13.1: Introduction

The reason of our thus treating composts of various soils and substances, is not only to dulcify, sweeten, and free them from the noxious qualities they otherwise retain. ... [Before composting, they are] apter to ingender vermin, weeds, and fungous ... than to produce wholsome [sic] plants, fruits and roots, fit for the table.

—J. Evelyn, 17th century

Decomposition of organic materials takes place naturally in forests and fields all around us. Composting is the art and science of combining available organic wastes so that they decompose to form a uniform and stable finished product. Composts are excellent organic amendments for soils. Composting reduces bulkiness of organic materials, stabilizes soluble nutrients and hastens the formation of humus. Most organic materials can be composted, and the process offers a win-win opportunity: reducing waste and improving soil.

In some ways, composting is microbe farming. If ingredients are combined to provide food (carbon and nitrogen), moisture, oxygen and shelter in proper proportions, a diverse cohort of organisms will efficiently process the feedstock. These microorganisms perform well at elevated temperatures with plenty of oxygen and moisture. They cover the range of warm (mesophilic) to hot (thermophilic) conditions. Thermophilic temperatures (from 110° up to 160°F) help kill off weed seeds and disease organisms, which sets composting apart from other decomposition processes. At temperatures below 110°F, the more prolific mesophilic organisms take over and the rate of composting again slows down, especially as it drops toward ambient temperatures, a process known as “curing.” At the other extreme, temperatures above 160°F can develop in compost piles; this overheating slows down the composting process by killing off most organisms and by possibly causing extreme drying. High temperatures, in combination with high ambient temperatures and aeration, can also cause spontaneous combustion in barns and at compost facilities. In general, the composting process is slowed by anything that inhibits good aeration or the maintenance of high enough temperatures and sufficient moisture. It has been found that mesophilic temperatures may be more effective at breaking down some pharmaceuticals.

Types of Composting

Some people talk about “low temperature” composting—including “sheet,” worm (vermicomposting) and small-pile composting—and “high temperature” composting. We like to use the term “composting” only when talking about the rapid decomposition that takes place at high temperatures.

Even Birds Do It

The male brush turkey of Australia gathers leaves, small branches, moss and other litter and builds a mound about 3 feet high and 5 feet across. It then digs holes into the mound repeatedly and refills them, helping to fragment and mix the debris. Finally, the pile is covered with a layer of sticks and twigs. The female lays her eggs in a hole dug into the pile, which heats to nearly 100°F around the eggs, while the outside can be around 65°F. The heat of the composting process frees the birds from having to sit on the eggs to incubate them.

—R.S. Seymour (1991)