The natural decomposition of organic materials takes place in three steps: degradation, conversion, and curing. The materials in a compost pile will go through these steps whether you ignore the pile for a year or turn it every few days.
During the degradation phase, the organic materials are broken down. At its simplest level, the phase involves microorganisms consuming proteins and carbohydrates in the materials. As the microorganisms feed and multiply, they create energy in the form of heat and release water and carbon dioxide. Then these microorganisms become dormant or are consumed by other microorganisms, whose populations are burgeoning. The temperature in the pile rises. It is during the conversion phase that humus—a rich, dark, earthy substance—is built.
The temperature of the pile drops, and microorganisms that work at lower temperatures take over and complete the decomposition. At this stage, the compost is considered to be fresh, or raw. It can be used now, although it will continue to decompose. When fresh compost is dug into the soil, the organisms that break it down further will consume some of the soil nitrogen, depriving plants of it. For that reason, fresh compost isn’t considered as valuable as compost that has cured.
During the curing process, microbial activity subsides; the pile cools down; and earthworms, insects, and mites gradually return. As the compost sits, it continues to decay. When you have a dark brown substance that looks and smells like humusy soil, your compost is mature. After it has cured even longer, the compost is aged. Beware of letting compost cure for too long—the longer it weathers, the more nitrogen it loses.