Composting is decomposition that we want. Gardeners and farmers pile up dead plant material and let bacteria and fungi break it down into compost, which returns minerals to the soil. The aim is to give the decomposers the best conditions, so the rate of decomposition is as fast as possible.
Temperature: warmth speeds up decomposition because enzymes work faster and microorganisms grow and respire more. A compost heap in cold weather rots slowly. Respiration by the microorganisms also releases heat, so a large heap warms itself. If it becomes too hot, the enzymes denature and the microorganisms die, and decomposition slows.
Water content: decomposers need moisture. A very dry heap rots very slowly. A waterlogged heap is also a problem, because water fills the air spaces and oxygen cannot get in, so decomposition becomes slower.
Oxygen: most decomposers in compost are aerobic, so they need oxygen for respiration. Turning the heap mixes in air and speeds up decomposition. Without oxygen, anaerobic microorganisms take over. They work more slowly and produce methane and other foul-smelling gases.