Aerobic decomposition can proceed at any moisture content between 30% and 100% if adequate aeration can be provided. In practical aerobic composting, however, a high moisture content must be avoided because water displaces air from the interstices between the particles and gives rise to anaerobic conditions. On the other hand, too low a moisture content deprives the organisms of the water needed for their metabolism, and inhibits their activity.

Maximum moisture content for satisfactory aerobic composting varies with materials used. If it contains considerable amounts of straw and strong fibrous material, the maximum moisture content can be much larger without destroying structural qualities or causing material to become soggy, compact, and unable to contain enough air in the interstices. But if it contains considerable quantities of paper and garbage, which have little structural strength when wet, or if it is granular, like ash and soil, less water is better. It is difficult to maintaining aerobic conditions at moisture content around 70%.

In University of California studies, fibrous materials containing a considerable amount of straw were composted aerobically with moisture contents of 85% to 90%, but other composts containing much paper became anaerobic in one day when the moisture content was about 70%.

If anaerobic composting is practiced, the maximum moisture content is not as important, since oxygen maintenance is not a factor. The upper limit of moisture, which may be from 80% to over 90%, is the amount of which excessive drainage from the compost will be produced. If the composting procedure has initial aerobic conditions to produce high temperatures lasting a few days for the destruction of pathogenic organisms, followed by anaerobic composting, the maximum initial moisture content may be as high as 65% to 85%, depending on the character of the composting materials.