An Agricultural Testament

by Sir Albert Howard

Part II
The Indore Process

Chapter 4
The Indore Process

THE Indore Process for the manufacture of humus from vegetable and animal wastes was devised at the Institute of Plant Industry, Indore, Central India, between the years 1924 and 1931. It was named after the Indian State in which it originated, in grateful remembrance of all the Indore Darbar did to make my task in Central India easier and more pleasant.

Although the working out of the actual process only took seven years, the foundations on which it is based occupied me for more than a quarter of a century. Two independent lines of thought and study led up to the final result. One of these concerns the nature of disease and is discussed more fully in Chapter 11 under the heading -- 'The Retreat of the Crop and the Animal before the Parasite'. It was observed in the course of these studies that the maintenance of soil fertility is the real basis of health and of resistance to disease. The various parasites were found to be only secondary matters: their activities resulted from the breakdown of a complex biological system -- the soil in its relation to the plant and to the animal -- due to improper methods of agriculture, an impoverished soil, or to a combination of both.

The second line of thought arose in the course of nineteen years (1905-24) spent in plant-breeding at Pusa, when it was gradually realized that the full possibilities of the improvement of the variety can only be achieved when the soil in which the new types are grown is provided with an adequate supply of humus. Improved varieties by themselves could be relied upon to give an increased yield in the neighbourhood of 10 per cent.: improved varieties plus better soil conditions were found to produce an increment up to 100 per cent. or even more. As an addition of even 10 per cent. to the yield would ultimately impose a severe strain on the frail fertility reserves of the soils of India and would gradually lead to their impoverishment, plant-breeding to achieve any permanent success would have to include a continuous addition to the humus content of the small fields of the Indian cultivators. The real problem was not the improvement of the variety but how simultaneously to make the variety and the soil more efficient.

By about the year 1918 these two hitherto independent approaches to the problems of crop production -- by way of pathology and by way of plant-breeding -- began to coalesce. It became clearer and clearer that agricultural research itself was involved in the problem; that the organization was responsible for the failure to recognize the things that matter in agriculture and would therefore have to be reformed; the separation of work on crops into such compartments as plant-breeding, mycology, entomology, and so forth, would have to be given up; the plant would have to be studied in relation to the soil on the one hand and to the agricultural practices of the locality on the other. An approach to the problems of crop production on such a wide front was obviously impossible in a research institute like Pusa in which the work on crops was divided into no less than six separate sections. The working out of a method of manufacturing humus from waste products and a study of the reaction of the crop to improved soil conditions would encroach on the work of practically every section of the Institute. As no progress has ever been made in science without complete freedom, the only way of studying soil fertility as one subject appeared to be to found a new institute in which the plant would be the centre of the subject and where science and practice could be brought to bear on the problem without any consideration of the existing organization of agricultural research. Thanks to the support of a group of Central Indian States and a large grant from the Indian Central Cotton Committee, the Institute of Plant Industry was founded at Indore in 1924. Central India was selected as the home of this new research centre for two reasons: (1) the offer on a 99 years' lease of an area of 300 acres of suitable land by the Indore Darbar, and (2) the absence in the Central India Agency of any organized system of agricultural research such as had been established throughout British India. This tract therefore provided the land on the one hand and freedom from interference on the other for the working out of a new approach, based on the humus content of the soil, to the problems underlying crop production. (An account of the organization of the Institute of Plant Industry was published as The Application of Science to Crop-Production by the Oxford University Press in 1929.)

The work at Indore accomplished two things: (1) the obsolete character of the present-day organization of agricultural research was demonstrated; (2) a practical method of manufacturing humus was devised.

The Indore Process was first described in detail in 1931 in Chapter 4 of The Waste Products of Agriculture. Since that date the method has been taken up by most of the plantation industries and also on many farms and gardens all over the world. In the course of this work nothing has been added to the two main principles underlying the process, namely, (1) the admixture of vegetable and animal wastes with a base for neutralizing acidity, and (2) the management of the mass so that the micro-organisms which do the work can function in the most effective manner. A number of minor changes in working have, however, been suggested. Some of these have proved advantageous in increasing the output. In the following account the original description has been followed, but all useful improvements have been incorporated: the technique has been brought up to date.

The Raw Materials Needed

1. Vegetable Wastes. In temperate countries like Great Britain these include -- straw, chaff, damaged hay and clover, hedge and bank trimmings, weeds including sea- and water-weeds, prunings, hop-vine and hop-string, potato haulm, market-garden residues including those of the greenhouse, bracken, fallen leaves, sawdust, and wood shavings. A limited amount of other vegetable material like the husks of cotton seed, cacao, and ground nuts as well as banana stalks are also available near some of the large cities.

In the tropics and sub-tropics the vegetable wastes consist of very similar materials including the vegetation of waste areas, grass, plants grown for shade and green-manure, sugar-cane leaves and stumps, all crop residues not consumed by live stock, cotton stalks, weeds, sawdust and wood shavings, and plants grown for providing compostable material on the borders of fields, roadsides, and any vacant corners available.

A continuous supply of mixed dry vegetable wastes throughout the year, in a proper state of division, is the chief factor in the process. The ideal chemical composition of these materials should be such that, after being used as bedding for live stock, the carbon: nitrogen ratio is in the neighbourhood of 33:1. The material should also be in such a physical condition that the fungi and bacteria can obtain ready access to and break down the tissues without delay. The bark, which is the natural protection of the celluloses and lignins against the inroads of fungi, must first be destroyed. This is the reason why all woody materials -- such as cotton and pigeon-pea stalks -- were always laid on the roads at Indore and crushed by the traffic into a fine state of division before composting.

All over the world one of the first objections to the adoption of the Indore Process is that there is nothing worth composting or only small supplies of such material. In practically all such cases any shortage of wastes has soon been met by a more effective use of the land and by actually growing plants for composting on every possible square foot of soil. If Nature's way of using sunlight to the full in the virgin forest is compared with that on the average farm or on the average tea and rubber estate, it will be seen what leeway can be made up in growing suitable material for making humus. Sometimes the objection is heard that all this will cost too much. The answer is provided by the dust-bowls of North America. The soil must have its manurial rights or farming dies.

2. Animal Residues. The animal residues ordinarily available all over the world are much the same -- the urine and dung of live stock, the droppings of poultry, kitchen waste including bones. Where no live stock is kept and animal residues are not available, substitutes such as dried blood, slaughter-house refuse, powdered hoof and horn, fish manure, and so forth can be employed. The waste products of the animal in some form or another are essential if real humus is to be made for the two following reasons.

(a) The verdict given by mother earth between humus made with animal residues and humus made with chemical activators like calcium cyanamide and the various salts of ammonia has always been in favour of the former. One has only to feel and smell a handful of compost made by these two methods to understand the plant's preference for humus made with animal residues. The one is soft to the feel with the smell of rich woodland earth: the other is often harsh to the touch with a sour odour. Sometimes when the two samples of humus made from similar vegetable wastes are analysed, the better report is obtained by the compost made with chemical activators. When, however, they are applied to the soil the plant speedily reverses the verdict of the laboratory. Dr. Rayner refers to this conflict between mother earth and the analyst, in the case of some composts suitable for forestry nurseries, in the following words:

The activator used in the first case was dried blood, in the second case an ammonium salt.

(b) No permanent or effective system of agriculture has ever been devised without the animal. Many attempts have been made, but sooner or later they break down. The replacement of live stock by artificials is always followed by disease the moment the original store of soil fertility is exhausted.

Where live stock is maintained the collection of their waste products -- urine and dung -- in the most effective manner is important.

At Indore the work-cattle were kept in well-ventilated sheds with earthen floors and were bedded down daily with mixed vegetable wastes including about 5 per cent. by volume of hard resistant material such as wood shavings and sawdust. The cattle slept on this bedding during the night when it was still further broken up and impregnated with urine. Next morning the soiled bedding and cattle dung were removed to the pits for composting; the earthen floor was then swept clean and all wet places were covered with new earth, after scraping out the very wet patches. In this way all the urine of the animals was absorbed; all smell in the cattle sheds was avoided, and the breeding of flies in the earth underneath the animals was entirely prevented. A new layer of bedding for the next day was then laid.

Every three months the earth under the cattle was changed, the urine-impregnated soil was broken up in a mortar mill and stored under cover near the compost pits. This urine earth, mixed with any wood ashes available, served as a combined activator and base in composting.

In the tropics, where there is abundance of labour, no difficulty will be experienced in copying the Indore plan. All the urine can be absorbed: all the soiled bedding can be used in the compost pits every morning.

In countries like Great Britain and North America, where labour is both scarce and dear, objection will at once be raised to the Indore plan. Concrete or pitched floors are here the rule. The valuable urine and dung are often removed to the drains by a water spray. In such cases, however, the indispensable urine could either be absorbed on the floors themselves by the addition to the bedding of substances like peat and sawdust mixed with a little earth, or the urine could be directed into small bricked pits just outside the building, filled with any suitable absorbent which is periodically removed and renewed. In this way liquid manure tanks can be avoided. At all costs the urine must be used for composting.

3. Bases for Neutralizing Excessive Acidity. In the manufacture of humus the fermenting mixture soon becomes acid in reaction. This acidity must be neutralized, otherwise the work of the microorganisms cannot proceed at the requisite speed. A base is therefore necessary. Where the carbonates of calcium or potassium are available in the form of powdered chalk or limestone, or wood ashes, these materials either alone, together, or mixed with earth, provide a convenient base for maintaining the general reaction within the optimum range (pH 7.0 to 8.0) needed by the microorganisms which break down cellulose. Where wood ashes, limestone, or chalk are not available, earth can be used by itself. Slaked lime can also be employed, but it is not so suitable as the carbonate. Quicklime is much too fierce a base.

4. Water and Air. Water is needed during the whole of the period during which humus is being made. Abundant aeration is also essential during the early stages. If too much water is used the aeration of the mass is impeded, the fermentation stops and may soon become anaerobic too soon. If too little water is employed the activities of the micro-organisms slow down and then cease. The ideal condition is for the moisture content of the mass to be maintained at about half saturation during the early stages, as near as possible to the condition of a pressed-out sponge. Simple as all this sounds, it is by no means easy in practice simultaneously to maintain the moisture content and the aeration of a compost heap so that the micro-organisms can carry out their work effectively. The tendency almost everywhere is to get the mass too sodden.

The simplest and most effective method of providing water and oxygen together is whenever possible to use the rainfall -- which is a saturated solution of oxygen -- and always to keep the fermenting mass open at the beginning so that atmospheric air can enter and the carbon dioxide produced can escape.

After the preliminary fungous stage is completed and the vegetable wastes have broken down sufficiently to be dealt with by bacteria, the synthesis of humus proceeds under anaerobic conditions when no special measures for the aeration of the dense mass are either possible or necessary.

Pits versus Heaps

Two methods of converting the above wastes into humus are in common use. Pits or heaps can be employed.

Where the fermenting mass is liable to dry out or to cool very rapidly, the manufacture should take place in shallow pits. A considerable saving of water then results. The temperature of the mass tends to remain high and uniform. Sometimes, however, composting in pits is disadvantageous on account of waterlogging by storm water, by heavy rain, and by the rise of the ground-water from below. All these result in a wet sodden mass in which an adequate supply of air is out of the question. To obviate such waterlogging the composting pits are: (1) surrounded by a catch-drain to cut off surface water; (2) protected by a thatched roof where the rainfall is high and heavy bursts of monsoon rain are the rule; or (3) provided with soakaways at suitable points combined with a slight slope of the floors of the pit towards the drainage corner. Where there is a pronounced rise in the water-table during the rainy season, care must be taken, in siting the pits, that they are so placed that there is no invasion of water from below.

To save the expense of digging pits and to use up sites where excavation is out of the question, composting in heaps is practiced. A great deal can be done to increase the efficiency of the heap by protecting the composting area from storm water by means of catch-drains and by suitable shelter from wind, which often prevents all fermentation on the more exposed sides of the heap. In temperate climates heaps should always face the south, and wherever possible should be made in front of a south wall and be protected from wind on the east and west. The effect of heavy rain in slowing down fermentation can be reduced by increasing the size of the heap as much as possible. Large heaps always do better than small ones.

In localities of high monsoon rainfall like Assam and Ceylon, there is a definite tendency to provide the heap or the pit with a grass roof so that the fermentation can proceed at an even rate and so that the annual output is not interfered with by temporary waterlogging. After a year or two of service the roof itself is composted. In Great Britain thatched hurdles can be used.

Charging the Heaps or Pits

A convenient size for the compost pits (where the annual output is in the neighbourhood of 1,000 tons) is 30 feet by 14 feet and 3 feet deep with sloping sides. The depth is the most important dimension on account of the aeration factor. Air percolates the fermenting mass to a depth of about 18 to 24 inches only, so for a height of 36 inches extra aeration must be provided. This is arranged by means of vertical vents, every 4 feet, made by a light crowbar as each section of the pit is charged.

Charging a pit 30 feet long takes place in six sections each 5 feet wide. The first section, however, is left vacant to allow of the contents being turned. The second section is first charged. A layer of vegetable wastes about 6 inches deep is laid across the pit to a width of 5 feet. This is followed by a layer of soiled bedding or farm-yard manure 2 inches in thickness. The layer of manure is then well sprinkled with a mixture of urine earth and wood ashes or with earth alone, care being taken not to add more than a thin film of about one-eighth of an inch in thickness. If too much is added aeration will be impeded. The sandwich is then watered where necessary with a hose fitted with a rose for breaking up the spray. The charging and watering process is then continued as before until the total height of the section reaches 5 feet. Three vertical aeration vents, about 4 inches in diameter, are then made in the mass by working a crowbar from side to side. The first vent is in the centre, the other two midway between the centre and the sides. As the pit is 14 feet wide and there are three vents, these will be 3 feet 6 inches apart. The next section of the pit (5 feet wide) is then built up close to the first and watered as before. When five sections are completed the pit is filled. The advantages of filling a pit or making a heap in sections to the full height of 5 feet are: (1) fermentation begins at once in each section and no time is lost; (2) no trampling of the mass takes place; (3) aeration vents can be made in each completed section without standing on the mixture.

In dry climates each day's contribution to the pit should again be lightly watered in the evening and the watering repeated the next morning. In this way the first watering at the time of charge is added in three portions -- one at the actual time of charging, in the evening after charging is completed and again the next morning after an interval of twelve hours. The object of this procedure is to give the mass the necessary time to absorb the water.

The total amount of water that should be added at the beginning of fermentation depends on the nature of the material, on the climate and on the rainfall. Watering as a rule is unnecessary in Great Britain. If the material contains about a quarter by volume of fresh greenstuff the amount of water needed can be considerably reduced. In rainy weather when everything is on the damp side no water at all is needed. Correct watering is a matter of local circumstances and of individual judgement. At no period should the mass be wet: at no period should the pit be allowed to dry out completely. At the Iceni Nurseries in South Lincolnshire in Great Britain, where the annual rainfall is about 24 inches and a good deal of fresh green market-garden refuse is composted, watering the heaps at all stages is unnecessary. At Indore in Central India where the rainfall was about 50 inches, which fell in about four months, watering was always essential except during the actual rainy season. These two examples prove that no general rule can ever be laid down as to the amount of water to be added in composting. The amount depends on circumstances. The water needed at Indore was from 200 to 300 gallons for each cubic yard of finished humus.

As each section of the pit is completed, everything is ready for the development of an active fungous growth, the first stage in the manufacture of humus. It is essential to initiate this growth as quickly as possible and then to maintain it. As a rule it is well established by the second or third day after charging. Soon after the first appearance of fungous growth the mass begins to shrink and in a few days will just fill the pit, the depth being reduced to about 36 inches.

Two things must be carefully watched for and prevented during the first phase: (1) the establishment of anaerobic conditions caused generally by over-watering or by want of attention to the details of charging; it is at once indicated by smell and by the appearance of flies attempting to breed in the mass; when this occurs the pit should be turned at once; (2) fermentation may slow down for want of water. In such cases the mass should be watered. Experience will soon teach what amount of water is needed at the time of charge.

Turning the Compost

To ensure uniform mixture and decay and to provide the necessary amount of water and air for the completion of the aerobic phase it is necessary to turn the material twice.

First turn. The first turn should take place between 2 and 3 weeks after charging. The vacant space, about 5 feet wide, at the end of the pit allows the mass to be conveniently turned from one end by means of a pitchfork. The fermenting material is piled up loosely against the vacant end of the pit, care being taken to turn the unaltered layer in contact with the air into the middle of the new heap. As the turning takes place, the mass is watered, if necessary, as at the time of charging, care being taken to make the material moist but not sodden with water. The aim should be to provide the mass with sufficient moisture to carry on the fermentation to the second turn. To achieve this sufficient time must be given for the absorption of water. The best way is to proceed as at the time of charging and add any water needed in two stages -- as the turning is being done and again next morning. Another series of vertical air vents 3 feet 6 inches apart should be made with a crowbar as the new heap is being made.

Second turn. About five weeks after charge the material is turned a second time but in the reverse direction. By this time the fungous stage will be almost over, the mass will be darkening in colour and the material will be showing marked signs of breaking down. From now onwards bacteria take an increasing share in humus manufacture and the process becomes anaerobic. The second turn is a convenient opportunity for supplying sufficient water for completing the fermentation. This should be added during the actual turning and again the next morning to bring the moisture content to the ideal condition -- that of a pressed-out sponge. It will be observed as manufacture proceeds that the mass crumbles and that less and less difficulty occurs in keeping the material moist. This is due to two things: (1) less water is needed in the fermentation; (2) the absorptive and water-holding power of the mass rapidly increase as the stage of finished humus is approached.

Soon after the second turn the ripening process begins. It is during this period that the fixation of atmospheric nitrogen takes place. Under favourable circumstances as much as 25 per cent. Of additional free nitrogen may be secured from the atmosphere.

The activity of the various micro-organisms which synthesize humus can most easily be followed from the temperature records. A very high temperature, about 65 deg C. (149 deg F.), is established at the outset, which continues with a moderate downward gradient to 30 deg C. (86 deg F.) at the end of ninety days. This range fits in well with the optimum temperature conditions required for the micro-organisms which break down cellulose. The aerobic thermophylic bacteria thrive best between 40 deg C. (104 F.) and 55 deg C. (131 deg F.). Before each turn, a definite slowing down in the fermentation takes place: this is accompanied by a fall in temperature.

As soon as the mass is re-made, when more thorough admixture with copious aeration occurs, there is a renewal of activity during which the undecomposed portion of the vegetable matter from the outside of the heap or pit is attacked. This activity is followed by a distinct rise in temperature.

The Storage of Humus

Three months after charge the micro-organisms will have fulfilled their task and humus will have been completely synthesized. It is now ready for the land. If kept in heaps after ripening is completed, a loss in efficiency must be faced. The oxidation processes will continue. Nitrification will begin, resulting in the formation of soluble nitrates. These may be lost either by leaching during heavy rain or they will furnish the anaerobic organisms with just the material they need for their oxygen supply. Such losses do not occur to anything like the same extent when the humus is banked by adding it to the soil. Freshly prepared humus is perhaps the farmer's chief asset and must therefore be looked after as if it were actual money. It is also an important section of the live stock of the farm. Although this live stock can only be seen under the microscope, it requires just as much thought and care as the pigs which can be seen with the naked eye. If humus must be stored it should be kept under cover and turned from time to time.


The output of compost per annum obviously depends on circumstances. At the Institute of Plant Industry, Indore, where the supply of urine and dung was always greater than that of vegetable waste, fifty cartloads (each 27 c. ft.) of ripe compost, i.e. 1,350 cubic feet or 50 cubic yards, could be prepared from one pair of oxen. Had sufficient vegetable wastes been available the quantity could have been at least doubled. The work-cattle at Indore were of the Malvi breed, about three-quarters the size of the average milking-cow of countries like Great Britain. The urine and dung of an average English cow or bullock, therefore, if properly composted with ample wastes would produce about sixty cartloads of humus a year, equivalent to about 1,600 cubic feet or 60 cubic yards.

As the moisture content of humus varies from 30 to 60 per cent. during the year, it is impossible to record the output in tons unless the percentage of water is determined. The difficulty can be overcome by expressing the output in cubic feet or cubic yards. The rate of application per acre should also be stated as so many cubic feet or cubic yards. Two cubic yards of compost weigh about one ton.

In devising the Indore Process the fullest use was made of agricultural experience including that of the past. After the methods of Nature, as seen in the forest, the practices which throw most light on the preparation of humus are those of the Orient, which have been described by King in Farmers of Forty Centuries. In China a nation of observant peasants has worked out for itself simple methods of returning to the soil all the vegetable, animal, and human wastes that are available: a dense population has been maintained without any falling off in fertility.

Coming to the more purely laboratory investigations on the production of humus, two proved of great value in perfecting the Indore Process: (1) the papers of Waksman in which the supreme importance of micro- organisms in the formation of humus was consistently stressed, and (2) the work of H. B. Hutchinson and E. H. Richards on artificial farm-yard manure. Waksman's insistence on the role of micro-organisms in the formation of humus as well as on the paramount importance of the correct composition of the wastes to be converted has done much to lift the subject from a morass of chemical detail and empiricism on to the broad plane of biology to which it rightly belongs. Once it was realized that composting depended on the work of fungi and bacteria, the reform of the various composting systems which are to be found all over the world could be taken in hand. The essence of humus manufacture is first to provide the organisms with the correct raw material and then to ensure that they have suitable working conditions. Hutchinson and Richards come nearest to the Indore Process but two fatal mistakes were made: (1) the use of chemicals instead of urine as an activator in breaking down vegetable wastes, and (2) the patenting of the ADCO process. Urine consists of the drainage of every cell and every gland of the animal body and contains not only the nitrogen and minerals needed by the fungi and bacteria which break down cellulose, but all the accessory growth substances as well. The ADCO powders merely supply factory-made chemicals as well as lime -- a far inferior base to the wood ashes and soil used in the Indore Process. It focuses attention on yield rather than on quality. It introduces into composting the same fundamental mistake that is being made in farming, namely, the use of chemicals instead of natural manure. Further, the patenting of a process (even when, as in this case, the patentees derive no personal profit) always places the investigator in bondage; he becomes the slave to his own scheme; rigidity takes the place of flexibility; progress then becomes difficult, or even impossible. The ADCO process was patented in 1916: in 1940 the method to all intents and purposes remains unchanged.

The test of any process for converting the waste products of agriculture into humus is flexibility and adaptability to every possible set of conditions. It should also develop and be capable of absorbing new knowledge and fresh points of view as they arise. Finally, it should be suggestive and indicate new and promising lines of research. If the Indore Process can pass these severe tests it will soon become woven into the fabric of agricultural practice. It will then have achieved permanence and will have fulfilled its purpose -- the restitution of their manurial rights to the soils of this planet. In the next four chapters the progress made during the last eight years towards this ideal will be described.


Howard, A., and Howard, G. L. C. The Application of Science to Crop-Production, Oxford University Press, 1929.

Howard, A., and Wad, Y. D. The Waste Products of Agriculture: Their Utilization as Humus, Oxford University Press, 1931.

Next: 5. Practical Applications of the Indore Process

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