Humification

Soil organic matter originates from plant residues (dead leaves, roots, etc.) and to a lesser extent from animal residues (waste products and dead animals). To find out more about what determines the amount of organic matter that accumulates in soil, look at the Soil Genesis module in the Soils and the Landscape section.

As residues are decomposed by soil organisms, they produce humus. In this reaction, the residues become finely divided and darker, something which will be familiar to anyone who has maintained a garden compost heap. It is the humus fraction of soil organic matter which contributes most to cation exchange capacity and stable soil structure.