Organic Matter and Soil Fertility
The ancients and the moderns are in the completest agreement as to the importance of organic matter in maintaining the fertility of the soil. This is evident when the methods of crop production in the time of the Romans are compared with the views now held by many of the leading experiment station workers in the United States and other parts of the world. In Roman times, the management of the manure heap had already reached an advanced stage. In 40 B.C. Varro drew attention to the great importance of the complete decay of manure before it was applied to the land. To bring this about, the manure heap, during the period of storage, had to be kept moist. In A.D. 90 Columella emphasized the importance of constructing the pits (in which farmyard manure was stored) in such a manner that drying out was impossible. He mentions the need of turning this material in summer to facilitate decay, and suggested that ripened manure should always be used for corn, while the fresh material could be applied with safety to grass land. The Romans therefore not only understood the importance of organic matter in crop production but had gone a long way towards mastering the principle that, to obtain the best results, it is necessary to arrange for the decay of farmyard manure before it is applied to arable land. It is interesting to turn from the writings of the ancients to the account of the symposium on 'Soil Organic Matter and Green-manuring' arranged by the American Society of Agronomy at Washington D.C. on 22 November 1928, the main results of which appeared in the Journal of the American Society of Agronomy of October 1929. Without exception, the investigators who took part in this conference laid the greatest emphasis on the importance of keeping up the supply of organic matter in the soil, and on discovering the most effective and the most economical method of doing this under the various conditions, as regards moisture, which the soils of the United States present.
During the 2,000 years which have elapsed since Varro wrote in 40 B.C. and the American investigators met in 1928, there has occurred only one brief period during which the role of organic matter was to some extent forgotten. This took place after Liebig's Chemistry in its Application to Agriculture and Physiology first appeared in 1840. Liebig emphasized the fact that plants derive their carbon from the carbon dioxide of the atmosphere and advanced the view that, in order that a soil may remain fertile, all that is necessary is to return to it, in the form of manure, the mineral constituents and the nitrogen that have been taken away in the crop. The discovery of the true origin of the carbon of plants not unnaturally suggested that the organic matter in the soil was of little consequence. Nitrogen and minerals only remained, the latter being found in the plant ashes. When therefore analyses of the crops had been made, it would be possible to draw up tables showing the farmer what he must add in the way of nitrogen and minerals in any particular case. These views and the controversies to which they gave rise, combined with the results of the Rothamsted experiments (started by Lawes and Gilbert in 1843) led to the adoption of artificial manures by many of the farmers of Europe. The Rothamsted experiments undoubtedly proved that if the proper quantities of combined nitrogen, phosphates and potash are added to the soil, satisfactory crops for many years can be obtained without the addition of organic matter beyond that afforded by the roots of the crops grown. Further, the results of hundreds of trials, in the course of ordinary farming practice, confirmed the fact that the judicious addition of nitrogenous artificial fertilizers can, in the great majority of cases, be relied on to increase the yield. It was only natural that results of this kind, combined with the important fact that the application of artificials often pays in practice, produced a marked effect on current opinion and also on teaching. For nearly a century after Liebig's ideas first appeared, the majority of agricultural chemists held that all that mattered in obtaining maximum yields was the addition of so many pounds of nitrogen, phosphorus and potassium to the acre. Beyond this the only other factor of importance was the liming of acid soils. The great development of the artificial manure industry followed as a matter of course.
The place of organic matter in the soil economy was forgotten. The old methods of maintaining soil fertility naturally fell into the background.
For a time all seemed to go well. It is only in comparatively recent years that experiment station workers have begun to understand the part played in crop production by the micro-organisms of the soil and to realize that the supply of artificials is not the whole story. Something more is needed. The need for the maintenance of the supply of organic matter soon became apparent. The view now beginning to be held is that, only after the supply of organic matter has been adequately provided for, will the full benefit of artificials be realized. There appears to be a great field for future experiment in the judicious use of artificials to land already in a fair state of fertility.
In all this however there was one important exception. In the Orient, the artificial manure phase had practically no influence on indigenous practice and passed unheeded. The Liebig tradition failed to influence the farmers of forty centuries. No demand for these products of the west exists in China. At the present day it would be difficult to purchase such a substance as sulphate of ammonia in the bazaars of rural India.
Soil Humus, its Origin and Nature
What is the origin and nature of the organic matter or soil 'humus' and what part does it play in soil fertility? These matters form the subject of the present chapter.
In the presentation which follows, the fullest use has been made of (1) one of the papers of Waksman (Paper No. 276 of the Journal Series, New Jersey Agricultural Experiment Station, Department of Soil Chemistry and Bacteriology, afterwards published in Soil Science, 22, 1926, p. 123) and (2) of the symposium on soil organic matter and green-manuring which appeared in the issue of the Journal of the American Society of Agronomy of October 1929. These important contributions to the subject have made it easy briefly to sketch the necessary scientific background for the presentation of the Indore process.
The organic matter found in the soil consists of two very different classes of material: (1) the constituents of plants and animals which have been introduced into the soil and are undergoing decomposition; various unstable intermediate products which have been formed under certain environmental conditions; substances like lignified cellulose which are more resistant to decomposition and which may persist in the soil for some time; and (2) number of valuable materials which have been synthesized by the numerous groups of micro-organisms which form the soil population. The soil organic matter is thus a heterogeneous mass of substances which is constantly undergoing changes in composition. When its composition reaches a certain stage of equilibrium, it becomes more or less homogeneous and is then incorporated into the soil as 'humus'. This definition of soil organic matter, which is due to Waksman, is of great importance. Soil organic matter or 'humus' is not merely the residue left when vegetable and animal residues decay. It contains in addition the valuable materials synthesized and left behind by the fungi and bacteria of the soil population. Moreover it is a product of the general soil conditions which obtain in any particular locality, and therefore varies in composition and character from one soil type to another. It is not the same all over the world. The soil humus for example of the black cotton soils of India is not identical with that of the alluvium of the Indo-Gangetic plain.
The various steps in the formation of soil organic matter are somewhat as follows. When the fresh remains of plants or animals are added to the soil, a portion of this organic matter is at once attacked by a large number of the micro-organisms present. Rapid and intense decomposition ensues. The nature of these organisms depends on the soil conditions (mechanical and chemical composition and physical condition) and on the soil environment (moisture content, reaction and aeration, and the presence of available minerals). The decomposition processes can best be followed by measuring one of the end-products of the reaction -- carbon dioxide. The rate of evolution of this gas depends on the nature of the organic matter, on the organisms which take part in the process and on the soil environmental conditions. As soon as the readily decomposable constituents of the plant and animal remains (sugars, starches, pectins, celluloses, proteins, amino-acids) have disappeared, the speed of decomposition diminishes and a condition of equilibrium tends to become established. At this stage only those constituents of the original organic matter, such as the lignins which are acted upon slowly, are left. These and the substances synthesized by the micro-organisms together form the soil humus and then undergo only a slow transformation during which a moderate but constant stream of carbon dioxide is liberated. At the same time the nitrogen of this soil humus is similarly converted into ammonia which, under favourable conditions, is then transformed into nitrate. It will be clear therefore that the soil organic matter or humus is a manufactured product and that its composition is not everywhere the same, but will vary with the soil conditions under which it is produced. Like all manufactured articles, it must be properly made if it is to be really effective. Too much attention therefore cannot be paid to its preparation.
After the production of humus and its incorporation into the soil mass, the next step is its utilization by the crop. This can only take place when this organic matter is decomposed by the micro-organisms of the soil. This process is very slow, as can be seen by placing a quantity of soil under favourable environmental conditions and measuring the rate of decomposition, either by the evolution of carbon dioxide or by the accumulation of ammonia and nitrate nitrogen. Since the ratio between the carbon and nitrogen content of the humus in normal cultivated soils is more or less constant, approaching 10:1, the evolution of carbon dioxide will be accompanied by the liberation of available nitrogen. This oxidation of the carbon and of the nitrogen is comparatively very slow, as only slow-growing groups of microorganisms are capable of attacking it. These organisms are aerobic and moreover can only work effectively when the general soil reaction is favourable. Their activities are therefore hastened in non-acid peat soils by draining, in acid peat soils by draining and liming, and in acid soils by liming.
It will be clear that the utilization of vegetable and animal wastes in crop production involves two definite steps: (1) the formation of humus and its incorporation into the soil and (2) the slow oxidation of this product accompanied by the production of available nitrogen. Both of these stages are brought about by micro-organisms for which suitable environmental conditions are essential. The requirements of the first phase -- the preparation of humus and its incorporation into the soil mass -- are so intense that if the process takes place in the soil itself, it is certain to interfere with the development of the crop. The needs of the second phase -- the utilization of humus -- are much less intense and can proceed in the soil without harm to the growing plant. From the point of view of crop production therefore, it will be a distinct advantage to separate these two stages and to prepare the humus outside the field. In this matter the Chinese have anticipated the teachings of western science. The cultivators of the Orient were the first to grasp and act upon the master idea that the growth of a crop involves two separate processes, the preparation of food-materials from vegetable and animal wastes which must be done outside the field, and the actual growing of the crop. Only in this way can the soil be protected from overwork.
The Formation of Humus as a Result of the Synthesizing Activities of Micro-organisms
Although the important part played by microorganisms in the formation of soil humus has only very recently been fully understood, nevertheless the older literature contains a number of useful contributions to the subject. Most of these early papers appeared towards the end of the last century; many of them related to other branches of knowledge and were not written from the point of view of agriculture. They have been summed up by Waksman, from whose paper the following account has been prepared. Post-Ramann and Muller considered that the 'humus' bodies obtained from soil often consist of the chitinous remains of insects and animal excrete. Wettstein and Winterstein showed that chitin is characteristic of various fungi and not of bacteria. Schmook advanced the view that the protein nitrogen in the soil was mostly present in the bodies of bacteria and protozoa. Trussov showed that the protoplasm of fungi is a source of humus in the soil. Schreiner and Storey suggested that various characteristic constituents of the soil are probably synthesized by micro-organisms.
The earlier work on this subject has been considerably developed, first by Falck and more recently by Waksman. Falck showed that organic matter in forest soils can be transformed into different types of humus in at least three ways: (1) The yearly additions of raw organic matter are completely decomposed by fungi (microcriny) accompanied by the synthesis of fungus protoplasm, which serves as an excellent fertilizer for the forest trees. In this process the celluloses are decomposed completely, whereas the lignins are more resistant. (2) The decomposition of the organic matter is begun by fungi and then carried on by lower invertebrates and bacteria (anthracriny). The fungus mycelium as well as the original organic matter are devoured by various larvae producing a dark 'humus' mass which, in the presence of bases, is oxidized by bacteria with the ultimate liberation of carbon dioxide and the formation of nitrate. (3) The formation of peat (anthrogeny), which Falck explains as resulting from the absence of an abundant fungus development. Waksman carried the subject still further and called attention to the similarity between the carbon-nitrogen ratio of the soil organic matter and that of the protoplasm of the soil fungi and other micro-organisms, and suggested that these probably make up a large part of the soil 'humus'. He further pointed out that when cellulose is added to the soil, it decomposes only in proportion to the available combined nitrogen present. This is because the decomposition is brought about by fungi and bacteria, both of which require combined nitrogen. The ratio between the amount of cellulose decomposed and the nitrogen required is about 30:1, so that, for every thirty parts of cellulose decomposed by the fungi and bacteria, one part of inorganic nitrogen (ammonium salt or nitrate) will be built up into microbial protoplasm. In the presence of sufficient combined nitrogen and under aerobic conditions, the decomposition of cellulose is very rapid. The same is true of vegetable wastes like straw, maize stalks, wood products and other materials rich in celluloses, pentosans and lower carbohydrates but poor in nitrogen. These facts explain the injurious effects on crop growth which follow the addition of straw and green-manure to the soil. The decomposition of these materials removes large quantities of combined nitrogen from the soil solution. This nitrogen is then temporarily stored in the form of microbial protoplasm, when for a time it is placed beyond the reach of the growing crop.
Since Waksman's paper appeared in 1926, an important contribution to this subject has recently been made by Phillips, Weite and Smith. The results of these investigators (which agree with our experience at Indore) has removed the impression that lignin is comparatively resistant to the action of micro-organisms. Under suitable conditions, soil organisms are capable of decomposing lignin as found in lignified plant materials (cornstalks, oat hulls, corn cobs and wheat straw), the rate of decomposition being as great as that of cellulose and pentosans.
The Role of Humus in the Soil
From the immediately practical point of view, the actual role of humus in the soil is of even greater interest than its formation, nature and decomposition. This material influences soil fertility in the following ways:
- The physical properties of humus exert a favourable influence on the tilth, moisture-retaining capacity and temperature of the soil as well as on the nature of the soil solution.
- The chemical properties of humus enable it to combine with the soil bases, and to interact with various salts. It thereby influences the general soil reaction, either acting directly as a weak organic acid or by combining with bases liberating the more highly dissociating organic acids.
- The biological properties of humus offer not only a habitat but also a source of energy, nitrogen and minerals for various micro-organisms.
These properties -- physical, chemical and biological -- confer upon humus a place apart in the general work of the soil including crop production. It is not too much to say that this material provides the very basis of successful soil management and of agricultural practice.
The Washington Symposium on Soil Organic Matter
Once the origin and nature of the soil organic matter is understood and the importance of this material in soil fertility is appreciated, the next step is to consider how best to make use of this information and to weld it into farming practice. With this object in view a symposium on soil organic matter and green-manuring was arranged at Washington D.C. on 22 November 1928, when the following papers were read and discussed:
- 'The Relation of Soil Type to Organic Matter.' C. F. Marbut.
- 'Organic Matter Problems in Humid Soils.' T. Lyttleton Lyon.
- 'Organic Matter Problems Under Dry-Farming Conditions.' J. C. Russell
- 'Organic Matter Problems in Irrigated Soils.' P. S. Burges
- 'Chemical and Microbiological Principles Underlying the Use of Green-Manures.' S. A. Waksman (by title only).
- 'Influence of Organic Manures on the Chemical and Biological Properties of Arid Soils.' J. E. Greaves.
- 'Green-Manuring and Its Application to Agricultural Practices.' A. J. Pieters and Roland McKee.
In dealing with the question of organic matter in humid soils, Lyon first presented a critical survey of the literature dealing with the losses of nitrogen in soils and concluded that:
- The loss of gaseous nitrogen may, under some conditions, cause a greater removal of nitrogen from a soil than occurs through absorption by crop plants.
- The conditions which favour a large loss of this kind are: (a) tillage or stirring the soil in any way, (b) absence of plant growth, (c) high nitrogen content of a soil, (d) application of large quantities of nitrogenous manures, and (e) possibly the application of lime to some soils.
- The loss of gaseous nitrogen does not take into account the amount fixed by soil organisms and therefore the calculated losses are less than actually occurred.
These losses of gaseous nitrogen from the soil may arise in five possible ways:
- There may be an escape of part of the ammonia during the process of ammonification.
- There may be a reduction of nitrates to form nitrogen as a result of alternating oxidation and reduction.
- There may be a loss of gaseous nitrogen in the oxidation of ammonia to nitrous acid since nitrogen is possibly an intermediate product in this process.
- A loss of nitrogen may result from the interaction of nitrous acid with the NH2 group of the amino-acids.
- A loss of gaseous nitrogen may occur as a result of the decomposition of ammonium nitrite in the process of nitrification.
In connexion with these losses of nitrogen it was pointed out in the discussion that the following two facts must be considered: (1) The ratio of carbon to nitrogen in the soils of the humid regions tends to maintain itself in the region of 10:1. If the organic residues left in the soil or applied to it afterwards have a higher carbon-nitrogen ratio than 10:1, an adjustment is soon effected, the extra carbon disappearing into the atmosphere as carbon dioxide. If the carbon-nitrogen ratio is less than 10:1, there is likely to be a loss of nitrogen before the ratio is adjusted. (2) The nitrogen content of any given soil tends to come to an equilibrium at a point which depends upon the nature of the soil, the effective climate and the cropping system. When therefore the nitrogen supply is increased in any way, the excess is soon dissipated when the soil comes under cultivation.
The information placed before the meeting by Russel (Nebraska) on the role of organic matter under dry-farming conditions was most instructive, and throws a flood of light on the consequences which are certain to follow the continuous cropping of virgin land without manure. A rapid and continuous fall in the total organic matter content, accompanied by loss of nitrogen, occurs together with a corresponding falling off in cropping power. Side by side, the water-holding capacity of these soils decreases, while the structure and tilth exhibit marked degeneration. All this has naturally led to attempts being made to restore the original content of organic matter. The results obtained, however, have been most disappointing, for the reason that most of these efforts have been directed towards the direct incorporation of green-manures and raw organic matter like straw into the soil under conditions of low rainfall. In many cases more harm than good has resulted. Russel concludes that the problem of the restoration of organic matter under dry-land conditions is extremely complicated and difficult and leans to the view that the solution of the problem might after all be found in the direction of nitrogenous fertilizers. Experience at Indore, however, suggests that all these difficulties could at once be avoided if the available supplies of green-manure, straw and other raw organic matter could first be composted outside the field before being applied to the land. The American farmers are obviously trying to overwork the soil and Mother Earth naturally objects.
The application of organic matter to the soil is followed by a number of important indirect results. These were dealt with by Greaves in a most interesting communication, in which the results obtained over a number of years on two different types of Utah soils were discussed. The first (Nephi) was typical dry-farm soil, the second was under irrigation (Greenville). In both the results were similar. The application of organic matter increased the ammonifying, nitrifying and nitrogen fixing processes of the soil. The gains in nitrogen, due to non-symbiotic nitrogen fixers, occurring under greenhouse conditions, varied from 0 to 304 lb. per acre foot of soil. The greatest gains occurred when legumes were used in the manure. The gain occurring in the soil under field conditions, and attributed to non-symbiotic nitrogen fixation, was 44 lb. per acre annually. Approximately 3,000 lb. of applied organic material were decomposed every year.
The last paper of the symposium dealt with the practice of green-manuring throughout the United States, with the various crops which are turned under, and with the great need for further exact experimentation on this question. Pieters and McKee state: 'In reviewing the experimental work that has been done with green-manures in the United States and the practices that are now followed it is evident that much work remains to be done before many questions can be settled or answered. Some of these fall clearly in the field of chemistry, others in physiology, and still others in bacteriology or other specialized fields of biology. Some, however, are strictly agronomic problems or so directly involved with crop production that their solution can perhaps best be undertaken by the agronomist or carried on with his active co-operation. It takes but a hasty survey to indicate the wide scope this work must cover in order to answer the specific questions for the many soil types, various climatic conditions, and for each of the large number of agronomic and horticultural crops involved.' In no case is there any reference in this paper to the growing of green-manures for the express purpose of providing material for composting, possibly because the need for this material has not yet been fully realized and because of the labour involved. Green-manuring in the United States, as in India and other parts of the world, is still in the empirical stage. Green crops are grown merely to provide a supply of organic matter for turning into the soil. What happens afterwards is a matter of chance. If the results are favourable, so much the better; if anything untoward occurs, one must hope for better things next time. That such an uncertain practice persists at all in the United States and that it appears to be spreading can only be explained by the great need of these depleted soils for fresh supplies of organic matter.
Liebig, J. -- Chemistry in its Application to Agriculture and Physiology, 1840.
Phillips, M., Wette, H. D., and Smith, N. R. -- The Decomposition of Lignified Materials by Soil Microorganisms,' Soil Science, 30, 1930, p. 383.
Russell, E. J. -- Soil Conditions and Plant Growth, London, 1927.
Russell, E. J. and Richards, E. H. -- 'The Changes taking place during the Storage of Farmyard Manure,' Journ. of Agric. Science, 8, 1917, p. 495.
'Symposium on Soil Organic Matter and Green-Manuring,' Journ. of the American Society of Agronomy, 21, 1929, p. 943
Waksman, S. A. -- 'The Origin and Nature of the Soil Organic Matter or Soil "Humus": 1 -- Introductory and Historical,' Soil Science, 22, 1926, p. 123.
Next: 3. The Sources of Organic Matter
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