What is compost?

When animals and plants die, they decompose surprisingly 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 colour 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.

Somewhere in this process—between the large amount of original material and the tiny fraction that is left as humus—lies what every gardener wants: compost.

The entire composting process can be seen on any woodland floor. On the surface are freshly fallen, unaltered leaves. Dig down a little way and the leaves become more fragmented and much less distinct. Eventually, you will come to a dark, crumbly material in which the original leaves are quite unrecognisable. This is usually called leaf mould, but it’s just a special variety of compost, made only from tree leaves rather than the usual mixture of materials.

In fact, the woodland floor tells us two important things about compost.

First, the transition from fresh material to finished compost is a gradual one, with the end point being entirely a matter of opinion.

Second, if you were to rummage around in the leaf mould, you would soon turn up a few clearly identifiable twigs. Larger pieces of tougher material (and twigs are both larger and tougher) decompose more slowly than smaller, softer material.

Compost is “finished” when the easily decomposed parts of the original material have disappeared, and the more resistant material has decomposed to the extent that it is no longer recognisable, yet the majority of the original organic matter and nutrients still remain. Nevertheless, deciding when this perfect state has been achieved remains a personal matter, largely affected by how much twiggy stuff you are prepared to tolerate in the finished product. I can’t tell you how to decide when your compost is finished, but I can tell you how to get there in a reasonable time.