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Compost: What It Is and How It Works

Compost can be simply described as decayed organic matter. The University of Alaska at Fairbanks has a more detailed definition, "Material derived from aerobic decomposition of recycled plant waste, biosolids, fish or other organic material."1 Aerobic decomposition basically means that oxygen was involved in the process.

Fresh Compost

Oxygen is important because, along with nitrogen and the carbon in the debris, it allows living organisms within the soil to break down organic materials. Some of the materials are exhaled in the form of CO2, a harmless gas (though it is said to be a factor in global warming), converted into protoplasm of the living organisms, or cast off as waste materials (yet these waste materials are quite nutritious to plants). The amount of nitrogen and oxygen in the matter can greatly vary the length of time it takes for the decay process to be completed. If there is too little nitrogen it takes several generations of organisms (dying and reproducing - and in the process recycling the nitrogen within their systems) in order to completely breakdown the organic materials.2

There are other elements and nutrients necessary in the making of compost, including phosphorus and potash. But these will not need to be added separately as they are normally of sufficient quantity in the materials to be composted.

As the composting process converts the carbon in the pile into CO2, heat is generated. So much heat, in fact, that temperatures within the compost pile can reach 170°F (about 77°C3). (Bacteria will cease to function at 160°F.) In decomposition, there are generally several different types of bacteria at work, each working best at different temperature ranges. So even during winter months, as long as the temperature stays above zero degrees F, decomposition will continue. The high temperatures to which a compost pile can reach has the benefit of killing off many bacteria that can be harmful to humans. It can also kill many weed seeds.

It is important to note that the aerobic process is not one that generates unpleasant odors (at least to most people). So if a pile seems to smell bad there may be other decaying processes going on, and in this case, it might indicate that it is a good time to turn the pile to introduce more oxygen. This other type of decay is anaerobic, and the results are generally not good for people or plants.

Most compost piles will also contain worms. Worms are incredibly efficient compost makers. We will discuss them at greater length in Vermicomposting.

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Next Page: How to Make Compost


  1. http://www.uaf.edu/ces/compost/definition.html - UAF Definition of Compost
  2. WSU - Aerobic Biology
  3. Temperature Conversion Tool


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