What is compost?

When animals and plants die, they decompose surprisingly quickly. The body of an adult human, if exposed to the elements, is reduced to a mere skeleton in about three weeks. The bones last longer, but not a great deal. Shakespeare was right when his gravedigger in Hamlet (Act V, Scene I) estimated there was not much left of a cadaver after eight or nine years. Even huge trees go the same way, and about as quickly. The end product of this composting process is a remarkably resistant and complex organic substance called humus, which is largely responsible for the brown color of the majority of soils in temperate regions. Humus is a mixture of the highly altered remains of the original organic matter—whether from plants or animals—that arrives at the soil surface, as well as new compounds made by bacteria and fungi. But only a small fraction of the original material is destined to become humus. Most simply disappears, turned back into the carbon dioxide (CO2), water, and mineral salts from which it was first made.