Organic Matter

organic matter, the carbon-containing matter formed in soil from the rotting of plant, animal, and microbial residues.

Normally most organic matter is in the top 10–20 cm/4–8 in of the soil, with some deeper as a result of deeply penetrating roots and its distribution by earthworms and other burrowing animals. On undisturbed soils, much of the store of readily available plant nutrients is associated with the surface layer and its organic matter, having been extracted from the subsoil over millennia and deposited at the surface in plant residues and the excreta of grazing animals. This applies especially to the less soluble nutrients such as phosphorus and the micronutrients (except boron), which do not appreciably leach down the profile except in very sandy soils.