An Agricultural Testament

by Sir Albert Howard

Chapter 6
Developments of the Indore Process


SINCE Schultz-Lupitz first showed about 1880 how the open sandy soils of North Germany could be improved in texture and in fertility by the incorporation of a green crop of lupine, the possibilities of this method of enriching the land have been thoroughly explored by the Experiment Stations. After the role of the nodules on the roots of leguminous plants in the fixation of atmospheric nitrogen was proved, the problems of green-manuring naturally centred round the utilization of the leguminous crop in adding to the store of combined nitrogen and organic matter in the soil. At the end of the nineteenth 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 little trouble, the leguminous nodule might be used as a nitrogen factory while the remainder of the crop could provide humus. All this might be accomplished at small expense and without any serious interference with ordinary cropping. These expectations, a natural legacy of the NPK mentality, have led to innumerable green-manuring experiments all over the world with practically every species of leguminous crop. In a few cases, particularly in open, well-aerated soils where the rainfall after the ploughing in of the green crop was well distributed and ample time was given for decay, the results have been satisfactory. In the majority of cases, however, they have been disappointing. It will be useful, therefore, to examine the whole subject and to determine if possible the reasons why this method of improving the fertility of the soil seems so often to have failed.

A consideration of the factors involved in the growth, decay, and utilization of the residues of a green crop will at once explain the general failure of green-manuring to increase the following crop and also put an end for all time to the somewhat extravagant hopes of repeating the German results, which succeeded because all the factors, including time, happened to be favourable. It is no use slavishly copying this method unless we can at the same time reproduce the North German soil and climatic conditions.

The chief factors in green-manuring are: (1) knowledge of the nitrogen cycle in relation to the local agriculture; (2) the conditions necessary for rapid growth and also for the formation of abundant nodules on the roots of the leguminous crop used for green-manuring; (3) the chemical composition of the green crop at the moment it is ploughed in; (4) the soil conditions during the period when decay takes place. These four factors must be studied before the possibilities of green-manuring can be explored.

The importance of the nitrogen cycle in relation to the local agriculture is a factor in green-manuring to which far too little attention has been paid. As will be shown more fully in Chapter 14, the full possibilities of green-manuring can only be utilized when we know at what periods of the year nitrate accumulations take place, how these accumulations fit in with the local agricultural practice, and when nitrates are liable to be lost by leaching and other means. If the crop does not make the fullest use of nitrate, this precious substance must be immobilized by means of green-manure or by means of weeds and algae. It must not be left to take care of itself. It must either be taken up by the crop or banked by some other plant.

The soil conditions necessary for the growth of the leguminous crop used as a green-manure have never been sufficiently studied. Clarke found at Shahjahanpur in India that it was advantageous to apply a small dressing of farm-yard manure to the land just before the green crop is sown. The effect of this is to stimulate growth and nodular development in a remarkable way. Further, the green crop when turned in decays much faster than when this preliminary manuring is omitted. It may be that besides stimulating nodular development the small dressing of farm-yard manure is necessary to bring into effective action the mycorrhizal association which is known to exist in the roots of most leguminous plants. This association is a factor which has been completely forgotten in green-manuring. There is no reference to it in Waksman's excellent summary on pp. 208-14 of the last edition of his monograph on humus. This factor will probably also prove to be important in the utilization of the humus left by green-manuring.

The living bridge between the humus in the soil and the plant must be properly fed, otherwise the nutrition of the crop we wish to benefit is almost certain to suffer.

As growth proceeds the chemistry of a green crop alters very considerably: the material in a young or in a mature crop, when presented to the micro-organisms of the soil, leads to very different results. Waksman and Tenney have set out the results of the decomposition of a typical green-manure plant (rye) harvested at different periods of growth. When the plants are young they decompose rapidly: a large part of the nitrogen is released as ammonia and becomes available. When the plants are mature they decompose much more slowly: there is insufficient nitrogen for decay, so the micro-organisms utilize some of the soil nitrates to make up the deficiency. Instead of enriching the soil in available nitrogen the decay of the crop leads to temporary impoverishment. These fundamental matters are summed up in the following Table:

Table 5
Rapidity of decomposition of rye plants at different stages of growth
(Waksman and Tenney)
Two grammes of dry material decomposed for 27 days
Stage of growth
CO2 given off
Nitrogen liberated as ammonia
Nitrogen consumed from the media
mg. C
mg. N
mg. N
Plants only 25-35 cm. high
Just before heads begin to form
Just before bloom
Plants nearly mature

The amount of humus which results from the decay of a green crop also depends on the age of the plants. Young plants, which are low in lignin and in cellulose, leave a very small residue of humus. Mature plants, on the other hand, are high in cellulose and lignin and yield a large amount of humus. These differences are brought out in Table 6.

Table 6
Formation of humus during decomposition of rye plants at different stages of development

(Waksman and Tenney)
Chemical constituents
At beginning of decomposition*
At the end of decomposition period**
Just before heads begin to form
% of original
Total water-insoluble organic matter
Protein insoluble in water
Plants nearly mature
% of original
Total water-insoluble organic matter
Protein insoluble in water
* 10 gm. material (on dry basis) used for young plants and 20 gm. for old plants.
** 30 days for young plants and 60 days for mature plants.

It follows from these results that if we wish to employ green-manuring to increase the soil nutrients quickly, we must always plough in the green crop in the young stage; if our aim is to increase the humus content of the soil we must wait till the green-manure crop has reached its maximum growth.

The soil conditions after the green crop is ploughed in are no less important than the chemical composition of the crop. The micro-organisms which decay the green-manure require four things: (1) sufficient combined nitrogen and minerals; (2) moisture; (3) air; (4) a suitable temperature. These must all be provided together.

The factor which so often leads to trouble is the poverty of the soil -- insufficient combined nitrogen and minerals. It follows, therefore, that when a mature crop is ploughed in the effect of its decay on the next crop will always depend on the fertility of the soil. If the soil is in a poor condition most of the combined nitrogen available will be immobilized for the decay of the green-manure; the next crop will suffer from starvation; green-manuring will then be a temporary failure. If, however, the soil is fertile or if we plough in freshly prepared humus with the green crop, the extra combined nitrogen needed for decay will then be present; the next crop will not suffer. Soil fertility in this, as in so many other matters, gives the farmer considerable latitude. All sorts of things can be done with perfect safety with a soil in good heart which are out of the question when the soil is infertile. A good reserve of fertility, therefore, will always be an important factor in green-manuring.

As the decomposition of a green crop is carried out by microorganisms, decay ceases if the moisture falls below a certain point.

Again, if the air supply is cut off by excessive rain after ploughing in or by burying the green crop too deeply, an anaerobic soil flora rapidly develops which proceeds to obtain its oxygen supply from the substratum. The valuable proteins are attacked and their nitrogen is released as gas. The chemical reactions of the peat bog replace those of the early stages of a properly managed compost heap. This frequently happens under monsoon conditions and is one of the reasons why green-manuring is so often unsatisfactory in tropical agriculture.

Finally, the temperature factor is important in countries like Great Britain which have a winter. Here green-manure crops must often be turned in during the autumn before the soil gets too cold, so that the early stages of decay can be completed before winter comes.

The uses of green-manuring in agriculture can now be considered. Generally speaking they fall into three classes: (1) the safeguarding of nitrate accumulations; (2) the production of humus, and (3) a combination of both.

The Safeguarding of Nitrate Accumulations

In studying this important matter we must at the outset consider how Nature, if left to herself, always deals with the nitrates prepared from organic matter by the micro-organisms in the soil. They are never allowed to run to waste but are immobilized by plants including the film of algae in the surface soil. These latter are easily decomposed: they are therefore exceedingly valuable agencies for safeguarding nitrates.

The farmer has at his command two methods of nitrate immobilization. He can either intercept his surplus nitrate accumulations by sowing a leguminous crop or by managing his weeds and soil algae so that they do the same thing automatically. In either case nitrates which would otherwise run to waste are converted into young fresh growth which cannot then be lost by leaching and which later on can be rapidly converted back into available nitrogen and minerals by the organisms in the soil. Obviously if weeds can be managed so that all nitrate accumulations can be utilized and the resulting growth can be turned under and decomposed in time for the next crop, there is no need to sow a leguminous crop to do what Nature herself can do so much better.

One of the best examples I have seen of the combined use of weeds and catch crops for immobilizing nitrates was worked out by Mr. L. P. Haynes on the large hop garden of Messrs. Arthur Guinness, Son & Co. at Bodiam in Sussex. Surface cultivation in this garden ceases in August soon after the hops form. A little mustard is then sown which, with the chickweed, soon produces a green carpet without interfering with the ripening of the hops. At picking time the mixed seedlings are well established, after which they have the nitrates formed at the end of the summer and in early autumn entirely to themselves. Growth is very rapid. During the autumn sheep are brought in to graze the mustard. Their urine and dung fall on the chickweed and so contribute a portion of the essential animal wastes. In the spring the easily decomposed chickweed is ploughed into the fertile soil and decayed in good time for the next crop of hops. The soil of this hop garden is now heavily charged with chickweed seeds so that the moment surface cultivation is stopped the following August a new crop starts. This management of a common weed of fertile soil to fit in with the needs of the hop appeared to me to be nothing short of a stroke of genius. It would be difficult to find a more efficient green-manure crop than the one Nature has provided for nothing. Could there be a better example of the use of a fertility reserve for rapidly decomposing a green crop in the early spring? The ground at Bodiam is hardly ever uncovered; it is occupied either by hops or by chickweed; one crop dovetails into the other; the energy of sunlight is almost completely utilized throughout the year; the invisible labour force of the hop garden -- earthworms and micro-organisms -- is kept fully occupied. As the use of artificials and poison sprays is reduced, there will be a corresponding increase in efficiency in this section of the unseen establishment.

Much more use might be made of this method of green- manuring in countries like Great Britain. In fruit, vegetable, and potato growing particularly, there seems no reason why an autumn crop of weeds should not be treated as green-manure on Bodiam lines. If the land is in good heart, the soil will have no difficulty in decaying the weeds. If the land is poor in organic matter, a dressing of freshly prepared humus of not less than 5 tons to the acre should be spread on the weeds before they are turned under.

The Production of Humus

The production of humus, by means of a green-manure crop, is a much more difficult matter than the use of this method for immobilizing nitrates. Nevertheless, it is of supreme importance in the maintenance of soil fertility. The factors involved in the transformation of green-manure into humus in the soil are the same as those in the compost heap. All factors must operate together. Failure of one will upset the process entirely. If this occurs the next crop will be sown in soil which has been placed in an impossible condition. The land will be called upon to complete the formation of humus and to grow a crop at the same time. This is asking too much. The soil will take up its interrupted task and proceed with the manufacture of humus. It will neglect the crop. The uncontrollable factor is the rainfall. It must be just right if humus manufacture in the soil is to succeed. In India, for example, during an experience of twenty-six years it used to be just right about once in six or seven years. It was completely wrong in the remainder. Often there was too much rain after ploughing in, when the aerobic phase never developed and bog conditions were established instead. At other times there was insufficient rain for the early fungous stage. Where, however, irrigation is available, any shortage of the Indian monsoon makes no difference.

In exceptional cases, however, it is possible to carry on the manufacture of humus in the soil without any risk of temporary failure. One British example may be quoted. On some of the large farms in the Holland Division of Lincolnshire peas are grown as a rotation crop with potatoes. The problem is to manufacture humus before the next crop of potatoes is planted. This has been solved. Early in July the peas are cut and carried to the shelling machines where the green seeds are separated and large quantities of crushed haulm are left. Immediately after the removal of the peas the land is sown with beans. The crushed pea haulm is then scattered on the surface of the newly sown land followed by a light dressing of farm-yard manure -- about 6 or 7 tons to the acre. The beans grow through the fermenting layer on the surface of the soil and help to keep it moist. While the beans are growing humus is being manufactured in a thin sheet all over the field. At the end of September, when the beans are in flower, this sheet composting on the ground is complete. The green crop is then lightly ploughed in together with a layer of freshly prepared compost. Humus manufacture is then continued in the soil. The beans under these conditions decay quickly; the process of humus manufacture is completed before the planting of the next potato crop.

The Safeguarding of Nitrates Followed by the Manufacture of Humus

The immobilization of nitrates by means of a green crop followed by the conversion of the green-manure into humus needs time and complete control of all the operations. An example of the successful use of this method is described in Chapter 14. Heavy crops of sugar-cane were produced at Shahjahanpur in the United Provinces by intercepting the nitrates accumulated at the break of the south-west monsoon by means of a leguminous crop and then converting this into humus with the assistance of the autumn accumulation of nitrate in the same soil.

It follows from the principles underlying green-manuring and the applications of these principles to agricultural practice that the ploughing in of a green crop is not a simple question of the addition of so many pounds of nitrogen to the acre but a vast and many-sided biological problem. Moreover it is dynamic, not static; the agents involved are alive; their activities must fit in with one another, with agricultural practice on the one hand and with the season on the other. If we attempt to solve such a complex on the basis of mere nitrogen content or on that of carbon: nitrogen ratios, we are certain to run counter to great biological principles and come into conflict with one rule in Nature after another. It is little wonder, therefore, that green-manuring has led to so much misunderstanding and to so much disappointment.

The Reform of Green-manuring

The uncertainties of humus manufacture in the soil can be overcome by growing the green crop to provide material for composting. This of course adds to the labour and the expense, but in many countries it is proving a commercial proposition. In Rhodesia, for example, crops of salt hemp are now regularly grown to provide litter, rich in nitrogen, for mixing with maize stalks so as to improve the carbon: nitrogen ratio of the bedding used in the cattle kraals. In this way the burden on the soil is greatly reduced; it is only called upon to decay what is left of the root system of the green crop at harvest time. Humus manufacture is shared between the soil and the compost heap.

In converting materials low in nitrogen (such as sugar-cane leaves and cotton stalks) into humus it is an immense advantage to mix these refractory materials with some leguminous plant in the green state. The manufacture of humus is speeded up and simplified; the amount of water needed is reduced; the land on which the green crop was raised benefits.


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.

Waksman, S. A., and Tenney, F. G. 'Composition of Natural Organic Materials and their Decomposition in the Soil', Soil Science, xxiv, 1927, p. 275; xxviii, 1929, p. 55; and xxx, 1930, p. 143.

Next: 7. Developments of the Indore Process, Cont.

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