How to encourage a hot compost pile

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Last Updated: 
February 19, 2003

If you are like most folks, you probably wonder whether your compost pile is really "working" or not. It probably is, according to Dan Sullivan, Oregon State University's organic waste recycling specialist. But the more relevant question might be, are you "fast" or "slow" composting?

Fast or hot compost is created by manipulating the decay process to make it process quickly, explained Sullivan.

"You do this by balancing food, water and air in the compost pile to favor the growth of high temperature microorganisms," he said. "A byproduct of their activity is heat."

A hot compost pile can heat rapidly to 120 to 150 degrees F, he said. The advantage of high temperature composting is that weed seeds and disease organisms are killed, while many beneficial fungi called mycorrhizae are not killed.

Once the hot phase is completed, lower temperature creatures such as worms, insects and other organisms can complete the decay process.

To construct a hot compost pile, home gardeners will need a combination of bulking agents and energy materials.

Bulking agents, sometimes called "brown materials" are dry, porous materials that help aerate the compost pile. They can be such things as wood chips, sawdust, grass hay, wheat straw and corn stalks.

They are too low in moisture and nutrients to decay quickly on their own.

Energy materials, sometimes called "green materials" provide the nitrogen and high-energy carbon compounds needed for fast microbial growth. They include grass clippings, fresh dairy, rabbit or chicken manure, fruit and vegetable waste and garden trimmings. If piled without bulking agents, energy materials are too wet and dense to allow much air into the compost pile. If you have too many energy materials, you may detect a rotten egg smell.

Some raw materials contain a balance of energy and bulking agent properties. These can be ground up tree and shrub trimmings, horse manure and bedding, deciduous leaves and legume hay. They will compost readily by themselves, and can be added to an existing pile to ensure the success of a hot compost pile.

To construct a hot pile, Sullivan recommends combining two parts by volume bulking agents with one part energy source. Some other hints for a hot compost pile:

  • Chop your raw materials into small pieces. For optimal hot composting, particles should be from one-eighth to one-half inch in diameter.
  • Mix the types of raw materials, rather than layering them.
  • A large pile holds heat better than a small pile. For hot composting, make the initial pile at least a cubic yard in volume.
  • Keep the pile moist, but not wet.
  • Turn the pile once a week to aerate it.
  • Compost ingredients have their own microorganisms. There is no need to add starters or soil. Some compost mixtures need supplemental nitrogen. Add nitrogen if your pile has mostly fibrous woody material.

Don't despair if your existing pile doesn't heat up, said Sullivan.

"If you cannot get the pile to heat, all is not lost," he said. "The pile will still break down slowly. Weed seeds and disease organisms may not all be killed but you will still have compost eventually."

Author: Carol Savonen
Source: Dan Sullivan