Special Properties of Organic Residues

Perhaps because the nature of soil organic matter is incompletely understood, exaggerated claims are sometimes made of its role. The use of composts derived from plant materials, with or without the addition of special amendments, with properties that verge on the magical, has been strongly advocated as a means of restoring the depleted supplies of organic matter in the soil.

It has even been asserted that crops grown in soil in which composts have been incorporated are more nutritious and less susceptible to disease. Such advocacy is usually coupled with an attack on commercial fertilizers, which are said to have been responsible for soil depletion, erosion, deficiencies of minor elements or vitamins, and most other agricultural ills. No sound scientific evidence is marshalled in support of such claims. Largely they amount to a criticism of the practice of supplying a portion of the nutrient needs of the crop by application of mineral—that is, commercial—fertilizers that do not at the same time have a beneficial effect on the physical condition of the soil. It is reasonable to inquire, however, whether there is evidence of the presence in composts or decomposing plant materials of any substances that directly affect the growth of plants other than by meeting the nutrient needs or providing a beneficient physico-chemical environment.

Traces of auxins (substances that affect the rate of growth of some plant tissues) have been found in soil. These probably come from applications of barnyard manure, originating apparently in the urine. Thiamine also has been found in both manure and soil, but it has not been established whether it is present as free thiamine or is in the microbial soil population. Plant-growth regulating substances have been found in animal products such as dried blood, bonemeal, or meat. Some persons have strongly advocated that animal products or animal residues always be added to composts, but there is no good evidence that they have value beyond the nitrogen and other nutrients that they contain.

In any event growth substances of the class of the auxins or heteroauxins have not been shown to exert any over-all growth stimulation of plants or to increase their yield or fruitfulness. They may, however, stimulate root growth of cuttings.

It cannot yet be said, then, that there is good evidence that composts or decomposed residues are characteristically endowed with substances that cause exceptionally good crop growth or quality.

It has often been noted that the incorporation of crop residues into soil is accompanied by an improvement in soil structure and an increase in the number and size of the stable soil aggregates. The duration of such improvement may be quite limited. Sod crops appear to be especially valuable in causing the development of an improved physical condition. The grass-derived prairie and chernozem soils developed a notably granular structure that for a period remains stable under cultivation, but that deteriorates slowly in intertilled crops.

There are probably several reasons why clay particles may be caused to stick together to form small aggregates that exhibit some stability when wet or when in water, but it is now certain that directly or indirectly one form of aggregation results from the activities of the soil organisms. To obtain this effect it is necessary that decomposible material be added to the soil. Crop residues affect soil aggregation only as they are decomposed. Soluble energy sources, such as glucose or aqueous plant extracts, have no direct effect on aggregation, but when they decompose they cause as marked an improvement in soil structure as may follow the incorporation of whole plant materials.

The implication of this fact is that it is not any residual fraction of the plant material that is responsible for the effects observed. It is now presumed that the micro-organic cell substance itself or products derived therefrom are largely responsible. The presence of ramifying fungal threads may temporarily serve to bind particles together, but such a mechanical effect might be expected to be of relatively short duration. The rate of formation of stable aggregates has been shown to be highest during the period when microbial activity is greatest, shortly after incorporation of food material. The effect of the organisms is due to products formed by and from them. Many soil bacteria, and particularly some of the aerobic cellulose-decomposers, produce gummy substances, which, it is suggested, may have a cementing action. Such substances would not necessarily be unavailable to other organisms; hence their effect might also be of short duration. There is, of course, the possibility that such gummy products might form some complex with the inorganic colloid that would be more stable.

An alternative opinion is that the cementing substances are the breakdown products of bacteria that develop on and utilize the fungal mycelium that usually appears soon after plant residues are added to soil.

A grass sod has long been recognized as particularly effective in causing the formation of a granular soil structure. This is claimed to be due to the activity of bacteria in the rhizosphere (the zone immediately surrounding the roots and rootlets), utilizing root excretions or sloughed off cellular material. Species of Pseudomonas have been. said to be particularly numerous, and products of break-down of certain of these species have been shown to exercise strong cementing action. The activity of rhizosphere bacteria on the fibrous ramifying root system of grasses is, accordingly, held to be responsible for the steady increase in soil aggregation that takes place when land is put in sod. Through the constant death and renewal of rootlets, such a crop also supplies energy material other than in the form of root excretions. Upon subsequent plowing there may be additional aggregation at the expense of the fungal tissues that then develop on the grass residues. The great difference between grass and other crops in extent of aggregation caused is, therefore, accounted for by the much denser and more extensive root system of the former and the fact that under sod excessively aerobic conditions do not prevail.