Humus

Definition

Humus is the dark, stable fraction of organic matter that remains after most plant and animal residues have decomposed. It binds mineral particles into crumbs, holds nutrients and water, and breaks down very slowly, which is why it gives topsoil its dark colour. It builds up only through years of steady inputs, yet can be lost quickly by intensive tillage, burning of residues or erosion. A dark, crumbly soil that smells earthy usually contains plenty of it. See also: Soil organic matter, Aggregate, Cation exchange capacity, and the guide "Nutrient Depletion, Organic Matter Loss and the Decline of Soil Life".