Aeration replaces oxygen-deficient air in the center of the compost pile with fresh air. Rapid aerobic decomposition occurs only when there is enough oxygen present. Air movement throughout the compost pile is affected by spaces between particles in the compost pile and by moisture content. If the material becomes water saturated, the air movement decreases. Regular mixing or turning of the pile fluffs up the material and increases air movement, enhances aeration and decreases compaction.
Moisture is needed for bacterial decomposition.
A moisture content of 40-60 percent provides adequate moisture without limiting aeration. Too much moisture causes nutrients to leach out, odors are produced and decomposition slowed. The material should feel damp to the touch, with just a drop or two of liquid expelled when the material is squeezed tightly. Adding dry material can fix excess moisture problems. Piles too dry can be watered while turning.