Soil Organic Matter

Abstract

Soil organic matter includes plant and animal residues at various stages of decomposition, ranging from fresh undecomposed materials through partially decomposed and short-lived products of decomposition to well-decomposed humus. Plant and animal residues contain sugars, celluloses, hemicelluloses, proteins, lignins, waxes, and lipids. When these materials are incorporated into soils, sugars, celluloses, and hemicelluloses, most proteins undergo rapid decomposition. With decomposition, the proportion of resistant compounds such as lignin gradually increases. Resistant decomposition products and newly synthesized substances accumulate in soil and form complexes with its mineral components. Humus is formed in the latter stage as a result of such continued decomposition, resynthesis, and complexation. Humus is a fairly stable substance and requires very long time, for example, centuries to millennia, to be completely decomposed. Most mineral soils contain less than 5 % by weight of soil organic matter (SOM), but this little quantity of SOM has tremendous biological significance. It provides food for microorganisms, stores nutrients, retains water, acts as mulch, and performs as a soil conditioner and aggregating agent. It makes the soil friable and fertile. Soil organic matter contributes significantly to the sequestration of carbon.