Composting is the process of producing compost through aerobic decomposition of biodegradable organic matter. The decomposition is performed primarily by aerobes, although larger creatures such as ants, nematodes, and oligochaete worms also contribute. This decomposition occurs naturally in all but the most hostile environments, such as within landfills, extremely arid deserts or cold weather such as boreal winters or polar regions, which prevent the microbes and other decomposers from thriving.

Composting can be divided into the two areas of home composting and industrial composting. Both scales of composting use the same biological processes, however techniques and different factors must be taken into account.

Composting is the controlled decomposition of organic matter. Rather than allowing nature to take its slow course, a composter provides an optimal environment in which decomposers can thrive. To encourage the most active microbes, a compost pile needs the correct mix of the following ingredients:

• Carbon
• Nitrogen
• Oxygen (in the case of aerobic composting)
• Water

Decomposition happens even in the absence of some of these ingredients, but not as quickly or as pleasantly. (For example, vegetables in a plastic bag will decompose, but the absence of air encourages the growth of anaerobic microbes, which produce disagreeable odors. Degradation under anaerobic conditions is called anaerobic digestion.)

Techniques

There are a number of different techniques for composting, all employing the two primary methods of aerobic composting:

• Active (or hot) composting allows aerobic bacteria to thrive, kills most pathogens and seeds, and rapidly produces usable compost. Aerobic bacteria produce less odour and fewer destructive greenhouse gases than their anaerobic counterparts. In addition, they are usually faster at breaking down material and the faster material is broken down, the faster compost is created for your garden.

Pasteurization in a hot compost (such as the Compost Oven) will occur in any garden compost bin if the temperature reaches above 55 C (131°F) for three or more days. To achieve it, you need to keep your garden compost bin warm, insulated and damp since this encourages the cultivation of actinomycetes, a vital bacteria in the pasteurization process.

The pasteurized soil naturally created through heat in the garden compost is valuable for the composting home gardener, since the pasteurisation process is otherwise both expensive and complicated, and adding chemicals to produce the pasteurization effect makes the compost less healthy.

• Passive (or cold) composting allows Nature to take its course in a more leisurely manner, while leaving many pathogens and seeds dormant in the pile.

Cold composting is the type of composting done in most domestic garden compost bins in which temperatures never reach above 30 C (86°F). Cold composting is characterised by individuals putting their kitchen scraps in the garden compost bin and leaving them untended. This "scrap bin," having a very high moisture content and without aeration, is likely to turn anaerobic and generate foul odours, including significant adverse greenhouse gas emissions.