Do you need to add water to a compost bin?

QUESTION: Do you need to add water to a compost bin?

ANSWER: It’s possible that you’ll need to occasionally add water to your compost bin to keep the microbes working at optimum capacity. Without the appropriate level of moisture, the microbes that decompose your compost materials won’t be able to do their job, and the process of decay that compost relies on will slow down to a crawl. This can happen, for example, if you’ve recently added a lot of dry carbon-rich ingredients, such as a big batch of dried leaves. 

“Green” or nitrogen rich materials typically have a lot of moisture, and that is the best way to keep your compost at the correct level of moisture. If you have the right balance of wet, nitrogen rich materials like kitchen scraps and fresh cut grass clippings. (Too much of this material will cause the opposite problem where your compost is too wet, and can then turn everything stinky.)

The compost materials should be kept moist enough to have the same dampness as a sponge that has been wrung out. If you aren’t sure whether your compost is moist enough, find some dried leaves in the pile and squeeze them tightly in your fist. The leaves should feel damp, and when you squeeze them tight, only one or two drops of water should trickle out. (Please take note that this “squeeze test” only works on dried leaves and should not be used on other compost ingredients to determine moisture level.)

The wrung-out sponge rule of thumb is all you really need to know to keep your compost moist enough. However, if you want to get technical about it, your compost ingredients should ideally be between 40 and 60 percent water, according to the Cornell Cooperative Extension.

Whenever your compost dips below the moisture level of a wrung-out sponge, simply sprinkle some water on the pile with your garden hose for a couple of minutes. If you’d like, you can make things even easier for yourself by setting your sprinkler system near enough to your compost bin that some of the water lands on your compost whenever you need to add some moisture. You can also help balance out compost that’s too dry by adding in more wet materials, such as coffee grounds, vegetable scraps, or fresh grass clippings.

Sometimes, the problem will be reversed, and your compost will need to dry off a bit. It’s inevitable that you’ll sometimes overshoot the desired level of moisture when watering, and the compost can get too wet if you’ve added lots of wet ingredients lately, too. Compost that is too wet won’t be getting the oxygen it needs because the spaces between things where air should be circulating is instead filled with water. 

When this happens, just prop the lid of your compost bin open on a warm, sunny day, and leave the lid off so the sun’s rays can dry your compost materials out until your compost is back to the moisture level of a wrung-out sponge. (If it rains while you’re letting your compost soak up some sun, make sure to close the lid during the rainy spell.)

Alternatively, you can stir in some more dry, carbon-heavy ingredients like dried grass clippings, wood chips, dried leaves, shredded newspaper, or shredded cardboard to help balance the excess moisture out and get the hydration level back where you want it. When you’re adding ingredients to dry things out a bit, you want to choose the coarsest, most porous dry materials you have available because the added permeability will allow air to circulate and dry the compost off as well as giving the excess water space to drain out.