Compost Moisture Levels

All life on this planet needs water. Monitoring your compost moisture is vital to the billions of living decomposers who are calling your compost home.

The ideal moisture level for compost is between 40% and 60%. In practical terms this feels like a sponge that you've squeezed most of the water out of. At this level there is a thin film of water covering all the soil and compost particles.

It's worth remembering that most of the food scraps and other greens or nitrogen rich material are quite wet. The other day I left a few mung bean sprouts on the counter. The next day they had almost shrivelled right out of existence. These greens generally have about 80-85% moisture levels. When you mix them with the dry, brown, carbon rich material they will soak up some of the excess moisture. However, you may need to wet down the dry brown material somewhat as you add it to the compost.

Too Wet Conditions

The problem with compost moisture levels above 60% is that the water starts to displace the air in your compost. This can create anaerobic conditions in the pile.

You'll know your moisture level is too high if your compost starts stinking. Anaerobic conditions smell like rotten eggs or ammonia. To lower your compost moisture levels try one or more of these strategies:

  • add air by fluffing things up by turning the compost.
  • use a pole or aeration tool to make water drainage and air channels.
  • if you live in a rainy area covering your compost will obviously help.
  • if you are not covering the compost make the top convex so that heavy rains can drain off the pile.

Too Dry Conditions

When the moisture levels fall below 40% decomposition will start to slow down. Below 15% and decomposition stops and I suppose mummification begins.

Here, in Southern Alberta it is pretty darn dry. In fact most compost problems here come from overly dry conditions. In dry areas or in drought years you might try some of the following strategies to keep compost moisture levels up:

  • build your compost in a pit to reduce evaporation
  • make the top of your compost concave to catch any rain that might fall
  • keep your compost covered to reduce evaporation
  • use some of your precious water to water to compost
  • if watering restrictions are severe collect and use grey water from your shower, bath or sink.
  • you may have dry spots in the middle of your compost so you'll need to either redo your pile and wet down all the sections or stick a hose down into the middle of the pile.


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