The Sources of Organic Matter
A number of sources of soil organic matter exist, namely: (1) the roots of crops left behind at harvest, including the weeds turned under in the course of cultivation; (2) the algae met with in large quantities in rice fields, on the surface of the soils of tropical countries during the rainy season and to some extent in all soils; (3) green-manure; (4) farmyard manure; (5) artificial farmyard manure. In addition to these supplies, certain by-products of industries, such as oil-cakes and wool-waste, are also employed as sources of organic matter. These, however, are small in total amount and need not be considered. Except in China and Japan and to a limited extent in India, little or no use is made of night soil in crop production.
The Root-Systems of Crops
It is not always realized that about half of every crop -- the root-system -- remains in the ground at harvest time and thus provides automatically a continuous return of organic matter to the soil. The weeds and their roots turned in during the ordinary course of cultivation add to this supply. When these residues, supplemented by the fixation of nitrogen from the atmosphere, are accompanied by skilful soil management, crop production can be maintained at a moderate level without the addition of any manure whatsoever. A good example of such a system of farming without manure is to be found on the alluvial soils of the United Provinces, where the field records of ten centuries prove that the land produces fair crops year after year without any falling off in fertility. A perfect balance has been reached between the manurial requirements of the crops harvested and the natural processes which recuperate fertility. A similar, although not so striking a result, is afforded by the permanent wheat plot at Rothamsted, where this crop has been grown every year on the same land without manure since 1844. This plot, which has been without manure of any kind since 1839, showed a slow decline in production for the first eighteen years after which the yield has been practically constant. Systems of soil management such as these provide, as it were, the base line for the would-be improver. Nothing exists in the world's agriculture below this level. At the worst, therefore, the organic matter of a soil, constantly cropped without manure, does not disappear altogether. The wheel of life slows down. It does not stop.
One source of readily decomposable organic matter, which is available in India just at the moment when the cold season crops need it, is to be found in the shape of a thick algal film on the surface of cultivated soils during the second half of the rains. This film has also been observed in Africa, Ceylon and Java, and is probably universal during the rainy season in all parts of the tropics. As is well known, there are two periods in India when the crop is in greatest need of combined nitrogen: (1) at the break of the monsoon in June and July, and (2) when the cold season crops are sown in October after the rains. These latter are planted at a time when the available nitrogen in the surface soil is likely to be in great defect. The land has been exposed to heavy rain for long periods; the surface soil is often waterlogged. Nitrates under such conditions are easily lost by leaching and also by de-nitrification. The conditions are therefore altogether unfavourable for any approach towards an ample supply of nitrate when sowing time comes round in early October. How do the cold weather crops obtain a sufficient supply of this essential food material? It is more than probable that the deficiency is made up for, in part at least, by the rapid decay of the algal film (which also appears to be one of the factors in nitrogen fixation) during the last cultivations preceding the sowing of the cold weather crop in October. It is possible that some changes may have to be made in soil management with a view to stimulating the growth of this algal film. One of the beneficial effects of growing a green-manure crop like sann hemp for composting, during the early rains, may prove to be due to the favourable environment provided for the rapid establishment of the algal film. On monsoon fallow land it will probably be found best to suspend surface cultivation during the second half of the rains when the film is most active. There is already among the cultivators of India a tendency to stop stirring the surface, from the middle to the end of the rains, even when this involves the growth of weeds. This coincides with the period when the algal film is most noticeable. The indigenous practices may therefore prove to be based on sound scientific principles. Here are ready to hand several interesting subjects which urgently call for study under actual tropical conditions. When this is undertaken, the investigation should include: (1) the conditions most favourable for the establishment of the algal film; (2) the part played by algae and associated bacteria in nitrogen fixation; (3) the role of algae in banking easily destroyed combined nitrogen during the rains; and (4) the supply of easily decomposable and easily nitrifiable organic matter for the use of the cold weather crops. In the rice fields of the tropics, the algal carpet is even more evident than on ordinary cultivated soils. The total weight of organic matter added every year to each acre of rice land in the shape of algal remains must be considerable and must serve as a useful addition to the store of organic matter. Apart from the fixation of nitrogen from the air, it may help to explain why such heavy crops of paddy can be obtained in India, year after year on the same land, without manure.
Since the investigations of Schulz-Lupitz first showed how open sandy soils in Germany can be rapidly improved in texture by the incorporation of green-manures, the future possibilities of this method of enriching the land became apparent to the investigators of the Occident. After the role of the nodules (found on the roots of leguminous plants) in the fixation of atmospheric nitrogen was proved, the problems of green-manuring have naturally centred round the utilization of the leguminous crop in adding to the store of organic matter and combined nitrogen in the soil. At the end of the last century it seemed so easy, by merely turning in a leguminous crop, to settle at one stroke and in a very economical fashion the great problem of maintaining soil fertility. At the expenditure of a very little trouble, the soil might be made to manure itself. A supply of combined nitrogen, as well as a fair quantity of organic matter, might be provided without any serious interference with ordinary cropping. These expectations have led to innumerable green-manuring experiments all over the world with practically every species of leguminous crop. The results however have left much to be desired. In a few cases, particularly on open soils and where the rainfall, after the ploughing in of the green crop, is well distributed, the results have been satisfactory. On rice lands, where abundance of water ensures the maintenance of swamp conditions, somewhat similar results have been obtained. In the vast majority of cases, however. green-manuring has been disappointing. As a general method of soil improvement, the game is hardly worth the candle. On the monsoon fed areas of India the rainfall is often so uncertain, after the green crop is ploughed in, that for long periods decay is arrested. Sowing time arrives at a stage when the soil contains a mass of half-rotted material, with insufficient nitrogen and moisture for the growth of a crop. Failure results. The crops raised after green-manure are worse than those obtained on similar land left fallow. For this reason green-manuring has not been taken up by the people in India, in spite of the experiments and propaganda of the Agricultural Department.
It soon became evident, during the early years of the present century in India, that no matter what the rainfall and the soil conditions may be, a definite time factor is in operation in green-manuring. A period of not less than eight weeks must elapse, between the ploughing in of the green crop and the planting of the next, if satisfactory results are to be obtained. This was well brought out in the green-manuring experiments on tobacco, carried out at Pusa between 1912 and 1915. Some years later, the explanation of this factor, as well as the general conditions necessary for the decay of a green-manure crop were furnished by the work done at the New Jersey experiment station by Waksman and his co-workers. The decay and incorporation of green-manure in the soil has been shown to be a very complex process, depending on: (1) the chemical composition of the plants which make up the green-manure, which in turn largely depends on the age of the crop when ploughed in; (2) the nature of the decomposition of the various groups of organic complexes in the plant by the different types of soil organisms, which in turn is influenced by such factors as moisture, aeration, and the supply of available nitrogen and phosphates needed by these organisms, and (3) the metabolism of the microorganisms taking part in the decay of the green crop.
The process of incorporation takes place on the following lines. When the green-manure crop is ploughed in, the first stages of decay are brought about by fungi, which require for their activities ample supplies of air, moisture and combined nitrogen, as well as the soluble and easily decomposable carbohydrates supplied by the green crop. If the supply of nitrogen provided by the green-manure is insufficient, the stores of soluble nitrates in the soil solution are utilized by the fungi. Decay is rapid provided all these essential factors are simultaneously arranged for. The result is that the whole energies of the soil at this period are given up to the needs of the fungi of decay, which synthesize large quantities of protoplasm from the materials supplied by the green crop and the soil solution. During this phase, most of the nitrogen present is built up into mycelial tissue, and is therefore not immediately available for the growth of crops. The next stage is the decay of the remainder of the green-manure, including the mycelial tissue itself, by various groups of bacteria, followed by the incorporation of the whole mass into the soil organic matter. This must first be nitrified before the soil solution and the crop can obtain any benefit from this form of manuring. Clearly all this takes time, and needs abundance of oxygen as well as a continuous supply of soil moisture. If any of the limiting factors -- nitrogen supply, air or moisture -- are in defect, it is obvious that the final stage of nitrifiable organic matter will not be quickly reached. The soil will not only contain a mass of undigested material, but will be poor in available nitrogen and perhaps low in moisture as well. Seeds sown in such a soil can only result in a poor crop. The investigations of the New Jersey experiment station explain the importance of the time-factor in green-manuring, and incidentally show that the ordinary green-manuring experiments in India cannot possibly succeed. The sooner they are discontinued the better. Nothing is to be gained by attempting the hopeless task of manufacturing soil organic matter under conditions which cannot be controlled.
The question at once arises as to whether the green-manuring process can be regulated in such a manner that the results can be relied upon? A number of attempts have been made in this direction in India, of which that carried out by Clarke at Shahjahanpur is the most promising. Green crops of sann hemp (Crotalaria juncea L.) have been successfully utilized for the growth of sugar-cane. The secret of the Shahjahanpur process is to provide ample moisture, by means of irrigation, for the first stages of the decay of the green-manure. The rainfall, after the hemp crop is ploughed in, is carefully watched. If it is less than five inches during the first fortnight of September, the fields are irrigated. This enables the first phase of the decay of the green crop by the soil fungi to be completed. Practically all the nitrogen is then in the form of easily decomposable mycelial tissue. During the autumn, nitrification is prevented by drying out the surface soil. The nitrogen is, as it were, kept in the bank till the sugar-cane is planted under irrigation in March. Nitrification then sets in and the available supplies of combined nitrogen are made use of by the sugar-cane. In this way crops of over thirty tons of cane to the acre have been grown without the addition of any manure beyond the hemp, grown on the same land the previous rains and treated in the manner indicated above. These results do not appear to have been obtained with any other crop than sugar-cane planted in March. It would be interesting to have figures for wheat, sown in October, i.e. about six weeks after the hemp was ploughed under. It is probable that even with irrigation, this interval is insufficient for the proper incorporation of the green crop into the body of the soil organic matter and its subsequent nitrification. In this case, the Shahjahanpur method, valuable and interesting as it is, can only have a limited application.
Is it possible to devise a method of green-manuring, by means of the leguminous crop, which avoids all risks, is certain, and also makes the fullest use of this system? There are two possible ways in which the growing of a leguminous green-manure crop may benefit the soil. These are: (1) the well-known advantages of such crops in the rotation in increasing the nitrogen supply and in stimulating the micro-organisms in the soil, and (2) the effects of incorporating the green crop into the store of soil organic matter. Lohnis, however, showed, in many green-manuring experiments with leguminous crops, that the same results were obtained when the crop was removed as when it was ploughed under -- a conclusion which is in full accord with Waksman's work. It follows from this that the double advantage of a leguminous green-manure crop can only be achieved provided full use of the crop itself can be found outside the field, either as fodder for animals, for making silage or as material for the manufacture of compost. This latter method has been successfully worked out at Indore, and will be described in the next chapter. The real place of the leguminous crop in green-manuring seems to be in providing material for the manufacture of organic matter in a compost factory, specially designed for the purpose.
The exact period in the life history of the green crop, when it should be reaped for composting, is an important matter. If the crop is cut before the grand period of growth is completed, the maximum amount of vegetable waste will not be obtained. On the other hand, an early harvest will yield a product rich in nitrogen and suitable for rapid decay (Appendix C). Late harvesting is also attended with disadvantages. If reaped after flowering begins, the green crop will have used up a good deal of the rich nodule tissue which will then be temporarily removed from the soil and will not benefit the next crop. Further, the older the crop, the more unfavourable the carbon-nitrogen ratio becomes. The best stage for removal will be just before flowering begins. At this point, most of the nitrates in the soil solution have been absorbed by the crop and have been banked, either in the form of an easily decomposable root-system or as compost material, the chemical composition of which is exactly what is needed to improve the carbon-nitrogen ratio of the other vegetable wastes of the farm. When the green crop is reaped at this stage the following advantages are obtained: (1) The nitrates of the soil solution are safely banked. (2) The next crop derives the maximum benefit from an easily decomposable and uniformly distributed root-system, rich in combined nitrogen, the decay and incorporation of which is well within the powers of the soil. (3) The store of vegetable waste for composting is increased in amount and improved in chemical composition by the uniform distribution of the combined nitrogen throughout the tissues of the green crop.
From the beginning of agriculture, the utilization of farm wastes, rotted by means of the urine and dung of animals, has been the principal means of replenishing soil losses. Even at the present day, in spite of the establishment of numerous experiment stations and the employment of an army of investigators, the methods in vogue in the preparation and storage of this product leave much to be desired. Even under the covered-yard system, when the dung and litter are left under the animals until a layer several feet thick is produced, and the product is protected from the weather, as much as fifteen per cent of the valuable nitrogen is lost. When the dung is carted out into a heap to ripen, as is the usual practice, the losses of nitrogen are even greater. Russell and Richards, who some years ago carried out an elaborate investigation on the storage of farmyard manure at Rothamsted, concluded that: (1) the system of leaving the manure under the beasts till it is required for the fields, as in the box or covered-yard system, is the best whenever this is practicable; (2) the ideal method of storage is under anaerobic conditions at a temperature of 26 degrees C.; (3) the manure heap, however well made and protected, involves losses of nitrogen; and (4) the best hope of improvement lies in storing the manure in watertight tanks or pits, so made that they can be completely closed and thereby allow the attainment of perfect anaerobic conditions. These investigations, published in 1917, clearly indicate that one of the reasons for the present imperfect management of farmyard manure lies in the fact that the conditions are sometimes aerobic, at others anaerobic, whereas they should be one or the other throughout. In other words, there is no proper management of the air supply. Moisture is not usually in defect, except in hot countries like India where there is abundant air but often little moisture. Taking Great Britain and India as extreme cases of the management of farmyard manure, we find one or other of the following conditions in operation. In Great Britain, the irregular air supply of the manure heap leads to serious losses of nitrogen.
The final product is not a fine powder but a partially rotted material, which cannot be incorporated into the pore-spaces of the soil until further decay has taken place. The soil therefore has to do a good deal of work before the farmyard manure, applied on the surface in lumps, can be uniformly distributed through and incorporated into the soil mass. In India, the storage of farmyard manure leads to the loss of so much moisture, that often insufficient decay takes place before it finds its way into the soil. Losses of nitrogen may be prevented in this way but the work thrown upon the soil is even greater than in temperate regions. Only in China and Japan is any real attempt made to prepare the manure for the use of the crop, and to relieve the soil from unnecessary work. What is needed throughout the world is a continuous system of preparing farmyard manure in which (1) all losses of nitrogen are avoided, and, (2) the various steps from the raw material to the finished product follow a definite plan, based on the orderly breaking down of the materials, and the preparation of a finished product, ready for immediate nitrification, which can easily be incorporated into the soil. At the same time, an attempt should be made to gain as much nitrogen as possible by fixation from the atmosphere. Only when all this is done will the preparation of farmyard manure be based on correct scientific principles.
Artificial Farmyard Manure
During the last ten years, an additional source of soil organic matter has been utilized, namely, artificial or synthetic farmyard manure. In 1921, the results of experiments, carried out by Hutchinson and Richards at Rothamsted on the conversion of straw into manure without the intervention of live stock, were published. In this pioneering work, which constitutes an important milestone in the development of crop production, a method was devised by which straw could be converted into a substance having many of the properties of stable manure. In the preliminary experiments, the most promising results were obtained when the straw was subjected to the action of a culture of an aerobic cellulose decomposing organism (Spirochoeta cytophaga), whose activities were found to depend on the mineral substances present in the culture fluid. The essential factors in the production of well-rotted farmyard manure from straw were found to be: air supply; a suitable temperature, and a small amount of soluble combined nitrogen. The fermentation was aerobic; the breakdown of the straw was most rapid in a neutral or slightly alkaline medium in the presence of sufficient available nitrogen. Urine, urea, ammonium carbonate and peptone (within certain concentrations) were all useful forms of combined nitrogen. Sulphate of ammonia by itself was not suitable, as the medium soon became markedly acid. The concentration of the combined nitrogen added was found to be important. When this was in excess, nitrogen was lost from the mass before decay could proceed; when it was in defect, a marked tendency to fix nitrogen was observed. The publication of this paper soon led to a number of further investigations, and to numberless attempts all over the world to prepare artificial farmyard manure from every kind of vegetable waste. The principles underlying the conversion are now well understood, and have recently been summed up by Waksman and his co-workers in the Journal of the American Society of Agronomy (21, 1929, p. 533) in a paper which should be carefully studied by all interested in this important subject. The principles underlying the conversion are so well put by these investigators that they are best given in the authors' own words:
'The problems involved in the study of the principles underlying the decomposition of mature straw and other plant residues in composts, leading to the formation of so-called artificial manure, involve a knowledge of: (a) the composition of the plant material; (b) the mechanism of the decomposition processes which are brought about by the micro-organisms; and (c) a knowledge of the metabolism of these organisms.
'Straw and other farm residues, which are commonly used for the purpose of composting, consist predominantly (60 per cent or more) of celluloses and hemi-celluloses, which undergo rapid decomposition in the presence of aufficient nitrogen and other minerals, of lignins (15 to 20 per cent) which are more resistant to decomposition and which gradually accumulate, of water-soluble substances (5 to 12 per cent) which decompose very rapidly, of proteins which are usually present in very small amounts (2.2 to 30 per cent) but which gradually increase in concentration with the advance of decomposition, and of the mineral portion or ash.
'The processes of decomposition involved in the composting consist largely in the disappearance of the celluloses and hemi-celluloses, which make up more than 80 per cent of the organic matter which is undergoing decomposition in the process of formation of artificial manures. These poly-saccharides cannot be used as direct sources of energy by nitrogen-fixing bacteria and their decomposition depends entirely upon the action of various fungi and aerobic bacteria. In the process of decomposition of the celluloses and hemi-celluloses, the micro-organisms bring about the synthesis of microbial cell substance. This may be quite considerable, frequently equivalent to a fifth or even more of the actual organic matter decomposed. To synthesize these large quantities of organic matter, the micro-organisms require large quantities of available nitrogen and phosphorus and a favourable reaction. The nitrogen and phosphorus are used for the building up of the proteins and nucleins in the microbial cells. Since there is a direct relation between the celluloses decomposed and the organic matter synthesized, it should be expected also that there would be a direct relation between the cellulose decomposed and the amount of nitrogen required. As a matter of fact, for every forty or fifty parts of cellulose and hemi-cellulose decomposed, one unit of available nitrogen has to be added to the compost.
'As the plant residues used in the preparation of "artificial manure" are poor in nitrogen, available inorganic nitrogen must be introduced for the purpose of bringing about active decomposition. This explains the increase in the protein content of the compost accompanying the gradual decrease of the celluloses and hemi-celluloses.
'In general, artificial composts can be prepared from plant residues of any chemical composition so long as the nature of these residues and of the processes involved in their decomposition are known. By regulating the temperature and moisture content and by introducing the required amounts of nitrogen, phosphorus, potassium and calcium carbonate, the speed of decomposition and the nature of the product formed can be controlled.'
It is not possible in the space available to summarize all the various experiments which have been made in Great Britain, the United States, India and other parts of the world on the actual conversion of vegetable residues into artificial farmyard manure. It will be sufficient to refer to typical examples of what has been done. The Rothamsted investigations have been continued and have led to a patented process, known as Adco, by which the requisite nitrogenous and phosphatic food for the micro-organisms, as well as a base for the neutralization of acidity, are added to the vegetable wastes in the form of powders. Full details and numerous illustrations are to be found in the various Adco pamphlets. The object of patenting the process is not profit for the inventors but the raising of funds for further research. All users of Adco therefore are not only provided with a useful mixture but also make a small contribution to the cost of fundamental research work. In India, the various experiments on the production of artificial farmyard manure from a large number of materials, such as prickly pear, fallen leaves, town refuse, mahua (Bassia latifolia L.) flowers, weeds, banana waste, leguminous plants such as sann hemp, green pea stalks and various weeds have recently been summed up by Fowler, whose paper (see Bibliography below) should be consulted for details. The materials employed for adding the necessary nitrogen and other materials for the micro-organisms were night-soil, cow-dung, cattle urine, activated sludge or chemicals like sulphate of ammonia and calcium cyanamide. A large number of experiments are described from which it is clear that very useful manures, containing from 1 to 4 per cent of nitrogen, were obtained, which in field trials with rice and maize gave results equal to or better than any other nitrogenous manure in common use. Attempts were made in the course of this work to determine the amount of nitrogen fixation from the air which occurs during the conversion of the vegetable waste. It was found, when proper care was taken to supply the necessary organisms, that a considerable amount of free nitrogen was actually absorbed. These results, which agree with others on the same point, are of considerable interest. If in the conversion of vegetable wastes into artificial farmyard manure additional nitrogen can be gained, obviously the ideal conditions have been discovered. Once such principles have been correctly ascertained and put into practice, it might then be possible to deal not only with the manure heap itself but also with green-manuring, so that actual fixation can be substituted for the losses of nitrogen which now occur.
As is to be expected in such a matter as this, the preparation of artificial farmyard manure has been in actual operation centuries before Hutchinson and Richards began their work at Rothamsted. King, in Farmers of Forty Centuries, describes the conversion by the Chinese peasants of clover (Astragalus sinicus) into manure by mixing the green crop with rich canal mud To all intents and purposes, this system closely resembles the Adco process. Once more the empirical methods, discovered during centuries of practice, have preceded the results obtained by the application of pure science. Nevertheless, although in a sense the Rothamsted workers have been anticipated, it is quite safe to say that but for their work, the utilization of green clover in China, although described in the literature of the subject, would have passed unheeded. It was the novelty of the Rothamsted investigations which has proved so useful and so stimulating.
A critical examination of the literature on the principles underlying the conversion into humus of the chief groups of crude organic matter -- green-manure, farmyard manure and vegetable wastes -- reveals one fundamental weakness, namely, the fragmentation, into a number of loosely related sections, of what is essentially one subject. Farmyard manure, green-manure and the preparation of synthetic farmyard manure are always dealt with as if they were separate things and not parts of one great project. Even Waksman (whose contributions to the principles underlying the conversion of vegetable wastes into humus cannot fail to compel the admiration of all investigators), when the time came to write up his work for the agronomists of the United States, contributed three separate papers to the Journal of the American Society of Agronomy -- one on farmyard manure, one on green-manure and the third on artificial farmyard manure -- instead of synthesizing all these related subjects into one single contribution. When we come to the practical side of the question, a similar fragmentation is apparent. Green-manuring is always a separate process. The manure heap and its utilization from the time of the Romans to the present day, forms a special section of the work of the farm. The manufacture of artificial farmyard manure is again split off as an isolated operation. This particularism, in the most recent papers, is reflected in the separate conversion of each kind of vegetable waste, although it follows, from considerations of chemical composition, that a mixture of residues is much more likely to possess a suitable carbon-nitrogen ratio than any single material. As will be evident from a study of Waksman's three papers referred to above, the principles underlying the decay of farmyard manure, of green-manure and the preparation of artificial farmyard manure are essentially the same, namely, the synthesis of humus, by means of fungi and bacteria, from crude vegetable matter, various nutrients, air, water and bases. This humus increases the supply of soil organic matter and is capable of rapid nitrification. What is needed is the welding of all the separate fragments of the subject into a well ordered system. One process is required, not several. The agriculturist of the future must be shown how to become a chemical manufacturer. Further, the method finally adopted must be so elastic that it can be introduced into almost any system of agriculture. Again, it must be simple, safe and must yield a continuous and uniform product, capable of being instantly utilized by the crop. No waste of valuable nitrogen should occur at any stage. If possible, matters should be so arranged that the fixation of atmospheric nitrogen takes place at all stages of the process -- in the compost factory and afterwards in the soil. In the next chapter, a continuous process of making humus is described which furfils the conditions just outlined. This includes, in a single process, the various fragments of the subject, such as the care of the manure heap, green-manuring, the utilization of all vegetable wastes as well as the urine earth from the cattle shed and the wood ashes from the labourers' quarters. By its means, the waste products of 300 acres of land are converted every year into about 1,OOO cart-loads of valuable humus, of uniform chemical composition and of uniform fineness. When this material is added to the soil there is a rapid increase in fertility. The practical results obtained at Indore prove that all that is needed to raise crop production to a much higher level throughout the world is the orderly utilization of the waste products of agriculture itself.
Bristol, B. M. -- 'On the Alga-flora of some dessicated English Soils: an Important Factor in Soil Biology,' Annals of Botany, 34, 192O, P.35.
Bristol, B. M. and Page, H. J. -- 'A Critical Enquiry into the Alleged Fixation of Nitrogen by Green Alga,' Annals of Applied Biology, 1O, 1923, p. 378.
Bristol-Roach, B. M. -- 'The Present Position of our Knowledge of the Distribution and Functions of Alga, in the Soil,' Proc. of the Inter. Congress of Soil Science, Washington, D.C., 1928, p. 30.
Carbery, M. and Finlow, R. S. -- 'Artificial Farmyard Manure,' Agric. Journ. of India, 23, 1928, p. 80.
Clarke, G., Banerjee, S. C., Naib Husain, M., and Qayum, A. -- 'Nitrate Fluctuation in the Gangetic Alluvium and Some Aspects of the Nitrogen Problem in India,' Agric. Journ. of India, 17, 1922, p. 463.
Clarke, G. -- 'Some Aspects of Soil Improvement in relation to Crop Production,' Proc. of the Seventeenth Indian Science Congress, Asiatic Society of Bengal, Calcutta, 1930, p. 23.
Dobbs, A. C-' Green-Manuring in India,' Bull. 56, Agric. Research Institute, Pusa, 1916.
Fowler, G. J. -- 'Recent Experiments on the Preparation of Organic Matter,' Agric. Journ. of India, 25, 1930, p 363.
Hall, A. D. -- The Book of the Rothamsted Experiments, London, 1905.
Howard, A. and Howard, G. L. C. -- 'The Improvement of Tobacco Cultivation in Bihar,' Bull. 50, Agric. Research Institute, Pusa, 1915.
Howard, A. -- Crop Production in India, a Critical Survey of its Problems, Oxford University Press, 1924.
Howard, A. and Howard, G. L. C. -- The Application of Science to Crop Production, an Experiment carried out at the Institute of Plant Industry, Indore, Oxford University Press, 1929.
Hutchinson, H. B. and Richards, E. H. -- 'Artificial Farmyard Manure,' Journ. of the Min. of Agric. (London), 28, 1921, p. 398.
King, F. H. -- Farmers of Forty Centuries, or Permanent Agriculture in China, Korea and Japan, London, 1926.
Lohnis, F. -- 'Nitrogen Availability of Green Manures,' Soil Science, 22, 1926, p. 253.
Lohnis, F. -- 'Effect of Growing Legumes upon succeeding Crops,' Soil Science, 22, 1926, p. 355.
Russell, E. J. -- Soil Conditions and Plant Growth, London, 1927.
Russell, E. J. -- 'The Present Status of Soil Microbiology,' Proc. of the Inter. Congress on Soil Science, Washington, D.C., 1928, p. 36.
Russell, E. J. and Richards, E. H. -- 'The Changes taking place during the Storage of Farmyard Manure,' Journ. of Agric. Science, 8, 1927, p. 495.
Russell, E. J. and others. -- The Micro-organisms in the Soil, London, 1923.
Waksman, S. A. -- 'Chemical and Microbiological Principles underlying the Decomposition of Green-Manures in the Soil,' Journ. of the Amer. Soc. of Agronomy, 21, 1929, p. 1.
Waksman, S. A., Tenney, F. G. and Diehm, R. A. -- 'Chemical and Microbiological Principles underlying the Transformation of Organic Matter in the Preparation of Artificial Manures,' Journ. of the Amer. Soc. of Agronomy, 21, 1929, p.533.
Waksman, S. A. and Diehm, R. A. -- 'Chemical and Microbiological Principles underlying the Transformation of Organic Matter in Stable Manure in the Soil,' Journ. of the Amer. Soc. of Agronomy, 21, 1929, p.795.
Next: 4. The Manufacture of Compost by the Indore Method
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