Aeration
C:N ratio
Moisture
Particle size
pH
Bin size

Particle size

Microbial activity generally occurs on the surface of the organic particles. Therefore, decreasing particle size, through its effect of increasing surface area, will encourage microbial activity and increase the rate of decomposition. However, when particles are too small, they pack together too tightly, inhibiting the air circulation through the pile. This decreases the oxygen available to microorganisms within the pile and ultimately decreases the rate of microbial activity.

Particle size also affects the availability of carbon and nitrogen. Large wood chips, for example, provide a good bulking agent that helps to ensure aeration through the pile, but they provide less available carbon per mass than they would in the form of wood shavings or sawdust.