humus

The dark, stable, fully decomposed organic matter in soil — the end product of years of biological breakdown of plant and animal material by fungi, bacteria, and invertebrates. Humus is not compost and not litter; it is what remains after decomposition has finished its work. It gives topsoil its dark color, its spongy texture, and its ability to hold water and nutrients. Humus is the wealth of the soil — accumulated slowly, spent quickly if mismanaged, and nearly impossible to replace on a human timescale.