Why a Compost Pile Heats Up On Its Own

Stick your hand into the center of an active compost pile on a cold morning and it is warm, sometimes hot enough to steam. Nobody plugged it in. The heat is a by-product of billions of microbes eating, and understanding it is the whole secret to fast compost.

When organic matter breaks down, microorganisms digest the carbon in it and release energy. Most of that energy escapes as heat. A small handful of scraps sheds that heat to the air as fast as it is made, so it never warms up. A large pile, roughly a cubic yard or more, traps the heat in its core because the outer layer insulates the inside. Once the core warms, a succession of microbes takes over.

The early stage runs on ordinary bacteria that work at room temperature. As they multiply and the core climbs past about forty degrees Celsius, thermophilic bacteria that prefer genuine heat take the lead and can push the center to sixty degrees or higher. At that temperature the pile breaks material down in weeks instead of months, and the heat is hot enough to kill most weed seeds and plant pathogens, which is why hot compost is considered cleaner than cold compost.

Three things keep that engine running. The first is the balance of ingredients. Microbes need both carbon, from dry brown material like leaves and cardboard, and nitrogen, from green material like grass clippings and food scraps. Too much brown and the process starves and stalls. Too much green and it turns slimy and smells of ammonia.

The second is air. The thermophilic microbes need oxygen, and they use it up fast in the core. Turning the pile, or building it with coarse material that leaves gaps, lets fresh air in. A pile that is never turned does not fail, it just slows down and may go sour in the airless middle.

The third is moisture. The microbes live in a thin film of water on each particle. The pile should feel like a wrung-out sponge, damp but not dripping. Too dry and the microbes go dormant, too wet and water fills the air gaps and drives out the oxygen.

When a hot pile cools down, it usually is not finished, it has just used up the easy food or run low on air or water. Turning it reintroduces oxygen and mixes un-decomposed edges into the center, and the temperature climbs again. After a few of these cycles the material no longer reheats, which is the pile telling you it is done. A good overview of the biology is on the Wikipedia compost article.