When the compost pile is lacking in oxygen, decomposition slows and odors occur.
Adequate moisture is essential for microbial activity that is responsible for decomposition.
A dry compost pile will decompose slowly. Proper moisture encourages the growth of microorganisms that break down the organic matter into humus. If rainfall is limited, water the pile periodically to maintain a steady decomposition rate. Add enough water so the pile is damp but not soggy. A rule of thumb is about as wet as a wrung out sponge.