Aeration
C:N ratio
Moisture
Particle size
pH
Bin size

Moisture

Compost microbes need water. Because of this, decomposition occurs most rapidly in the thin liquid films found on the surfaces of the organic particles.

About 50% moisture is optimal for composting. Too little moisture (<30%) inhibits bacterial activity. Too much moisture (>65%) chokes off the diffusion of air into the pile which results in slow decomposition and the production of odor in anaerobic pockets.

To gauge moisture content, the compost mixture should feel damp to the touch with about as much moisture as a wrung-out sponge. Or use the "squeeze test": Take a handful of compost mixture and squeeze hard; If you get just a drop or two of water from the mixture, then moisture content is just right; If you get no water, then it's too dry; If you get a small stream, then it's too wet.