Compost Moisture Needs
Prepared by Maher Jabado, Making the world greener

(Materials & methods to ensure quality compost)


Just like people, compost organisms need water to live. Some microorganisms use the film of water to move—slipping and sliding to another section of the pile. Biological activity stops when the pile dries out.

If adequately aerated, composting material with moisture content between 30% and 100% will be aerobic. In practical aerobic composting, however, high moisture content must be avoided because water displaces air from the interstices between the particles causing anaerobic conditions. However, too low moisture content deprives organisms of water needed for their metabolism, and inhibits their activity.

Maximum moisture content for satisfactory aerobic composting varies with materials used. If straw and strong fibrous materials are used, 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 lots of paper or grass clippings, which have little structural strength when wet, or if granular, like ash and soil, less water is better.

Ideally, home compost piles whould contain 40 - 60% moisture. It should feel as moist as a wrung out sponge. Dry carbon layers can be watered as the pile is built, then with each turning, add more water as necessary.

In University of California studies, fibrous materials containing an abundance 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.