The curing phase is an integral part of the composting process, especially for those wanting to make quality composts. Lignin is degraded most rapidly during curing, and compost is enriched with complex humus compounds.

Curing begins when temperatures are sustained below 45°C even after turning. It is complete when temperatures decline to within 5°C or so of ambient. Curing completes the composting process and produces a mature end-product. Immature compost can be toxic and have negative effects on plant growth, which are alleviated by a suitable curing phase of 3–6 weeks.

The curing phase can be shorter (e.g. 3–4 weeks) when composting is just to dispose of waste. In this case, the main purpose of curing would be to make to compost safe to store elsewhere on the farm.

Compost is cured in a separate functional area of the composting facility to avoid recontamination with pathogens. The use of the same machinery with cured compost directly after handling the raw material is to be avoided for the same reason (unless it is thoroughly washed).

Turning is less frequent during curing – perhaps once a week to begin with but as the temperature drops below 40°C, the pile may be left undisturbed for long periods provided that aerobic conditions are maintained. Limiting piles to about 2.5m in height and 6m in width is usually enough to prevent the buildup of anaerobic conditions during curing. If the pile continues to reheat above 45°C after turning, more frequent turnings may be needed to maintain aerobic conditions. Alternatively, consider reducing the size of the pile.

Properly cured compost has a low rate of microbial activity, and should therefore not develop anaerobic conditions during storage. Very large storage piles increase the risk of fire from spontaneous combustion. The height of fully cured compost in storage should not exceed 3.5m, though the reach of front-end loaders or other equipment often determines maximum pile height.