Soil organic matter, perhaps, contains most of the naturally occuring organic compounds. In addition to the organic constituens present in undecayed plant and animal tissues, soil organic matter contains living and dead microbial cells, microbially synthesized compounds and an array of derivations of these materials. However, soil organic matter needs to be chemically characterized for understanding its role in soil productivity. The major constituens of organic matter are polysaccharides, lignins and proteins. Polysaccharides are natural carbohydrates such as cellulose,the hemicelluloses, starch and pectic substances. Lignins are complex, highly resistant materials, which occur in the woody tissues of plants. Proteins, the principal nitrogen-containing constituents, are formed by linkages of many amino acids amino acids, and most living matter consists of proteins.
Most of the benefits which accrue to soils from organic matter are due to the ceaseless decomposition of plant and animal residues, into simple inorganic compounds such as carbon dioxide, water, nitrate, phosphate, and sulfate. The polysaccharides, which constitute the bulk of mature plant tissues, are decomposed by several species of soil microbes. The products are carbon dioxide and water, and new celss of molds, actinomycetes, and bacteria. The result is that soil organic matter contains very little polysaccharide of plant origin, though it does contain some polysaccharide of microbial origin. Lignins being far more resistant to microbial attack than polysaccharide, accumulate as the decomposition proceeds but don’t remain unaltered. When some of the side chains are split off and methoxyl groups are cleaved, the number of acidic groupings such as carboxyl, increase. These changes modify the properties of lignin; which can now retain more nutrient elements. Plant proteins contain the nitrogen essential for building microbial cells. Thus, nitrogen accumulates in soil organic matter not because plant protein is resistant to microbial attack but because nitrogen is an important constituent of microbial cells, and is used and reused as old cells die and new ones are formed. Organic nitrogen in soils appears gradually in soluble inorganic forms because clay minerals exert a protective action on protein molecules, trapping them within the lattice of the clay crystal and also because of the formation of organic complexes resulting from reactions between proteins and lignin.
Virmani, SMM, et., al. 1994. Stressed Ecosystems and Sustainable Agriculture. Science publisher: Lebanon USA