Soil Organic Matter. The organic fraction of the soil. The broadest definition includes undecayed plant or animal residues, living soil organisms, and plant roots as categories of soil organic matter. Many definitions restrict the term to humus and organic materials that are at least partially decomposed, and whose origin (plant stems, leaves, animal remains) is no longer recognizable. Organic matter that bacteria, fungi, earthworms, and other soil organisms can decompose is a critical component of the soil food web, which is important in nutrient cycling and all other soil biological processes.