top 9 inches 1856. [•152] 1 1879. •205 1888. •235 1912. •338 where only little calcium carbonate is present ; whether this is due to any specific action, or to the changed physical con- ditions brought out by decalcifying a soil, is not clear. Gains of nitrogen also take place on land covered with perennial grasses and clovers even when the crop is mown or grazed. On clay pastures dressings of basic slag have been found to increase the nitrogen content of the soil, whilst potassium salts, such as kainit, have had the same effect on sandy soil. In all these cases leguminous plants are present in greatest extent where the gains in nitrogen are greatest, but they are not necessarily the only nitrogen fixers. 1 Estimated. 1 84 SOIL CONDITIONS AND PLANT GROWTH Advantage is taken of this recuperative effect in all rotations by alternating the periods of arable cultivation with periods of " rest " in grass and leguminous crops. In the old Norfolk rotations one year in four was given up to clover,1 in modern rotations the clover or "seeds" mixture is some- times left for two or three years before it is ploughed up, so that the enrichment may become more marked. Mr. Mason at Eynsham Hall 2 considerably enriched in nitrogen some poor Oxford clay by the growth of lucerne. But the gain in nitrogen does not go on indefinitely ; in course of time a point of equilibrium is reached, higher or lower according to the soil conditions, where further gains are balanced by losses, so that the nitrogen content remains constant. Thus there is an upper as well as a lower limit to the nitrogen content of the soil, the actual values depending on the soil conditions. Between these limits the nitrogen con- tent may be maintained at any desired level, high when the ground is left in grass and leguminous crops, low when the ground is continuously cultivated. Unfortunately, on our present knowledge it is impossible to maintain a high content of nitrogen on cultivated land except at a wasteful expenditure of nitrogenous manure. Tentative determinations of some of these limits are ; — Black Organic Soils (containing more than 10 per Cent, of Organic Chalk Soils.s Loams.3 Sands.3 Matter). Upper limit I •42 •25 •20 Lower limit •25 •13 •09 •03 The reactions involved in all these changes are obviously complex, but they have been partially disentangled, and we can now pass on to a more detailed consideration of the separate changes. 1 It was known to the Romans that vetches were a good preparation for wheat (cf. Virgil, Georgics, Book I., lines 73 et seq.). 2jfourn. Roy. Agric. Soc., 1904, Ixv., 106-124. 3 Containing less than 10 per cent, of organic matter CARBON AND NITROGEN CYCLES IN THE SOIL 185 The Formation of Ammonia. Ammonia is in all probability an intermediate product in the formation of nitrates. It is formed in the soil from the proteins of plant residues or manures, and the process is effected mainly by micro-organisms, but not entirely, for it still continues at a diminished rate in presence of antiseptics. The reaction has not yet been completely elucidated. There is evidence of the production of amino acids which subsequently hydrolyse, or oxidise.1 Although amino acids are in general fairly stable, several reactions are known whereby they may be decomposed with production of ammonia : — R-CH-NH2-COOH + H2 = R-CH2COOH + NH3. R-CH-NH2-COOH + O2 = RCOOH + CO2 + NH3.2 R-CH-NH2-COOH + H2O = RCH-OHCOOH + NH3. R-CH-NH2-COOH + H2O = RCH2'OH + CO2 + NH3.3 It is not, however, known how they break down in the soil. Miyake (202^) has shown that the rate of production of ammonia can be expressed by the equation for autocatalytic reactions, which means that it is much affected by the ac- cumulation of the products. Both Gainey 4 and Neller 5 have studied the connection between the production of ammonia in soil and the evolution of CO2. The investigations by Marchal (190) in 1893 °f tne method of ammonia production in the soil are so complete that little has since been added to the facts he ascertained. Miintz and Coudon (207) had established the micro-organic nature of the process by showing that it was stopped by sterilisation. Marchal, therefore, made systematic bacteriological and myco- logical analyses of soils, and studied the action of the organ- isms thus obtained on solutions of albumin. Of the dozen ' See R. H. Robinson and H. V. Tartar, J. Biol. Chem., 1917, 30, 135-144. 2 Dakin, Jour. Biolog. Chem., 1908, iv., 63 ; Oxidation and Reduction in the Animal Body (Longmans, 1912). 3Ehrlich, Zeitsch. Verein. Riibenzucker Ind., 1905, 539-567. 4 Gainey, Soil Sci., 1919, 7, 293. . R. Neller, Soil ScL, 1918, 5, 225. 1 86 SOIL CONDITIONS AND PLANT GROWTH or so varieties that invariably occurred, practically all de- composed the albumin and formed ammonia. One of the mycoides group proved very vigorous and was studied in some detail. The process was considered to be a simple oxidation necessary to the life of the organism ; oxygen was absorbed and carbon dioxide evolved, the ratio NH3 : CO2 produced being i : 8*9. For complete oxidation of the carbon, hydro- gen, and sulphur of the albumin molecule the ratio would be I : io*3 ; but the change was known to be incomplete, and small quantities of leucine, tyrosine, and fatty acids could also be detected. Free oxygen, however, was not essential. When grown in a culture solution containing sugar and nitrate the organism took its oxygen from the nitrate, but it still produced ammonia. The energy relationships thus indicated were wholly over- looked by investigators for nearly a quarter of a century : it was not till 1916 that Doryland (86) showed their significance. The organism produces ammonia, not because it must, but because it can do so, and can leave ammonia in excess of its needs : its prime requirement is energy. If sources of energy other than proteins are supplied, e.g. carbohydrates, these may be used and then there is a decrease in ammonia production which may finally fall to nothing or become negative :• the ammonia producers then become ammonia absorbers just like the higher plants, and indeed they compete with growing crops. It follows that no simple numerical expression can be given for the ammonification of organic matter : the true constant is the requirement of energy. This might be ex- pressed in terms of protein if no other carbon compounds were present, but it is improbable that' any equivalent could be worked out for the complex mixture in ordinary soils.1 1 For other experiments on the effect of carbohydrate in reducing ammonia production see H. M. Jones, J. Infect. Dis., 1919, 19, 33, showing that Bac. proteus does not produce a proteolytic enzyme in presence of available carbo- hydrate ; J. G. Lipman and Blair, New Jersey Bull., 247, 1912, and Annual Kept., 1914, p. 220, ammonia producing power of soil is diminished by carbohydrates ; S. A. Waksman, J.Amer. Chem. Soc., 1917, 39, 1503, similar effects are produced in cultures of aspergillus ; I. J. Kliger, your. Bact., 1, 663, and Herman and Rettger, Jour. Bact., 1918, 3, 389. CARBON AND NITROGEN CYCLES IN THE SOIL 187 Subsequent developments have been entirely on the bacteriological side. A number of organisms are now known to produce ammonia from complex nitrogen compounds, but soil bacteriologists, notably J. G. Lipman and his school at New Jersey, have generally preferred to study the group as a whole, rather than isolate and study individual members. The method consists in inoculating soil into various arbitrary culture media, each designed to favour one group only of organisms. Some of the results obtained are discussed in Chapter VII. ; they show the method has value as a bacterio- logical test, but it has thrown little or no light on the processes going on in the soil. Indeed, so dependent is bacterial activity on temperature, concentration, reaction of medium (whether acid or alkaline), and other conditions, that it may be doubted whether any method of study, except in the actual soil itself, will further our knowledge of the reaction very much. Of the few attempts to study the individual species of organisms concerned in ammonification H. J. Conn's is per- haps the most notable. Contrary to the accepted view he claims that ammonia formation is mainly brought about by non-spore formers: B. mycoides^ generally regarded as one of the most common ammonia producers in the soil, h€ dismisses as ineffective. He maintains that of the eight ammonifiers studied by Marchal only one, B. fluorescens liq. (a non-spore former), is a typical soil organism. He describes in detail two organisms, Ps. fluorescens and Ps. caudatus^ which, while not very numerous in unmanured soil, multiply vigorously on addition of farmyard manure and also produce ammonia (70*:). Waksman has adduced evidence (see p. 259) that fungi, and especially actinomycetes, are active ammonia producers in soil. Nitrification. The ammonia formed by the action of soil bacteria, or added in manures, is changed to carbonate, which is then rapidly converted by Nitrosomonas into nitrite, and this by 1 Really a group, not a single organism. 1 88 SOIL CONDITIONS AND PLANT GROWTH Nitrobacter into nitrate, the changes proceeding so rapidly that only traces of ammonia or nitrite are ever found in normal arable soils (241 /). We may, therefore, infer that the production of nitrates is the quickest of the three reactions, the production of nitrites is slower, while the formation of ammonia is the slowest of all and sets a limit to the speed at which they can take place. Thus a measure of the speed at which nitrates are formed in soil does not measure the rate of nitrification, as is sometimes assumed, but the rate of ammonia production.1 The essential facts of nitrification are readily demonstrated by putting a small quantity of soil — '2 to '5 grm. — into 50 c.c. of a dilute solution of ammonium sulphate containing nutrient inorganic salts and some calcium or magnesium carbonate, but no other carbon compound.2 After three or four weeks at 25° the ammonia has all gone and its place is taken by nitrates. The conversion is almost quantitative, only an in- significant quantity of nitrogen being retained by the organisms. The course of the oxidation is unknown, and nothing intermediate between ammonia and nitrous acid has been detected. On general grounds one would look for hydroxy- lamine. A. Bonazzi 3 has made the significant observation that nitrififation in culture solutions is intensified by thorough aeration or continuous motion of the culture medium. Miyake has shown (202^) that the reaction resembles autocatalysis in its course. Omelianski could obtain no evidence of an oxidase in Nitrosomonas (2\gc). The action of both organisms seems to be entirely specific. Nitrosomonas oxidises ammonium carbonate and nothing else ; it will not touch nitrates, urea, or the substituted ammonias. Even 1 See also P. L. Gainey, Soil Sci., 1917, 3, 399-416. 2 Omelianski (2196) used 2 grms. each (NH4)2SO4 and NaCl, i grm. KHgPO4, -5 MgSO4, -4 FeSO4 in i litre of water, and added *5'grm. MgCO3 for each 50 c.c. of solution used. Nitrite formation goes on in this solution. For nitrate production he used i grm. each NaNO2and Na2CO3, '5 each KH2PO4 and NaCl, -4 FeSO4 and -3 MgSO4 in i litre of water. Ashby (ja and b) found that both processes went on simultaneously when he diluted the first of these solutions to one quarter the strength. 3 A. Bonazzi, y. Bact., 1919, 4, 58. CARBON AND NITROGEN CYCLES IN THE SOIL 189 ammonium salts are nitrified only in presence of a carbonate that can change them into ammonium carbonate (296). Nitro- bacter is equally specific, oxidising nitrites only and not ammonia. Addition to the solution of almost any carbon compound other than calcium or magnesium carbonates retards the rate of nitrification, glucose and peptone being particularly harm- ful (311^). Carbon dioxide suffices as the source of carbon for the growth of the organism. Godlewski1 showed that nitrification proceeds in solutions free from organic matter so long as the air supplied contained carbon dioxide, but stops as soon as the carbon dioxide is removed by passage over caustic potash. But the synthesis of complex cell substances from carbon dioxide is an endothermic process requiring a supply of energy. In the case of the green plant — the only other living thing known to utilise carbon dioxide — the energy comes from light, the transformer being chlorophyll. Here, however, light is out of the question, and is even fatal to the organism. Winogradsky (311*2) suggested that the necessary energy is afforded by the oxidation of ammonia and of the nitrite, and he traced a definite relationship between the amount of ammonia oxidised and the carbon assimilated. Experiment i. Experiment 2. Experiment 3. Experiment 4. Ammonia oxidised (ex- pressed as nitrogen) . Carbon assimilated 722-0 mg. I9'7 M 506-1 mg. I5'2 „ 928-3 mg. 26-4 „ 815-4 mg. 22-4 „ N Ratio -£ 36-6 „ 33-3 „ 35*2 „ 36-4 „ In these experiments mixed cultures were used, the nitrate producers predominating. More recently Coleman (68), using pure cultures of nitrate producers, obtained ratios varying from 40 to 44 for the second stage of the process. No useful hypothesis has yet been put forward to account 1 Quoted in Lafar, Tech. Mykologie, 1906, Bd. 3, 165. 190 SOIL CONDITIONS AND PLANT GROWTH for these remarkable facts. The whole subject deserves serious attention from some competent chemist. It was somewhat hastily inferred that organic matter would have a retarding effect in the soil just as it has in culture solutions. From the outset, however, certain facts were known to be against this view : thus, there was a good deal of organic matter in the old nitre beds (235) and also in rich gardens, and yet nitrification went on vigorously in both cases. An exception was therefore made in favour of " humus " (208). Later on Adeney (2), and again Miss Chick (66), found another exception : the organic matter of the filter beds used in sewage purification. Richards finds that nitrification pro- ceeds very vigorously during the activation of sludge by aeration. Coleman has now shown (68), and Stevens and Withers (271^) have confirmed it, that only in culture solutions is organic matter injurious : in the soil it does no harm, and may even help the process. Thus quantities of dextrose that stopped nitrification entirely in Winogradsky and Omelianski's culture solutions were found to act beneficially in soil under normal conditions of temperature and moisture content. The discrepancy cannot y^t he explained. Sucrose, lactose; and certain other non-nitrogenous compounds had no effect, but nitrogenous compounds were distinctly injurious. The organisms will not tolerate an acid medium ; a suf- ficient excess of calcium carbonate is therefore necessary both in culture solutions and in soils. Nor will they tolerate free am- monia. In culture solutions the nitrate producer is somewhat sensitive even to ammonium salts, indeed both Warington (296) and Omelianski (219$) suppressed it by maintaining a sufficient concentration of ammonium sulphate ; Lohnis has shown, however (i8i 345)- 2 A list is given by C. B. Lipman in Joum. Biol. Chem., 1911, x., 169-182. 3 A. Koch has collected instances in Lafar, Tech. Mykologie, Bd. iii., p. 15. 4 B. Moore and T. A. Webster, Proc. Roy. Soc., 1920, B 91, 201. CARBON AND NITROGEN CYCLES IN THE SOIL 201 Generally, there is a gain of nitrogen ; losses are, however, often recorded (248, 151, etc.), whilst a certain loss of nitrate invariably occurs (p. 266). A. Koch (151^) added successive small doses of dextrose to 500 grms. of loam, mixed with sand and spread on plates to secure copious aeration, kept uniformly moist and at 20° C. Nitrogen fixation began very soon and reached its maximum after eighteen weeks, when losses set in ; the results are given in Table LIV. TABLE LIV. — NITROGEN FIXED IN SOIL BY BACTERIAL ACTION IN PRESENCE OF DEXTROSE. KOCH (1516). Increments of Dextrose per Total Dextrose Supplied in grms. per 100 grms. of Soil after Mgs. N fixed per 100 grms. of Soil after Soil. 5 Weeks. 8 Weeks. 18 Weeks. 26 Weeks. 5 Weeks. 8 Weeks. 18 Weeks. 26 Weeks. June 26. July 2O. Oct. 3. Nov. 30. June 26. July 20. Oct. 3. Nov. 30. "2 I'O 1-6 3-6 5'2 8-3 14-9 I7-8 18-9 *5 2'5 4-0 9-0 I3-0 20'I 32-5 36'8 31'6 I'D 5'0 8-0 18-0 26'0 35-8 57'2 587 527 i*5 7'5 I2'O 27*0 37*5 4°'5 667 68-5 66-8 2-0 8-0 14-0 26-0 36*0 43'9 78-8 80-0 78-8 For each grm. of dextrose supplied in the small doses about 8 milligrams of nitrogen were fixed during the first eight weeks ; but only 4 or 5 milligrams later on. In larger dozes the sugar was less effective, only 5 to 6 milligrams of nitrogen being fixed per grm. of sugar at first and 3 milligrams later. Pot experiments showed that the nitrogen thus added to the soil became available for plant food. Dextrose and sucrose first depressed the crop, then caused an increase, and finally left the soil richer in nitrogen at the end of the experiment than at the beginning (Table LV.). But if the soil temperature fell too low nitrogen fixation ceased : it was not observed at 7° C. although it appeared to go on at 15° C. The optimum temperature lies between 25° and 30° C. Pfeiffer and Blank (224^), however, were unable to obtain any beneficial results from sugar. The Rothamsted trials 202 SOIL CONDITIONS AND PLANT GROWTH showed increases for autumn applications but decreases for spring dressings (140*?). TABLE LV. — EFFECT OF DEXTROSE AND SUCROSE ON THE PRODUCTIVENESS AND NITROGEN CONTENT OF THE SOIL. KOCH (1516). Sugar added per 100 grms. of Soil. Crops Obtained. Total N Removed in Crop. Nitrogen Left in Soil, Spring, 1906. Oats, 1905. Sugar Beets, 1906. Dry Matter. Yield of N. Dry Matter. Yield of N. Grms. Total N per Cent. N as Nitrates, Parts per Million. 2 per cent, dextrose . 2 „ „ cane sugar IOO 32-8 33*3 377 IOO 62-5 587 78-1 IOO 186 179 283 IOO 190 195 339 0-59I4 0-6814 0-680 i '0092 •C93 •105 •105 •119 IO 17 15 37 Increased yields of sugar cane have followed the applica- tion of molasses to soils at the Station Agronomique and on Mr. Ebbels' estate l in Mauritius, where the residual effect is well shown, and also in Antigua.2 Peck in Hawaii, on the other hand, observed marked losses of nitrate, as also did Harrison in British Guiana.3 An increase in crop following the application of sugar or starch to the soil is not evidence of nitrogen fixation, but might equally well be adduced to show that sugar and its decomposition products are direct plant nutrients. Only when an actual gain in nitrogen is demonstrated by analysis does the proof become satisfactory. As a practicable scheme the addition of sugar to the soil would be out of the question for field work. Beijerinck (15) has shown, however, that certain compounds producible in the decomposition of cellulose also serve as sources of energy to azotobacter, and Pringsheim (232) found that the same holds true for clostridium also. Hutchinson (140^) has shown that leaves, stubble, etc., 1 See The Agricultural News, 1908, vii., 227; 1910, ix., 339; and 1911, x., 179. 2 See Manurial Experiments with Stigar Cane in the Leeward Islands, 1908-09 and igog-io. (Pamphlets 64 and 68, West Indian Department of Agriculture.) 3 West India Bull., 1913, 13, 136. This contains an interesting discussion on the losses of nitrogen from soil. CARBON AND NITROGEN CYCLES IN THE SOIL 203 serve to increase nitrogen fixation, though under other conditions they can, like sugar, depress the nitrate content of the soil. Doryland1 has discussed the possibility of using various waste substances as energy supply, and Emer- son2 has discussed the prospect of soil inoculation with azotobacter. Richards (2380) has made the interesting observation that animal faeces also serve for the organism both in culture and in more natural conditions, but there is a sharp connection between the diet and the effect. Horses fed on oats gave faeces which induced the greatest fixation : horses in grass came next : cattle receiving cake were next, while the faeces from cattle fed on grass proved unsuitable. The difficulty of material might therefore be overcome because large quantities of cellulose are available on the farm in the form of straw. But there still remains the question of temperature. Azotobacter, as we have seen, requires more warmth than many other organisms, and according to Koch's experiments ceases to work at 7° C. Thiele read tempera- tures daily for three years of arable and grass soils at different depths at Breslau (281)^ and concluded that only rarely were they favourable for azotobacter. But it is impossible to argue from a culture solution to the soil, and, indeed, Lohnis has shown that the mixed cultures of the soil are almost as effective at 10° as at 20°. 3 It seems legitimate to conclude that azotobacter fixes nitrogen in well-aerated soils sufficiently provided with calcium carbonate, potassium salts and phosphates, carbon- aceous material of the right kind and moisture, so long as the temperature is high enough. Where the air supply is diminished owing to the close texture of the soil there is still the possibility of fixation by clostridium. Ashby (jc) found 1 Sci. Proc. Soc. Amer. Bacteriologists, 1918. 2 Iowa Research Bui., 45, 1918. 3 Mi'tt. Landw. Inst., Leipsic, 1905, vii., 94. IO°-I2° C. 20°-22° C. 30°-32° C. 3-15 mg. 4*55 mg. 4*27 mg. nitrogen fixed. 204 SOIL CONDITIONS AND PLANT GROWTH that the relative distribution of azotobacter and clostridium at Rothamsted depended on the amount of calcium carbonate in the soil ; wherever any notable quantity was present, azoto- bacter invariably occurred : otherwise clostridium alone was found. This result appears to be general.1 Remy suggested in 1906 that azotobacter was a good organism for the bacterial diagnosis of soils — absence showing some harmful factors and a rich development showing favourable conditions. Christensen (670) gave definite form by designing a workable method (see p. 242), while Gainey 2 has ascertained that azotobacter occurs in soils with a PH value 6*0 or more, but not in those with PH value 5 -9 or less. Nitrogen Fixation by Bacteria in Symbiosis with Leguminosae. After Hellriegel and Wilfarth's great discovery of the relationship between bacteria and leguminosae (p. 24) many unsuccessful attempts were made to isolate and study the organisms by the methods then in vogue. In 1888 Beijerinck (13) broke away from the ordinary meat-bouillon-gelatin plate and substituted a slightly acid medium made up of infusion of pea leaves, gelatin (7 per cent), asparagine (-25 per cent.) and sucrose (-5 per cent). Growth readily took place and the colonies yielded rods I p wide and 4 to 5 p long, some of which showed signs of bacteriod formation, and " swarmers " O'9 IM long and O'i8 //, wide, these being among the smallest soil organisms known.3 The life cycle has been shown by Bewley and Hutchinson (36) to include non-motile and motile stages : conditions were ascertained under which one passed into the other. 1 Hugo Fischer (1905) found azotobacter on the limed plots at Bonn- Poppelsdorf but not on the unlimed. Burri (1904) found it in only one-third of the Swiss soils examined. 9 Journ. Agric. Res., 1918, 14, 265. E. B. Fred and A. Davenport found the limits in culture solutions to lie between 6*5 and 8'6 (Journ. Agric. Res., 1918, 14. 3i7). 3 Golding has shown that they will even pass through a porcelain filter and has prepared pure cultures in this way. CARBON AND NITROGEN CYCLES IN THE SOIL 205 C. B. Lipman and L. W. Fowler1 have succeeded in isolat- ing the organism from soil, and T. F. Manns and Goheen 2 also claim to have done so. The mode of entry into the pea was studied by Prazmow- ski (227), and later by Nobbe and Hiltner (215^). The organ- ism— presumably in the swarmer stage — attacks the root hair, secreting a substance which causes the root tip to curl up ; the membrane of the hair becomes swollen and the bacteria then penetrate.3 A filament, known as the infection thread, and shown to be formed of rapidly multiplying bacteria, gradually extends up into the root where the nodule begins to form ; beyond this, the organisms do not penetrate. The morphological changes have been described by Marshall Ward (294), Miss Dawson (80) and others. Soon the organisms sur- round themselves with slime and appear as bacterial rods, which may then change to the characteristic branched or Y-shaped bacteriods and assimilate free nitrogen. There appear to be some 6 or seven different kinds of nodule organisms which can be distinguished by the agglutination test4 and which show characteristic discrimination between the various legumin- ous plants. Hiltner (135) regards them as parasites attracted chemo- tactically to the root hair by root excretions, but prevented from getting too far into the plant by excess of the attract- ing material, which now becomes a deterrent. He grades them according to their virulence, the less virulent either being unable to enter the plant, or, if they do enter, being quickly resorbed, or only fixing little nitrogen ; the more virulent, on the other hand, bring about energetic fixation. As evidence he adduces the well-known fact that infection proceeds best in plants weakened by nitrogen starvation, and scarcely takes place at all in plants growing vigorously on 1 Science, 1915, 41, 256 and 725, where earlier isolations by Nobbe, etc., are discussed. 2 Delaware Ag. Expt. Sta. Bull., 115, 1916. 3L. Hiltner (13501). 4 M. Klimmer and R. Kruger, Cent. Bakt. Par., 1914, 40, 256-265. 206 SOIL CONDITIONS AND PLANT GRO WTH rich soils. The parasitism is beneficial to both parties : the plant gains nitrogen and the organism gains carbohydrates. In its general outlines the process has been reproduced artificially. Leguminous plants can be fed with nitrogen compounds and made to grow perfectly without the organism. On the other hand, the organism can be grown on artificial media containing carbohydrates,1 made to pass through all its stages from s warmers to bacteroids, and to fix nitrogen.2 The change to bacteroids is conditioned by the presence of carbo- hydrates or of small quantities of various acids, such as are known to occur in the plant (276). The fixation of nitrogen rapidly comes to a stop unless the resulting compound is removed, as in the plant. Golding has attained this end by an ingenious filtering device, and has thus succeeded in fixing considerable quantities of nitrogen. He has also shown that the reaction of the medium during actual fixation is alkaline, but changes to acid when fixation is stopped by the accumula- tion of nitrogen compounds. An actual loss then seems to set in (109). The chemistry of the process is unknown ; even the changes in the carbohydrates of the culture medium have not been worked out. Nitrogen fixation is known to take place in the nodule, which thus becomes richer in nitrogen than the rest of the root,3 and its final product is supposed to be a soluble protein which is passed on to the plant. Phosphates, calcium compounds, and carbon compounds, such as sugars, 1 Harrison and Barlow (125) used maltose : other observers have used an infusion of the host plant. Neumann suggests pentosans (213). 2 See also (13). 3 Stoklasa's analytical results with yellow lupines (Landw. Jahrb., 1895, xxiv., 827) are: — Blossom Formed. Seed Beginning to Form. Seed Ripe. Nitrogen in nodule, per cent. ,, in rest of root, per cent. 5'2 1-6 2'6 1-8 17 I '4 Whiting (Illinois Bull., 179, 1915) has discussed this question also. CARBON AND NITROGEN CYCLES IN THE SOIL 207 organic acids, etc., arid according to Olaru,1 manganese com- pounds, have a marked effect in stimulating nodule formation in soil cultures.2 The amount of nitrogen fixed in this way is so large that it is easily measured on the field. When the host plant dies, or is ploughed into the ground, the nitrogen compounds speedily change into nitrates. A uniform piece of ground at Rothamsted was divided into two parts ; on one a crop of clover was taken, on the other barley was grown. After the crops were removed samples of soil were taken for analysis, and then barley was grown in both plots. The analytical results were : — Plot where Clover was Grown. Plot where no Clover was Grown; Nitrogen in crop (1873), Ib. per acre . I5I-3 37'3 (in clover) (in barley) Nitrogen left in soil after crop was removed (1873), per cent. . •1566 •1416 Nitrogen in crop (1874), Ib- per acre . 69-4 39'i (in barley) (in barley) These facts are well known to the practical man, and are utilised for increasing the nitrogen supply of cultivated soils and for reclaiming barren sands and clays (pp. 314 and 184). Leguminosae are among our commonest plants, both wild and cultivated. Wherever they grow they lead to enrichment of the soil in organic nitrogen compounds through the operation of the nodule organisms. The difference between the action of this organism and that of azotobacter is that it gets its carbohydrates from the plant, and is, therefore, independent of soil organic matter. Thus, it operates perfectly well in the poorest soils, provided potassium salts, phosphates and calcium carbonate are present in sufficient quantity for the host plant, 1 Compt. Rend., 1915, 160, 280. 2 J. K. Wilson, N.Y., Cornell, Agric. Expt. Sta. Bull., 386, 1917. W. A. Albrecht, Soil Sci., 1920, 9, 275, and various papers from A. L. Whiting's labora- tory, Illinois. This may explain the action of farmyard manure on the clover crop. (E. J. Russell, Journ. Bd. Agric., 1919, 26, 124.) 208 SOIL CONDITIONS AND PLANT GROWTH while azotobacter (except where it is associated with algae, a case that requires further investigation) requires a supply of organic matter in the soil, and therefore works only in fairly rich soils where its effects are more difficult to measure. Few improvements in agriculture have produced more marked effects than the extension of leguminous cropping. Where a leguminous crop has not hitherto been commonly grown it may be necessary to introduce the appropriate organism, as has been successfully done in Canada by Harrison and Barlow (126), in the United States by the Bureau of Plant Industry, and on the North German moors by Hiltner (135 ; see also 215 c and d]. In Great Britain these inoculations have not proved useful, and they have never come into farming practice : the high hopes sometimes enter- tained that the whole problem of nitrogenous manuring — the most costly item in the farmer's fertiliser bill — might reduce itself solely to bacterial inoculation have never been realised. Other Nitrogen Fixing Organisms. — Other organisms have been described which have the power of fixing gaseous nitrogen : among them Phoma, which in culture solutions proved about half as effective as azotobacter1 ; this organism belongs to the mycorrhiza group and may play a part in plant nutrition. Denitrification. If the air supply of the soil is cut off by water-logging, or in the laboratory by means of an air-pump, the nitrates rapidly disappear, whilst nitrites, ammonia, or gaseous nitrogen are formed. The conditions can be so arranged that the decomposition of nitrate-bouillon by soil shall give rise to notable quantities of gaseous nitrogen, nitrous oxide (18), or nitric oxide (168 and 277^). The reduction of nitrates to nitrite has long been known. As early as 1867 Schonbein2 stated that it could be brought 1 B. M. Duggar and Davis, Annals Missouri Botanical Gardens, 1916, 3, 4I3-437- 2C. F. Schonbein, Beitrage zur Physiologischen Chemie, Zeitsch.f. Biologic, 1867, 3, 325-340- CARBON AND NITROGEN CYCLES IN THE SOIL 209 about by " frische Conferven, wie sie so haufig in stehendem Wasser vorkommen " ; after 10 to 15 minutes' boiling, how- ever, the property was lost. Meusel in 1875 098) showed that it was bacterial, and could be stopped by antiseptics. The property appears to be generally possessed by bacteria and was shown by no fewer than 85 out of 109 kinds investi- gated by Maassen (184). The formation of gaseous products is effected by a smaller but still considerable number of organisms ; these were first investigated by Gayon and Dupetit (103), and by Deherain and Maquenne (Si a). The physiological significance of the reduction appears to be that nitrates can supply oxygen to the organisms when free oxygen is no longer obtainable. It is not simply a re- action between the organism and the nitrate : easily oxidisable organic matter must be present at the same time. The partially decomposed organic matter of the soil — the " humus " — does not seem to be very serviceable (274). There is a very sharp contrast between the bacterial pro- duction and the bacterial destruction of nitrates. Nitrate production is the work of one organism only at each stage, and the end result is a single product quantitatively equivalent to the original ammonia ; no single chemical process oxidises ammonia in this complete manner. The bacterial reduction of nitrates, on the other hand, gives no single product, but a number of products not in any simple ratio, whilst the chemical reduction can readily be made to go quantitatively to ammonia. Whether denitrification goes on to any* extent in properly drained agricultural soils is doubtful, because the three essential conditions, lack of air, presence of much easily decomposable organic matter and of nitrate are rarely obtained. In 1895 Wagner and Maercker startled the agricultural world by an- nouncing that unrotted.dung destroys the nitrates in the soil and reduces the crop yield (291). Their experiments were criticised by Warington (297) who pointed out that their dress- ings of dung were enormous and their results would not apply to ordinary farm practice. But it may well occur in rich soils 2 to SOIL CONDITIONS AND PLANT GROWTH and it goes on to a marked extent in wet soils. Nagaoka (211, see also 740) has shown that nitrate of soda frequently depresses, instead of increasing, the yield of rice, sagittaria and juncus on the swamp soils of Japan, an action which he attributes to the formation of poisonous nitrites. Organic manures or sulphate of ammonia are always used on such soils. Kelley has obtained similar results in Hawaii.1 Denitrification will also take place in peaty soils, and this has led to a very interesting observation as to the effect of lime. So long as these soils are left in their natural acid state nitrification cannot take place and therefore there is no denitri- fication. But as soon as lime is added in sufficient quantities to give a neutral reaction nitrification begins : part of the nitrate is then reduced to nitrite by microbic activity, then a chemical reaction sets in between the nitrite and the peat whereby some nitrogen is lost and some transformed into in- soluble compounds. Thus large doses of lime may produce injurious' effects on peat soils (Arnd (6)). Assimilation of Ammonia and Nitrates by Bacteria and other Micro-organisms. Probably most of the bacteria and moulds occurring in soil are capable under suitable conditions of assimilating ammonia.2 The process has not been observed in ordinary arable soils rather poor in organic matter ; Schlosing /m? (245*:) recovered as nitrate 98 per cent, of the added ammonium compounds, so also did Russell and Hutchinson. In peaty soils, however, the assimilation of added ammonia appears to be more pro- nounced, amounting to nearly 30 per cent, in Lemmermann's experiments (170). Certain 'organisms are capable of taking up nitrates : there is evidence (p. 1 79) that the action normally occurs in soils. Algae would be expected to behave in this way, and certain bacteria are known to do so in presence of easily decomposed organic matter (38) and air, in which respect the action differs 1 Hawaii Bull., No. 24. 2 Observed by Bierema (38) in 1909, and much investigated subsequently. CARBON AND NITROGEN CYCLES IN THE SOIL 211 markedly from denitrification proper ; it apparently goes on when sugar is added to the soil (151^). But such assimilation does not necessarily involve any loss of nitrogen, for as the organisms die they are probably decomposed with formation of ammonia and nitrates once again. Doryland (86) has shown that this assimilation of am- monia and nitrates depends on the amount of energy material available, and may become considerable in favourable circum- stances (pp. 1 86, 266). The Sulphur Cycle. Sulphur enters into the composition of several plant con- stituents, and it appears to undergo a series of changes in the soil whereby it is converted into sulphate, in which form it is readily taken up once more by plants. The conditions of this change have been investigated by Brown and his co-workers (60 b and c) who call the process "sulfofication".1 Effect of Bacteria on Soil Phosphates. It is frequently suggested that the phosphates in the soil are made more soluble through the activity of bacteria, but there is no sufficient proof. British agriculturists in the eighteenth century recognised that bones acted better as manure after fermentation than before, but the older chemists attributed the action to the decomposition of the organic matter and consequent greater accessibility of the phosphate. More recent bacteriological investigations by Stoklasa 2 and by Sackett, Patten, and Brown,3 however, show that certain bac- teria have the power of dissolving both bone and mineral phosphates in culture solution, but the mechanism of the pro- cess is not clear. It is uncertain whether this action goes on in the soil : direct analysis has failed to demonstrate it and the evidence from field experiments is conflicting (p. 191). 1 See also Kappen and Quensell, Landw. Versuchs-Stat., 1915, 86, and J. W. Ames and T. E. Richmond, Soil Sci., 1918, 5, 311. 2 Stoklasa, Duchacek, and Pitra, Centr. Bakt. Par., 1900, 6, 526 and 554. 3 Sackett, Patten, and Brown, Michigan Special Bull., 43, 1908. 14* 212 SOIL CONDITIONS AND PLANT GROWTH Hart well and Pember,1 and Tottingham and Hoffman,2 failed to find any increased solubility even when rock phosphate was composted with stable manure.3 Decomposition of Organic Compounds. Many organic compounds surfer decomposition in the soil, including some toxic to the higher forms of life and others usually regarded as very stable. Among those readily de- composed are phenol * potassium sulphocyanide, naphthalene, pyridine and vanillin 5 : apparently the decomposition is brought about in each case by micro-organisms. Some of these toxic substances may arise during the ordinary decomposition processes in the soil,6 and if so it is obvious that the micro-organisms which further break them down to innocuous compounds are playing an important part in soil fertility. Swamp and Paddy Soils. In the East — in India, Japan, etc. — considerable quantities of rice are grown on swamp soils, and the biochemical changes differ considerably from those in normal soils. As already pointed out, nitrification does not go on but the converse process, denitrification, occurs, so that if nitro- genous artificial manures are to be used nitrates are out of the question and organic manures and ammonium salts only are possible (p. 210). 1 Hartwell and Pember, Rhode Island Bull., 151, 1912. 2 Tottingham and Hoffman, Wisconsin Research Bull., 29, 1913. 3 For a summary of the recent work see Soil Sci., 1919, 7, 141. Solution of the phosphate proceeded when sulphur was added (J. C. Lipman and McLean, Soil Sci., 1918, 5, 243 and 533 ; also O. M. Shedd, Journ. Ag. Res., 1919, 18, 329). 4N. Sen Gupta, Journ. Ag. Science, 1921, II. For the organisms see G. J. Fowler, Ardern, and Lockett, Proc. Roy. Soc., 1911, 83, 149-156, and Wagner, Bied. Zentr. Agrik. Chem., 1915, 44, 212. 6 W. J. Robbins and A. B. Massey, Soil Sci., 1920, 10, 237, also Ala. Expt. St. Bui., 195, 1917, and 196, 1917 (M. J. Funchess). 6 E.g. Liechti and Mooser (Land. Jahrb. Schweiz, 1906, i) estimate that 34-83 kilo of phenol per hectare (30-74 Ib. per acre) are formed during the de- composition of an ordinary dressing of liquid manure. Of benzoic acid no less than 400-500 kilos are supposed to be formed. CARBON AND NITROGEN CYCLES IN THE SOIL 213 Green manuring is, however, commonly adopted and con- siderably benefits the crop : the decompositions taking place when the green crop is ploughed in have been studied by W. H. Harrison and Aiyer (126). In the body of the soil marsh gas, hydrogen, and CO2 are evolved, as would be expected from the anaerobic decomposition of cellulose. But at the surface of the soil the change is entirely different and the gases consist of oxygen and nitrogen only. The difference was traced to a film of organisms which have the power of converting the marsh gas into COg1 and this into oxygen. The oxygen is directly beneficial to the plant by providing for the aeration of the root. The production of oxygen was suppressed when the film was killed by adding copper sul- phate : marsh gas and hydrogen then appeared at the surface. It is only so long as the film is working that green manur- ing is beneficial to the crop. The plant roots must have oxygen and the film supplies it. The green manure, there- fore, not only supplies ammonia, but also dissolved oxygen. It would be interesting to have studies of the conditions obtaining in water cress-beds. The Effect of Adding Plant Residues to the Soil. The addition of plant residues to the soil is a normal oc- currence and a recognised method of manuring land. The effects produced depend on the proportions of carbohydrate and protein present in the residues.2 If the conditions are favourable to the activity of micro- organisms the result of the addition is to cause an increase in numbers and in activity of the micro-organisms : this is shown by increases in oxygen absorption and CO2 evolution. The increased numbers may have any of the following effects : — 1 The biological oxidation of marsh gas has been described by Kaserer and Sohngen and by I. Giglioli and G. Masoni (Pisa: Chim. Agric. Stud. e. Ri- cherche, 1909-14, Part 22, 76-108). 2 For further discussion see H. B. Hutchinson (1400); P. E. Brown and F. E. Allison (6oe) ; P. Felber, Mitt. Landw. Hochschule f. Bodenkulture in Wien, 1916, 3, 23. 214 SOIL CONDITIONS AND PLANT GROWTH 1. If the material is rich in energy supply but not in nitrogen (e.g. carbohydrates) the organisms may assimilate nitrates or ammonia already existing in the soil and thus re- duce the amounts of these substances present (p. 266). These circumstances are favourable to the nitrogen-fixing bacteria, and if the temperature is sufficiently high there may be an increase in the amount of nitrogen fixed (p. 203). The as- similation effects are temporarily harmful but the fixation is ultimately beneficial to the growing plant. 2. If the material is rich in nitrogen (e.g. protein), the organisms will produce considerable quantities of ammonia in obtaining their energy supplies. This effect is wholly benefi- •cial to the plant (p. 185). 3. If the air supply is insufficient — either generally or locally — but other conditions are favourable, the organisms will obtain some of their oxygen from the nitrates present in the soil, and gaseous nitrogen will be evolved. This effect is wholly harmful to the plant (p. 208). 4. If, however, conditions are unfavourable (e.g. if there is acidity or too low a temperature), the organisms will not act ; the material will only partly decompose and it will ac- cumulate as peat (p. 133). Summary of the Changes Taking Place and the Agencies Involved. It is unfortunate that no synthesis of a soil has yet been effected, and consequently the preceding analysis of the changes taking place cannot be tested by reconstructing the whole pro- cess out of its constituent parts. On the whole the evidence is satisfactory as to the general course of the changes, but in- sufficient for sorting them out quantitatively and precisely. The following scheme summarises them as completely as is possible at present : — l 1 It will be noticed that these processes show certain resemblances to those of sewage purification beds, as worked out by Adeney (2) and Fowler (Bacteria logical and Enzyme Chemistry, London, 1911). The decomposition in the septic tank, however, and especially in the percolating filter and the contact bed, ap- CARBON AND NITROGEN CYCLES IN THE SOIL 215 Carbohydrates x + Protein Cellulose \ 1 \ i \ AminoAcids Other Compounds Acia ^x" \. "flumm " 1 NH3 llydroxy Acids Nitrites Calcium, Jolts Calcut i ™olls MtLs c}co, CO* co, an Oils Waxes There is good reason to suppose that all of these changes are effected by micro-organisms, but the evidence is by no means conclusive. The obvious test of working with sterilised soil is out of the question, because soil can only be sterilised by drastic methods that wholly change its character. The fact that antiseptics put an end to most of the reactions always used to be regarded as sufficient proof of their biological nature, but this argument has lost much of its force since Bredig and others1 have shown that indisputably dead materials like spongy platinum partially lose their power of bringing about chemical changes when treated with antiseptics. Now the soil is a spongy mass, measurably radio-active (p. 89), con- taining numerous colloidal bodies conceivably capable of act- ing as catalysts, and it is possible to imagine a series of catalysts that would bring about all the known changes and be put out of action by antiseptics. Hypotheses of this nature have indeed been put forward from time to time and reactions have been discovered which cannot be attributed to micro- organisms. Cowie has shown 2 that the first stage of the de- composition of cyanamide in the soil, the formation of urea, is not due to organisms, but to some mineral constituent.3 pears extremely rapid to the agricultural chemist; changes such as nitrification, for which he is accustomed to allow days or even weeks, being brought about in two or three hours even at the low temperature. For the decomposition of fats, see O. Rahn, Centr. Bak. Par., Abt. II., 1906, 15, 52-61 ; 422-429. 1C/. Bredig and Ikeda, Zeit. Physikal Chem., 1901, xxxvii., 1-68. ^jfourn. Agric. Sci., 1920, 10: see also Lohnis, Zeitsch.f. Garungsphysiol, 1914, 5, 16-25. 3 For a study of solid catalysts see E. F. Armstrong and Hilditch, Proc. Roy. Soc.r 1919, 96 A, 137. 216 SOIL CONDITIONS AND PLANT GROWTH Soil rapidly decomposes hydrogen peroxide. Some decom- position of protein and of phenol and hydrolysis of cane sugar also occurs under conditions where living organisms seem excluded. There is some evidence that soil can bring about most of the chemical actions that a large number of organisms can effect, such as ammonification, but not changes peculiar to one or two organisms only like nitrification and nitrogen fixation. Sestini,1 indeed, supposed that ammonia is oxidised catalytically by the ferric oxide always present to nitrites and nitrates, while Loew (i8o<^) states that nitrogen can be catalytically "fixed" and converted into nitrates; Russell and Smith,2 however, failed to reproduce these changes catalytically. Indeed, the chief argument in favour of the bacterial hypothesis is that all known soil processes can be reproduced in the laboratory by soil bacteria acting under conditions comparable with those known to obtain in nature, whilst they have not been produced by catalysts. The biological hypothesis, therefore, remains the simplest and most satisfying, but there is room for more evidence before it can be regarded as positively established.3 It is certain that living bacteria occur in the soil in addi- tion to those present as spores. Some idea of the relative pro- portions of these two forms was obtained by making gelatin plate cultures of soil before and after treatment with toluene, which destroys living forms but not spores, or at any rate not all spores (Table LVL). Spores form only about 25 to 30 per cent, of the total numbers, and for some unknown reason do not accumulate. Conn (see p. 187) does not consider, however, that they represent organisms of importance in the soil. The bacterial numbers are seen to be very high, but even these figures do not represent the true totals, and no medium has yet been devised that allows of the growth of all the organisms known to occur in the soil. 1 Landw, Versuchs-Stat., 1904, 60, 103-112. *Journ. Ag. Sci., 1905, I, 444-453. 3 For a recent discussion see The Proof of Microbial Agency in the Chemical Transformation of Soil (H. J. Conn, Science, 1917, 46, 252). CARBON AND NITROGEN CYCLES IN THE SOIL 217 TABLE LVI. — NUMBERS OF ACTIVE BACTERIA AND SPORES OCCURRING IN SOILS AND CAPABLE OF GROWTH ON GELATIN PLATES. RUSSELL AND HUTCHINSON (24ic). MILLIONS PER GRAM OF DRY SOIL. Arable Soil. Rich Green- house Soils. Forms killed by toluene (i.e. active forms) Spores surviving toluene .... Total growing on gelatin . , . 7 '4 2-9 39*6 i3'3 10-3 52'9 The active forms must be held responsible for some at least of the oxygen absorption, carbon dioxide evolution and decomposition going on. Under comparable soil conditions a distinct relationship exists between the productiveness of the soil and the amount of bacterial activity, although it cannot be expressed in any definite form. Counts of the numbers of bacteria by any particular method fail to give results sharply connected with productiveness (although there is a general relationship) because the organisms are of the most varied description (ill), and of widely different efficiency as food makers. Nor, on the other hand, have the methods of physi- ological grouping helped much, since they necessitate growth in culture media wholly different from the soil under tempera- ture and water conditions that never obtain in nature. Not until the fundamental difficulty has been overcome of syn- thesising a soil identical with natural soil will it be possible fully to interpret the many interesting observations that soil bacteriologists are now accumulating. The subject is further discussed in Chap. VII. CHAPTER VI. THE BIOLOGICAL CONDITIONS IN THE SOIL. IT is now necessary to study the soil conditions that determine the growth not only of plants but also of the micro-organisms that, as we have seen, make new plant food out of old plant residues, and render possible the continuance of vegetable life on the earth. These conditions are water supply, air supply, temperature, food supply, and absence of injurious factors. The Water Supply of the Soil. The water content of the soil is the difference between the amount supplied and the amount lost. The supply may come from rain, from irrigation, and from the subsoil : the loss is by evaporation or drainage from the soil or transpiration through the plant. The broad general facts are that a soil tends to maintain its water supply within fairly definite seasonal limits, any excess draining rapidly away (unless there is some mechanical hindrance), and any deficiency below the lower limit setting in only slowly. This is well shown in the curves of Fig. 17 (p. 131), where the winter level is never greatly exceeded and the summer level is fairly well maintained. The fluctuations, however, are of great importance. The moisture range depends on the structure of the soil. The balance retained in the soil is obviously determined by the resistance the soil can offer to the forces involved in drainage or evaporation, which depends on its composition, , its texture and its colloidal matter. To a considerable degree I evaporation can be controlled by cultivation : this forms the * basis of the so-called Dry Farming.1 1 See British Ass. Repts., 1914, p. 645. THE BIOLOGICAL CONDITIONS IN THE SOIL 219 The main facts in regard to water supply are as follows : — 1. A sandy soil is liable to great fluctuations in moisture content, easily becoming very wet or nearly dry. Movement from underground sources of supply is rapid, but except where it is gravitational, i.e. due to seepage from higher ground, it does not appear to extend far. A chalk soil behaves similarly. The fluctuations are reduced and the tendency to become dry is considerably lowered when organic matter or clay is added to the sand. 2. A heavy clay soil fluctuates proportionately much less in water content, and its maximum and minimum contents are both high. Water travels only very slowly : it is common to see the soil of a field cracking with drought almost up to the edge of a stream. 3. Feat soils like clay soils have a high maximum and a high minimum water content. They show in an exaggerated degree a tendency seen in all soils — a difficulty in becoming remoistened after the soil has become dry. These relationships are well shown in Table LVII. TABLE LVII. — MOISTURE CONTENT l OF SANDY, LOAMY, AND CLAY SOILS AT WOBURN, LYING NOT FAR APART, AND UNDER APPROXIMATELY EQUAL RAINFALL CONDITIONS. RUSSELL. Sandy Soil (Clay =5-0 per Cent.). Loamy Soil (Clay=9-3 per Cent.). Clay Soil (Clay -43-0 per Cent) Highest observed . Lowest observed . Mean of all observations 14*0 I'l Q-o 16-5 6-0 12 'O 35'o 15-8 27*0 For biological purposes a better idea of the meaning of these results is obtained by translating them into volumes. The soil is a porous mass and a large part of it is not occu- pied by solid matter at all but by air and water. Comparison of the specific gravity of the soil particles determined by the specific gravity bottle — the so-called " true specific gravity " — 1 This determination is made by drying at 40° C, 220 SOIL CONDITIONS AND PLANT GRO WTH with the specific gravity of the mass — called the apparent den- sity— obtained by weighing a block of soil of which the volume in situ is known, shows that the solid matter forms 50 to 65 per cent., leaving 50 to 35 per cent, of pore space. Organic matter increases the pore space in consequence of its " lighten- ing" action (p. 130). This is shown in Table LVIII. TABLE LVIII.1 — PORE SPACE, WATER CONTENT, AND AIR CONTENT OF CERTAIN SOILS. RUSSELL. ' Specific Gravity of Dry Soil. Volume Occu- pied in Natural State by • Volume of Water. Volume of Air. c ii i 5 O <«0 iS N/ I" < « 2 o. awai •a g 232 SOIL CONDITIONS AND PLANT GRO WTH (5) The top 6 inches of soil has a higher mean tempera- ture than the air both in summer and in winter. At 6 inches the warmer part of the day centres round 5.30 p.m., and the cooler part round 9^30 a.m. (6) The warming of the soil in spring is facilitated by drying ; the cooling in autumn is increased by clear nights and diminished by rain (Keen and Russell, 146^). TABLE LX. — TEMPERATURES OF SOIL AT DIFFERENT DEPTHS UNDER VARYING CONDITIONS. RUSSELL. Effect of Weather. Air Temp. Temperature of Bare Soil. Untouched. Surface Stirred by Hoes. Hot sunny day, 2oth June, 1910 Cold cloudy day, 2yth June, 1910 30° 18° i inch. "35° 17-5° 3 inch. 30-5° I67° 6 inch. 27° I5-8° } inch. 3I'5° 17° 3 inch. 29-8° 16-3° 6 inch. 26-5° 15-5° Effect of Vegetation. Warm Weather, 5th Oct., 1910, Air Temperature, 17°. Cold Weather, 4th Jan., ign, Air Temperature, 3-5°. i inch. 3 inch. 6 inch. i inch. 3 inch. 6 inch. Bare soil 17° I67° 15-5° 3° 2-5° 2'5° Soil covered with living vegeta- tion (grass land) . I5'5° 15° 14-5° 3° 3° 3° Soil covered with dead vegeta- tion (mulched land) 15-5° 15° I4'5° 2-5° 2'0° 2-0° Records extending over periods of some months have been published by Wollny (318) and by Thiele (281). British data generally refer only to 6 inch or 1 2 inch readings ; they have been collected and worked up by Mawley, by Mellish,1 and by Franklin.2 Systematic readings are taken at the Radcliffe Observatory, Oxford,3 at Kew, and also at Rothamsted, where a continuous self-recording thermometer is installed. Detailed 1 See Quart. Journ. Roy. Meteor. Soc., 1899, xxv., 238-265. iProc. Roy. Soc., Edin., 1920, 40, 10, 56. 3 See A. A. Rambaut, Radcliffe Observations, 1901, 48, 1-245, and 1911, 51, 103-204; also Phil. Trans., 1901, 195 A, 235-8. THE BIOLOGICAL CONDITIONS IN THE SOIL 233 records of soil temperatures at East Lansing,' Mich., have been taken by Bouyoucos (490), who has also discussed their effect on the physical properties of the soil. Food Supply. In spite of numerous investigations our knowledge of the plant food in the soil is very limited. On physiological grounds it is supposed that the whole of the nutrient material coming from the soil enters the plant in the dissolved state, but whether the actual soil solution is all the plant gets, as Whitney and Cameron suppose (see p. 173), or whether the carbon dioxide respired from the roots1 effects the solution of more material than is already dissolved, has not been ascer- tained. The soil solution may safely be regarded as the mini- mum food supply, which is reinforced to an unknown extent by the soluble substances in the soil. Nitrogen nutrition presents a tolerably simple case because plants growing on cultivated soils probably absorb all their nitrogen as nitrates, which are readily and completely dissolved by water, whilst plants in undisturbed soil — grass land, etc. — probably utilise ammonium compounds as well. Potassium and phosphorus nutrition present greater difficulties because very little is known about the compounds of these elements in the soil. This particular problem is of such technical im- portance that it has been necessary to do something empirically, and by common agreement the small fraction of the phosphorus and potassium compounds soluble in dilute acids is called "available" food material, while the rest is said to be 1 At one time it was supposed that special acids were excreted by plant roots to dissolve insoluble food materials in the soil. This idea, which was a sur- vival of the mediaeval view that plants are wholly analogous to animals, persisted into our own times, but has been shown to be untenable by Czapek (Biochemie der Pftanzen, Bd. 2, pp. 872 et seq.}. So far as is known CO2 is the only acid ex- creted. The evidence is of the negative kind and is therefore not entirely satis- fying, so that the problem is periodically brought up again ; recently, for instance, Pfeiffer and Blanck stated that other acids also are given off (224^) Cf. (275), and footnote, p. 33. For a recent discussion, with references, see H. Kappen, Untersuchungen an Wurzelsaften (Landw. Versuchs-Stat., 1918,91, 1-40). 234 SOIL CONDITIONS AND PLANT GROWTH " unavailable ".l Here, however, the agreement ends, for no two dilute acids give the same results and no two Associations of Agricultural Chemists recommend the same dilute acid. Dyer's I per cent, citric acid (91) is adopted in Great Britain, and its use has been justified by Wood's investigations (319) and by those of Hall and Flymen (i2o#).2 N/2OO hydro- chloric acid has been recommended in the United States, 2 per cent, hydrochloric acid by Nilson in Sweden, aspartic acid in Hawaii, and so on. Mitscherlich (201 c) uses a saturated solu- tion of CO2 as being the nearest approach to natural con- ditions. Some of the younger workers in the United States are using the water extract as the nearest feasible approach to the soil solution.3 A 2 per cent, citric acid solution has been suggested by Bergu.4 Bogdanow 5 in his investigations of Russian soils used 2 per cent, acetic acid. The German "Verband landwirtschaftlicher Versuchsstationen " recom- mends both 25 per cent, and 10 per cent, hydrochloric acid (see 224/~). Lastly Ramann proceeds in a different manner altogether and adopts a method depending on the interchange of bases. The results obtained by different acids are shown in Table LXI. Similar results have been obtained by Engels (95) (see also p. 344). An empirical method can be retained only so long as it justifies itself by results. Few agricultural chemists of repute would be prepared to draw up a scheme of manuring on the basis of soil analysis alone, though probably most would like to have some analytical data before them. Even strong hydrochloric acid dissolves only a part of the potash and phosphoric acid, the remainder not coming out till after treatment with hydrofluoric acid. The different action of the various dilute acids has been 1 Daubeny (78) originated this distinction, using the terms " active " and " dormant ". 2 Also those of O. Lemmermann, A. Einecke, and L. Fresenius in Germany (Landw. Versuchs-Stat., 1916, 89, 81-195). 3J. S. Burd (62a). 4 Landw. Ver such s-S tat., 1901, 55, 19 ; also by Engels (95). 5 Expt. Station Record, 1900, u, 130; 1901, 12, 725. THE BIOLOGICAL CONDITIONS IN THE SOIL 235 adduced as evidence of the existence in soils of a considerable number of phosphorus and potassium compounds of varying degrees of solubility, but no such assumption is necessary. It more probably represents the division of these compounds be- tween two solvents, the weak acid and the colloidal complex in which they are present in the soil (see p. 172). TABLE LXI. — AMOUNTS OF K2O AND P2O5 EXTRACTED BY ACIDS FROM ROTH- AMSTED SOILS, PER CENT. OF DRY SOIL. HALL AND FLYMEN K20. Strong HC1. Dilute Acids Equivalent to i per Cent. Citric Acid. Citric Acid. HC1. Acetic Acid. Carbonic Acid. Broadbalk, unmanured . ,, minerals only ,, dung 0-380 0-463 0'453 0-0043 0-0458 0-0400 0-0147 0-0522 0*0684 0-0082 0-0307 0-0451 O'OIII O'O2I5 0*0380 P205. Broadbalk, unmanured . ,, minerals only ,, dung 0*114 O-228 0-209 0-0080 0*0510 0-0477 O-OO2I 0-0360 O-O224 o-ooii 0-0098 0-0166 0-0005 0*0058 0-0095 More definite information can be obtained about the nitrogen compounds. The amount of ammonia and of nitrate can be ascertained in any desired depth of soil. On cultivated land the amount is not generally more than enough for one year's crop, any balance being liable to be washed out in winter, so that the plant depends in spring on the activities of the decomposition processes for a regular supply of nitro- genous food. This is one of the factors that produce the marked retardation of plant growth in spring when the soil is wet and cold, especially after a wet winter when the washing- out process is complete, and it further accounts for the re- markable benefits produced by even small additions of nitrate of soda or sulphate of ammonia to the soil at this period. In dry regions the accumulation of plant food and other soluble decomposition products in the soil may be too great to admit of plant growth, and bare patches or regions arise 1 For other results see F. Miinter, Zur chemischen Bodenanalyse (Landw. Versuchs-Stat., 1919,91, 181-189). 236 SOIL CONDITIONS AND PLANT GRO WTH known as " alkali soils " from the circumstance that sodium and potassium carbonates are often present. In wetter climates the soluble substances tend to be washed out more completely, but notable quantities often persist in heavy clay soils, especially where the drainage is bad, and may produce injurious effects on vegetation. In discussions of the1 plant food in soils it is often assumed that the only significant plant nutrients are nitrates, phos- phates, sulphates and salts of potassium, calcium, magnesium and iron, commonly called the " essential" plant foods. Perfect plants can be raised in water cultures containing only these substances. Reference to Fig. 12 (p. 68), however, shows that productiveness is not always maintained indefinitely simply by supplying these salts, and not nearly so well as when the complex farmyard manure — excreta, litter, etc. — is used. It is known also that certain other compounds besides the "essential" foods may increase plant growth (p. 56). Further, Russell and Petherbridge have shown that on heating soil to 100° something is formed that stimulates root production to a remarkable degree. Are these effects the results of nutrition or of stimulation ? Are the manganese, fluorine, etc., compounds and the beneficial soil constituents indispensable nutrients of which only traces are required, or are they, as Armstrong expresses it, "condi- mental " foods ? It would be attractive to think that some of the vague physiological conditions that trouble the grower are to the plant what beri-beri and similar diseases are to the animal — the result of withholding some essential or useful " accessory substance". Bottomley1 considers that certain substances obtained in the bacterial decomposition of peat are of this nature. Maze has published some remarkable results 2 show- !W. B. Bottomley, Proc. Roy. Soc., 1914, 88, 237-247, and 1920, B 91, 83 ; Florence A. Mockeridge, Biochem. Journ., 1920, 14, 432. The substances are called auximones. See also J. F. Breazeale, Journ. Ag. Research, 1919, 18, 267, who found that citrus seedlings grew better in peat extract than in distilled water. 2 Maze, Compt.Rend., 1915, 160, 211. THE BIOLOGICAL CONDITIONS IN THE SOIL 237 ing that maize fails to complete its growth in water cultures containing all the recognised nutrient salts if these are chemi- cally pure, but it grows normally as soon as tap water is intro- duced. No combination of added salts has as good an effect as the tap water (Table LXIL). TABLE LXII. — GROWTH OF MAIZE IN WATER CULTURES. MAZE, 1915. Culture solution containing compounds of boron and aluminium (tap water) 43 „ „ „ „ „ „ (distilled water) 24 ,, ,, „ „ ,, ,, aluminium and arsenic . 16-5 ,, ,, „ ,, ,, „ „ arsenic and iodine 36*6 A considerable amount of work has obviously to be done before the problem can even be clearly stated. And against all this is the fact, abundantly demonstrated by Dr. Brenchley, that barley and peas make full growth in nutrient solutions containing only the ordinary " essential " foods (54 • | | "3 o.« || |f I 5 1 •3 &£ 1-1 &% J CB o (top g") No manure . Condition poor . Cropped Fallow 23-12 23 15-8 15 } 990 2500 { til 12-30 20-36 7-12 7-12 O'2-0'62 0'2-O'4 Farmyard manure Condition good . Cropped Fallow 30-15 30 I7-8 17 1 2200 5000 / IO-2O i 20-35 25-50 50-84 15-25 15-25 o'5-i'o3 0*2-0-6 Ordinary arable field Cropped 25-12 I5-8 I5OO 3700 10-15 25-36 15-25 o'5-i'o4 ^Thus Burmeister (Fuhl. Landw. Zeit., 1914, 63, 547-556 ; see Rome Bull., 1914, 1691) found that couch (Triticum or Agropyron repens) increased the yield of oats, and Dr. Brenchley found that certain weeds had the same effect on the yield of wheat per plant (New Phytologist, 1917). 2 Running on occasions up to 1*8. 3 Occasionally up to 2-3. 4 Occasionally up to 2*5. * The concentration of the dissolved matter is of the order of o'2 per cent, and the osmotic pressure about i atmosphere. CHAPTER VII. THE MICRO-ORGANIC POPULATION OF THE SOIL AND ITS RELATION TO THE GROWTH OF PLANTS. THE soil is inhabited by a great variety of micro-organisms, but their precise relationship to the growing plant is difficult to determine because we know so little about them. The micro-organic population is certainly highly complex : it is known to contain many kinds of bacteria, moulds, protozoa and other organisms, and new members are discovered almost every month. Usually they are picked out by some culture method, and their physiological effect is studied in an arbitrary culture solution : sometimes the results are applied straightway to the soil without further ado. The method is defective for two reasons. Firstly, micro-organisms are considerably in- fluenced by the medium in which they happen to find them- selves, and may effect one change under one set of circumstances but quite another change under other circumstances. Secondly, most micro-organisms exist in two states : an active or trophic state, and a resting state, and it is reasonable to suppose that the resting forms are comparatively unimportant. Probably in many cases no sharp line exists between the two, the active forms changing to the resting stage or back again as the soil conditions alter ; but it is never safe to assume without proof that any organism discovered by culture methods is active in the soil. The main difficulty in applying the results is that soil cannot be sterilised l because of its chemical instability, nor can it be made up artificially ; in consequence one cannot begin with a sterile soil and inoculate into it a particular set 1 Intermittent sterilisation at 80° causes less decomposition, but it does pro- duce change (D. A. Coleman, H. C. Lint, and N. Kopeloff, Soil Sci., 1916, 1, 259). 250 THE MICRO-ORGANIC POPULATION OF THE SOIL 251 of organisms so as to observe their behaviour under natural conditions. These difficulties have not proved insuperable, and a number of organisms have been discovered which nor- mally lead a trophic life in the soil. These organisms may be regarded as the micro-organic population of soil. The essential conditions for the life of a micro-organic population are adequate supplies of food and energy materials, sufficient air and water, a proper range of temperature, and absence of harmful factors. With the exception of the energy supply these requirements are the same as for higher plants ; and as many of the nutrients are common to the plant and to micro-organisms it is not surprising that soils well suited to the growth of plants often carry a large population of micro-organisms also. Investigators have long realised that soils contain numbers of bacteria, and in Chap. V. some of the changes they bring about are described. Evidence has been accumulated that other groups of micro-organisms are normally present also — fungi, algae, protozoa, nematodes, etc. In previous editions of this book the organisms were classified according as they were or were not useful to plants. This scheme is useful as a first approximation, but it does not admit of fuller development : it is therefore discarded in the present edition. The members of the soil population must be regarded as leading their own lives and, with few exceptions, possessing powers of adaptation which enable them to draw sometimes on one compound and sometimes on another for food and for energy, and therefore not necessarily always producing the same substances. The mycoides, for ex- ample, where they are active, can produce ammonia from protein and would then be helpful to plant growth ; under other circumstances, however, they assimilate ammonia and therefore compete with growing plants. Certain broad relationships have been established between the various members of the soil population, and between the population as a whole and the growing plant : there is evidence for the following propositions : — 252 SOIL CONDITIONS AND PLANT GROWTH 1. The population at any given moment is as large as the conditions allow, and is limited by presence of a harmful factor or by lack of some essential factor such as energy supply, food supply, air, water, or temperature. Improvement in the supply of the limiting factor, or removal of the harmful factor, allows an increase in numbers of the populaton. 2. Competition therefore is an important factor in de- termining the relative numbers of the various groups : any increase in the numbers of one group may lead to a decrease in the numbers of others having similar requirements. Changes in numbers of any one group, therefore, are not necessarily sharply related to changes in external conditions, and when the external conditions are brought back to their original level the numbers of any one group may not return to what they were before. 3. Owing to the great differences in size the numbers of the various groups do not allow a ready basis of comparison. The most suitable basis for quantitative comparisons between one group and another is their respective energy requirements : these are comparable. Bacteria, fungi, protozoa, and nematodes are all consumers : algae only are producers of energy materials. The relationships of the soil population to the growing plant are as follows : — 1. The energy supplies in the soil come from the residues of green plants. Since energy supply is probably the most important factor limiting the numbers of the soil population it follows that the soil population is to this extent dependent on the plant. 2. But the plant is also dependent on the soil population since the soil population in its search for energy supplies decomposes so much protein substance with formation of ammonia and of nitrate as to leave over a surplus for the plant. Further, certain members of the population also de- compose materials such as straw, etc., which possess definite structure and have undesirable physical effects on the soil, while other members decompose toxins such as phenol, which they accumulated would be harmful to the plant. THE MICRO-ORGANIC POPULATION OF THE SOIL 253 3. During the growth of the plant some at any rate of the soil organisms appear to be adversely affected : some of the activities of the growing plant seem to be detrimental to the micro-organic population. 4. While sharp distinctions cannot be drawn some group- ings of the micro-organic population are more conducive to the production of plant nutrients than others, and some organisms are definitely harmful to the plant Organisms Acting Directly on the Plant. 1 I ) Parasitic and Disease Organisms. The study of these organisms has developed into a special branch of Economic Biology, and we need therefore only briefly refer to them here. The commonest are the eel- worms, the myxomycete Plasmodiophora, some of the " wilts," and certain organisms that attack potatoes. Of the numerous kinds of eel- worms occurring in the soil, about six are known to attack and enter the plant, and da considerable direct injury, besides opening the way for the entrance of fungi, bacteria, etc. The commonest are Heterodera radicicola, which causes swellings or " knots " on the roots of tomatoes, cucumbers and other plants, and Tylenchus dipsaci (syn. devastatrix] which attacks oats, causing tulip root, and clover, bringing on one form of clover sickness. In some soils, especially those short of lime, another pest is common : the myxomycete Plasmodiophora, which enters the roots of swedes,, turnips and other plants of the Brassica tribe, causing the disease known as finger-and-toe. (2) Symbiotic Organisms. — In normal conditions legu- minous plants possess nodules on their roots which contain numbers of bacteroids living in association with the plant. This organism, Bacillus radicicola, enters the plant roots at an early stage and, having brought about the formation of the nodule, proceeds to manufacture nitrogen compounds for the plant from the gaseous nitrogen of the air (see p. 204) Certain trees and shrubs (notably beech, heather, etc.) become associated with mycorrhiza, fungi which grow on their 254 SOIL CONDITIONS AND PLANT GROWTH roots and aid in the nutrition of the plant. These were first investigated by Frank and have received considerable attention from mycologists.1 Organisms Capable of Bringing About Changes of Im- portance to the Plant. i. Alga. All soils hitherto examined contain algae including diatoms, green and blue-green algae, and the flora seems to be par- ticularly well-developeo! in cultivated soils. F. Esmarch2 studied the soil flora of the German African colonies and of Schleswig-Holstein, W. W. Robbins3 that of the Colorado, and J. B. Petersen 4 the diatoms of the Danish soils ; Dr. B. Muriel Bristol of Rothamsted 5 the flora of certain English soils. The algae occur to a depth of 12 inches and in some cases even 2 feet : the species in the lower layers are almost identical with those at the surface. It is now known that certain algae can develop in darkness provided suitable nutrients are present, and assuming these conditions there is nothing against the possibility of a trophic alga-flora in the soil. Esmarch attempted to obtain positive evidence by burying algae in the soil and making microscopic examinations after given intervals of time ; in these circum- stances the blue-green algae retained their colour for some weeks, but ultimately became yellowish, and the filaments disintegrated. 1 B. Frank's paper is in Botan. Ztg., 1891, 9, 244-253, where references to his earlier papers are given. Recent investigations have been made by M. C. Rayner, Annals of Botany, 1915, 29, 97-133. 2F. Esmarch, Beitrag zur Cyanophyceen-Flora unserer Kolonien (Jahrbuch der Hamburglschcn wisscnsch. Anstalten, xxviii., 3 Beiheft, 62-82, 1910). Unter- suchungen fiber die Verbreitung der Cyanophyceen aufund in verschiedenen Boden (Hedwigia, Band lv., Heft 4-5, September, 1914). 3 W. W. Robbins, Alga in some Colorado Soils (Bulletin 184, Agri. Exp. Sto., Colorado, June, 1912). 4J. B. Petersen, Danske aerofile alger (D. Kgl. Danske Vidensk. Selsk. Skrifter, 7, Raekke, Naturv. og Mathem., Afd. xii, 7, 1915). 5 B. M. Bristol, On the Alga-Flora of some Desiccated English Soils (Annals of Botany, 1920, 34, 35-80). THE MICRO-ORGANIC POPULATION OF THE SOIL 255 Considerable numbers of species have been found, but estimates of total numbers are rendered uncertain by the breaking up of the filaments during the laboratory opera- tions. There has been some speculation as to the possible function of algae in the soil. Assuming that they continue to assimilate CO2 they are, of course, accumulators of energy and of carbo- hydrate material, and while they may thus function at the surface in the light it is difficult to see how they can be other than consumers in the dark recesses of the soil. The possible effects of this accumulation of organic matter in newly formed soils is discussed by Treub l in his account of the recolonisation of Krakatoa after the eruption of 1886, and by Fritsch.2 There is also the possibility that algae may take part in nitrogen fixation in the soil. It was at first thought by Frank 3 and by Schloesing and Laurent 4 that algae could themselves fix nitrogen, and this view has recently been revived by Moore and Webster.5 Kossowitsch,6 however, maintained that they can- not, bacteria alone having this power. He observed, however, that the presence of algae facilitated the process, and concluded that symbiosis occurs bet ween algae and nitrogen-fixing bacteria, the algae supplying the necessary carbohydrate energy material and the bacteria furnishing the nitrogen required for growth. Nakano 7 shows that the relationship with azotobacter holds not only for blue-green but also for certain green algae. 1 Treub, Notice sur la Nouvelle Flore de Krakatau (Ann. yard. hot. Buiten- zorg, Vol. 7, 1888, p. 213). 3F. E. Fritsch, The Role of Algal Growth in the Colonisation of New Ground and in the Determination of Scenery (Georg. Journ., November, 1907). 3 B. Frank, Ueber den experimentallen Nachweis der Assimilation freien Stickstoffs durch erdbodenbewohnende Algen (Ber. der D. Bot. Ges., vii., 1889, pp. 34-42). 4 Schloesing, fils, and E. Laurent, Recherches sur la fixation de Vazote libre par les plantes (Ann. de VInstitut Pasteur, vi., 1892, p. 65-115). 5 Proc. Roy. Soc., 1920, B. 91, 201. 6 P. Kossowitsch, Untersuchungen iiber die Frage, ob die Algen freien Stickstoff fixtren (Bot. Zeit., 1894, Heft 5, pp. 98-116). 7 H. Nakano, Untersuchungen iiber die Entwicklungs- und Ernahrungs- physiologie einiger Chlorophyceen (Jour, of Coll. of Science, Imperial Univ., Tokyo, vol. xl., 1917, Art. 2, p. 66, etc.). 256 SOIL CONDITIONS AND PLANT GROWTH Bouilhac l and Giustiniani 2 showed that in sand free from organic matter and nitrogen compounds, soil bacteria and algae not only develop normally, but also enrich the soil with nitrogen sufficiently to support the growth of higher plants ; while still more recently Pringsheim 3 has shown that the ability of bacteria to fix nitrogen is closely dependent upon the presence of blue-green algae (see p. 200). A third possibility discussed by Gautier and Drouin 4 is that algae may assimilate ammonia or nitrate in the soil and convert it into complex organic substances. The existence of an action of this kind in normal soils has been indicated by Russell and Richards (24 1£). Harrison and Aiyer (126) maintain that algae serve a special purpose in swamp soils, taking in CO2 and giving out oxygen, which is then available for the plant roots. 2. Fungi. It has long been known that fungi occur in the soil, and as long ago as 1886 no less than eleven different species were isolated by Adametz.5 Subsequent workers have greatly en- larged the list and more than 200 species have now been de- scribed as soil fungi. The investigations have fallen into several groups. Some workers, as Butler6 in India, Hagem7 in Norway, and Lendner* 1 R. Bouilhac, Sur la fixation de Vazote atmospherique par Vassociation des algues et des bacteries (Compt. Rend., cxxiii., 1896, pp. 828-830). 2R. Bouilhac and Giustiniani, Sur line culture de sarrasin en presence d'un melange d'algues et de bacteries (Compt. Rend., cxxxvii., 1903, pp. 1274-6). 3 E. Pringsheim, Kulturversuche mit chlorophyllfiihrenden Mikro-organ- ismen., III. Zur Physiologie der Schizophyceen (Cohns, Beitrage z. Biol. d. Pflanzen, Bd. xii., pp. 99-107). 4 Gautier and Drouin, Recherches sur la fixation de Vazote par le sol et les vegetaux (Compt. Rend., cvi., 1888, pp. 754, 863, 944, 1098, 1174, 1232, 1605). 5 L. Adametz, Untersuchungen uber die niederen Pilze der Ackerkrume (Inaug. Diss., Leipzig, 1886). 6E. J. Butler, An Account of the Genus Pythium and some Chytridiacece (Mem. Dept. Agric., India, Bot. Ser., 5, I, 1-160, 1907). 7 O. Hagem, Untersuchungen tiber norwegische Mucorineen (Videns. Skrift, I., Math. Nat. Kl., 1907, No. 7 ; Part II., 1910, No. 4). 8 A. Lendner, Les Mucorinees de la Suisse, Berne, 1908. THE MICRO-ORGANIC POPULATION OF THE SOIL 257 in Switzerland, have confined themselves mainly to particular types, and have not attempted a comprehensive survey of the whole field. Others have tried to ascertain how far facultative parasitic fungi could live saprophytically in the soil : apparently certain disease-producing Fusaria and Pythium can do this. Pratt 1 has isolated fungi which cause disease in potatoes from virgin desert lands and also from Idaho soils that have never been cropped with potatoes. The more serious problem of studying the fungus flora of the soil as a whole has been attempted in Holland by Ou de- mans and Koning (221), in the United States by Waksman (292^:), and in England by Miss E. Dale (75). It is now under investigation at Rothamsted by W. B. Brierley and Miss S. T. Jewson. The great difficulty is the lack of suitable methods of in- vestigation : neither the morphologists nor the physiologists have yet developed precise, simple methods. The fungi are invariably isolated from the soil by means of cultural media, but there is no means of ensuring that the medium used is suitable for the development of all forms that may be present : many of the parasitic forms and others in addition are doubt- less missed. Further, the morphological characters vary with different media : 2 hence it is impossible to describe the organisms as they exist in the soil. Nor are the physiological properties any more definite : they depend on the condition of the organism — the newly germinated fungi behaving differently from the older mycelium. In these circumstances rigid identification is a matter of great difficulty and it is impossible to reprehend too strongly the practice, so tempting to pioneers, to describe forms as new species, unless careful examination of their behaviour on different media has shown that they really are new. Again, there is no method of estimating even approxi- 1 O. A. Pratt, Journ. Ag. Res., 1918, 13, 73-100. 2 For a discussion of the phenomena see W. B. Brierley, Some Concepts in Mycology— An Attempt at Synthesis (Trans. Brit. Mycol. Soc., VI., part 2, 1919). 17 258 SOIL CONDITIONS AND PLANT GROWTH mately the numbers of various fungi in the soil. When used for fungi, the plating methods devised for bacteria and for protozoa suffer from the drawback that they involve the break- ing up of pieces of mycelium and the scattering of clusters of spores, with the result that a single fragment of fungus in the soil might appear as hundreds or even thousands of individuals on the plates. Finally, there is great difficulty in ascertaining the physio- logical effects of the soil fungi. Because a fungus brings about a particular reaction in a culture medium it by no means follows that the same reaction will occur in the soil where the conditions and the compounds are wholly different. In these circumstances the only safe method is to ac- cumulate observations and interpret them cautiously. The existence of a fungus flora is undoubted, and it is largely con- fined to the top 6 inches of soil. Formerly it was supposed (and the view was emphasised by Ramann, 2330) that fungi predominated in acid soils, and bacteria in neutral soils, but the evidence is not beyond criticism : fungi can, however, certainly produce and tolerate acidity. Little can be stated as to relative numbers in grass and arable land, or in poor and rich soils. The species most frequently found in temperate regions, and therefore presumed to be commonest, are the Penicillia and Mucors : in addition Fusarium, Aspergillus, Trichoderma and Cladosporium are common. In the warmer Southern States, however (e.g. Texas), Waksman (292^) found Aspergillus more frequently than Penicillium. In the absence of quantitative methods of estimating numbers or of physio- logical methods of estimating activity it is impossible to say how fungal activity varies with soil conditions. Like all other living organisms fungi require sources of energy and of nitrogen. All species can obtain their energy supply .from various carbohydrates, and most species can obtain it from cellulose, in which respect fungi differ from many bacteria. McBeth and Scales (185^) found the most active cellulose decomposers to be the Penicillia, Aspergilli, Trichodermae and Verticillia : on the other hand, the Mucorales THE MICRO-ORGANIC POPULATION OF THE SOIL 259 apparently effect little or no decomposition of cellulose (292^). The chemical changes involved are unknown, but it is unlikely that the reaction would proceed by itself in the soil ; in all probability the soil bacteria would participate. When ample supplies of carbohydrate (including cellulose) are present, the most economical nitrogen nutrients are the amino-acids (Czapek).1 In absence of sufficient supplies of carbohydrate, Waksman shows that the fungi can obtain both energy and nitrogen from soil protein compounds, but in this case more degradation occurs than is necessary to supply the nitrogen required, and consequently ammonia remains over (292^). Under suitable conditions certain fungi (e.g. Tricho- derma Koningi) can produce ammonia more rapidly than bacteria (McLean and Wilson, 1 88). Fungi have no power of oxidising ammonia to nitrates. They can, however, readily assimilate both ammonia and nitrates, and they have been supposed by Ehrenberg (93^) to cause considerable locking up of these compounds in the soil (see p. 210). It was at one time thought that fungi could fix gaseous nitrogen, but Duggar and Davis,2 in a careful series of ex- periments, were unable to obtain evidence that any fungus excepting Phoma Betae possessed this property.3 It is impossible on present knowledge to assess the im- portance of fungi in soil fertility. The decomposition of cellu- lose is undoubtedly beneficial and probably justifies the old view that fungi are the humus formers of the soil (Ramann, 233) : on the other hand, the locking up of nitrogen compounds is a disadvantage which, however, would be considerably counterbalanced if the substance of the mycelium were readily decomposable by bacteria on the death of the organism. 3. Actinomyces. This group of organisms, sometimes included in the fungi and sometimes in the bacteria, is of frequent occurrence in the 1 Beit. Chem. Physiol. u. Path., 1901-2, I, 538 ; 2, 557 ; 3, 47. 2 Ann. Mo. Bot. Gard., 1916, 3, 413-437. 3 According to Ternetz (Jahrb. f. wiss. Bot., 1907, 44, 353-408) Phoma radicis can also assimilate gaseous nitrogen. 17* 260 SOIL CONDITIONS AND PLANT GROWTH soil. Conn (job) estimated that they were one-quarter as numerous as the bacteria in arable soil and one-half as numerous in grassland. It is very doubtful, however, whether any counting method is reliable. Waksman and Curtis (292^) have made an extensive study of this group 1 and have de- scribed thirty or forty soil species. Apparently the actinomyces decompose cellulose though they do not readily produce ammonia from protein. It is said also that they can reduce nitrates to nitrites. 4. Bacteria. From time to time indications have been obtained that some of the soil bacteria bring about changes harmful to the plant, but the evidence is insufficient to justify any detailed discussion. It has been supposed that plant toxins are produced (p. 247), that soil nitrates are assimilated (p. 210), and that the food, air, and water which should otherwise be available to the plant are taken up by the micro-organisms.2 The subject that has been most frequently investigated is the part played by soil bacteria in the decomposition of the organic matter of the soil and the production of nitrates. Both changes are brought about by bacteria and they are of such obvious advantage to the plant as to suggest that re- lationships ought to exist between bacterial activity and the growth of plants. Several methods have been adopted to trace such relationships. The direct method consists in picking out definite organisms and studying them in conditions calculated to throw light on their action in the soil. This has proved very difficult, and has been successfully achieved only by a few of the best bacteriologists ; instances are afforded by the work of Winogradsky (pp. 187, 196), Beijerinck (p. 197) and others. Three indirect methods have therefore been used : — 1For a morphological study see Charles Drechsler (Bot. Gaz., 1919, 67, 65-83, 147-168). 2 See, e.g., Dachnowski (Ohio), Expt. Sta. Record, 1910, 23, 122. THE MICRO-ORGANIC POPULATION OF THE SOIL 261 (1) Soil is inoculated into various media each arranged to bring out one group of organisms, and the amount of decom- position is taken as a measure of the number and vigour of the members of the group. This is often called the method of physiological grouping. (2) Platings on gelatin or agar media are made of soil suspensions suitably diluted, and the colonies which develop are counted. The results are expressed as millions of bacteria per grm. of soil. (3) Chemical analyses are made at stated intervals to determine the rate of progress of the various changes going on — the absorption of oxygen, the evolution of carbon dioxide, the production of nitrates, etc. The difficulty with the first or direct method is to imitate the soil conditions, and the history of the subject affords many instances of the danger of getting away from them : for example, Krzemieniewski's work on the nitrogen-fixing organisms may be quoted (p. 199). It is impracticable, for reasons already given, to keep to the soil as the medium for work, and most investigators have therefore used the indirect methods. Physiological Grouping. — This method was introduced by Remy (2370) and developed by Lohnis (182); it has become very popular. Four distinct media are in use, arranged respectively to favour nitrification, ammonia pro- duction, nitrogen fixation, and denitrification. The experi- ments are easy to carry out, but they require skilful interpretation, and the results may prove treacherous unless carefully handled. The fundamental objection to the method is that the reaction goes on in a medium very different from ordinary soil, so that it throws no light on the relationships obtaining in the soil itself. The results really only prove that the bacteria from one soil will flourish better in a certain artificial medium than those from another. The medium for studying nitrification is usually that sug- gested by Omelianski or Ashby's modification (p. 1 88); it is inoculated with a definite weight of soil and incubated : the 262 SOIL CONDITIONS AND PLANT GRO WTH nitrates produced after a certain time are determined. By working under uniform conditions (which each investigator fixes for himself) the results obtained are comparable for the series of soils under investigation.1 The amount of nitrate ^produced by unit weight of soil is called the "nitrifying power ". The actual figure is obviously arbitrary, depending on the conditions selected, and it has meaning only in relation to the other soils in the same set of experiments. Several investigators, however, have found that nitrifying power shows some relation to plant growth, the soils most favourable to plants having, on the whole, the highest nitrifying power. This result has been obtained by Gainey in Kansas 2 ; C. B. Lipman in California 3 ; G. P. Given in Pennsylvania 4 ; Percy Brown in Iowa (6o#) ; P. S. Burgess in Hawaii (63); at Rothamsted by Ashby (7#) ; and at Fallen, Nevada, by Keller- man and Allen 5 ; some of the data are given in Table LXVII. TABLE LXVII.— NITRIFYING POWERS OF VARIOUS SOILS OF KNOWN PRODUCTIVENESS. Kellerman and Bacterial Ashby, Rothamsted (ja). Nitrifying Power. (Fallen, Nevada) (Order of Nitrifying Power. Numbers. Millions per Productiveness). grm. Agdell Field, A. 3, \ Very productive 54 0'02 most productive J 93 Pro- r Plot 40 20 0'2I Agdell Field, A. 2, \ intermediate J 38 ductive J Plot 190 plots t Plot 290 36 30 0-003 0'27 Agdell Field, A. i, ~\ poorest J 26 (Plot ro Plot 30 4 3 0-44 0-16 Plot 1 80 5 0-06 Withers and Fraps (313) have modified the method, and use sterilised soil as the medium for the growth of the 1 For a statistical study of the magnitude of the error see D. D. Waynick, Univ. Calif. Pub. Ag. Sci., 1918, 3, 243. 2 Soil Sci., 1917, 3, 339-416. sProc. Soc. Prom. Agr. Sci., 1914, 33-39, and Cal. Bull., 260, 107-127. *Penn. Kept., 1912-13, 204-206. 5 Bacteriological Studies of the Soil of the True kee-C arson Irrigation Project, Karl F. Kellerman and E. R. Allen (U.S. Dept. of Agric. Bureau of Plant In- dustry, Bull. No. 211, 1911). See also Ehrenberg (936). THE MICRO-ORGANIC POPULATION- OF THE SOIL 263 organisms. A careful distinction must be made between the nitrifying power ascertained from culture media and the rate at which nitrates accumulate in the soil. The experiments in culture media measure the rate of nitrification under the circumstances of the experiment : the accumulation of nitrate in the soil, on the other hand, measures the rate of ammonia production (p. 1 88). The "ammonifying power" or " putrefactive power" is determined by inoculating soil into a I per cent, peptone solution, and determining the ammonia formed after incubation at 20°. Remy found that certain soils known to give good crop returns for organic manures also possessed high putre- factive power. He incubated for four days, but Russell and Hutchinson (241^) obtained better results by taking definite intervals and plotting curves showing the respective rates of ammonia production by the different soils. Lohnis has used this method a good deal (i8i£) as also has J. G. Lipman, who, however, modifies it considerably, and among other things uses sterilised soil as the medium and substitutes dried blood or cotton-seed meal l for peptone (176). Percy Brown (60) used a similar modification in his studies of Iowa soils, and found that the "ammonifying power" ran along with the "nitrifying power" and, in four out of the six plots, with the crop-producing power also (Table LXVIIL). Other workers have observed a general similarity between ammonifying power and productiveness which, however, frequently breaks down in individual cases. Further, the differences between good and poor soils are not particularly marked and would often be considered to lie within the error of the experiment. The relationship with productiveness is therefore less definite than in the case of nitrification.2 1 Sackett (Colorado Bull., 184, 1912) has shown that these cannot be used indiscriminately ; in some soils dried blood is ammonified more rapidly than cotton-seed meal while in others the reverse holds. 2 For evidence to this effect see P. S. Burgess (63), Gainey, Soil Sci., !9i7» 3. 399; Kelley, Science, 43, 30-33, and Hawaii Bull., 37, 1915, p. 52 ; Stevens and Withers (271) ; J. G. Temple, Ga. Bull., 126, 1919. 264 SOIL CONDITIONS AND PLANT GRO WTH TABLE LXVIII. — BACTERIAL ACTIVITY IN SOILS OF KNOWN PRODUCTIVENESS. P. E. BROWN ( oa). IOWA SOILS. Plot No. Yield of Maize. Bushels per Acre. History. Ammonifying Power. Nitrifying Power. Nitrogen- Fixing Power. Bacterial Numbers. Millions per grm. 607 527 2 year rotation, maize, oats, clover ploughed in .... 175 7'I I43 2-8 604 507 3 year rotation, maize, oats, and clover I89 12-6 2O'6 3'3 602 46-0 2 year rotation, maize and oats . I78 8-1 J7'5 2-6 go i 43'2 2 year rotation, maize, oats, rye ploughed in 175 67 *4*3 ' 2'5 601 35'5 Continuous maize . . 171 5*o 9'5 2'I 609 32-5 2 year rotation, maize, oats, cowpeas ploughed in 1 80 11-9 18-2 2'7 P. S. BURGESS (63). HAWAII SOILS. (Plots Grouped in Order of Merit.) Best. Very Good — not Placed in Order. Poorer. Least. Yield of sugar: Plot No. 2 18463 5 7 9 Ammonification : dried blood: Plot No. 2 19468 3 7 5 Mgrms. of added nitrogen ammoni- fied after 10 days 100*2 76*4 75*9 74-5 68-6 57-7 57*1 47*6 26-6 Alfalfa meal 2 9364 i 5 Mgrms. of added nitrogen an moni- ried after 10 days 14-8 14*0 12*9 ii*2 9'8 9*5 67 — I-4 Nitrification : dried blood 2 34681 7 5 9 Mgrms. of added ni- trogen nitrified after 30 days 20'8 20*0 18-0 17-2 16*8 15-2 13*6 4-0 4-0 Alfalfa meal 2 74386 1 5 9 Mgrms. of added nitrogen nitrified after 30 days 15-2 — 12*8 12*0 10*0 9-6 9*0 7*2 4 '5 Nitrogen rendered water soluble : dried blood 9 26348 7 r 5 Mgrms. of nitrogen re- covered after 20 days 31-I 29*4 27-0 24-4 23-9 22'8 22*3 18*3 10-3 Nitrogen fixation (man- nite solution) 2 86431 7 9 5 Mgrms. nitrogen fixed 1 per grm. of mannite II*2O io'64 7*28 7-00 6*44 5-60 3*92 3-60 2 'So Good Azotobacter surface membrane. Doubtful Azotobacter. No Azoto- bacter. THE MICRO-ORGANIC POPULATION OF THE SOIL 265 " Nitrogen-fixing power " is measured by inoculating soil into Beijerinck's or some similar solution (p. 197) and in- cubating for a definite time. This reaction proceeds only slowly. The results usually show a general resemblance to those given by nitrification experiments.1 Bacterial Counts. — The method of counting the number of colonies that develop on gelatin or agar plates is admittedly faulty, but it has the advantage of showing whether the numbers are high or low and whether they are increasing or decreasing. It has, unfortunately, three serious defects. No medium is known that brings out all the soil organisms, so that the results are invariably low,2 and their quantitative appearance is wholly illusory. No medium even distantly resembles the soil in composition or in structure, so that the flora developing on the plates does not necessarily reflect the flora' active in the soil ; in particular it is impossible to tell which of the forms developing on the plate are active and which are spores in the soil. Account is seldom taken of the kinds of bacteria on the plates; in practice it proves far too laborious to attempt any but the simplest identifications. This disregard of the nature of the bacteria constitutes a fundamental distinction from the method dependent on physi- ological grouping, and the two methods do not always give similar results. The counts show fairly correctly whether any given treatment of the soil has raised or has lowered the number of bacteria, but unless the change has been drastic they do not show whether all varieties have been equally affected. Thus they have always to be combined with deter- minations of the amounts of ammonia and nitrate in the soil. Chemical Analysis in Conjunction with Bacterial Counts. — The third method of making bacteriological counts in con- junction with chemical analysis, has been largely used in the Rothamsted laboratories. Increases in bacterial numbers are so often associated with increased production of nitrate as 1 E.g. see papers by P. E. Brown ; also P. S. Burgess. 2 Comparisons of various media have been made by Cook (Soil Sci., 1916, I, 153-101). The more uniform the results the better the medium. 266 SOIL CONDITIONS AND PLANT GRO WTH to justify the assumption that the two phenomena are causally connected. Two cases have, however, been studied where no such relationship exists. (i) The soil treatment, while raising the total numbers, has either acted differentially on the organisms and did not encourage the ammonia producers to develop, or it has caused them to effect some decomposition that does not give rise to ammonia. (See p. 186.) The addition of certain organic compounds to the soil has this effect (Table LXIX.). TABLE LXIX. — EFFECT OF CERTAIN ORGANIC SUBSTANCES ON BACTERIAL NUMBERS AND ON NITRATE PRODUCTION. Bacterial Numbers Ammonia and After 50 Days I Nitrate Present (millions per grm.). After 50 Days. Q , AAA A In In In In Observers. Control Treated Control Treated Soil. Soil. Soil. Soil. Russell and Cane sugar (0-25 per cent.) 21 51 32 20 Hutchinson (24Oa) Amyl alcohol (0*1 per cent.) Phenol (M/2OO per kilo) l . 30 27 85 IOI 37 30 35 33 Buddin (640.) Hydroquinone (M/2OO per kilo) .... 16 55 35 44 »» (2) Even when the ammonia-producing organisms are caused to multiply they do not increase the stock of ammonia and nitrates in the soil beyond a certain limiting amount. Thus partial sterilisation increases bacterial numbers and usually increases the amount of ammonia and nitrate also, but it fails to do this after a certain quantity has accumulated (Fig. 24). On looking over the figures in Tables LXVII. and LXVIII. (p. 262) it is evident that the numbers of bacteria revealed by this method bear no relationship to the amount of crop growth. Other field experiments have given similar results. Yet in laboratory and pot experiments bacterial counts have often See p. 212. THE MICRO-ORGANIC POPULATION OF THE SOIL 267 Bacterial Numbers. NH3 and Nitrate. Limit 200 100 200 100 UNTREATED SOIL .''' UNTR E ATEDSpjL 19 40 70 130 19 40 70 130 Days Case i. — Small amounts of NH3and nitrate initially present. A relation- ship is indicated between bacterial numbers and the rate of production of NH, and nitrate. -250 150 50 Bacterial Numbers. TOLUENED SOIL 500 300 UNTREATED SOIL 100 NH3 and Nitrate. Limit • 14 14 . - 110 Days Case 2. — Large amounts of ammonia and nitrate initially present. No relationship like that in Case i is indicated. FIG. 24. — Relation between bacterial numbers and amounts of nitrate and ammonia formed. 268 SOIL CONDITIONS AND PLANT GRO WTH proved most valuable ; numerous instances are given in the Rothamsted papers on soil sterilisation (241). This discrepancy between field and laboratory experience is cleared up by a closer examination of the nature of the relationship between bacterial activity and plant growth. The connection, as already pointed out, lies in the fact that bacteria decompose the organic matter of the soil and make new plant food out of old plant residues. If the factor limiting plant growth happens to be the supply of nitrogenous plant food or the rate of decomposition of the plant residues we may expect to find a close connection between bacterial activity and soil fertility ; if, on the contrary, the limiting factor is something else — such as water supply, lack of phos- phates, etc. — no such connection is necessary. Even here, however, a connection may exist, for bacteria are living things, affected by the same circumstances that influence plants. Three distinct cases, therefore, arise : — (1) Bacterial activity may show no sort of relationship with soil fertility, because fertility is limited by some factor other than the nitrogen supply or rate of decomposition of plant residues. (2) Bacterial activity may be directly related to soil fertility but the relationship is only accidental, both bacteria and plants being affected by the same limiting factor. (3) Bacterial activity may be directly related to soil fertility and the relationship is causal, fertility being limited by the amount of ammonia and nitrate produced by the bacteria or by other decompositions which they effect. Instances of the first are common in arid and semi-arid districts. The second case is not unfrequent. An admirable illustra- tion is afforded by the experiments of Crowther and Ruston on the effect of acid rain-water on plant growth (71). The pots were watered with solutions of sulphuric acid, some being of the same order of concentration as the Leeds rain-water. The acid depressed the growth not only of plants but of bacteria also, and the effect is very similar in both cases (Table LXX.). THE MICRO-ORGANIC POPULATION OF THE SOIL 269 TABLE LXX. — EFFECT OF ACIDULATED WATER ON THE GROWTH OF PLANTS AND BACTERIA. CROWTHER AND RUSTON (71). ields Dry , Grms. I ft It bo c ^ ||Ss Absorp- mm. i and Nit- in Soil at icperiment lillion. >4 C p l£ I2 ll'ls. y i^s S"^0^ • ° Z 0 III Soil watered with Garforth rain-water, neutralised 147 13-9 I'O2 4-6 5*2 12 9*3 i part H2SO4 per 100,000 I2'0 I2'I 0'80 3'3 i '3 8 6-2 2 „ 8-0 11*2 0*85 3-0 i-i 6 8-2 4 » 3'9 I0'5 0-52 2-8 07 4 8-1 8 „ 3'7 io'3 0-36 2*4 O'l 4 8-4 16 „ nil io'3 0-28 1-9 o'O4 3 8-1 32 „ , nil 8-1 0-13 1-8 0-015 0 9'4 At first sight this looks like a close relationship between bacterial activity and plant growth. But the figures in the last column show that the failure of the crop is not due to the failure of the bacteria to produce ammonia and nitrate, for relatively large amounts of these substances are left at the end of the experiment. In similar manner the growth both of bacteria and of plants may be helped by the same cause. This is more fully discussed in the following section. Speaking generally, it is found that the bacterial numbers increase as the intensity of the farming increases. Thus Stoklasa and Ernest (2730) found only I to 2 million organisms per grm. in their barley land, 3 to 5 millions on the better treated sugar-beet land, and 7 to 8 millions on the clover land. Again, the addition of plant residues or of farmyard manure to the soil increases the bacterial numbers by furnishing the organisms with addi- tional food ; l it also increases the crop. Moorland soils contain only few bacteria and are very unsuited to the growth of most plants. But after cultivation and treatment with lime and manures they become much better media both for plants and bacteria. Fabricius and von Feilitzen (97^) found 1 Examples are given by P. E. Brown, Iowa Research Bull., No. 13, 1913, and by Heinze, Landw. Jahrbiicher, 1910, 39; Erganz. Bd. 3, 314-343. 270 SOIL CONDITIONS AND PLANT GROWTH OT million of bacteria per grm. in the raw moorland soil, but 7 millions in similar soil that had been cultivated and manured. So close is the similarity between ordinary crops and azotobacter in relation to soil acidity that an azotobacter test is used in Denmark for determining the need of lime (p. 242). It may often be difficult in practice to determine whettfer the relationship between the bacterial numbers and plant growth is causal or accidental, but the principle is perfectly clear ; the relationship is causal only when the plant growth is limited by the supply of compounds produced by bacterial activity, or the rate at which plant residues or harmful substances such as phenol, thiocyanates, etc., are decomposed by bacteria. The recognition of this central principle greatly facilitates investigation, for it shows the futility of haphazard attempts to correlate bacterial activity and plant growth over a set of soils that are not strictly comparable. The better course is to narrow down the problem and confine it to the elucidation of the connection between bacterial activity and nitrate production. Effect of Soil Conditions on Bacterial Numbers and on Nitrate Production. On general grounds it might be expected that the soil bacteria would be affected by external conditions in much the same way as plants, and to a considerable extent this happens. Most of the effects observed with growing plants have been paralleled in the case of the soil bacteria. There are, however, nearly always differences between the effects produced on the bacterial numbers and on the amount of work done : one may be increased but not the other. Discussion of the effects of external' conditions on bacterial numbers is much hampered by the paucity of data. Observers have often been content to make counts once in twenty or thirty days, and they have usually assumed that changes in numbers are slow. Cutler and Crump have shown (73^) that this supposition is unfounded : changes proceed rapidly from day to day. I OOP Soil + Ammonium Sulphate. Temp. 13° C. 15% 66 locr Additional Nitrate found in Soil &lone. (Lo&rn Gottingen) 60 40 20 3% 5% 26 66 FIG. 25. — Effect of moisture on nitrification process in the soil (Traaen). 272 SOIL CONDITIONS AND PLANT GRO WTH Inspection of their curves shows that reliable conclusions cannot be drawn when counts are taken at long intervals. Effect of Temperature. — Bacteria being living organisms it is natural to expect that their activity increases with the temperature up to a certain point. The amount of nitrate does show this expected increase but the bacterial numbers do not, there being no steady rise as the temperature of storage increases (Table LXXL). Field observations lead to the same conclusion (p. 275). TABLE LXXL — EFFECT OF TEMPERATURE OF STORAGE ON BACTERIAL NUMBERS AND NITRATE PRODUCTION. RUSSELL AND HUTCHINSON (2410). Bacteria, Millions per Grm. of Dry Soil. Nitrate and Ammonia, Parts per Million of Dry Soil. 1 emperature of Storage. At After After At After After Start. 10 Days. 50 days. Start. 10 Days. 50 days. 7°-I2° 16 16 16 I? 18 22 20° 12 21 16 3° 30° 15 14 24 36 40° 9 M 55 76 Effect of Moisture. — Increasing moisture supply causes in- creases in bacterial numbers, but they are not regular (24 if). Field observations give the same result (p. 275). The rate of nitrate production increases up to a certain point, beyond which it decreases, presumably because of the lack of air. Traaen's results are plotted in Fig. 25.1 On the practical side it is found that application of irrigation water in arid regions has a distinctly beneficial effect on the ammonifying and nitrifying powers, both of cropped and fallow soils, and on the numbers of organisms on fallow soils. Excess of water, however, washes out the resulting nitrate from the soil and so deprives the plant of the advantage it would otherwise gain. (J. E. Greaves, R. Stewart, and C. T. Hirst ,113^).) Effect of Added Organic Matter. — As shown on p. 252 the 1 Centr. Bakt. Par., 1916, 45, 119. Other results are plotted in Greaves and Carter's paper (113^), where also a full bibliography is given. THE MICRO-ORGANIC POPULATION OF THE SOIL 273 effect of added organic matter is to increase supplies of energy and therefore to increase bacterial numbers. The question whether nitrate supplies will be increased depends on the pro- portion of nitrogen present in the added matter : if it is rich in nitrogen a considerable amount of nitrate may be formed ; l if it contains no nitrogen there arises an actual loss of nitrate though there may be a fixation of gaseous nitrogen. The special case of farmyard manure has been much studied.2 Effects of Lime, Calcium Carbonate, and Magnesium Car- bonate.— Numerous papers have been written on this subject and the results are at first sight somewhat contradictory. Much of the work has been done on acid soils, where, as might be expected, benefit has been derived from neutralisa- tion. F. E. Bear3 finds that calcium carbonate greatly in- creases the numbers of bacteria in acid soils once the neutral point is passed, but not before : it much increases ammonifi- cation and nitrification up to the neutral point, but to a less extent afterwards. In neutral soils, however, less concordant results have been obtained ; some investigators have observed detrimental effects from further addition of calcium carbonate,4 while others have obtained only beneficial results,5 bacterial numbers, ammonifying power and nitrifying power all being increased. Magnesium carbonate may be more effective than calcium carbonate in small quantities, but it is toxic in larger amounts.6 Lime in excess of a certain amount acts as a sterilising agent (139^). 1 For the effect of the N : C ratio see P. E. Brown and F. E. Allison, Soil Sci., 1916, i, 49-75 ; and also H. B. Hutchinson and J. Clayton (i^gf). 2 P. 193, also T. G. Temple, Georgia Rpt., 1916 ; Greaves and Carter, Journ. Ag. Res., 1916, 6, 889. 3 Soil Sci., 1917, 4, 433. 4 E.g. J. G. Lipman, P. E. Brown, and I. L. Owen, Centr. Bakt. Par., 1911, 30, 156-181. 5 S. S. Peck, Hawaiian Sugar Planters' Chem. Bull., 34, 1911 ; P. E. Brown, Iowa Research Bull., 2, 1911; Bull., 44, 1918; J. E. Greaves, Soil Sci.,igi6, 2, 443-480; H. L. Fulmer, Journ. Agric. Res., 1918, 12, 463-504, although he obtained harmful effects on nitrification. 6J. G. Lipman and P. E. Brown, N.J. Agric. Expt. Sta. 2$th Ann. Rept., 141-204 : C. B. Lipman and P. S. Burgess, Journ. Agric. Sci., 1914, 6, 484-498 ; H. L. Fulmer, Journ. Ag. Res., 1918, 12, 463. 18 274 SOIL CONDITIONS AND PLANT GROWTH Effect of Salts.1 — Alkali salts adversely affect bacteria somewhat as they do green plants : in India a sufficient de- gree of similarity exists to allow the wheat-yielding power of an alkali soil to be estimated from such bacterial activities as CO2 production, ammonification, etc.2 The prob- lem has been much investigated in the Western United States where alkali salts are apt to cause trouble. System- atic investigations have been made by C. B. Lipman in California (175^) and by J. E. Greaves and his co-workers in Utah (i 1 3). Typical results are given in Table LXXIL The ammonifying organisms are usually stimulated by small con- centrations of " alkali salts" but adversely affected by larger ones, though they are less susceptible than wheat seedlings The effect is not constant but varies with differences in soil and conditions.3 As a general rule chlorides are the most toxic salts, while nitrates, sulphates, and carbonates are suc- cessively less toxic ; it is suggested that the electronegative ion plays the more important part (113). Nitrifying organ- isms, however, are more susceptible than ammonia producers and the effects are determined by the specific compound rather than by one ion. There is also a well-marked anta- gonism of ions (e.g. Ca and K ; Mg and Na ; K and Na ; (Na2)CO3 and (Na)Cl) 4 as in the case of green plants (p. 79). Salts of arsenic,5 copper,6 lead, zinc, and iron appear to be capable, in suitable concentration, of stimulating nitrifying organisms but not ammonification. Nutritive salts have a marked effect. Phosphates notably increase all types of bac- terial activity in the soil : 7 potassium salts have acted well in 1 For a summary of the extensive literature see J. E. Greaves, Soil Sci., 1916, 2, 443-480. a J. H. Barnes and Barkat Ali, Ag. Journ. India, 1917, 12, 368. 3W. P. Kelley, Journ. Agric. Research, 1916, 7, 417-437. 4 C. B. Lipman, Bot. Gaz., 1909, 48, 106, and Centr. Bakt. Par., 1914, 41, 430-444 (with P. S. Burgess) ; see also J. E. Greaves, Soil Sci., 1920, 10, 77-102. 5J. E. Greaves, Science, 1917, p. 204. 6C. B. Lipman and P. S. Burgess, Univ. Cal. Pub. Agric. Sci., 1914, I, 127-139. 7 E. B. Fred and E. B. Hart, Wisconsin Research Bull., 35, 1915 ; and Centr. Bakt. Par., II., 1916, 45, 379 ; G. P. Koch, Journ. Biol. Chem., 1917, 31, 411 ; see also p. 203. THE MICRO-ORGANIC POPULATION OF THE SOIL 275 certain soils,1 but not on all. Nitrates increase the amount of ammonification,2 the growth of the organisms causing decay (176^), the nitrogen fixing azotobacter (p. 199) and of nitrate assimilating organisms (113^), though in higher quantities they become toxic. TABLE LXXII. — PERCENTAGES OF VARIOUS SALTS IN LOAM SOIL WHICH ARE NECESSARY TO REDUCE AMMONIFICATION, GERMINATION AND DRY MATTER PRODUCED IN WHEAT TO ABOUT HALF NORMAL. J. E. GREAVES, E. G. CARTER, AND H. C. GOLDTHORPE (lisa AND c). Wheat Seedling to Half Normalj Ammonification to Half Normal. Nitrification to Half Normal. Sodium chloride O*2O 0-117 0-234 Calcium ,, •30 *222 Potassium „ •25 •298 •298 Magnesium ,, •40 •38l •006 Potassium nitrate •40 •607 •IOI Sodium ,, •30 •850 •170 ,, sulphate "55 •852 •568 Magnesium „ •70 •963 •36l Sodium carbonate •30 I-I66 '212 Magnesium nitrate "45 1-187 •074 Potassium sulphate •60 I '394 •349 „ carbonate 70 1-520 •I3« Effect of Dissolved Oxygen Supply, — Russell and Apple- yard (241^-) found that rainfall has a more definite effect than moisture in increasing bacterial numbers and biochemical changes in the soil. They attribute this action to the oxygen dissolved in the rain-water which renews the dissolved atmos- phere in the soil and gives the organisms a new lease of activity. > Effect of Previous Treatment of the Soil. — Prolonged drought affects the soil even after it has passed away and the soil has become moistened. The rate of production of nitrate and the bacterial numbers both increase (2^c and also Prescott 3). Field Observations. — The general phenomena observed in the laboratory can be seen also in the field, but it is less easy to disentangle the various factors. aj. Dumont, Compt. Rend., 1897, 125, 469-472. 2 D. A. Coleman, Soil Sci., 1917, 4, 345. 3 A note on the Sheraqi soils of Egypt (jfourn. Ag. Sci., 1920, 10, 177.) 18* 276 SOIL CONDITIONS AND PLANT GRO WTH As in the laboratory experiments (p. 272) the bacterial numbers show no relation to the temperature and moisture content of the soil. Numerous counts were made by Hiltner and Stormer (135^) from plots of cropped ground, and of un- manured and dunged fallows. Some of these are recorded in Table LXXIII. TABLE LXXIII. — BACTERIA IN CROPPED AND FALLOW SOILS, MILLIONS PER GRAM. HILTNER AND STORMER (135^). 1901. 1902. 10 May. 27 Aug. 18 Oct. i Feb. 12 June. 18 Aug. Cropped land, grass and clover Cultivated fallow, unmanured . „ „ dunged1 8-0 II*O 4-2 10-5 II'O 6-6 4'i 9*3 8-1 57 7'2 4'9 8-4 The only marked effect is that of the dung ; the net result of the clover and grass has only been small in spite of the organic residues shed by the roots. On no plot has the warm summer weather increased the bacterial numbers. Later on Engberding (94) made a more extensive series of counts of the bacteria in plots of ground under known treat- ment and published his results in very complete form, giving details of temperature, moisture content, etc. Here again no connection could be traced between temperature or moisture content and bacterial numbers. A very similar result was obtained by W. E. King and Doryland (148). Neither tem- perature nor moisture changes explained the observed fluc- tuations in bacterial numbers.2 Some of Waksman's results (292*2) with a garden soil at New Jersey are given in Table LXXIV. Russell and Appleyard (241^) have studied the problem in some detail at Rothamsted, and have determined the bacterial numbers, and the nitrate in the soil, and the amount 1 Dung applied in July at the rate of 130 to 140 Centner pro Morgen (10 to ii tons per acre). 2 In these experiments, however, one cubic millimetre only of the soil sus- pension was used for the plate cultures. THE MICRO-ORGANIC POPULATION OF THE SOIL 277 of CO2 in the soil air, at frequent intervals for a period of two seasons on several of the plots. The experimental data do not represent the amounts of production but only of accumulation ; nevertheless the curves are found to give useful information as to production. TABLE LXXIV. — BACTERIA IN GARDEN SOIL, NEW JERSEY, MILLIONS PER GRAM. WAKSMAN (2920). $** 3 M « vd 00 co tx co 0 • i VO "* Date of Sampling ^S .Q JS £ d >> >• bo £ > o»a fe £ 1 •< § j O z Nos. of bacteria, millions per grm. 87 6-0 8-8 4'3 10-7 4'8 6-9 7'8 5'o 8-8 5 '9 6-9 9'2 Moisture content, per cent. 15 14 13 8 9 9 9 9 12 10 9 9 19 These figures are for the top inch : samples were taken at six different depths with similar results. In the first instance it is observed that the curves are all sufficiently alike to justify the view that in the soil, as in the laboratory, the phenomena of nitrate production and CO2 evolution are closely related to the numbers of bacteria. When the bacterial numbers rise there is a rise in the amount of COg and of nitrate. But the nitrate curve does not sharply agree with the others : it is displaced, showing a lag of two or three weeks. The general results are illustrated by the curves in Fig. 26. During the winter months there is very little activity. But as soon as the temperature rises above 5° C. change sets in : bacterial numbers, COg,1 and nitrates all increase. The rise, however, is not long sustained ; it is followed by a fall, not- withstanding the continuance of favourable temperature con- ditions. To some extent this is due to lack of moisture, for the curves now begin to resemble the soil moisture curves. It is also due to lack of something supplied by rain — presum- ably dissolved oxygen — for the rainfall curves more closely fit the CO2 and bacterial number curves. This period of summer 1 The very sharp rise of CO2 in May and August appears to be associated with the crop (241^). 278 SOIL CONDITIONS AND PLANT GRO WTH •Bug-peg UJBJQ jad suoi||i|fl xd I I I ' il CO Nirrar Bacre JE?_. :|UI jei|doj. • J^T-Wg j9]_3JEj£iN JSB Z|N OO>«Or-»«OVO*t THE MICRO-ORGANIC POPULATION OF THE SOIL 279 sluggishness is followed by one of considerable activity in autumn and this in turn by the period of winter inertness. The growing crop appears to affect the biochemical changes (P- 296). These periods of spring activity, summer sluggishness, and autumn activity seem to be fairly general, and they have been recorded elsewhere. Using Remy's method of physiological grouping Lohnis and Sabaschnikoff at Leipzig (181^) obtained a curious and wholly unexpected set of curves suggesting some remarkable seasonal relationships. The urea-decomposing power, nitrify- ing power, nitrogen-fixing power, and to a less extent the denitrifying power, all reached a maximum in spring, a minimum in summer and a maximum again in September. Miintz and Gaudechon (209) also showed by a somewhat different method that the nitrifying power is at a maxi- mum in spring. Conn (70) obtained a similar curve for the bacterial numbers in his plots : the numbers of bacteria being high in February when the land was frozen, they fell in summer but rose again in autumn (Fig. 27). Brown and Smith at Iowa (6o 2. Where the gravel or rock is not too near the surface, systematic green manuring with lupines and other crops fertilised by potassium salts and calcium carbonate will often effect sufficient improvement to make cultivation profitable. Examples are afforded by the Schultz-Lupitz estate, Germany (255), and Dr. Edwards' experiments at Capel St. Andrews, Suffolk, and at Methwold. On such land an industrious cultivator may make a living but not a fortune. Under favourable conditions recourse may be had to 1 See F. W. Oliver and E. J. Salisbury, Topography and Vegetation of Blakeney Point (Trans. Norfolk and Norwich Nat. Soc., 1913, 9, 485-543) THE SOIL IN RELATION TO PLANT GROWTH 315 dressings of clay (as in Lincolnshire) or to warping (in the Fens). Barren conditions also result when, by reason of a thin parting of clay or its low situation, water cannot run away but accumulates and forms a marsh. Reclamation in such cases is possible as soon as a way out has been found for the water.1 Loams. — As the proportion of fine material in the soil increases and that of coarse material falls off, a gradual change in the character of the soil sets in, till finally, but without any sharp transition, a new type is reached known as a loam. The increase of fine material somewhat retards the movements both of air and of water, so that loams are characterised by a more uniform water content throughout the mass than sands. On the other hand, loams show less tendency to become water-logged or to allow plants to become parched in very dry weather than clays. The soil decompositions proceed normally, rapidly producing plant food, with little tendency to "sour"2 or other abnormal conditions so long as sufficient calcium carbonate is present. In consequence, most plants will grow on loams, even some of those supposed to be specially associated with some other soil type. Thus, where a chalk and a loam soil meet, it is not uncommon to find the chalk plants, e.g. traveller's joy (Clematis Vitalba\ guelder rose, etc., wandering on to the loam, and it is much more difficult to find the line of separation of the soils than where the chalk abuts on to a sand or a clay. For the same reason loams allow of very wide choice in the systems of husbandry, and, as they become very fertile under good management, they are usually in this country all cultivated. Closer observation over a limited area shows, however, that a given class of loam is more suited to one crop than to another ; the ecologist recognises differences in the sub-associations or facies, and the practical man will distinguish between a potato soil, a barley soil, a wheat soil, etc. : distinctions due, no doubt, to water 1 For a fuller discussion of land reclamation see E. J. Russell, Journ. Roy. Ag. Soc., 1919, 80, 112-132. 2 See p. 118. 3i6 SOIL CONDITIONS AND PLANT GRO WTH and air relationships, and arising from differences in the com- pound particles. Unfortunately no method of investigating the compound particles has yet been devised, and a study of the ultimate particles by a mechanical analysis is alone possible. But even though the differences are thus attenuated they can still be traced, as shown by the analysis in Table LXXXII. of soils in Kent, Surrey, and Sussex, known to be well suited to the particular crops. Low amounts of clay and fine silt, and high amounts of coarse sand, whenever the clay begins to approach 12 per cent., characterise the potato soil ; these are the most porous of the series, allowing free drainage and aeration. Barley tolerates heavier and- shallower soils. Fruit and hops both TABLE LXXXII.— MECHANICAL ANALYSES OF SOILS WELL SUITED TO CER- TAIN CROPS IN THE SOUTH-EASTERN COUNTIES ; LIMITS OF VARIATION. HALL AND RUSSELL (nza). Potatoes. Barley. Fruit. Hops. ! Wheat. Waste Land. Fine gravel 0-1-3 0-2-2-5 0-3-2-3 0-3-2-3 0-4-6 i-7 Coarse sand 2-47 i-53 0-8-9-5 07-9-5 | 0-13 16-69 Fine sand . 23-68 20-45 3°-55 25-39 : is-31 i8-6'4 Silt . 3-5-2I-4 5-33 13-44 20-45 11-35-5 2-7 Fine silt . 5-9 3-5-16-4 6-1 1 6-n 9-5-24 2 Clay . S'5-12'6 4-19 10-5-14-6 H'5-15 13-24 i or less require deep soils, and seem to find their most favourable circum- stances only in a restricted class of soils : the fruit soils gener- ally contain rather more sand and less silt than the hop soils. But the fruits differ among themselves ; the best nursery stock is raised on soils of the potato class, where the conditions are for some unknown reason very favourable to fibrous root development ; strawberries prefer the lighter and apples the heavier kinds of fruit soil. Even different varieties of the same plant show distinct preferences for one class of soil over another : the finest varieties of hops are found only on the typical hop soils, and have to be replaced by coarser varieties directly it is desired to grow hops on heavier soils. Prefer- ences for certain soil conditions are also shown by varieties of THE SOIL IN RELATION TO PLANT GROWTH 317 the common crops, oats, barley, wheat, etc. ; unfortunately these can only be discovered by direct field trials, and even then the results only hold so long as similar conditions prevail and may often be reversed in a different climate or season. Still more subtle differences may be observed : one and the same variety of a crop will acquire one habit of growth on one soil and a different habit on another. Wheat growing under the best soil conditions will produce stiff straw and ears well set with corn, so that a crop of 50 or 60 bushels per acre may be raised without difficulty ; on soil rather different in type, and especially under somewhat different climatic conditions, only 30 or 40 bushels can be raised, because the ears are less thickly set and the straw is too weak to carry a heavier crop, becoming " laid " directly an attempt is made to increase pro- duction by increased manuring.1 Whether some unknown nutrient is absent from these soils, or whether the adjustment of the air and water supply is wrong, is not known ; but the limitation of yield arising from this unsuitability of soil con- ditions is one of the most serious problems of our time. Another instance may be given : in Romney Marsh pastures commonly occur carrying a vegetation of rye grass and white clover, with crested dog's-tail and agrostis, easily capable of fattening sheep in summer without any other food. All round these pastures are others, with the same type of vegetation, but the plants grow more slowly, produce more stem and less leaf, are less nutritious and incapable of fattening sheep. The soils are identical in mechanical analysis and in general water and temperature relationships, although certain differences have been detected (\2ic). Again: grass grown on Lower Lias pastures in Somersetshire and Warwickshire causes acute diarrhoea (" scouring ") in cattle, whilst grass on adjoining alluvial pastures does not (106). Harmful effects of a wholly different nature are recorded from certain Swiss pastures.2 1 Further illustrations are given by the author in Science Progress, 1910, v., 286. 2 Jahrb. Schweiz., 1898, 104-5. 318 SOIL CONDITIONS AND PLANT GROWTH Some of the South African grazing lands tend to give the animals a serious disease, Lamziekte, which is attributed to a harmful quality in the herbage and not to any particular plant or organism.1 Lastly : potatoes grown in the Dunbar district are remarkable for their quality, they will stand boiling and subsequent warming-up without going black. The same varieties of potatoes grown in the same way in the Fens blacken badly under the same treatment, and consequently command a much lower price in the market (jd). Instances might be multiplied ; enough have been given to show that the plant responds in a remarkable degree to variations in soil conditions. Our knowledge of these variations is frag- mentary and wholly empirical, and would be much furthered by close and detailed study, jointly by a botanist and a chemist, of the factors causing differences in plant associations in two nearly similar habitats. The agricultural treatment of loams, as already indicated, admits of considerable variety. The old plan was to apply a good dressing of dung every third or fourth year and a smaller intermediate dressing ; clover was also grown every fourth year, and, on light loams, the root crop was eaten by animals on the land. At long intervals lime was applied and some- times bones. The modern movement is towards specialisation, each man producing the crops he can best grow and manag- ing them in the way he finds most profitable, but the system usually involves feeding a good deal of imported food to sheep and cattle, either on the land, or in yards, and utilising the excretions as manure, buying nitrate of soda, sulphate of ammonia, and manufacturers' waste products (generally those derived from imported animal or vegetable products) to supply more nitrogen, and buying also imported phosphates and potassium salts. Thus the fertility of highly-farmed countries like England tends to increase at the expense of new countries that export large amounts of animal and vegetable produce. But the transfer is prodigiously wasteful ; aSee Ingle, Journ. Agric. Sci., 1908, 3, 22-31; A. Theiler, also H. H. Green, $th and 6th Veterinary Reports, Union of South Africa, 1918. THE SOIL IN RELATION TO PLANT GROWTH 319 enormous losses arise in virgin countries through continuous cultivation (p. 181), and at this end in making dung (p. 193), and especially through our methods of sewage disposal. It seems inevitable that these losses must make themselves felt some day, unless the movement for the conservation of natural resources ever becomes a potent factor in international life. Soil Fertility and Soil Exhaustion. From the preceding paragraphs it is clear that fertility is not an absolute property of soils, but has meaning only in relation to particular plants. Plant requirements vary ; a soil may be fertile for one plant and not for another ; every soil might conceivably prove fertile for something. But in practice the agriculturist can find use only for a very limited number of plants ; he, therefore, has to select those combining the double features of saleability in his markets and suitability to his conditions of soil and climate. To a certain extent it is possible to bridge the gap between plant requirements and soil conditions ; the former may be permanently altered by breeding if suitable plants cannot be found by selection, and the latter may be changed by such processes as draining, liming, etc. When all has been done that is economically possible there may still remain a divergency between the conditions ideal for the plant and those it finds in the soil ; this divergency is the measure of the infertility of the soil for the crop. The problem has to be simplified by restricting attention to the common agricultural crops and interpreting fertility to mean the capacity for producing heavy crops regardless of any subtle distinctions of quality. Four factors then come into play: -an adequate supply of air and water to the roots, a sufficiently rapid production or solution of food material, a sufficiently rapid movement of nutrient substances to the plant roots, and absence of harmful agencies. These have already been discussed in Chapters III. and VI., where also it is shown that the three are not independent, but related to one 320 ^ SOIL CONDITIONS AND PLANT GROWTH another, inasmuch as they are all directly bound up with the nature of the compound particles, and, therefore, with the ultimate particles as revealed by mechanical analysis, and with the amounts of calcium carbonate and of organic matter. We have seen that the compound particles can be altered considerably by human efforts, within limits fixed by the properties of the unalterable ultimate particles. In trying to improve a soil, therefore, four courses are open : — 1. The water supply may be increased by deepening the soil, e.g. by breaking a " pan," by enriching the lower spit, or other device, while the air supply can be increased by drain- age. 2. The compound particles may be built up by proper cultivation and the addition of organic matter (e.g. dung, green manuring, etc.) and of calcium carbonate. 3. Sufficient calcium carbonate must be added for the needs of the crop and the micro-organisms — nothing but a field trial can determine what this is. 4. The food supply can be increased by the addition of fertilisers, the ploughing-in of green leguminous crops, feeding cake on the land, etc. Conversely the "exhaustion" of soil is limited in our climate to the removal of organic matter, calcium carbonate, and some of the food (often the nitrogen compounds), and the destruction of the desirable compound particles ; the ultimate particles, and all the possibilities they stand for, remain untouched. A distinction is therefore made between the temporary fertility or "condition" within the cultivator's control, and the "inherent" fertility that depends on the unalterable ultimate particles. Of course, the distinction is very indefinite and, in practice, wholly empirical, no proper methods of estimation having yet been worked out, but it is of importance in compensation and valuation cases. Serious soil exhaustion did not arise under the old agricultural conditions where the people practically lived on the land and no great amount of material had to be sold THE SOIL IN RELATION TO PLANT GROWTH 321 away from the farm. Phosphate exhaustion was the most serious occurrence because there was no way of meeting it, and as the original supplies were not as a rule very great, it must have become acute by the end of the eighteenth century in England, for remarkable improvements were, and still are, effected all over the country by adding phosphates. Then began a process, which has gone on to an increasing extent ever since, of ransacking the whole world for phosphates ; at first the search was for bones, even the old battle-fields not being spared, if we may believe some of the accounts that have come down; later on (in 1842) Henslow discovered large deposits of mineral phosphates, to which more and more attention has been paid. Phosphate supplies may yet become the factor that will determine the course of history. The crowding of the population into cities, and the enormous cheapening of transport rates, led during the nineteenth century to the adoption in new countries, particu- larly in North America, of what is perhaps the most wasteful method of farming known : continuous arable cultivation without periodical spells of leguminous and grass crops. The organic matter was rapidly oxidised away, leaching and erosion increased considerably when the cover of vegetation was removed, while the compound particles that had slowly been forming through the ages soon broke down. Nothing was returned to the soil, the grain and other portable products were sold and the straw burnt. The result has been a rate of exhaustion unparalleled in older countries, and wholly beyond the farmer's power to remedy, consequently he left the land and moved on. The excellent experimental studies of Hop- kins (138) at the Illinois Experimental Station, of Whitson (307) at Wisconsin, and other American investigators, have shown that additions of lime, of phosphates, and sometimes of potassium salts, with the introduction of rotations including grass and leguminous crops, and proper cultivations will slowly bring about a very marked improvement. 21 CHAPTER IX. SOIL ANALYSIS AND ITS INTERPRETATION. WHEN an agricultural chemist is asked to analyse a soil he is expected to give some information about the crops to which it is best suited, and the manures that must be applied ; he has, therefore, a much more complex problem than the mineralogical or geological chemist who simply has to report on the actual constituents he finds. It has been shown in the last chapter that the vegetation relationships of soils are not determined solely by the nature of the soil, but also by its position, subsoil, climate, and other circumstances, so that it is manifestly impossible for the chemist to make a satisfactory report on a sample of soil on the basis of analytical data only. He cannot even give a complete account of the soil itself, since he has no method of estimating the compound particles on which the water and air supply, the temperature and the cultivation properties depend, but he can only get at the ulti- mate particles out of which they are built. Soil analysis is, therefore, restricted to: (i) comparisons between soils, showing which are fundamentally identical and in what respects others differ ; (2) the tracing of such corre- lations as exist between the chemical and physical properties of the soils of a given area and the crops and agricultural methods generally associated with them. In order to carry out either of these satisfactorily it is necessary to make a systematic soil survey, a task that is certainly laborious but by no means impossible, since each agricultural chemist usually confines his attention to a definite region — a county or so — and is not called upon to deal with outside soils. In planning a soil survey it must be remembered that the 322 SOIL ANALYSIS AND ITS INTERPRETATION 323 basis of the whole work is empirical : the agricultural and vegetation characteristics have first to be ascertained by field trials, and then systematised and amplified by aid of the laboratory data. It is necessary to begin, therefore, by going over the whole region very carefully and dividing it up in areas within which similar agricultural or vegetation character- istics prevail. In general the areas differentiated in the geological drift maps will be found identical with the vegeta- tion areas, especially if the drift map is interpreted in the light of the Memoirs of the Geological Survey. But as agri- culture is influenced by altitude and rainfall, the investigator must also use an ordinary contour map and a rainfall map, which, unfortunately, he must construct for himself from data in British Rainfall. Within each vegetation area a number of soil samples must then be taken from points representing the area as closely as possible : situated, for example, on level regions or long gentle slopes, and not on made ground, steep slopes, or places of local disturbance, etc. In seeking for typical places the investigator is likely to meet with a good deal of discouragement ; farmers will tell him that half a dozen or more different kinds of soil occur on their particular farms, while the vegetation of a wild area may show considerable changes. But these differences often arise from differences in the compound particles and not in the ultimate particles ; small variations in the amount of calcium carbonate or of organic matter, or differences in the water supply, or management, may considerably affect the ease of cultivation and the vegetation relationships and give the im- pression of a wholly different type of soil. So little does cropping, cultivation, etc., affect the ultimate particles that it is quite immaterial for the purpose of a soil survey whether the sample is taken from pasture land or arable land, but it is well to take a number of samples from both. With a little practice abnormal places are easily avoided. It is necessary to take a large number of samples, not only to get at the type but also to trace out the causes of the phenomena noted by the farmer, and to discover the main factors determining the cultivation 324 SOIL CONDITIONS AND PLANT GROWTH and vegetation relationships of the soil. Very full inquiries must be made on the spot as to the agricultural value of the land, the crops and manures most suitable, its behaviour during drought and wet weather, and any special points to be observed during cultivation. Information is also wanted about the most troublesome weeds, the native vegetation, hedgerow and other timber, etc., and note must be taken of the position of the soil in regard to water supply, the nature of the strata down to the permanent water table, etc. The most reliable information is obtained only by properly con- ducted manurial trials. It is usual to take the sample to a depth of 9 inches and a lower sample to a depth of 18 inches, but if any marked change occurs in the soil the sample should only be taken to the point where it sets in. The subsoil sample does not char- acterise the formation any better than the surface sample, but it affords a useful check and helps in detecting abnormalities. The vegetation areas correspond with the geological forma- tions only so long as the lithological characters remain con- stant. Some formations are very uniform, e.g. the Folkestone beds of the Lower Greensand, but, in general, certain changes are observed. Where the formation has been laid down in an estuary of no very great size, the coarser particles are deposited near the old shore and the finer particles farther out, so that a gradual change from finer to coarser soil is observed in travelling along the formation, necessitating a soil division into two or three vegetation areas ; the Hythe beds of the Lower Greensand and the London clay afford illustrations. Considerable trouble often arises when the formation consists of a number of strata of sands and clays, and the outcrops are so narrow that no one type persists over any large area. The simplest method of procedure is to map out any uniform areas that possess sufficient agricultural importance, and then group the remaining less important soils simply into gravels, sands, loams, and clays. In dealing with drift soils, it is well first to map the uniform areas and then look out for lines of uniformity and make up regions within which the agricultural character- SOIL ANALYSIS AND ITS INTERPRETATION 325 istics vary between a higher and a lower limit. The investi- gator must be guided by the importance of the region from the particular point of view in deciding how closely he is to map out the country. Absolute uniformity cannot be expected over any con- siderable area, and even such uniformity as existed has often been upset by subsequent rearrangements of the soil which result in the original surface being covered up with a later deposit or being washed away, leaving the original subsoil to become the new soil. Such changes are readily detected by mechanical analysis of the surface and subsoils ; examples are given in Table LXXXIII. At Merton the subsoil on the lower ground appears to have been the original surface soil, because of its identity with the surface soil and its marked difference from the subsoil of the land higher up ; it has been covered with a deposit identical with and presumably derived from the higher lying soil. The same thing has happened at Hamsey Green. At Woodchurch, however, it appears that the old surface of 69, now the subsoil, has been covered with rather a lighter soil ; it is equally possible, however, in this particular case that 70 has lost its original surface soil, the present TABLE LXXXIII. — VARIATION IN SOIL DUE TO WASHING OR FLOODING. Formation . London Clay. Clay-with-Flints. Weald. Locality Merton, Surrey, Hamsey Green, Surrey, Woodchurch, Kent. Lower Ground. On the Hill, Soil 109. Soil iio, 200 yards Soil 69. Soil 70. away. •3 1 3 i i 1 i 1 «! J i 1 «J u >*S E 1 1 1 0) CO 03 CO 01 w 01 CO 01 o> 01 01 Fine gravel I-7 i'3 i*5 0-3 1-6 3'i 17 i '4 0'5 0-6 0-9 07 Coarse sand 18-4 23-6 16-9 8-4 9'5 67 5*3 7'1 2'5 1-9 I'l I'l Fine sand . 127 11-3 12-4 127 22-3 28-0 287 25-1 147 13-0 9'3 9-0 Silt . 16-6 18-0 16-6 13*4 25'4 22-5 26-3 17-6 24-2 27-8 25'9 18-8 Fine silt . ii'i 11-4 IO'I 9'8 9-9 I2'6 IO'2 9-s 23-7 2VS 24-4 26-s Clay . 24-6 24-9 267 41*7 16-0 16-4 16-4 28-3 2O*I 28-9 28-6 37-8 326 SOIL CONDITIONS AND PLANT GROWTH surface being the bared subsoil. This kind of variation is common on clay soils and often leads to differences in agri- cultural value that are fairly marked, but not sufficient to affect the type. Normally the subsoil contains more clay than the surface soil (as in no, 69, and 70), and any deviation should be carefully investigated. The characterisation of soil types is usually effected by mechanical analysis and determinations of calcium carbonate and organic matter. A representative set of soils should, however, be subjected to chemical analysis, the clay fractions being, if possible, broken up by ammonium fluoride and analysed completely. Soils about which precise information has been obtained by manurial and other trials should be very completely examined in order that they may serve as standards in the analysis of other soils from the same area. The problem set by the farmer is wholly different. He does not want to know to what type his soil belongs, but how he must manure it, etc. If the analyst has an adequate know- ledge of the soil type and the locality he can readily ascertain in what respects the soil differs from the type, and then, from the known results of manurial and other trials on that type, he can give the information wanted with a reasonable degree of probability ; otherwise his report can only be a matter of guesswork. In short, the farmer's problem can be satisfac- torily solved, and the manurial trials fully interpreted, only when a complete soil survey has been made. The analyst must consider the soil from three points of view: (i) its physical properties, especially those relating to the ease of movement of the soil water ; (2) its store of plant food, actual and potential ; (3) the rate at which potential food can be converted into actual food. The Interpretation of Mechanical Analyses.1 The properties of the various fractions have already been given in Chapter III., but some little practice is necessary 1 See Appendix for Methods of Analysis and pp. 98 and 102 for details as to dimensions and composition of fractions. SOIL ANALYSIS AND ITS INTERPRETATION 327 before they can be used for the interpretation of an analysis. A few illustrations are therefore given from Hall and Russell's survey of Kent, Surrey, and Sussex (12 la): the data are set out in Table LXXXIV. The Chilworth soil contains so little clay and fine silt and so much coarse sand that it has very little power of retaining water. As it lies too high to obtain any seepage water from the neighbouring formations it is dependent on the immediate rainfall, and is therefore not in cultivation but has always been heath land. Owing to its bad constitution and its high situa- tion it could not by any known method be made suitable for farming. TABLE LXXXIV. — MECHANICAL ANALYSES OF SOILS AND THEIR INTER- PRETATIONS. Formation Folkestone Beds. Thanet Beds. Brick Earth. London Clay. Weald Clay. Alluvium. Locality . Chil- worth. Shal- ford. Gold- stone. Bar- ton. Ick- ham. Oving. Tol- worth. Shaddox- hurst. Ewhurst. Gravel I'2 2'5 O'2 0'2 0-3 0-9 0-4 0'2 07 O'l Coarse sand 65'9 S2-6 15 '3 2-3 0-7 !'3 12-8 «'5 I'O 0'5 Fine sand . 237 26'2 44*9 347 247 1 6-0 25*5 II'O 19-8 193 Silt . 2-4 4-8 17*3 36-2 44-8 35'5 11-3 19-6 28-4 I3-0 Fine silt 2'0 3*5 6-3 6-3 8-6 i3'3 ii'i 26-8 I2'I 20 '0 Clay . 0-Q 3'8 8-9 "'5 14-7 i5'9 23-7 22*1 19-7 26-9 Calcium car- bonate nil 0-3 0-08 0-18 0-4 o'75 2'O O'l6 o 05 0-28 Loss on igni- tion . 2-6 33 3'i 4 '3 4-6 6-5 5'6 9-8 I0'2 11-3 The Shalford soil lies lower down and has a better water- supply, less coarse sand, and more clay and fine silt. But its water-holding capacity and its retentive power for manures are still very low ; artificial manures are of much less value than organic manures, and the best treatment of the land is to grow green crops and fold them off to sheep. It is better suited to special purposes like the production of malting barley or market-garden crops than to ordinary mixed farming. The Goldstone soil contains more clay and fine silt, and 328 SOIL CONDITIONS AND PLANT GROWTH has therefore better power of retaining water and manures, and is more productive and more generally useful. But as the coarse sand exceeds the clay in amount it is still distinctly light ; it responds better to organic than to artificial manures and suffers rather in droughty weather in spite of lying not far above the marshes. It contains 45 per cent of fine sand and therefore tends to cake on the surface after rain and to form steely lumps if worked when wet. Under proper management, however, it produces good crops and is equally suited for ordinary arable and for fruit or potato cultivation. The next three soils may be taken as illustrations of the very best loams in the three counties. Silt forms the largest fraction and therefore the soils possess sufficient, but not too great, a power of retaining water. The fine silt is always lower than the clay ; the latter varies between 1 2 and 1 6 per cent., a very satisfactory amount where the rainfall is not too high. As there is a considerable amount of fine sand and no excess of fine silt and clay, the absence of coarse sand is no disadvantage. The Tolworth soil is highly productive arable land but almost too heavy for profitable cultivation ; only by dressings of dung (fortunately obtainable cheaply from town) can it be kept workable. It contains rather too much clay and would no doubt have gone down to grass had there not been so much coarse sand present. The Shaddoxhurst soil is bad. It contains much clay and still more fine silt, consequently its texture is not improved as much as might be expected by liming. There is practically no coarse sand and not much fine sand to keep the soil open, it has always and deservedly been in bad repute. It is best as pasture land, and, after drainage and treatment with basic slag, it may be made useful but never very good. The Ewhurst soils are both in pasture, being too heavy for arable cultivation on account of their high clay and low coarse sand content. The first has the better constitution ; silt is the predominant feature, the clay is not too high, nor is the fine silt. It has all the characteristics of a good, heavy, SOIL ANALYSIS AND ITS INTERPRETATION 329 soil, and is indeed known to be an excellent bullock pasture. The second is not so good ; it contains too much clay and fine silt, and too little silt and coarse sand. It has no great agricultural value. Factors Modifying the Interpretation of a Mechanical Analysis. The Amount of Organic Matter. — Organic matter at the proper stage of decomposition has the effect of binding a loose soil and lightening a heavy one ; thus it reduces the difference between a light sand and a heavy clay, bringing them both closer to the loams. When 10 to 15 per cent, of organic matter is present it so impresses its properties on the soil that the mechanical analysis loses much of its significance, and all the analyst can do is to point out what the soil would become if by persistence in certain methods of management the organic matter were reduced below a certain point. It is, however, essential that the organic matter should be properly decomposed. Barren sandy wastes not infrequently contain 5 to I o or even 15 per cent, of organic matter, but much of it is simply dried bracken or other vegetation that has not broken down and has no value, but rather the reverse, in improving the physical conditions. If the drainage is bad a good deal of peat may form ; further, the water fills up the soil, making its condition bad whatever its composition may be. The Amount of Calcium Carbonate. — In interpreting a mechanical analysis it must be remembered, as shown on p. 117, that I or 2 per cent, of calcium carbonate may greatly modify the clay properties and give a considerable degree of friability to a soil which otherwise would be very intractable. When the percentage rises to much higher amounts the soil becomes very chalky and the mechanical analysis loses its meaning, just as when much organic matter is present. Water-supply and the Interpretation of Mechanical Analysis. It has already been pointed out that a mechanical analy- sis can be interpreted and discussed with any degree of 330 SOIL CONDITIONS AND PLANT GRO WTH completeness only in terms of the water-supply ; the rainfall, the coolness of the climate, the presence of moving under- ground water, and the nature of the subsoil all have to be taken into account. Effect of the Rainfall. — The effect of a high rainfall is to bring into prominence the "sticky" properties of the fine fractions, and to put into the background their water-holding capacity. Thus a light soil under a high rainfall behaves like a heavier soil under a low rainfall ; it is as well supplied with water and on the whole behaves in the same kind of way on cultivation. For example, the Stedham soil (Table LXXXV.) is rather lighter than the Swanley soil, and yet in virtue of its extra rainfall is more useful for farming purposes ; * indeed, the Swanley soil is essentially a market-garden soil, requiring large dressings of dung for successful cultivation. The North Chapel soil is physically as good as the East Far- leigh soil but agriculturally much inferior ; owing to the higher rainfall it becomes somewhat too sticky to cultivate profitably and so is in rather poor grass ; the East Farleigh soil, on the other hand, is from a highly fertile hop garden. TABLE LXXXV. — WATER-SUPPLY AND INTERPRETATION OF MECHANICAL ANALYSIS. Swanley. Stedham. North Chapel. East Farleigh. Tol- worth. Nut- field. Dunbar. Fine gravel . I'2 1*4 0-Q 2-3 0'6 2-Q 3-0 I'O Coarse sand IO'2 9'3 ix'4 9'5 37-8 46-6 33 '« 237 Fine sand . 58-6 68-5 43*2 30-6 33'i 22*9 28-0 38-2 Silt . I3'3 3-6 13-0 197 77 3*5 5'5 6-8 Fine silt 5'i 5'6 IO'2 in 4*7 8-8 10-8 n-8 Clay . 5-5 5'5 io'9 I3'3 7-6 6-9 6-6 9'5 Loss on ignition . 2-9 3H 5'i 5'6 3-6 3-6 6-9 6'2 Calcium carbonate '02 •03 •80 I'O •27 *2I •15 •3i Rainfall in inches (approximate) . 24 33 3° 24 28 27*5 25 25 Coolness of Climate. — Soils containing so much coarse sand or fine sand that they would scorch or burn in a dry warm 1 An example is given by G. W. Robinson, Journ. Ag. Sci.t 1917, 8, 370. SOIL ANALYSIS AND ITS INTERPRETATION 331 district may prove very suitable for cultivation in a cooler district where evaporation is lessened Potato soils afford some good illustrations ; potatoes require a light soil, but it must be cool and moist. The Nutfield soil (Table LXXXV.) fulfils these conditions ; it is on a slope facing northwards not very far above a stream, and, therefore, does not quickly dry out, hence it is very good for main crop potatoes. The Tolworth soil, on the other hand, although similar in com- position, is so placed that it quickly dries and is of much less value. Some of the potato soils of Dunbar, analysed by S. F. Ashby (jd\ have all the appearance of soils readily drying out, but in their cool climate this property does not show itself to an injurious extent. Effect of Underground Water. — When the underground water is near the surface, but sufficiently far below to allow of proper root development, the most important property of the soil becomes its power of lifting the water by surface action up to the roots. The silt and sands are in such cases the useful constituents, the clay and fine silt being less neces- sary. The Weybridge soil (Table LXXXVI.), at about 3 feet below its surface, has a current of underground water which is brought to the roots by the fine and coarse sand. It therefore grows excellent wheat crops. The Bagshot sands, however, although similar in physical type, have in general no such water-supplies and are sterile because they lack the clay which, in their circumstances, could alone confer an adequate power of holding water from one shower to the next. TABLE LXXXVI. — UNDERGROUND WATER AND MECHANICAL ANALYSIS. Weybridge. Bagshot Beds. Shalford. Lydd. Fine gravel Coarse sand i'3 38-4 •I to -6 20 30 2'5 52-6 O'l 0-9 Fine sand 39 '9 45 65 26'2 66-7 Silt 5'6 5 10 4-8 7-2 Fine silt 5-i 5 10 3'5 11-4 Clay . 3-8 3 7 3-8 3'9 The Shalford soil is a light sand with too, little power of 332 SOIL CONDITIONS AND PLANT GROWTH retaining water for pastures to last through a hot summer, consequently the grass land, except near the brooks, is parched and scorched. The Lydd soil is certainly somewhat finer grained, but not so very different that one would expect to find it much better for pasture purposes, yet it produces one of the best pastures in Romney Marsh, not only carrying but fattening sheep throughout the summer. It has, however, a constant supply of water 3 or 4 feet below the surface, while at Shalford the water level is much lower down. This underground flow is one of the factors concerned in the proverbial fertility of valleys. Soils lying towards the bottom of a long slope receive not only the rainfall but also the water steadily drifting downwards to the stream or marsh at the bottom, and this advantage is further enhanced by the gradual transport of soil down the slope which increases the depth through which the plant roots can range. Effect of the Subsoil. — In general the subsoil is rather heavier in type than the surface soil, especially in the case of clays; examples are given in Table LXXXVII. The excep- tions to this rule may arise through periodical flooding with water containing much clay in suspension, or through the oc- currence of a bed of sand just below the surface. TABLE LXXXVII.— NATURE OF THE SUBSOIL. Loddington. Harting. Dicker. Shopwyke. Wye. Sur- Sub- Sur- Sub- Sur- Sub- Sur- Sub- Sur- Sub- face. soil. face. soil. face. soil. face. soil. face. soil. Fine gravel 3 '5 2'6 0-6 0'2 I'O 0-6 0-6 O'l I'O 0*2 Coarse sand I0'2 9-8 3 '3 3-3 2'0 IT 0-8 0-4 3-0 1-9 Fine sand . 33*5 30-2 31-6 33'9 26-6 23-2 25-0 21'9 27-2 25'3 Silt . 14-6 i7'5 I7'3 21-3 23'0 I5'1 27-3 38-0 40-0 41-4 Fine silt . 14-9 15-5 I4'5 13*4 17-8 21-9 16-4 I5-2 8-9 9-6 Clay . I2'2 I5-3 12-3 16-0 17-9 25*7 IIT 157 II-2 I4'5 Two cases described on page 314 may be illustrated here. The bad effect of a layer of impermeable material near the surface is shown by the Loddington soil (Table LXXXVII.), SOIL ANALYSIS AND ITS INTERPRETATION 333 typical of an area near Maidstone (Cox Heath), much of which was waste land. . Its sterility was due to no fault in the soil, which is obviously of excellent type, but to a thin layer of rock lying near the surface. When this was removed a very good soil was obtained. The Harting soil lies on the Upper Greensand in West Sussex ; the rock comes close to the surface, restricting both the root range of the plant and the water supply where it lies horizontally, but proving much less harmful where it dips at any considerable angle. The soil itself is good, although it has rather too much fine silt, and it becomes very productive when the effect of the rock is counteracted. The Dicker soil, while not of the best type, as its fine silt is too high, is far from being hopeless, but it unfortunately lies on a deep bed of stiff clay which keeps it wet in winter and parched in summer. It is therefore very poor, and even with the best management never gives great results. The second case, over-drainage, is illustrated by the Shopwyke soil in the same table. It is a fair soil, containing too much fine silt to be in the first rank, but it is spoiled by lying on a deep bed of gravel only nine inches or a foot below the surface — the subsoil sample could only be taken in one or two instances. Consequently it dries out badly in summer and does not repay much expenditure in the way of manures. The Wye soil is given as an instance of the normal case where a soil becomes rather heavier in its lower depths, with the result that the movement of water is somewhat impeded without being stopped. Thus the subsoil furnishes a reserve of water for the surface, yet even in wet weather it does not hold up too much water. The Wye soil contrasts with the Dicker soil, the usual case in a clay, where the subsoil contains much more true clay than the surface. Chemical Analysis of Soils.1 Recourse is had to chemical analysis to discover the amounts of potential and actual plant food in the soil, and the 1 See Appendix for methods of analysis. 334 SOIL CONDITIONS AND PLANT GRO WTH rate at which potential food is likely to become available. But as the problem is vague, so the methods are empirical and the interpretation of the results often very difficult. The following discussion is confined to English soils and English methods. No claim is made that these methods are superior to those of other countries or that they give absolute information about the amounts of plant nutrients in soils. They are intended only to facilitate comparison of given soils with a standard soil, and they are probably neither better nor worse than other analytical methods which might be pro- posed. In the United States chemical analysis is not in much favour with the younger school of investigators, though determinations are made of the components of the water extracts of soils.1 For a discussion of the chemical and physical properties of soils formed direct from granite in Aberdeen, or from the paleozoic soils of North Wales, the reader is referred to the papers by Hendrick and Ogg (i32)2 and G. W. Robinson (240). Organic Matter. — The analyst should note whether the organic matter is fairly well decomposed, whether it still shows definite plant structure, and whether or not it is acid to litmus paper. He can then interpret his observations as shown on pages 128 et seq. Nitrogen. — Unlike the other soil constituents nitrogen and carbonates are determined absolutely. The amount of nitrogen is closely related to the loss on ignition, of which in a large proportion of cases it is about 3 per cent. As a guide to fertility it is therefore subject to the same limitations ; a high nitrogen content may be associated either with a rich soil containing abundance of valuable non-acid organic matter, 1 See, e.g., J. S. Burd, Journ. Agric. Res., 1918, 12, 297-310, and Soil Sci., 1918, 5, 405-419, who finds water more useful than citric acid and holds hydro- chloric acid worthless as a means of assessing the crop producing power of a soil (see p. 234). a See also Aberdeen Bulls., i, 3, and 10, where the relationship between soil analysis and manurial results is discussed. SOIL ANALYSIS AND ITS INTERPRETATION 335 or with a soil where the conditions are so unfavourable that organic debris does not decompose (e.g. acid soils), or only forms accumulations of peat. On the other hand, some of the best loams, where the conditions are most favourable to rapid decomposition and nitrification, contain %but little nitrogen. A few typical examples are given in Table LXXXVIII. TABLE LXXXVIII. — NITROGEN AND Loss ON IGNITION. Fertile Arable Soils. Poor Arable Soils. Barren Wastes. Loss on ignition 4^5 6*58 37° 4-65 4*13 6-23 3'6o 5'i4 5 '94 7-00 5'8i Nitrogen . •120 '22O •133 •141 •128 •143 •182 •152 •130 •195 -167 Loss on ignition in subsoil 3-00 4'94 2'8l 3*29 374 5-50 2-58 4-14 — — 2-70 Nitrogen in sub- soil •078 •139 •08 1 •097 *II2 •104 •061 •096 — — •058 Soils containing much calcium carbonate are as a rule rich in nitrogen, partly no doubt because of the rather high nitrogen content of the rock and partly also because they are folded, green manured, cropped with leguminous plants like sainfoin, lucerne, etc., all of which tend to increase the nitrogen supply. The nitrogen in some chalk soils is given in Table LXXXIX. ; all are arable soils, excepting the last, which is open downland. TABLE LXXXIX. — NITROGEN CONTENT OF CERTAIN CHALK SOILS. Nitrogen in surface soil . •25 •194 •33i •258 •249 •419 ,, subsoil . •128 •130 •162 •192 •196 •180 Calcium carbonate in sur- face soil 18-1 370 497 66-0 65-6 44-0 Calcium carbonate in sub- soil .... n'37 14-9 61-3 55'2 54'8 71-6 Carbonates. — The analyst is often asked whether or not a particular soil contains sufficient calcium carbonate, and in endeavouring to answer this question he must bear in mind the twofold function of this substance, to prevent "sourness" (p. 1 1 8), and to flocculate the clay. Where only a small amount of clay — say 8 per cent, or less — is present the floccu- lating action is less needed and a smaller amount of calcium 336 SOIL CONDITIONS AND PLANT GROWTH carbonate suffices. The Stedham soil (Table LXXXV.) is an example ; it is near the bottom of a slope along which water containing calcium bicarbonate in solution is drifting, and therefore shows no tendency to become sour. The 0-03 per cent, of calcium carbonate present, hopelessly inadequate as it appears, suffices for its needs and no increase in crop is ob- tained by applying lime. The Lydd soil (Table LXXXVI.) contains only 'O2 per cent, but is also well supplied with cal- careous water from below and shows no sign of sourness. Similar soils that have not this advantage of position stand in great need of lime even when err per cent, is present. As the amount of clay increases, the need for lime becomes greater because flocculation is now wanted ; soils with 20 per cent, or more of clay may need two or three times as much lime as sandy soils. It is impossible to fix limits that shall hold universally. Some soils free from lime appear to need none : Hendrick and Ogg's Craibstone soil has already been men- tioned (p. 1 1 8). Gregoire1 also found soils practically free from carbonate and yet not acid, and presumably not needing lime. Before an analyst recommends lime or chalk on a sandy soil he should satisfy himself that the need is indicated by the vegetation, and before he states that lime is not necessary on a clay he should be quite sure that further additions would have no beneficial flocculating effect The need of lime is indicated : — I. If clover fails to start well, or to stand the winter, or if it looks bad in spring. Cases have been examined by the writer where clover or lucerne were failing in patches in the field and weeds were consequently getting a firm hold. The amounts of lime in the soil were : — On the good parts : Suffolk lucerne, o'8, Norfolk lucerne, 0-6, Herts clover, 0*2 per cent, calcium carbonate. On the bad patches : Suffolk lucerne, 0*07, Norfolk lucerne, o'2, Herts clover, o'oi per cent, calcium carbonate. Ann. Stat. Agron., Gcmbloux, 1913, 2, 87. SOIL ANALYSIS AND ITS INTERPRETATION 337 2. If swedes, turnips, or cabbages get finger-and-toe rather badly. The Armstrong College experiments have shown that 2 tons per acre of ground lime, or 3^ tons per acre of ground limestone, afford suitable dressings in this case.1 3. If mayweed springs up vigorously among the wheat, or if spurrey, sorrel, or bent grass become prevalent. Land that has been wet through the winter ought to have lime in the spring : otherwise uneven patches may arise in the field on which weeds develop and the crop ripens unevenly. Neither lime nor limestone, however, should be applied to potatoes or oats unless actual trials have shown that benefit will be obtained ; as a general rule these two crops respond less than others : and in the Kilmarnock trials,2 lasting over eight years, potatoes were actually injured by lime, though oats benefited by it (p. 244). Basic slag reduces the need for lime, but superphosphate does not. Sulphate of ammonia increases the need for lime. Instances of soils known to respond to lime are given in Table XC. TABLE XC. — CALCIUM CARBONATE CONTENT OF SOILS KNOWN TO RESPOND TO LIME. Sandy Soils. Loams and Clays. No. of Soil-3 Percentage of Clay. Percentage of Calcium Carbonate. No. of Soil.3 Percentage of Clay. Percentage of Calcium Carbonate. 126 7-8 •04 207 II'I •02 675 8-9 •08 119 10-4 •03 193 6-0 •18 118 II'S •18 189 3'8 '35 152 I2'2 •26 215 I3-0 "45 127 I3-3 I'OO 1 Armstrong Coll. Bull., No. 12, 1915. 2 West of Scotland Agric. Coll. Bull., No. 55, 1911 (pp. 193-222). 3 The numbers are those used in Soils and Agriculture of Kent, Surrey, and Sussex (Hall and Russell). 22 33* SOIL CONDITIONS AND PLANT GRO WTH Alumina. — In general, in the much weathered soils of the south of England the alumina is approximately equal in amount to one-third of the clay fraction, indicating that the acid treatment breaks down some definite group of silicates associated mainly with the clay fraction in the soil.2 Ex- amples are given in Table XCI. TABLE XCI. — ALUMINA AND CLAY CONTENTS OF VARIOUS SOILS. Formation. Bagshot Sands. London Clay. Thanet Beds. Sand- gate Beds. Folke- stone Beds. Weald Clay. Percentage of clay in soil 3-6 4'Q 7-1 36-8 2l'3 "'5 7'I 15*3 6-q 33T8 Percentage of A12O3 in soil •92 i '43 1-94 H'75 678 3-46 2-66 5^4 rqq 10-45 . ALO3 Ratio -f-3 . clay •25 •29 •27 "SI •31 •31 '37 '33 •28 '31 Exceptions to the rule occur when much fine silt is present, the alumina then being markedly less than one-third of the clay (Table XCIL). TABLE XCII. — ALUMINA AND CLAY CONTENTS OF SILTY SOILS. Formation. Weald Clay. Lower Wealden Beds. Upper Oreensand. Gault. Percentage of fine silt in soil „ clay in soil . A1203 . . . ALOo Rstin 27-4 21'5 5'02 •23 35-8 22'I 5'42 •25 25^ 19-4 5-68 •29 15-8 5 '4 •17 •03 2i'5 12-5 1-66 '13 I4'3 97 2-38 •24 15-9 I3'I 2-48 •18 !4'5 12-3 2'39 -19 14-0 n-8 5'ii "43 clay Iron Oxide. — The iron oxide is present in quantities com- parable with those of alumina, but no close relationship is observable, nor does the amount of iron oxide afford any indication of the fertility of the soil. F. V. Button finds no more iron in the fertile red soils of Devon than in the infertile grey soils. Light soils, good or bad, contain about I to 2-5 1 The percentage of A13O3 given here represents the amounts extracted by HC1 and are not comparable with those on p. 102 obtained after treatment with ammonium fluoride. SOIL ANALYSIS AND ITS INTERPRETATION 339 per cent., good loams and poor clays contain 3-5 to 5 per cent. Larger amounts of iron oxide are not common. Soils containing ferrous compounds are generally infertile.1 Lime and Magnesia. — About 'I to -5 per cent, of magnesia is found in the soils we have examined, and in general the ratio falls between I and 3, but ratios of 4 and 5 are magnesia not uncommon, while on chalk soils they may rise very high. No connection could be traced between the ratio magnesia and the productiveness of the soil ; indeed, Table XCIII. shows that very good and very poor soils may have practically identical ratios. TABLE XCIII. — MAGNESIA RATI° IN VARI°US SOILS. Barren Wastes. Poor Cultivated Soils. Fertile Soils. No. of Soil. CaO. MgO. Ratio CaO. MgO' No. of Soil. CaO. MgO. Ratio CaO MgO' No. of Soil. CaO. MgO. Ratio CaO Mgo' 170 •0.5 •06 I'D 45 "43 •23 I-9 183 •56 •40 «'4 IQ2 •13 •08 1-6 242 •30 •13 2 '3 222 •46 •28 1-6 168 •21 •13 1-6 106 •48 •21 2-3 152 I'O2 HI 2*5 50 •15 •08 1-9 255 •89 •27 3-4 122 •60 •22 2-7 197 •21 •08 2-6 196 "43 •12 3'6 211 179 •40 4'5 91 •08 •03 2-7 287 I-I9 •29 4-0 72 1-94 •42 4-6 24I '22 •07 3-1 127 2-14 •40 5H 13 •58 •14 4'i Potash. — In the south of England soils the amount of potash is closely associated with that of alumina, being commonly about one-tenth ; it is, therefore, about one-thirtieth of the clay. Some examples are given in Table XCIV. The " available " potash 2 shows no kind of regularity, but varies between 5 and 50 per cent, of the quantity ex- tracted by strong acids. In deciding whether or not sufficient is present, attention must be paid to the soil, the crop and the aFor the effect of soil iron compounds on soluble phosphates see A. de Jongh, Int. Mitt. Bodenkunde, 1914, 4, 32-45. 2 I.e. extracted by i per cent, citric acid. 22 * C. 340 SOIL CONDITIONS AND PLANT GRO WTH TABLE XCIV. — POTASH AND CLAY CONTENTS OF VARIOUS SOILS. No. of Soil. Percentage of A12O3. Percentage of K2O. Rati°^§? Percentage of Clay. Rati° Cla7 112 4-07 '45 •II I3-I •034 120 2-84 •31 •II 10*4 •029 IOO 3'83 •40 '10 11*2 •035 133 3-67 '44 •12 11-7 •037 103 3-66 •30 •08 11-9 •025 161 7*97 I -08 •14 27-7 •039 67 n'75 1-44 •12 36-8 •039 118 •404 '12 ii'5 •035 79 5^4 •40 •08 I5-3 •O26 43 10-45 •76 •07 33-8 •022 147 7-88 •96 •12 22-5 •043 rainfall. Thin chalky soils, sandy soils, and soils rich in organic matter are peculiarly responsive to potassic manures, whilst clay soils generally are not. Carbohydrate-making crops, like sugar-beet, mangolds, and potatoes also invariably want more potash than they find in the soil or in dung. Potassic manures also tend to prolong the life of the plant, and, therefore, to increase the yield in dry districts where the conditions all tend to early stoppage of growth. Illustrations are afforded in Table XCV. where soils in dry districts, known to respond profitably to potassic manures, are compared with soils in places of much higher rainfall where potassic manures do not prove profitable. TABLE XCV. — "AVAILABLE" POTASH IN SOILS OF KNOWN BEHAVIOUR TOWARDS POTASSIC MANURES. Soils Responding to Potassic Manures. Soils not Responding to Potassic Manures. East Kent. Surrey. Sussex. West Sussex. Kent. Newing- ton. Barton. Redhill. Patch- ing. Oving. Rogate. Stedham. Yalding. Available K2O KZO extracted by cone. HC1 Clay . Rainfall •013 *2OO 6'o 22 '5 •015 •404 "'5 23 •oio •181 7-8 277 '007 •260 25*5 28-6 •014 '43 15 '9 28 '024 •18 67 33 *OIO •14 5*5 33 •044 '59 9-1 24 SOIL ANALYSIS AND ITS INTERPRETATION 341 All are arable soils. The chalk pastures on the South Downs usually contain less than -oi per cent, of available potash (e.g. the Patching soil), and they respond to potassic manures. It will be observed that -015 per cent, is insuffi- cient in East Kent where the rainfall is 23 inches, whilst -oio per cent, suffices in West Sussex under 10 inches higher rain- fall and generally better water-supply in the soil. Phosphoric Acid. — Generally speaking, the largest amount of phosphoric acid is found in chalk soils, 0*2 to 0-25 per cent, being present; about 0-15 to 0-2 per cent is found in good loams ; sandy loams contain about O'l per cent, while poor clay pastures and poor sands contain still less. Little if any direct connection can be traced between the phosphoric acid and the productiveness ; in general it tends to increase as the clay, fine silt, and silt increase — the poor clay pastures form a readily explained exception — but it does not appear to be closely associated with any one fraction like the potash. The amounts of " available " phosphoric acid vary enormously ; Kentish hop gardens commonly contain from '05 to '18 per cent; well-farmed arable soils contain some '015 per cent, while in poor worn-out pastures the quantity may sink as low as *OO2 per cent In most cases these quantities are insuffi- cient for some of the crops grown, especially where high quality or feeding value is aimed at ; hop growers regularly} and, they maintain, profitably, apply phosphates to gardens already containing '05 per cent, of available phosphoric acid, whilst arable farmers use them for swedes when -015 per cent, or sometimes even more, is present 'J'he exceptions to this rule are the light soils sufficiently provided with moisture and a forward climate ; on these the need for phosphates appears to be less. But in all cases where much purchased food is fed on the land phosphates appear to be of advantage to the suc- ceeding crop. Rainfall does not appear to have so marked an effect in controlling the need for phosphates as it has for potassic manures. The explanation is to be found in the fact that phosphates are useful both in dry and in moist situations : 342 SOIL CONDITIONS AND PLANT GROWTH they tend to promote root development, an obvious advantage in a dry soil where the plant will fail unless the roots strike into the deeper, moister layers ; they also stimulate the vital processes going on at the end of the season and are thus valuable in wet, cold districts. But rainfall and water-supply are important factors in determining the choice of phosphates on arable land ; basic slag proves less useful than super- phosphates on dry soils, but it is sometimes nearly as useful on moister soils or under higher rainfall. The amount of chalk in the soil is not the determining factor, but the moistness ; if, as often happens, a chalky soil is dry, superphosphates will prove the more useful ; where the soil is moister, basic slag is as good, and of course cheaper. Evidence is accumulating that mineral phosphates are often of value. On grass land basic slag is often more effective than superphosphate. The Relative Advantages of Mechanical and Chemical Analysis. The fundamental distinction between mechanical and chemical analysis is that the former deals with the whole of the soil, which it sorts out into fractions of varying sizes, while the latter deals only with the part that is readily dis- solved by acids. Mechanical analysis, therefore, gives a picture of the whole (albeit very incomplete), while chemical analysis does not ; it is in consequence eminently suited for the purpose of a soil survey, the chief object of which is to classify and describe the soils. Further, it enables the in- vestigator to explain to some degree the observed water re- lationships of the soil when sufficient is known about the water-supply, and also to account for many of the peculiarities observed in cultivation. It enables him to say, as far as can be said on our present knowledge, whether any observed de- fects are due to defects in the soil or its situation, or to the system of management that has been adopted. As it cannot be interpreted fully without a knowledge of the amounts of organic matter and calcium carbonate present, these two quan- tities must be determined in every sample. SOIL ANALYSIS AND ITS INTERPRETATION 343 In the preceding discussion the soils have all belonged to one type — the much-weathered soils of the South of England. We have seen that in this case there is a close correlation be- tween the potash, the alumina and the clay. For purposes of a survey it seems superfluous to determine these two bases in every sample taken. The iron oxide shows a general but by no means a close correlation with the others ; but no con- nection could be traced between iron oxide and fertility in the soils examined by the author, the iron oxide being almost always less than 5 per cent, in amount. Nor did it appear that the ratio of lime to magnesia in these soils was significant. The nitrogen is closely correlated with the organic matter, i.e. the loss on ignition. The total phosphoric acid shows no great variations in the different soils, but the available phosphoric acid, like the available potash, varies greatly with the manage- ment of the soil. Thus the figures obtained by chemical analysis, apart from the loss on ignition and the calcium car- bonate, fall into two groups : the nitrogen, potash and alumina, which are so closely correlated with quantities already deter- mined in the mechanical analysis that their separate deter- mination is almost superfluous ; and the iron oxide, magnesia, lime, etc., which do not give sufficiently useful indications to be worth determining in every case. Since chemical analysis fails to describe the soil with sufficient completeness for agricultural purposes Hall and Russell recommend that for purposes of a survey a large number of soils should be sub- mitted to mechanical analysis, including the determination of organic matter and of calcium carbonate, and then a carefully chosen representative set should be analysed chemically so as to characterise the type ; these can further serve as standards with which farmers' samples can be compared by the citric acid method. They agree with Whitney that mechanical analysis should form the basis of the survey, because it alone takes account of those physical functions — the regulation of the water-supply and therefore of the temperature, of the air supply, ease of cultivation, etc. — that play so large a part in determining the value of a soil, 344 SOIL CONDITIONS AND PLANT GRO WTH But, on the other hand, mechanical analysis is restricted in its application and breaks down altogether on chalk soils, acid humus or peat soils, and neutral humus soils, while it gives useful indications only on the mineral soils, i.e. sands, loams and clays. Agricultural soils belong so largely to this group that the method is really applicable in by far the great majority of cases. Among the mineral soils the chemical grouping cuts across the mechanical classification. When the clays are separated out and subjected to hydrofluoric acid treatment or fusion with alkalis and then analysed, they are seen to fall into two or more types, as shown in Table XXIX. (p. 102). Further evidence of dissimilarity among the clays is obtained by a study of the results of the acid extraction of the soil ; in general the alumina is about one-third of the clay in amount and the potash is roughly one-tenth of the alumina ; in other cases, however, very different relationships obtain. For the comparison or characterisation of types chemical analysis becomes of considerable importance. For this pur- pose Hissink (136) and von 'Sigmond (266*2) have each em- phasised the value of extraction by strong HC1, though they do not agree as to the interpretation of the results. Hissink claims to be able, by suitable modification of the analytical process, to differentiate van Bemmelen's three groups : — (1) "Complex A," the " weathered silicates" capable of absorption and exchange of bases. (2) " Complex B," the less active "kaolin silicates."1 (3) Inert "un weathered silicates" incapable of absorption and base exchange effects. The advantages of such discrimination are manifest, but von 'Sigmond is not prepared to admit that it can be done. He considers, however, that strong hydrochloric acid has a practical value for describing soils. Other chemists do not altogether agree : E. A. Mitscherlich sees no advantage in the method2 (see p. 234). 1 For a discussion of these see R. Gans, Int. Mitt. Bodenkunde, 1913, 3, 546. *Ibid., 1914, 4, 327. SOIL ANALYSIS AND ITS INTERPRETATION 345 SOIL SURVEYS. Soil surveys of the following counties have been pub- lished :- ENGLAND. BEDFORD. — Rigg, Th. The Soils and Crops of the Market-garden District of Biggleswade. Journ. Ag. Sri., 1916, vii., 385. CAMBRIDGE. — Foreman, F. W. Soils of Cambridgeshire. Journ. Ag. Sri., 1907, ii., 161. DORSET. — Gilchrist, D. A., and Luxmoore, C. M. The Soils of Dorset. Reading Coll. Dorset C. Council, 1907. KENT, SURREY, AND SUSSEX.— Hall, A. D., and Russell, E. J. Agriculture and Soils of Kent, Surrey, and Sussex. Bd. Ag. and Fisheries, 1911. NOTTINGHAM. — Goodwin, Wm. The Soils of Nottinghamshire. Mid. Ag. and Dairy Coll. Kingston, Derby. NORFOLK. — Newman, L. F. Soils and Agriculture of Norfolk. Trans, Norfolk and Norwich Nat. Soc., 1912, ix., 349-393. SHROPSHIRE. — Robinson, G. W. A Survey of the Soils and Agriculture of Shropshire. County of Salop Higher Education Com., n.d. WALES. Robinson, G. W. Studies on the Paleozoic Soils of North Wales. Tourn. Ag. Sci., 1917, viii., 338. Robinson, G. W., and Hill, C. F. Further Studies on the Soils of North Wales. Journ. Ag. Sri., 1919, ix.. 259. Griffith, J. J. Influence of Mines upon Land and Livestock in Cardiganshire, fourn. Ag. Sri., 1919, ix., 366. 346 SOIL CONDITIONS AND PLANT GRO WTH SCOTLAND. Hendrick, James, and Ogg, W. G. Studies of a Scottish Drift Soil. I. The Composition of the Soil and of the Mineral Particles which compose it. fourn. Ag. Set., 1916, vii., 458. IRELAND. Kilroe, J. R. Soil Geology of Ireland. Dept. Ag. and Tech. Instntctio n for Ire land ^ 1907. See also publications of Dept. of Ag. and Tech. Inst.for Ireland. In France the surveys have been made on a petrogeological basis as initiated by M. Risler in 1856. The work is particularly associated with the Station Agronomique de PAisne et Laon. Many of the maps are beautifully drawn : for an example see Journ. Min. Ag., 1920, 27, 57. In the United States an extensive soil survey is organised by the Dept. of Agriculture and in many of the individual States by the Colleges or AgriculturaLExperiment Stations. For the 'soil map of Germany see Int. Mitt. Bodenkunde, 1917, 7» x- APPENDIX I. THE METHODS OF SOIL ANALYSIS. How to Take the Sample of Soil. — Owing to the variation in composition of the soil at different depths it is particularly necessary that the sample should always be taken to the same depth and with a tool making a clean vertical cut. Samples taken with a spade are of very doubtful value and do not justify any lengthy examination. A suitable tool consists of a steel tube 2 ins. in diameter and 1 2 ins. long, with a f-in. slit cut lengthwise and all its edges sharpened fixed on to a vertical ^— steel rod, bent at the end to a ring 2 ins. in diameter, through which passes a stout wooden handle (Fig. 31). A mark is made 9 ins. from the bottom so that the boring process can be stopped as soon as this depth is reached. On withdrawing the tool the core of soil is removed by a pointed iron rod. Five or six samples should be taken along lines crossing the field so as to get as representative a sample as possible ; the whole bulk must then be sent to the labora- tory. Samples should not be taken from FJG. 31.— Tool for taking freshly ploughed or recently manured land. Soil SamPles- In very stony soils it is easier to use a 2 -in. auger, but this does not, in our experience, yield as satisfactory a sample as the tool shown here. A spud is useful for rapid preliminary inspection of waste land to ensure that the sample is normal ; much disturbance of these soils is sometimes caused by rabbks. For precautions to be taken in drawing the sample see Russell, Journ. Bd. of Agric., 1916, 23, 342, and for a discussion of the magnitude of the experimental error see Robinson and Lloyd, Journ, 347 348 SOIL CONDITIONS AND PLANT GROWTH Agric. Sri., 1915, 7, 144-153, and Leather, Trans. Chem. Soc., 1902, 8l, 883-886. For American methods of sampling see Bot. Gazette^ 1919, Feb., p. 173. C. B. Lipman and D. E. Martin1 show that no further or unusual precautions need be taken when the sample is required for bacteriological purposes. The Analysis. — On arrival at the laboratory the soil is spread out to dry, and is then pounded with a wooden pestle and passed through a 3 mm. sieve. The stones that do not pass through, and the fine earth that does, are separately weighed, and the proportion of stones to 100 of fine earth is calculated. Subsequent analytical operations are made on the fine earth. Moisture. — Four or five grms. of the soil are dried at 100° C. till there is no further change in weight. Organic Matter. — No accurate method of estimation has yet been devised. It is usual to ignite at low redness the sample dried as above. The loss includes organic matter, water not given off at 1 00° C., and carbon dioxide from the carbonates ; allowance may be made for the latter, but not for the combined water. The carbon is sometimes determined either by the ordinary combustion or by some wet combustion method. Methods have also been described for determining " humus," but they have not come into general use. For ordinary purposes it is sufficient to take the loss on ignition as organic matter. Total Nitrogen. — Kjeldahl's method is almost invariably adopted. About 25 to 30 grms. of soil are ground up finely in an iron mortar ; 10 to 15 grms. are heated in a Kjeldahl flask with 20 to 25 c.c. of strong sulphuric acid for three-quarters of an hour; 5 grms. of potassium sulphate are added, and then a crystal of copper sulphate. The heating is continued till all the black colour has gone. Then cool and dilute the mixture, transfer the fluid part to a distillation flask, but leave as much as possible of the sand behind, wash well to remove all the adhering liquid. Add saturated soda solution till the liquid is strongly alkaline, distil, and collect the ammonia in standard acid. Nitrates must be determined in a sample taken direct from the field and dried without any delay at 55° C. ; 200 to 500 grms. of the dried soil are pressed firmly on to a Buchner funnel fitted to a filter flask, and distilled water is poured on. The first 300 c.c. of water 1 Soil Sd., 1918, 6, 131-36. APPENDIX 1 349 passing contains practically all the nitrates, but it is safer to wash more fully. The solution is poured into a flask covered by an inverted porcelain crucible-lid, 10 c.c. of 4 per cent, caustic soda and i or 2 c.c. of 3 per cent, potassium permanganate are added, and the whole is then boiled down to some 75 c.c. and kept just boiling for about two hours. If the permanganate is com- pletely decolorised a little more is added until there is no appreci- able change in half an hour. The excess is then destroyed by cautious addition of sodium sulphite solution and the solution is diluted to 200 c.c. and distilled down to 50 c.c. with the addition of i grm. powdered Devarda alloy, 10 c.c. more of 4 per cent, caustic soda and half a grm. of recently ignited lime. The condenser should be of pure tin with a short length of hard glass tubing at its lower end to dip into the N/5o sulphuric acid in the receiver. Shortly before the end of the distillation the cooling water is emptied out so that steam passes through. A large volume of hydrogen and spray is given off at the beginning of the operation, so special attention must be given to trapping. The titration is carried out as described under Ammonia. In the United States the more rapid , phenol sulphonic acid method is used. (See C. B. Lipman and L. T. Sharp, Univ. Cal. Pub. Ag. Set., 1912, I, 23-37.) For statistical discussion of errors see D. D. Waynick, Univ. Cal. Pub. Ag. Sci., 1918, 3, 243-270. Ammonia is determined by aeration. The apparatus consists of a glass tube sloping at an angle of about 40° ; its dimensions are about 83 cm. by 2 '2 cm. and it has a pear-shaped bulb blown on it at about one-third of its length from the upper end, the small end of the bulb being downwards. It is closed at the lower end by a rubber stopper, through which a glass inlet tube for air passes close to the wall of the larger tube at the under side. The upper end of the large tube is connected by a rubber stopper with a short tube packed with cotton-wool to stop spray, and this in its turn is con- nected with an absorber containing from 5 to 10 c.c. of N/5o sulphuric acid diluted with water and a few drops of a 0*05 per cent, solution of methyl red. The freshly taken soil is passed as quickly as possible through a 3 mm. sieve, stones above this size being rejected; 25 grms. are placed at the lower end of the tube on a loose cotton- wool plug, and 50 c.c. of a solution containing 108 grms. of sodium carbonate crystals and 150 grms. of sodium chloride 350 SOIL CONDITIONS AND PLANT GROWTH per litre are added, together with a few drops of paraffin lamp-oil to stop foaming. The upper stopper is replaced and a current of purified air is sucked through the apparatus until the whole of the ammonia has been carried over into the absorber. With the amounts given a total of 1000 litres of air is sufficient ; a good filter pump should aspirate this amount in from 3-^ to 4 hours. The current should not fall below 250 litres per hour. The acid is then poured into a hard glass flask, titrated with N/5o caustic soda till nearly neutral, boiled to expel carbon dioxide, cooled, and titrated to a clear yellow1 (Fig. 32). 40 50 Scale of CenMmetres. FIG. 32. Carbonates are determined by treating a weighed quantity of the soil with dilute sulphuric acid and estimating the carbon dioxide evolved. Collins' calcimeter is a satisfactory instrument.2 Lime Requirement. — Hutchinson and McLennan's method for the determination of the lime requirement of the soil is as follows : 1 See D. J. Matthews, Journ. Agric. Sci., 1920, 10, 72. 2y0'/ra. Chem. Soc. Ind.t 1906, 25, 518. The apparatus is made by Messrs. Brady and Martin, Newcastle-on-Tyne. APPENDIX t 351 10 to 20 grms. oi the soil are placed in a bottle of 500 to 1000 c.c. capacity, together with 200 to 300 c.c. of approximately N/5o solu- tion of calcium bicarbonate, and the air in the bottle is displaced by a current of carbon dioxide in order to avoid possible precipita- tion of calcium carbonate during the period of the determination. The bottle is then placed in a shaking machine for three hours, after which time the solution is filtered and an aliquot portion of the filtrate is titrated against N/io acid, using methyl orange as indicator. The difference in strength of this filtrate and that of the initial solution represents the amount of calcium carbonate absorbed, each cubic centimetre of N/io acid being equal to 5 mgs. calcium carbonate. For a criticism of the method see E. A. Fisher (99) and for a modification see F. J. Warth and M. P. Saw, Chemical Mem. Dept. Ag. India, 1919, 5, 157-172. For an account of the numerous other methods see H. R. Christensen (67^). Hydrogen Ion Concentration, PH value (see p. 115, and E. A. Fisher (99)). Mineral Substances. — Complete analysis of a soil after the silicates have been decomposed and the silica volatilised by treat- ment with hydrofluoric acid is only rarely attempted. The British method, adopted by the Agricultural Education Association, is thus described by Hall : "20 grms. of the powdered soil are placed in a flask of Jena glass, covered with about 70 c.c. of strong hydrochloric acid, and boiled for a short time over a naked flame to bring it to constant strength. The acid will now contain about 20*2 per cent, of pure hydrogen chloride. The flask is loosely stoppered, placed on the water bath, and the contents allowed to digest for about forty- eight hours. The solution is then cooled, diluted, and filtered. The washed residue is dried and weighed as the material insoluble in acids. The solution is made up to 250 c.c., and aliquot portions are taken for the various determinations. The analytical operations are carried out in the usual manner, but special care must be taken to free the solution from silica or organic matter" (The Soil). As a rule only potash and phosphoric acid are determined, but where necessary other bases are estimated in the usual way. Hissink x has discussed the value of strong HC1 as a solvent in soil analysis. American methods are described by F. E. Bear and llnternat. Mitt. Bodtnkunde, 1915,5, 1-24. For determination of iron see Mori son and Doyne, Journ. Agric. Sci., 1914, 6, 97. 352 SOIL CONDITIONS AND PLANT GROWTH M. Salter,1 and Continental methods by von 'Sigmond (266(2). See also p. 344. Potash. — 50 to 100 c.c. of the solution are evaporated to dry- ness, after addition of 0*5 grm. of pure CaCO3 if the original soil did not effervesce on addition of HC1. Two courses are then open : — (a) The residue is gently ignited over a Bunsen burner until completely charred, and is then extracted with water until all the potassium chloride has dissolved (Neubauer's method2 (212)). To the clear filtrate 5 c.c. of platinum chloride (containing '005 grm. Pt per c.c.) are added and the mixture slowly concentrated on the water bath to a very small bulk. The potassium platino-chloride is filtered off in a Gooch crucible, washed with 80 per cent, alcohol, dried and weighed. or (b) Add 10 c.c. of 5 per cent, baryta solution, evaporate to dryness, ignite and take up with water as in (a), add 2*5 c.c. per- chloric acid (sp. gr. 1*12), concentrate until dense fumes are given off, allow to cool, add 20 c.c. 95 per cent, alcohol, and stir. Decant off the clear alcohol, add 40 c.c. alcohol containing o-2 per cent, perchloric acid, transfer to a tared filter paper, wash with 50 to 100 c.c. of 95 per cent, alcohol till the runnings are no longer acid, dry at 100° and weigh as KC1O4. Phosphoric Acid. — The charred residue from which the potassium chloride has been removed is digested for half an hour on a sand bath with 50 c.c. of 10 per cent. H^SC^ and filtered; the filtrate is treated with 25 c.c. cone. NH4NO3 solution and warmed to 55° C. ; 25 c.c. ammonium molybdate, previously warmed to 55° C., is added and the whole allowed to stand for two hours and filtered. Wash with 2 per cent. NaNO3 till the washings are neutral, transfer the precipitate and filter paper to the beaker used for the precipitation, and add a known volume of standard alkali so that the precipitate completely dissolves. Measure the excess by titration, using phenol- N phthalein as indicator, i c.c. of — alkali = -0003004 grm. P2O5.3 10 1 West Virginia Bui., 159, 1916. 2 The older method due to Tatlock is still sometimes used. It is described by Dyer (91). 3 This volumetric method was originally described in Bull. 46 (revised), United States Division of Chemistry (Washington, 1898). A careful examination has been made by Prescott and the conditions laid down under which it gives satisfactory results (Journ. Agric. Sci., 1914,6, 111-120). Prescott's modifica- tion is given here. The method is applicable for the " available" P2O5, but in APPENDIX I 353 Available Potash and Phosphoric Acid. — Dyer's directions are as follows : 200 grms. dry soil are placed in a Winchester quart bottle with 2 litres of distilled water, in which are dissolved 20 grms. of pure citric acid. The soil is allowed to remain in contact with the solution at ordinary temperatures for seven days, and is shaken a number of times each day. The solution is then filtered, and 500 c.c. taken for each determination ; this is evaporated to dryness, and gently incinerated at a low temperature. The residue is dissolved in hydrochloric acid, evaporated to dryness, redissolved, and filtered ; in the filtrate the potash is determined. For the phosphoric acid determination the last solution is made, as before, with nitric acid. Mechanical Analysis. — The object is to obtain information about the size of the ultimate particles of which the soil is composed ; the compound particles are therefore broken down by treatment with hydrochloric acid, and afterwards with ammonia. Direct measure- ment of the ultimate particles is found to be impracticable ; indirect methods have to be adopted, depending on the time taken to fall through a column of water of given height. When a body falls through a vacuum the time taken is independent of its size or weight, but if air or any other fluid is present the case becomes more com- plicated and the proper mathematical relationship has been found 2 / \ by Stokes to be v = — — - -- — , where v — velocity of the falling particle, o- its density, a its radius (assuming it to be a sphere), and p the density and 17 the coefficient of viscosity of the medium (Trans. Camb. Phil. Soc.t 1851, vol. ix., p. 8). The numerical values at 16° C. are : g = 981, o- = 2-5, p = i, 17 = 'on, and the equation therefore reduces to v = #2 x 29730, Jv or a = -z— cm. 172 The calculated and observed values are found to agree fairly well, differences being due to the fact that the particles are not true spheres, and to the existence of convection currents produced by changes of temperature. The effect of variations in temperature is discussed by Robinson in Jour -n. Agric. Sa'., 1914, 7, 142. The method a lopted by the Agricultural Education Association . Agric. Sa'., 1906, i., 470) is as follows: — this case the residue from the citric acid extraction has first to be heated two hours at 120° to 160° to render the silica insoluble. The older method is de- scribed by Dyer (91). 23 354 SOIL CONDITIONS AND PLANT GROWTH (1) Ten grms. of the air-dry earth, which has passed a 3 mm. sieve, are weighed out into a porcelain basin and triturated with 100 c.c. of N/5 hydrochloric acid, further acid being added if much calcium carbonate is present. After standing in contact with the acid for one hour, the mixture is transferred to a dried, tared filter which is washed until acid-free, dried and weighed. The loss repre- sents hygroscopic moisture and material dissolved by the acid. (2) The soil is washed off the filter with ammoniacal water on to a small sieve of 100 meshes to the linear inch, the portion passing through being collected in a beaker marked at 10, 8*5, and 7*5 cm. respectively from the bottom. The portion which remains upon the sieve is dried, weighed, and divided into " fine gravel " and " coarse sand " by means of a sieve with round holes of i mm. diameter. The portion which does not pass this sieve is the " fine gravel ". This is dried and weighed. The difference gives the "coarse sand ". If required, both these fractions can also be weighed after ignition. (3) The portion which passed the 100 mesh sieve is triturated with a rubber pestle (made by inserting a glass rod as handle into an inverted rubber stopper), and the beaker filled up to the 8*5 cm. mark and allowed to stand for twenty-four hours. The ammoniacal liquid which contains the "clay" is then decanted off into a Win- chester quart bottle, the operation being repeated as long as any matter remains in suspension for twenty-four hours. The liquid containing the "clay " is either evaporated in bulk or measured, and, after being well shaken, an aliquot portion taken and evaporated. In either case the dried residue consists of "clay" and "soluble humus". After ignition the residue gives the " clay," and the loss on ignition the " soluble humus ". Here minimum value of v = o'ooooi cm. per second, and the minimum diameter of the particles = '0013 mm. (4) The sediment from which the " clay " has been removed is triturated as before in the beaker, which is filled to the 10 cm. mark and allowed to stand for 100 seconds. The operation is repeated until the "fine sand" settled in 100 seconds is clean, when it is collected, dried, and weighed. Here minimum value of v = o-i cm. per second; the calculated minimum diameter = "037 mm. (5) The turbid liquid decanted from the " fine sand " is collected in a suitable bottle, allowed to settle, and the clear liquid syphoned or decanted off. The sediment is then washed into the marked APPENDIX I 355 beaker and made up to the 7*5 cm. mark. After stirring, it is allowed to settle for twelve and a half minutes, and the liquid decanted off. The operation is then repeated as before till all the sediment sinks in twelve and a half minutes, leaving the liquid quite clear. The sediment obtained is the " silt," which is dried and weighed as usual. The liquid contains the " fine silt," which, when it has settled down, can be separated by decanting off the clear liquid, and dried and weighed. For silt minimum value of v = o'oi cm. per second, minimum diameter of particles = 0*012 mm. For fine silt the diameter ob- viously lies between this value and the one found for clay. When it is desired to compare the results with American data the fine silt can be divided into two groups : settling for two hours five minutes brings out a group croi to 0*005 mm- diameter, and the remainder lies between 0*005 and 0*002 mm. diameter. The fractions can then be made to correspond fairly closely with those adopted in the United States. (6) Determinations are made of the " moisture" and "loss on ignition " of another 10 grms. of the air-dry earth. The sum of the weights of the fractions after ignition + loss on ignition + moisture + material dissolved in weak acid should approximate to i o grms. (7) It is advisable to make a control determination of the "fine gravel " in a portion of fifty grms. of the air-dry earth. The soil should be treated with acid, as in (i), and after that is removed by decantation may be at once treated with dilute ammonia and washed on the sieve with i mm. round holes. The " fine gravel " left on the sieve is then dried and weighed. The American method is somewhat different. The breaking down of the aggregates is brought about by physical means — e.g. violent shaking — and sedimentation is hastened by a centrifugal apparatus which is a distinct improvement on the British method : the centrifuge was also used by Beam in Khartoum.1 Hilgard does not adopt a sedimentation method but proceeds in the converse manner; he collects and weighs the particles carried off by successive streams of water of varying velocity. Full details are given in Bull. 24, Bureau of Soils, 1904, and in Wiley's Agricultural Analysis, vol. i., where the continental methods are also described. An entirely new method has been described by Oden (218^). 1 W. Beam, The Mechanical Analysis of Arid Soils (Cairo Scientific Journal, 191 1, 5, 107-119). Pub. Wellcome Tropical Research Lab., Khartoum. 356 SOIL CONDITIONS AND PLANT GROWTH Methods of Sand and Soil Cultures. — See Hellriegel (130^) and — Sand, McCall, A. G., Journ. Amer. Soc. Agron., 1915, 7, 249- 282 ; Soil Sri , 1916, 2, 207-253 ; Journ. Amer. Soc. Agron., 1918, IO, 127-134. Hoagland, D. R., Journ. Ag. Res., 1919, 18, 177- Comparison of Sand and Water Cultures, Hoagland, D. R., Journ. Ag. Res., 1919, 18, ii. * S! VO CO Oi OO CO VO Oi ^i- ro Oi 0 Tt- 01 0~ VO M ^ H VO ^ t^ CM o vO c 0 bfl c/3 .n Ovo H VO M in S o\ in H VO £ in ** 0 VO l^ CM m *" CO CO vO OO Oi M 01 '£ "i u g . H Tl- m invo H in H VO 0 CO M CO ^ OI CM M CO CO M invo M t u jo s o — ' O N 01 O CO * o vo 0 Tt" H 01 H Th in O H H M 01 VO 00 ir CM 0 & TJ c <" T -d ^ CM 01 0 t-s ^ 0 *• TJ- ro Oi OO CO H TT r>- H OlOO ^ Tt- «n O in rt •ojp M vO ^ 0 covo Oi CM Tf CM VO o> CM 0 tr VO Tf H vo VO 01 H OJJ c Q, a< <*-! o 1 • vo m M O O1^ Oi vo » ^ -t- 01 CM l^ p 1^00 m 00 Tt 01 CO CM in ff CO CL, 2 1 — CO CO "* «*• N 0 co m CO H cc rt- in m co Oi VO CM Oi ^ CM^ 01 VO £ u . . 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H Joo in >«t X m q; 0 "*• 01 ir o in Oi CO 00 OO •n 1 -| 0 0 CM Tl- Oi CM vg cgct O Tj- in O Tj- vo O CO t^ i O co 00 CO CM CM CO H >n 0 * 1 * 1 H CO OO w CO H 5 H Th co rr. OiCO £ "' C/J en P O CO 3 & CO 3 4 n o c o CO CO c 0 CO 1 ! Amounts of Var 0 co bO to rt .r C-, 2 u 1 0 il bo co rt PQ G, 0 u 1 0 -H 3 m cf £ 'rt 2 bbto co" o" u 3 3 H X ed 1 57 ts ffi 1 U •g Beans, grain, 30 b ,, straw U 0 — < 11 .1 1 c. 2 0 "c5 0 co" . 11 co~ -S D «. C/3 0 15 0 -H Mangolds, roots, 2 leaf . G. O U Potatoes, tubers, t A SELECTED BIBLIOGRAPHY.1 Page of text on which reference is made. 1. Abbe, Cleveland, "Relation between Climates and Crops," Weather Bureau Bull. 36, U.S. Dept. of Agric. 2. Adeney, W. E., Appendix VI. to Fifth Report of Royal Commission on Sewage Dispjsal, 1908, pp. 13-20 190, 194, 214 3. Alway, F. J., (a) "The Loess Soils of the Nebraska Portion of the Transition Region," Soil Sci., 1916, i, 299-316, 405-36 . . . 226 (b) " Studies on the Relation of the Non-available Water of the Soil to the Hygroscopic Coefficient," Nebraska Research Bull., No. 3. 1913 223,226 (c) Alway, F. J., and McDole, G. R., " Relation of the Water-retaining Capacity of Soil to its Hygroscopic Coefficient," y. Ag. Re- search, 1917, ix., 27 226 (d) Alway, F. J., and Neller, J. R., " A Field Study of the Influence of Organic Matter upon the Water-holding Capacity of a Silt-loam Soil," ibid., 1919, xvi., 263 . 137 4. Ames, J. W., and Schollenberger, C. J., " Liming and Lime Require- ment of Soil," Ohio Bull., No. 306, 1916 119 5. Armstrong, H. E., and E. F., (a) " The Origin of Osmotic Effects". — III., "The Function of Hormones in Stimulating Enzymic Change in Relation to Narcosis and the Phenomena of Degenerative and Regenerative Change in Living Structures," Proc. Roy. Soc,, 1910, Ixxxii., B, 588-602 . 87 (b) " The Function of Hormones in Regulating Metabolism," Annals of Botany, 1911, xxv., 507-19 87 6. Arnd, Th. (of Bremen), (a) " Ueber schadliche stickstoffumsetzungen in Hochmoorboden als Folge der Werkung starker Kalkgaben," Landw. Jahrbucher, 1914, xlvii., 371-442 ..... 210 (b) " Beitrage zur Kenntnis der Mikrobiologie unkultivierte und kulti- vierte Hochmoore," Centr. Bakt. Par., II., 1916, xlii., 554 . . 309 (c) " Zur Kenntnis der Nitrification in Moorboden," ibid., 1919, xlix., i-5i 309 7. Ashby, S. F., (a) " The Comparative Nitrifying Power of Soils," Trans. Chem. Soc., 1904, Ixxxv., 1158-70 188,262 (b) " Some Observations on Nitrification," Journ. Agric. Sci., 1907, ii., 52-67 262 (c) "Some Observations on the Assimilation of Atmospheric Nitrogen by a Free-living Soil Organism — Azotobacter Chroccoccum of Bei- jerinck," ibid., 1907, ii., 35-51 203 (d) " A Contribution to the Study of Factors Affecting the Quality and Composition of Potatoes," ibid., 1905, i., 347-57 . . . 3l8» 331 1 Most of the papers and books quoted here can be seen in the Rothamsted Expt. Station Library, or in the Patent Office Library, London, and some can be borrowed from the Library of the Ministry of Agriculture. 359 360 SOIL CONDITIONS AND PLANT GROWTH Page of text on which reference is made. 8. Bacon, Francis, Sylva Sylvarum, 1627 2 9. Baule, B., " Zu Mitscherlichs Gesetz der physiologischen Beziehungen," Landw. Jahrb., 1917, li., 363; ibid., 1920, liv., 494-505 ... 34 10. Baumann, A., "Untersuchungen iiber die Humussauren," I., Mitt. d. k. bayr. Moorkulturanstalt, 1909, Heft 3, 53-123 . . in, 130, 140 Baumann, A., and Gully, E., " Untersuchungen liber die Humus- sauren," II., ibid., 1911, Heft 4. (For a more accessible summary, see Gully, 116 (b) in, 130, 140 11. Bear, F. E., "A Correlation between Bacterial Activity and Lime Re- quirement of Soils," Soil Sci., 1917, iv., 433-462 .... 243 12. Beckley, V. A. (a) " The Preparation and Fractionation of Humic Acid," Jonrn. Ag. Sci., 1921, xi., 66-68 141 (b) "The Formation of Humus," Journ. Ag. Sci., 1921, xi., 69-77. . 142 13. Beijerinck, Martinus W., " Die Bacterien der Papilionaceen-Knollchen," Botan. Ztg., 1888, xlvi., 725-35, 741-50, 757-71, etc. "Kunstliche Infection von Vicia Faba mit Bacillus radicicola, Ernahr- ungsbedingungen dieser Bacterie," ibid., 1890, xlviii., 837-43. " Over ophooping van atmospherische stikstof in culturen van Bacillus radicicola," Versl. en Mededeel. d. Akad. von Wetensch. Amsterdam. Afd. Naturkunde, 1891, viii. (3), 460-75 204 14. Beijerinck, Martinus W., " Ueber oligonitrophile Mikroben," Centr. Bakt. Par., Abt. II., 1901, vii., 561-82 197 15. Beijerinck, Martinus W., and van Delden, A., " Ueber die Assimilation des freien Stickstoffs durch Bakterien," ibid., 1902, ix., 3-43 . 197, 202 16. Beijerinck, Martinus W., "Fixation of Free Atmospheric Nitrogen by Azobacter in Pure Culture, Distribution of this Bacterium," Proc. k. Akad. Wetensch. Amsterdam., 1908, xi., 67-74 .... 197 17. Beijerinck, Martinus W., " Ueber Chinonbildung durch Streptothrix chromogena und Lebensweise dieses Mikroben," Centr. Bakt. Par., Abt. II., 1900, vi., 2-12 143 18. Beijerinck, Martinus W., and Minkman, D. C. J., " Bildung und Ver- brauch von Stickoxydul durch Bakterien," ibid., 1910, xxv., 30-63 . 208 19. Bemmelen, Jakob M. van, "Die Absorptionsverbindungen und das Ab- sorptionsvermogen des Ackererde," Landw. Versuchs-Stat., 1888, xxxv., 67-136 130, 140, 155 20. Bemmelen, Jakob M. van (a) " Das Absorptionsvermogen der Ackererde," ibid., 1878, xxiii., 265-304 155 (b) " Die Zusammensetzung der Ackererde nach Anleitung der in den vorigen Abhandlungen mitgeteilten Analysen von gewohnlichen und vulkanischen Thonboden," ibid., 1890, xxxvii., 347-73 . . . 105 21. Bemmelen, Jakob M. van, " Die Absorption von Stoffen aus Losungen," Zeit. Anorg. Chem., 1900, xxiii., 321-72 130 22. Bemmelen, Jakob M. van, " Beitrage zur Kenntnis der Verwitterungs- produkte der Silikate," etc., ibid., 1904, xlii., 265-314 . . . 101 23. Bemmelen, Jakob M. van, " Die verschiedenen Arten der Verwitterung der Silikatgesteine in der Erdrinde," ibid., 1910, Ixvi., 322-57 . . 101 24. Bemmelen, Jakob M. van, " Ueber das Vorkommen, die Zusammenset- zung und die Bildung von Eisenanhaufungen in und unter Mooren," ibid., 1899, xxii., 313-79 163 A SELECTED BIBLIOGRAPHY 361 Page of text on which reference is made. 25. Bemmelen, Jakob M. van, " Die absorption von Wasser durch Ton," Zeit. Anorg. Chem., 1904, xlii., 314-24 163 (Van Bemmelen's papers are collected by Wo. Ostwald and published by Steinkopff of Dresden in " Die Absorption," pp. 548; a Gedenk- buch is also published by the same firm.) 26. Berthelot, Marcellin, "Fixation directe de 1'azote atmospherique libre par certains terrains argileux," Compt. Rend., 1885, ci., 775-84 25, 195 27. Berthelot, Marcellin, "Sur le dosage du carbon e organique contenu dans les sols qui fixent 1'azote libre," ibid., 1886, cii., 951-54 . . 25 28. Berthelot, Marcellin, ibid., 1888, cvi., 569-574, " Sur quelques conditions generates de la fixation de 1'azote par la terre vege'tale" ... 26 29. Berthelot, Marcellin, " Recherches nouvelles sur la fixation de 1' Azote, par la terre vegetale et les plantes, et sur 1'influence de l'electricit£ sur ce phenomene," Ann. Chim. Phys., 1890, Series 6, xix., 434-92 26 30. 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Wolff, E., and Kreutzhage, C., " Bedeutung der Kieselsaure fur die Entwicklung der Haferpflanze nach Versuchen in Wasserkultur," Landw. Versuchs-Stat., 1884, xxx., 161-97 77 316. Wolkoff, M. I., " Effect of Ammonium Sulphate in Nutrient Solu- tion on the Growth of Soy Beans in Sand Cultures," Soil Sci., 1918, v., 123-50 80 317. Wollny, E., (a) " Untersuchungen iiber den Kohlensauregehalt der Bodenluft," Landw. Versuchs-Stat., 1880, xx*, 373-91 . . .175 (b) " Ueber die Thatigkeit mehrerer organismen in der Ackererde," Bied. Zentr. Agric. Chem., 1884, xiii., 796-814 ... 26, 175 318. Wollny, E., Die Zersetzung der Organischen Stoffe, 1897 . . .139 (A summary of Wollny' s work. For the papers by himself and his students see his Journal Forschungen auf dem Gebiete der agricultur Physik, 1878 et seq.) 319. Wood, T. B., "The Available Potash and Phosphoric Acid in Soils," Trans. Chem. Soc., 1896, Ixix., 287 234 320. Wood, T. B., and Berry, R. A., "Soil Analysis as a Guide to the Manurial Treatment of Poor Pasture," Journ. Agric. Sci., 1905, i., 114-21 234 25 386 SOIL CONDITIONS AND PLANT GROWTH Page of text on which reference is made. 321. Woodward, J., " Thoughts and Experiments on Vegetation," Phil. Trans., 1699, xxi., 382-92 4 322. Woronin, M., " Ueber die bei der Schwarzerle (Alnus glutinosa) und der gewohnlichen Garten-Lupine (Lupinus mutabilis) auftretenden Wurzelanschwellungen," Memoires Acad. Scl., St. Petersburg, 1866, (7), x., No. 6 25 323. Zyl, J. P. van, " Ueber die Bodenlosung : ihre Gewinnung, Zusam- mensetzung und Anwendung bei der Schammanalyse," y. fur Landw., 1916, Ixiv., 201-75 125 AUTHOR INDEX. The numbers in square brackets marked B refer to numbers in the bibliography to which the reader should turn for fuller reference. ABBE, Cleveland, [B i]. - Abbott, J. B., 112. Acqua, C., go. Adametz, L., 256. Adeney, W. E. [B 2], 190, ig4, 214. Ageton, C. N. [B 105], 304. Aiyer, S. [B 126], 213, 256, 300. Albrecht, W. A., 207. Ali, Barkat, 274. Allen, E. R., igg, 262. Allison, F. E. [B 60], 169, 213, 273, 2go. Alves, A. [B 1516], 201. Alway, F. J. [B 3], 137, 221, 223, 226. Ames, J. W. [B 4], ng, an. Amos, A., 84, 157. Andre, G. [B 30, 31], 138, 146, 152, 2g7- Appleyard, A. [B 241^], 227, 228, 275, 276, 2g7. Ardern, E., 212. Aristotle, 152. Armsby, H. P., 156. Armstrong, E. F. [B 5], 87, 215. — H. E. [B 5], 87, 236. Arnd, Th. [B 6, 278], 210, 307. Ashby, S. F. [B 7], 188, 203, 261, 262, 3i8, 331. Ashley, H. E., 170. Aso, K. [B 277]. Atkins, W. R. G., 2 .'2. Atterberg, A., g8. BACH, N., 161. Bacon, Francis, [B 8], 2. Bahr, F. [B 93], 113, 143. Bailey, C. H., 44. Baker, T. T., go. Bancroft, W. D., 148. Barger, G., 195. Barlow, B. [B 125], 206, 208. Barnes, J. H., 274. Bassett, H., 120. Bauer, H. [B 233^], 124. Baule, B. [B 9], 34. Baumann, A. [B 10], in, 130, 140. Beam, W., 355. Bear, F. E. [B 11], 243, 273, 351. Beckley, V. A. [B 12], 141, 142. Bedford, Duke of, [B 225]. Beijerinck, Martinus W. [B 13-18], 26, 143, igg, 202, 204, 208, 260. Bell, J. M. [B 65*]. Bemmelen, Jakob M. van, [B 19-25], 27, 101, 105, 130, 140, 148, 155, 344- Benz, Max, [B 310]. Bergu, G., 234. Berman, N., 186. Berry, R. A. [B 320], 234. Berthelot, Marcellin, [B 26-33], 25, 26, 138, 195- Bertholon, Abbe, [B 34], 87. Bertrand, Gabriel, [B 35], 76. Berzelius, J. J., 12. Bewley, W. F. [B 36], 72, 204. Bialoblocki, J. [B 37], 35. Bierema, S. [B 38], 210. Birner, H. [B 3g]. Bizzell, James A. [B 183], 243, 2g8, 2gg. Blackman, F. F. [B 40], 28, 35, 38. - V. H., 28, 88, gi. Blanck E. [B 170, 22^c-f\t 34, 45, 101, 201, 210, 233, 234, 305. Blair, A. W., 181, 186, 243. Bocker, F., 152. Boedecker, C., 155. Boerhaave, H. [B 41], 5. Bogdanow, S., 77, 234. Bolley, H. L., 282, 2g5. Bonazzi, A., 188. Bottomley, W. B. [B 42], 236, 24g. Bouilhac, R. [B 43, 44], 200, 256. Boussingault, J. B. [B 45-48, 88], 12, 15, 20, 175, 178, 192, 227. 387 388 SOIL CONDITIONS AND PLANT GROWTH Bouyoucos, G. J. [B 49], 125, 173, 225, 233- Boyle, Robert, [B 50], 3, 4. Breazeale, J. F. [B 51, 650, 179], 73, 79, 139, 152, 236, 238, 248. Bredemann, G. [B 52], 200. Bredig, G., 215. Brenchley, Winifred E. [B 53, i2i