Heat Composting


To the surprise of some gardeners, a properly-made compost pile gets really hot really fast. Within a few hours of mixing dead leaves with grass clippings and a little water, the compost can easily reach a temperature of 160 degrees Fahrenheit. But if the pile is too dry, or the balance of brown and green materials is wrong, it will just sit there for weeks without heating up. It will break down into compost in about a year, but if you build the pile correctly it will heat up and decompose much faster.

What is Compost Made Of?

Compost is a mixture of carbon, nitrogen, water, and oxygen. Carbon is brown, dead material such as leaves, straw, or sawdust. Nitrogen comes from fresh, green materials like grass clippings, salad trimmings, garden waste, and animal manures. Water may need to be added, and oxygen is part of the air that gets mixed into the pile. Decomposition will happen faster if materials are shredded before being added to the pile.

The C-to-N ratio and Moisture

By volume, carboniferous materials should exceed nitrogenous materials by about a 2-to-1 or 3-to-1 ratio. The moisture level should be about that of a wrung-out sponge. If the pile is too dry or does not have enough nitrogen it will just sit there and not heat up, so add more green materials. Too much water will exclude oxygen and the pile will start to stink as anaerobic bacteria take over --- more dry carbon material should be added and mixed in immediately.

Why Heat is Good

A compost pile contains a huge colony of microscopic creatures that break down plant materials. The heat they create as they do this is hot enough to kill most weed seeds and disease pathogens. This heat-loving (thermophilic) microbial life itself needs that warmth and moisture to survive and keep working.

Keeping it Warm

Microbial activity begins when the materials are first mixed together. Heat will be generated and will continue for a few days. When the pile cools slightly, it should be turned so that material on the outside gets into the middle and everything in the middle ends up on the outside. It will heat up again. After turning weekly three or four times, the pile will no longer heat up and can be left to continue decomposing.

The Curing Phase

As the temperature drops, another large group of microorganisms and invertebrates (worms, springtails, and beetles, for example) takes over. These will continue to break down the material. If the C-N ratio and moisture level is correct, and the pile has been properly turned, finished compost should be available for use within six to eight weeks. The compost may need to be screened through a wire-mesh sieve to remove large pieces before it is spread on the garden. Chunks can be returned to the pile where they will continue to decompose. They will also help to "inoculate" the pile with beneficial bacteria.

Keywords: compost, thermophilic, microorganisms, C:N ratio

About this Author

Peter Garnham has been a garden writer since 1989. Garnham is a Master Gardener and a Contributing Editor for "Horticulture" magazine. He speaks at conferences on vegetable, herb, and fruit growing, soil science, grafting, propagation, seeds, and composting. Garnham runs a 42-acre community farm on Long Island, NY.