The increased surface area means that the microorganisms are able to digest more material, multiply more quickly, and generate more heat. It is not necessary to increase the surface area when composting, but doing so speeds up the process. Insects and earthworms also break down materials into smaller particles that bacteria and fungi can digest. The decomposition occurring in the compost pile takes up all the available oxygen. Aeration is the replacement of oxygen to the center of the compost pile where it is lacking.
Efficient decomposition can only occur if sufficient oxygen is present.
This is called aerobic decomposition. It can happen naturally by wind, or when air warmed by the compost process rises through the pile and causes fresh air to be drawn in from the surroundings. Composting systems or structures should incorporate adequate ventilation. Turning the compost pile is an effective means of adding oxygen and brings newly added material into contact with microbes. If the compost pile is not aerated, it may produce an odor symptomatic of anaerobic decomposition.