Compost-ology hose
Moisture & Aeration

A balance of air and water must be maintained for rapid decomposition to take place.

  • Too much air circulating in the pile can make the pile too dry for bacteria and other decomposers to function. Inadequate moisture is the most common problem limiting backyard compost piles.
  • If the pile is too wet, anaerobic bacteria, which thrive in the absence of air, can take over the pile. Anaerobic decomposition is slow, produces an odor similar to rotten eggs and by-products that are toxic to plants. Compost piles can also become anaerobic due to the settling of materials during decomposition, which prevents air flow. This is a common problem with composting grass clippings.

The optimal moisture level for composting is 40 to 60 percent - about as moist as a wrung-out sponge. This level of moisture provides organisms a thin film of water on materials, while still allowing air into pore spaces.

  • If a pile is too wet or compacted, it should be turned (pulled apart and restacked) or "fluffed" with a garden fork, to allow air back into the pile. Mixing in coarse materials, such as leaves, also helps to aerate compost piles.
  • If a composting pile becomes dry, it needs to be pulled apart and watered as it is restacked. Watering an intact pile is not an effective way to moisten a dry pile because dry organic materials often shed water like a rain coat. Dry materials should be gradually wetted with a fine spray and mixed until they glisten with moisture.

Cover compost piles once materials are uniformly moist to retain moisture and prevent nutrients from leaching out in rainfall. Plastic sheeting, burlap scraps, straw or plywood laid directly on a pile can help to maintain moisture.