• A biodegradable material is capable of being completely broken down under the action of microorganisms into carbon dioxide, water and biomass. It may take a very long time for some material to biodegrade depending on its environment (e.g. wood in an arid area versus paper in water), but it ultimately breaks down completely. Many contaminating materials not dealt with in common composting are in fact "biodegradeable", and may be dealt with via bioremediation, or other special composting approaches.

• A compostable material biodegrades substantially under specific composting conditions. It is metabolized by the microorganisms, being incorporated into the organisms or converted into humus. The size of the material is a factor in determining compostability, and mechanical particle size reduction can speed the process. Large pieces of hardwood may not be compostable under a specific set of composting conditions, whereas sawdust of the same type of wood may be. Some biodegradeable materials are only compostable under very specific conditions, usually with an industrial process.

Importance

Composting upcycles

Long used in subsistence farming and home gardening for creating garden-ready soil, composting is becoming increasingly important and better understood as a tool for reducing municipal solid waste, and reducing the amount of green waste going into landfills. The decomposition of organic material sent to landfills is a principal cause of methane, an important greenhouse gas, making reduction of organic waste being landfilled a key element in the fight against climate change. In suburban and rural areas, much of the organic waste could be removed from the waste stream by promoting home composting, where consumers compost their yard waste and kitchen scraps on their own land, regardless of whether the material is ever actively re-used as "soil". In urban areas with dwellings predominantly lacking individual yard space, there are indoor small scale composting alternatives, such as vermicomposting and bokashi composting.

Materials

Composting organisms require four equally important things to work effectively:

• Carbon ("C" or carbohydrates), for energy - the microbial oxidation of carbon produces the heat.
• High carbon materials tend to be brown and dry.
• Nitrogen ("N" or protein), to grow and reproduce more organisms to oxidize the carbon.
• High nitrogen materials tend to be green (or colorful, like fruits and vegetables) and wet.
• Oxygen, for oxidizing the carbon, the decomposition process.
• Water, in the right amounts to maintain activity without causing anaerobic conditions.