Aeration
C:N ratio
Moisture
Particle size
pH
Bin size

pH

As bacteria and fungi digest organic matter, they release organic acids. In the early stages of composting, these acids often accumulate. The resulting drop in pH encourages the growth of fungi and the breakdown of lignin and cellulose. Therefore it's best not to add lime to adjust pH as some books suggest because it will slow down this process. Adding lime also converts ammonium nitrogen to ammonium gas, creating an odor problem.

If the system becomes anaerobic, acid accumulation can lower the pH to 4.5, severely limiting microbial activity. In such cases, aeration usually is sufficient to return the compost pH to acceptable ranges.

In any case, the organic acids become further broken down during the composting process and as the compost matures it will approch a neutral pH of 7.