Organic waste needs water to decompose. A pile of dried leaves exposed to rain decomposes in two or three years, whereas the same pile may last for decades if it’s kept dry. Too much water, however, is just as detrimental to the composting process as a lack of water. In an overly wet pile, water replaces the air, creating an anaerobic environment and slowing decomposition.
Ideally, the materials going into a compost pile should have a moisture content of 40 to 60 percent. They should feel slightly moist but not wet—like a sponge after it’s been soaked and thoroughly wrung out. The easiest way to achieve that moisture level is to expose the materials to rain for a while.
After they absorb some rain and drain off the excess, they’ll have just about the right amount of moisture for composting. During dry periods, use a garden hose to moisten the materials. If you’re using very wet materials, mix them with dry materials as you’re building the pile. If all the material is very dry, soak it as you construct the pile.
Because of their heat, compost piles tend to dry out. On cool fall mornings you can see steam rising from active piles. Water is escaping from the pile at that rate all the time, but as invisible water vapor. Cool, moist air condenses the water vapor to make it visible.
If you live in a dry climate, build piles with a concave top to catch water. In wet climates, make the top convex to shed excess rain.
Whenever you turn a pile, check the moisture level and add water if necessary. The easiest way to add water is to put a sprinkler on the pile as you turn it. Wet passive piles when necessary by setting a sprinkler on them.