Aeration
C:N ratio
Moisture
Particle size
pH
Bin size

Aeration

Oxygen is essential for the metabolism and respiration of aerobic microorganisms (from the Greek aero for "air" and bios for "life") and for oxidizing the various organic molecules present in the material. At the beginning of the compost process, the oxygen concentration in the pore spaces is about 15-20% (similar to that of air) and the carbon dioxide concentration varies from 0.5-5.0%. As composting progresses, the oxygen concentration falls and the carbon dioxide concentration increases. Anaerobic (an or "without" air) conditions will develop if the oxygen concentration drops below 5%. While anaerobic activity is characterized by undesirable odors, a small amount of this activity in the compost pile is acceptable and common. The compost pile acts as a bio-filter to trap and degrade the odorous compounds produced as a by-product of anaerobic decomposition.

Some compost systems are able to maintain adequate oxygen passively, through natural diffusion and convection. Other systems require active aeration, provided by blowers or through turning or mixing the compost ingredients.