Compost: What Is It? Part 2
By John Ferguson
Summer 1997

In general, composting is the controlled conversion of dead organic material into a stable form by the action of beneficial microbes, generating sufficient heat to kill all weed seeds and harmful pathogens (bad microbes), producing a stable end product.  This end product varies greatly in value, quality, and in possible applications from soil amendment to bioremediation or air filtration.

There are three major techniques (physical and chemical processes) used to produce compost: aerobic, anaerobic, and vermicomposting.  Depending on the type of raw source materials (feedstock) and the processing variables (time available, quality desired, economic constraints, etc.), one process may be better suited than the other, as each method has its positive and negative attributes.

Aerobic composting occurs in the presence of oxygen from the air.  Carbon from the source material by the action of microbes is combined with oxygen, producing carbon dioxide (CO2) and energy (heat).  Nitrogen, sulfur, and other nutrients in the material are stabilized and converted into amino acids, proteins, and humic substances which are beneficial for plant growth.

Anaerobic composting means ‘without oxygen’.  A different set of microbes digest the source material producing methane and other gases, including odors.  A common example of anaerobic composting occurs when a plastic bag of grass clippings is stored for a few days and then opened.  The odor that one smells is from nitrogen and sulfur compounds (nutrients that are typically lost).  This type of composting is generally used for waste reduction and not agriculture or horticulture.

Vermicomposting occurs by the action of certain types of earthworms eating and digesting the source material.  The manure produced is called vermi-compost and is generally the highest quality and most valuable type of compost.  Demand for special variations of this type of compost has driven the price as high as $2,000/ton for use as a bio-stimulant in some areas of the world.  In the US, most vermicomposts are sold bulk wholesale for $35-$100 per cubic yard.  Many professionals on the west coast are starting to use vermicompost in potting mixes and are finding it very cost effective.  Researchers at Ohio State University are  finding many uses and benefits from vermicompost in horticulture and agriculture.  Additionally, different species of earthworms produce vermicompost with different properties and uses.

Aerobic composting is the most common process used commercially and in our backyards.