The Science of Composting

In order for composting to be successful, the environment (e.g. compost bin) must provide the correct conditions and be stocked with the correct materials to promote the growth of microbes. It is the microbes which will breakdown the material to produce the finished product - compost.

• Microbes are microscopic organisms such as bacteria and fungi present in the waste stream they are capable of decomposing organic material. They consume organic material and produce heat, carbon dioxide, water vapor and compost.
• The most essential nutrients for the microbes are carbon and nitrogen. Woody materials tend to be high in nitrogen and green materials tend to be high in carbon. The optimum carbon to nitrogen ration for composting is about 25: 1.
• The metabolic activity of the microbes when digesting the organic material causes the temperature of the compost pile to rise. The optimum temperature for composting is about 38 - 55 degrees Celsius. Once the organic matter is consumed the temperature of the pile cools off.
• Aerobic microbes (those who require oxygen) compost at a much faster rate than anaerobic microbes (those who do not require oxygen). If oxygen levels fall below five percent the aerobic microbes die and the rate of decomposition slows considerably. A slow rate of decomposition can result in serious odor problems.
• The compost pile should have the consistency of a damp sponge. The optimal moisture level for composting is 40 - 60 percent, by weight. If the pile is too dry the availability of nutrients is limited. Conversely, if the pile is too moist the amount of oxygen available is reduced.
• During composting the pile first becomes slightly acid, then strongly alkaline and is nearly neutral when finished. The pH of finished compost is usually about 7.0 -7.5.
• The time taken to produce finished compost varies with the conditions, nutrients supplied and the amount of care. Under active composting the process may take as little as one month, while utilizing the passive process it may take six months to a year until finished compost is produced.

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