Soil organic matter

Definition

Soil organic matter is all the living and dead organic material in soil, from fresh residues and roots to stable humus. It underpins structure, nutrient supply and biological activity, and it feeds the soil organisms that keep nutrients cycling. Even when it makes up a small share of the soil by weight, its influence is large. It declines when soil is tilled intensively, left bare or eroded, and it rebuilds only slowly with consistent inputs such as residues, compost and living roots. See also: Humus, Aggregate, Cover crop, and the guide "Nutrient Depletion, Organic Matter Loss and the Decline of Soil Life".