COMPOSITION OF NATURAL ORGANIC MATERIALS AND THEIR DECOMPOSITION IN THE SOIL

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Abstract
Decomposition of corn stalks, rye straw, oak leaves and alfalfa plants, was studied under aerobic conditions in presence of optimum moisture, at 25-27[degree] C. The processes were followed by determining the disappearance of total organic matter and of the more important groups of organic complexes. Under like conditions, the nature and rapidity of decomposition of different plant materials are markedly influenced by their chemical composition. Decomposition of the sugars, celluloses, hemicelluloses, fats and proteins account for most of the decomposed plant materials. Lignins are more resistant and tend to accumulate. Under aerobic conditions, however, there is a decided reduction in total lignin content, indicating that its resistance to decomposition is only relative. With low N content, decomposition of plant materials is accompanied by a relative and absolute increase in crude protein. This is due to the synthesizing activities of the microorganisms. Celluloses and hemicelluloses, including pentosans, disappear more rapidly than can be accounted for by reduction in total organic matter. This loss is balanced by accumulation of lignins and proteins. Among the other synthesized products, hemicelluloses occupy a prominent place. This is partly accounted for by the fact that toward the end of decomposition more hemi-cellulose than cellulose is left. Addition of available inorganic N salt hastens decomposition of celluloses and hemicelluloses, hence of those plant materials rich in celluloses and hemicelluloses and poor in proteins. Residues after plant materials have decomposed for 12-14 months at optimum temp. and moisture and under aerobic conditions, possess all the properties of soil "humus." This is true especially of corn stalks which have undergone most rapid decomposition. This "humus" is made up chiefly of lignins or modified lignin complexes largely of plant origin, of proteins and other complex organic N compounds largely of microbial origin, of hemicelluloses partly of plant and partly of microbial origin, of a small amount of celluloses undergoing decomposition and of varying concentrations of other organic complexes in process of decomposition, resistant to decomposition, or products of decomposition. This residual material is not in equilibrium but undergoes continuous change, the rate of change becoming constantly slower, approaching that of the soil "humus.".