Composition of Organic Matter
Soil organic matter plays important role in the maintenance and improvement of soil properties. It is a dynamic material and is one of the major sources of nutrient elements for plants. Soil organic matter is derived to a large extent from residues and remains of the plants together with the small quantities of animal remains, excreta, and microbial tissues. Soil organic matter is composed of three major components i.e. plants residues, animal remain and dead remains of microorganisms. Various organic compounds are made up of complex carbohydrates, ( Cellulose, hemicellulose, starch) simple sugars, lignins, pectins, gums, mucilages, proteins, fats, oils, waxes, resins, alcohols, organic acids, phenols etc. and other products. All these compounds constituting the soil organic matter can be categorized in the following way.
Organic Matter (Undecomposed)
Water Soluble eg. Nitrates, ammonical compounds, amides, amino acids etc.
Insoluble eg. Proteins nucleoproteins, peptides, alkaloids purines, pyridines chitin etc.
Carbohydrates eg. Sugars, starch, hemicellulose, gums, mucilage, pectins, etc.
Micellaneous: eg. Lignin, tannins, organic acid, etc.
Ether Solube: eg. Fats, oils, wax etc.
The organic complex / matter in the soil is, therefore made up of a large number of substances of widely different chemical composition and the amount of each substance varies with the type, nature and age of plants. For example cellulose in a young plant is only half of the mature plants; water-soluble organic substances in young plants are nearly double to that of older plants. Among the plant residues, leguminous plants are rich in proteins than the non-leguminous plants. Grasses and cereal straws contain greater amount of cellulose, lignin, hemicelluloses than the legumes and as the plant gets older the proportion of cellulose, hemicelluloses and lignin gets increased. Plant residues contain 15-60% cellulose, 10-30 % hemicelluslose, 5-30% lignin, 2-15 % protein and 10% sugars, amino acids and organic acids. These differences in composition of various plant and animal residues have great significance on the rate of organic matter decomposition in general and of nitrification and humification (humus formation) in particular. The end products of decomposition are CO2, H2O, NO3, SO4, CH4, NH4, and H2S depending on the availability of air.