CONTENTS.
| CHAP. | PAGE | |
|---|---|---|
| I. | Development of the Idea of a Soil Population | [1] |
| Sir E. John Russell, F.R.S., Director. | ||
| II. | Occurrence of Bacteria in Soil—Activities connected with the Acquirement of Energy | [20] |
| H. G. Thornton, B.A., Head of the Department of Bacteriology. | ||
| III. | Conditions affecting Bacterial Activities in the Soil—Activities connected with theIntake of Protein Building Materials | [39] |
| H. G. Thornton, B.A., Head of the Department of Bacteriology. | ||
| IV. | Protozoa of the Soil, I. | [66] |
| D. W. Cutler, M.A., Head of the Department of Protozoology. | ||
| V. | Protozoa of the Soil, II. | [77] |
| D. W. Cutler, M.A., Head of the Department of Protozoology. | ||
| VI. | Soil Algæ | [99] |
| B. Muriel Bristol, D.Sc., Algologist. | ||
| VII. | Soil Fungi—The Occurrence of Fungi in the Soil | [118] |
| W. B. Brierley, D.Sc., Head of the Department of Mycology. | ||
| VIII. | Soil Fungi—The Life of Fungi in the Soil | [131] |
| W. B. Brierley, D.Sc., Head of the Department of Mycology. | ||
| IX. | The Invertebrate Fauna of the Soil (other than Protozoa) | [147] |
| A. D. Imms, D.Sc., Head of the Department of Entomology. | ||
| X. | The Chemical Activities of the Soil Population and their Relation to the GrowingPlant | [164] |
| Sir E. John Russell, F.R.S., Director. | ||
| Index | [181] |
CHAPTER I.
THE DEVELOPMENT OF THE IDEA OF A SOIL POPULATION.
From the earliest times agriculturists have been familiar with the idea that decomposition of vegetable and animal matter takes place in the soil, and that the process is intimately connected with soil fertility.
By the middle of the nineteenth century three different ways were known in which the decomposition occurred. One had been since early times specially associated with soil fertility, in that it gave rise to humus, the black sticky substance in farmyard manure or in soil—which was supposed up to 1840 to be the special food of plants. No good account of the process or of the conditions in which it occurred is, however, given by the older writers.
A second resulted in the formation of nitrates. This process became known as nitrification: it was described by Georgius Agricola (1494-1555) in his book “De Re Metallica,” and it was of great importance in the seventeenth and eighteenth centuries, because it was used for the manufacture of gunpowder in the great wars of that period. The conditions for the making of successful nitre beds were so thoroughly investigated that little fresh knowledge was added to that of 1770[A] until quite recently. This process, however, was not usually associated with soil fertility, although both Glauber (1656) and Mayow (1674) had insisted on the connection.
[A] See the remarkable collection of papers entitled “Instructions sur l’établissement des nitrières,” publié par les Régisseurs-généraux des Poudres et Salpêtre. Paris, 1777.
A third type of decomposition was brought into prominence by Liebig in 1840.[7] [B] Reviewing the decomposition of organic matter in the light of the newer chemistry, he concluded that the process was a slow chemical oxidation, to which he gave the name “Eremacausis.” He recognised that humus was formed, but he regarded it only as an intermediate product, and emphatically denied its importance in soil fertility. The true fertility agents, in his view, were the final products—CO2, potassium and other alkaline salts, phosphates, silicates, etc. He went on to argue brilliantly that instead of applying farmyard or similar manures to the soil it was altogether quicker and better to apply these mineral compounds obtained from other sources than to wait for the slow process of liberation as the result of decomposition. For some reason, difficult to understand, he overlooked nitrification and the part that nitrates might play in soil fertility. Lawes and Gilbert[6] were much attracted by this new idea; they showed that it was incomplete because it took no account of the necessity for supplying nitrogen compounds to the crop. When ammonium salts were added to Liebig’s ash constituents the resulting mixture had almost as good a fertilising effect as farmyard manure. Lawes at once saw the enormous practical importance of this discovery, and set up a factory for the manufacture of artificial fertilisers. He did not, however, follow it up more closely on the scientific side.
[B] The numbers refer to the short bibliography on [p. 18].
Both Lawes and Gilbert were in constant touch with the idea of decomposition in the soil, and they attached so much importance to nitrogen compounds in plant nutrition that it is not easy to understand how they missed the connection with nitrification. But they did so, and like other English and German workers of the day, considered that plant roots assimilated their nitrogen as ammonia. For the first ten years of the history of Rothamsted only few experiments with nitrates were made, and not till thirty-five years had elapsed were they systematically studied.
It was by Boussingault[2] and in France that the connection between nitrification and soil fertility was first recognised. The news came to England, but it was not accepted, although Way, one of the most brilliant agricultural chemists of his time, showed that nitrates were formed in soils to which nitrogenous fertilisers were added, and that they were comparable in their fertiliser effects with ammonium salts.[12] “The French chemists,” he wrote in 1856, “are going further, several of them now advocating the view that it is in the form of nitric acid that plants make use of compounds of nitrogen. With this view I do not at present coincide, and it is sufficient here to admit that nitric acid in the form of nitrates has at least a very high value as manure.” Indeed, Kuhlmann went so far as to argue that the nitrates found in the soil were there reduced to ammonia before assimilation by plants could take place. The water-culture work of the plant physiologists of the ’sixties finally showed the correctness of the French view.
Even when the importance of nitrification was realised its mechanism was not understood: some thought it was chemical, some physical. Again the explanation came from France. Pasteur in 1862 had expressed the view that nitrification would probably be a biological action, since purely chemical oxidation of organic matter was of very limited occurrence. “Pénétrés de ces idées,” as Schloesing tells us, he and Müntz in a memorable investigation cleared up the whole problem, and in 1877 opened the way to a most fruitful field of research.[10] The formal description is given in his papers in the “Comptes Rendus,” but a more lively account is given in his lectures before the École d’application des Manufacteurs de l’état, which, though not printed, were collected and issued in script by his distinguished son, and a copy of this work is among the treasures of the Rothamsted Library.
He had been asked to study the purification of sewage, and he and Müntz showed that it was bound up with nitrification. The process was slow in starting, then it proceeded rapidly. Why, they asked, was the delay? There should be none if the process were physical or chemical, and the fact that it occurred strongly suggested biological action. The process was stopped by chloroform vapour, but could be restarted after the removal of the vapour by the addition of a little fresh soil.
The importance of this work in connection with soil fertility was immediately realised by Warington, who had recently come to Rothamsted.[11] He quickly confirmed the result, and made the valuable discovery that two stages were involved—the conversion of ammonia to a nitrite by one organism, and of the nitrite to nitrate by another. He made long and persistent attempts to isolate the organisms from the soil, using the best technique of his time, but though he found many bacteria none of them could nitrify ammonium salts; yet the soil did it easily. For years he continued his efforts to find the nitrifying organism, but always failed. His health was not good, his life at Rothamsted was not happy owing to disagreements with Gilbert, and although his other research work was succeeding, this investigation on which he had set his heart was not coming out; bacterial technique was not yet sufficiently far advanced. Ten bitter, disappointing years passed, and the crown of disappointment came when Winogradsky, a young bacteriologist in Paris, changed the technique and succeeded at once in isolating both the nitrite and the nitrate-forming organisms.[13]
The numerous bacteria found by Warington in the soil suggested the presence of a soil population, and this idea was greatly strengthened by another line of investigation which was being followed up in France. Boussingault had shown that soils absorb oxygen and give out carbon dioxide; Schloesing extended this discovery, as also did Wollny. It was concluded that oxidation was the result of the activities of the soil organisms in decomposing the organic matter of the soil, and thus preparing the way for the nitrifying organisms.
A third important function of soil bacteria was revealed by Berthelot.[1] It was known that considerable loss of nitrogen from the soil took place as the result of the conversion of nitrogen compounds into nitrates, which were subsequently washed out in the drainage water. It followed inevitably that the stock of nitrogen compounds in the soil must long ago have become exhausted had there been no addition of nitrogen compounds to the soil. Berthelot argued that there must be fixation of atmospheric nitrogen, and, following the prevailing trend of thought in France, he attributed it to bacteria. He confirmed the anticipation by exposing soil to air in such conditions that dust, rain, etc., were excluded, and he found an increase in the percentage of nitrogen.
Looking back over the work, it is difficult to understand the result. The fixation of nitrogen is a process that absorbs energy, and should have necessitated some source of energy, which apparently was not supplied. But in spite of this drawback the investigation was immediately fruitful in that it gave the key to another problem which had long puzzled agriculturists.
It had long been known that the growth of leguminous crops, unlike that of others, enriched the ground,[C] and Lawes and Gilbert had shown that this was due to an increase of soil nitrogen. But no explanation could be found till Hellriegel and Wilfarth solved the problem.[4] In studying the nitrogen nutrition of gramineous and leguminous crops, they discovered that the gramineous plants died in absence of nitrate, and in its presence made growth which increased regularly with nitrate supply; while leguminous plants sometimes died and sometimes flourished in absence of nitrate, and behaved equally erratically with increasing nitrate supply. When the plants flourished nodules were invariably present on the roots, but not otherwise. They concluded, therefore, that the nitrogen nutrition of leguminous plants differed from that of the gramineæ, and depended on some factor which sometimes came into their experiments and sometimes did not, and, in any case, was associated with the nodule. Knowing that the nodules on the roots of leguminous plants contained bacteria-like bodies, and remembering Berthelot’s results, they explored the possibility of bacterial fixation. They sterilised the sand and found that peas invariably failed to develop nodules and died, but after adding a little garden soil nodules were found and vigorous growth was obtained.
[C] “Of the leguminous plants the bean best reinvigorates the ground ... because the plant is of loose growth and rots easily, wherefore the people of Macedonia and Thessaly turn over the ground when it is in flower” (i.e. dig it into the ground if the soil is poor). Theophrastus, “Enquiry into Plants,” bk. viii. 2, and bk. ix. I. This book is of profound interest to agriculturists and botanists. An excellent translation by Sir Arthur Hort is now available. (Loeb’s Classical Library.)
Chemical analysis showed considerable fixation of gaseous nitrogen, which Hellriegel associated with the nodule organism. This has proved to be correct, and the fixation of nitrogen by bacteria is now a well-recognised process, the conditions of which are being thoroughly worked out. Two types of organisms are known—those associated with leguminous plants, and those living in a free and independent state in the soil. Of the latter the Clostridium, isolated by Winogradsky, is anaerobic, and the Azotobacter of Beijerinck is aerobic. The essential conditions are that a source of energy must be supplied—usually given as sugar—that the medium must not be acid, and that sufficient phosphate must be present.
All this brilliant work had been accomplished in the short space of the ten years 1880 to 1890. The inspiration had in each instance come from France, and is traceable direct to Pasteur, although coming long after his own work on bacteriology. It is impossible for us now to realise the thrill of wonder and astonishment with which students, teachers, and writers of those days learned that the nutrition of plants, and therefore the growth of crops and the feeding of themselves, was largely the result of the activity of bacteria in the dark recesses of the soil. It is not surprising that the ideas were pushed somewhat too far, that the soil population was regarded as solely bacterial, and that important chemical and physical changes were sometimes overlooked.
Gradually there came the inevitable reaction and a somewhat changed outlook. Continued examination showed the presence in soil of almost every kind of bacteria for which search was made. Some of them were almost certainly in the resting condition as spores, and the new generation of workers had an uneasy feeling that the case for the overwhelming importance of bacteria in the economy of the soil was not too well founded. It was shown that the decomposition of nitrogen compounds to form ammonia would take place without micro-organisms if, as presumably would happen, the plant enzymes continued to act after they got into the soil. Even the oxidation of ammonia to nitrate—the great stronghold of the biological school—was accomplished by chemical agents. The fixation of nitrogen in soil conditions was beyond the power of chemists to achieve, and here it was universally agreed that bacteria were the active agents. And finally, chemists were themselves bringing into prominence a set of bodies—the colloids—whose remarkable properties seemed indefinitely expansible, and were in addition sufficiently incomprehensible to the ordinary student to attain much of the magnificence of the unknown.
All the time, however, a faithful body of workers was busy exploring the ground already won, improving the technique, making counts of the numbers of bacteria in the soil, and trying to measure the amount of bacterial activity. Much of the value of this work was limited by the circumstance that the bacteria were regarded as more or less constant in numbers and activities, so that a single determination was supposed to characterise the position in a given soil.
This was the condition of the subject when it was seriously taken up at Rothamsted. Before turning to agriculture, the writer had been studying the mechanism of certain slow chemical oxidations, and one of his first experiments in agriculture was to examine the phenomena of oxidation in soil. The results accorded with the biological explanation of Schloesing: when the soil was completely sterilised oxidation almost ceased. But the striking discovery was made, as the result of an accident to an autoclave, that partial sterilisation increased the rate of oxidation, and therefore presumably the bacterial activity. This remarkable phenomenon had, however, already been observed, and it had been shown that both bacterial numbers and soil fertility were increased thereby. A full investigation was started in 1907 by Dr. Hutchinson and the writer.[9] From the outset the phenomena were recognised as dynamic and not static, and the rates of change were always determined: thus the bacterial numbers, the nitrate and ammonia present were estimated after the several periods. Close study of the curves showed that the chemical and bacterial changes were sufficiently alike to justify the view that bacteria were in the main the causes of the production of ammonia and of nitrate; although non-biological chemical action was not excluded, there was no evidence that it played any great part. Thus the importance of micro-organisms in the soil was demonstrated.
The factor causing the increased bacterial numbers after partial sterilisation was studied by finding out what agents would, and what would not, allow the numbers to increase, e.g. it was found that the bacterial increases became possible when soil had been heated at 56° C., but not at 40° C. Again, it was shown that the high numbers in partially sterilised soils rose for a time even higher if a little fresh untreated soil were incorporated into the partially sterilised soil, but afterwards they fell considerably. Putting all the results together, it appeared that some biological cause was at work depressing the numbers of bacteria in normal soils, but not—or not so much—in the partially sterilised soils. Studied in detail, the data suggested protozoa as the agent keeping down bacterial numbers, and they were found in the untreated, but not in the treated, soils. The hypothesis was therefore put forward that bacteria are not the only members of the soil population, but that protozoa are also present keeping them in check, and therefore adversely affecting the production of plant food.
This conclusion aroused considerable controversy. It was maintained that protozoa were not normal inhabitants of the soil, but only occasional visitants, and, in any case, they were only there as cysts; the soil conditions, it was urged, were not suitable to large organisms like protozoa. The objection was not to be treated lightly, but, on the other hand, the experiments seemed quite sound. As neither Dr. Hutchinson nor the writer were protozoologists, Dr. T. Goodey and (after he left) Mr. Kenneth R. Lewin were invited to try and find out, quite independently of the partial sterilisation investigation, whether protozoa are normal inhabitants of the soil, and if so, whether they are in a trophic condition, and what is their mode of life and their relation to soil bacteria. Had it turned out that protozoa had nothing to do with the matter, search would have been made for some other organism. Goodey showed that the ciliates were not particularly important; Lewin soon demonstrated the existence of trophic amœbæ and flagellates. Unfortunately he was killed in the war before he had got far with the work. After the Armistice, Mr. Cutler accepted charge of the work: he will himself relate in [Chapters IV.] and [V.] what he has done.
At first sight it might be thought comparatively easy to settle a question of this kind by examining soil under a microscope or by sterilising it and introducing successively bacteria and known types of protozoa. Unfortunately neither method is simple in practice. It is impossible to look into the soil with a microscope, and methods of teasing-out small pieces of soil on a slide under the high, or even the low power, give no information, because the particles of soil have the remarkable power of attracting and firmly retaining protozoa, and no doubt bacteria as well; indeed, for protozoa (which have been the more fully investigated) there seems to be something not unlike a saturation capacity (see [Fig. 9], p. 78). Further, complete sterilisation of soil cannot be effected without at the same time altering its chemical and physical properties, and changing it as a habitat for micro-organisms. Cutler has, however, overcome the difficulties and shown that the introduction of protozoa into soils sterilised and then reinfected with bacteria considerably reduces the numbers of these organisms.
The method adopted, therefore, is to take a census of population and of production. Counting methods are elaborated, and estimates as accurate as possible are made of the numbers of the various organisms in a natural field soil at stated intervals. Simultaneously, wherever possible some measure is taken of the work done. The details of the census are finally arranged in consultation with the Statistical Department, to ensure that the data shall possess adequate statistical value. From the results it is possible to adduce information of great value as to the life of the population, the influence of external conditions, etc.
The most important investigation of this kind carried out at Rothamsted was organised by Mr. Cutler.[3] A team of six workers was assembled, and for 365 days without a break they counted every day the ciliates, the amœbæ, the flagellates, and the bacteria in a plot of arable ground, distinguishing no less than seventeen different kinds of protozoa. The conclusions arrived at were carefully tested by the Statistical Department.
Of the protozoa the flagellates were found to be the most numerous, the amœbæ came next, and the ciliates were by far the fewest. The numbers of each organism varied from day to day in a way that showed conclusively the essentially trophic nature of the protozoan population. The numbers of amœbæ—especially Dimastigamœba and of a species called α—were sharply related to the numbers of bacteria: when the amœbae were numerous the bacteria were few, and vice versa. Detailed examination showed that the amœbæ were probably the cause of the fluctuations in the bacterial numbers, but Mr. Cutler has not yet been able to find why the amœbæ fluctuated; it does not appear that temperature, moisture content, air supply or food supply were determining causes. The flagellates and ciliates also showed large fluctuations, amounting in one case—Oicomonas—to a definite periodicity, apparently, however, not related to bacterial numbers, or, so far as can be seen, to external conditions of moisture, temperature and food supply, and showing no agreement with the fluctuations of the amœbæ. However, one cannot be certain that lack of agreement between curves expressing protozoan numbers and physical factors implies absence of causal relationships: the observations (though the best that can yet be made) are admittedly not complete. If we saw only the end of the bough of a tree, and could see no connection with a trunk, we might have much difficulty in finding relationships between its motion and the wind; whatever the direction of the wind it would move backwards and forwards in much the same way, and even when the wind was blowing along the plane of its motion it would just as often move against the wind as with it.
Meanwhile evidence was obtained that the twenty-four hour interval adopted by the protozoological staff was too long for bacteria, and accordingly the Bacteriological Department, under Mr. Thornton, refined the method still further. Bacterial counts were made every two hours, day and night, for several periods of sixty or eighty hours without a break. The shape of the curve suggests that two hours is probably close enough, and for the present counts at shorter intervals are not contemplated. But there is at least one maximum and one minimum in the day, although the bacterial day does not apparently correspond with ours, nor can any relationship be traced with the diurnal temperature curve.
The nitrate content of the soil was simultaneously determined by Mr. Page and found to vary from hour to hour, but the variations did not sharply correspond with the bacterial numbers; this, however, would not necessarily be expected. The production of nitrate involves various stages, and any lag would throw the nitrate and bacterial curves out of agreement. There is a suggestion of a lag, but more counts are necessary before it can be regarded as established.
Examination of these and other nitrate curves obtained at Rothamsted has brought out another remarkable phenomenon. No crop is growing on these plots, and no rain fell during the eighty hours, yet nitrate is disappearing for a considerable part of the time. Where is it going to? At present the simplest explanation seems to be that it is taken up by micro-organisms. A similar conclusion had to be drawn from a study of the nitrogen exhaustion of the soil. The whole of the nitrate theoretically obtainable from the organic matter of the soil is not obtained in the course of hours or even days; in one of our experiments at Rothamsted nitrification is still going on, and is far from complete, even after a lapse of fifty-three years. The explanation at present offered is that part of the nitrate is constantly being absorbed by micro-organisms and regenerated later on.
Now what organisms could be supposed to absorb nitrates from the soil? Certain bacteria and fungi are known to utilise nitrates, and one naturally thinks of algæ as possible agents also. Dr. Muriel Bristol was therefore invited to study the algæ of the soil. Her account is given in [Chapter VI.] She has found them not only on the surface, but scattered throughout the body of the soil, even in the darkness of 4 inches, 5 inches, or 6 inches depth, where no light can ever penetrate, and where photosynthesis as we understand it could not possibly take place. Some modification in their mode of life is clearly necessary, and it may well happen that they are living saprophytically. Dr. Bristol has not yet, however, been able to count the algæ in the soil with any certainty, although she has made some estimates of the numbers.
The quantitative work on the soil population indicates other possibilities which are being investigated. There is not only a daily fluctuation in the numbers, but so far as measurements have gone, a seasonal one also. There seems to be some considerable uplift in numbers of bacteria, protozoa, and possibly algæ and fungi in the spring-time, followed by a fall in summer, a rise in autumn, and a fall again in winter. At present we are unable to account for the phenomenon, nor can we be sure that it is general until many more data are accumulated.
In the cases of the protozoa and the algæ, there was a definite reason for seeking them in the soil.
Another section of the population, the fungi, was simply found, and at present we have only limited views as to their function. The older workers considered that they predominated in acid soils, while bacteria predominated in neutral soils. Present-day workers have shown that fungi, including actinomycetes, are normal inhabitants of all soils. The attempts at quantitative estimations are seriously complicated by the fact that during the manipulations a single piece of mycelium may break into fragments, each of which would count as one, while a single cluster of spores might be counted as thousands. Little progress has therefore been made on the quantitative lines which have been so fruitful with protozoa. Dr. Brierley gives, in [Chapters VII.] and [VIII.], a critical account of the work done on fungi.
In addition to the organisms already considered there are others of larger size. The nematodes are almost visible to the unaided eye, most of them are free living and probably help in the disintegration of plant residues, though a few are parasitic on living plants and do much injury to clover, oats, and less frequently to onions, bulbs, and potatoes. Further, there are insects, myriapods and others, the effects of which in the soil are not fully known. Special importance attaches to the earthworms, not only because they are the largest in size and in aggregate weight of the soil population, but because of the great part they play in aerating the soil, gradually turning it over and bringing about an intimate admixture with dead plant residues, as first demonstrated by Darwin. Earthworms are the great distributors of energy material to the microscopic population. Systematic quantitative work on these larger forms is only of recent date, and Dr. Imms, in [Chapter IX.], discusses our present knowledge.
TABLE I.
Soil Population, Rothamsted, 1922.
(The figures for algæ and fungi are first approximations only, and have considerably less value than those for bacteria and protozoa.)
| Numbers per Gram of Soil. | Approximate Weight per Acre of— | |||||||
|---|---|---|---|---|---|---|---|---|
| Living Organisms. | Dry Matter in Organisms. | Nitrogen in Organisms. | ||||||
| Bacteria— | lb. | lb. | lb. | |||||
| High level | 45,000,000 | 50 | } | 2 | 0·2 | |||
| Low level | 22,500,000 | 25 | ||||||
| Protozoa— | ||||||||
| Ciliates— | ||||||||
| High level | 1,000 | — | — | — | ||||
| Low level | 100 | — | — | — | ||||
| Amœbæ— | ||||||||
| High level | 280,000 | 320 | } | 12 | 1·2 | |||
| Low level | 150,000 | 170 | ||||||
| Flagellates— | ||||||||
| High level | 770,000 | 190 | } | 7 | 0·7 | |||
| Low level | 350,000 | 85 | ||||||
| Algæ (not blue-green) | [100,000 | ] | 125 | 6 | 0·6 | |||
| Blue-green | Not known. | — | Say 6 | Say 0·6 | ||||
| Fungi— | ||||||||
| High level | [1,500,000 | ] | 1700 | } | 60 | 6·0 | ||
| Low level | [700,000 | ] | 800 | |||||
| 93 | 9·3 | |||||||
| = 4 parts nitrogen per 1,000,000 of soil. | ||||||||
| Larger Organisms. | ||||||||
|---|---|---|---|---|---|---|---|---|
| Numbers per Acre.[D] | Approximate Weight per Acre of— | |||||||
| Living Organisms. | Dry Matter in Organisms. | Nitrogen in Organisms. | ||||||
| Ma- nured. | Un- ma- nured. | Ma- nured. | Un- ma- nured. | Ma- nured. | Un- ma- nured. | Ma- nured. | Un- ma- nured. | |
| Oligochaeta (Limicolae)— | lb. | lb. | lb. | lb. | lb. | lb. | ||
| Nematoda, etc. | 3,609,000 | 794,000 | 9 | 2 | 3 | 1 | — | — |
| Myriapoda | 1,781,000 | 879,000 | 203 | 99 | 85 | 42 | 4 | 2 |
| Insects | 7,727,000 | 2,475,000 | 34 | 16 | 14 | 6 | 1 | 1 |
| Earthworms | 1,010,000 | 458,000 | 472 | 217 | 108 | 50 | 10 | 5 |
| Total | 210 | 99 | 15 | 9 | ||||
| Total organic matter (dry weight) in this soil = 126,000 lb. per acre. | ||||||||
| Total nitrogen = 5700 lb. per acre. (1 lb. nitrogen per acre = 0·4 parts per1,000,000 of soil.) | ||||||||
| [D] To a depth of 9 inches. The weight ofsoil is approximately 1,000,000 kilos. | ||||||||
Are there any other members of the soil population that are of importance? As already shown, the method of investigating the soil population in use at Rothamsted is to find by chemical methods the changes going on in the soil; to find by biological methods what organisms are capable of bringing about these changes; and then to complete the chain of evidence by tracing the relationships between the numbers or activities of these organisms and the amount of change produced. The list as we know it to-day is given in [Table I.]
The method, however, does not indicate whether the account is fairly complete, or whether there are other organisms to be found. We might, of course, trust to empirical hunting for organisms, or to chance discoveries such as led Goodey to find the mysterious Proteomyxan Rhizopods, which cannot yet be cultured with certainty, so that they are rarely found by soil workers. It is possible that there are many such organisms, and it is even conceivable that these unknown forms far outnumber the known. The defect of the present method is that it always leaves us in doubt as to the completeness of the list, and so we may have to devise another.
Reverting to [Table I.], it obviously serves no purpose to add the numbers of all the organisms together. We can add up the weights of living organisms, of their dry matter or nitrogen, so as to form some idea of the proportion of living to non-living organic matter, and this helps us to visualise the different groups and place them according to their respective masses. But a much better basis for comparing the activities of the different groups would be afforded by the respective amounts of energy they transform, if these could be determined. It is proposed to attempt such measurements at Rothamsted. The results when added would give the sum of the energy changes effected by the soil population as we know it: the figure could be compared with the total energy change in the soil itself as determined in a calorimeter. If the two figures are of the same order of magnitude, we shall know that our list is fairly well complete; if they are widely different, search must be made for the missing energy transformers. There are, of course, serious experimental difficulties to be overcome, but we believe the energy relationships will afford the best basis for further work on the soil population.
Finally, it is necessary to refer to the physical conditions obtaining in the soil. These make it a much better habitat for organisms than one might expect. At first sight one thinks of the soil as a purely mineral mass. This view is entirely incorrect. Soil contains a considerable amount of plant residues, rich in energy, and of air and water. The usual method of stating the composition of the soil is by weight, but this is misleading to the biologist because the mineral matter has a density some two and a half times that of water and three times that of the organic matter. For biological purposes composition by volume is much more useful, and when stated in this way the figures are very different from those ordinarily given. [Table II.] gives the results for two Broadbalk arable plots, one unmanured and the other dunged; it includes also a pasture soil.
The first requirement of the soil population is a supply of energy, without which it cannot live at all. All our evidence shows that the magnitude of the population is limited by the quantity of energy available. The percentage by weight of the organic matter is about two to four or five, and the percentage by volume runs about four to twelve. Not all of this, however, is of equal value as source of energy. About one-half is fairly easily soluble in alkalis, and may or may not be of special value, but about one-quarter is probably too stable to be of use to soil organisms.
A second requirement is water with which in this country the soil is usually tolerably well provided. Even in prolonged dry weather the soil is moist at a depth of 3 inches below the surface. It is not uncommon to find 10 per cent. or 20 per cent. by volume of water present, spread in a thin film over all the particles, and completely saturating the soil atmosphere.
TABLE II.
Volume of Air, Water and Organic Matter in 100 Volumes of Rothamsted Soil.
| Solid Matter. | Pore Space. | In Pore Space. Values Commonly Obtained. | |||
|---|---|---|---|---|---|
| Mineral. | Organic. | Water. | Air. | ||
| (1) | 62 | 4 | 34 | 23 | 11 |
| (2) | 51 | 11 | 38 | 30 | 8 |
| (3) | 41 | 12 | 47 | 40 | 7 |
(1) Arable, no manure applied to soil. (2) Arable, dung applied to soil. (3) Pasture.
The air supply is usually adequate owing to the rapidity with which diffusion takes place. Except when the soil is water-logged, the atmosphere differs but little from that of the one we breathe. There is more CO2, but only a little less oxygen.[8] The mean temperature is higher than one would expect, being distinctly above that of the air, while the fluctuations in temperature are less.[5]
The reaction in normal soils is neutral to faintly alkaline; pH values of nearly 8 are not uncommon. Results from certain English soils are shown on [p. 18].
The soil reaction is not easily altered. A considerable amount of acid must accumulate before any marked increase in intensity of pH value occurs; in other words, the soil is well buffered. The same can be said of temperature, of water, and of energy supply. Like the reaction, they alter but slowly, so that organisms have considerable time in which to adapt themselves to the change.
Hydrogen Ion Concentration and Soil Fertility.
| pH | |||||
|---|---|---|---|---|---|
| Alkaline | 10 | Sterile: Alkali soil. | |||
| 9 | |||||
| 8 | Fertile: Arable. | ||||
| Neutral | 7 | ||||
| 6 | |||||
| 5 | Potato Scab fails. Nitrification hindered. Barley fails. | ||||
| 4 | |||||
| Acid | 3 | Sterile: Peat. | |||