Does compost need to breathe?

QUESTION: Does compost need to breathe?

ANSWER: Air circulation is a vital component of producing healthy compost, but it isn’t the compost itself that needs to breathe——but the microbes that live in the compost and break the ingredients down into the nutrient-rich final product do require oxygen to survive. When you turn or stir the compost, you’re aerating the soil to ensure all those little microbes get plenty of the air they need to breathe.

However, your hands-on contribution to aeration is not required. It will take quite a bit longer for your food scraps to be transformed into “black gold” if you aren’t turning and mixing the compost, but it will still get done. The only real necessities for aeration are air holes in the bottom of your compost bin, although it really helps to have air holes in the sides as well.

There are tools on the market designed specifically to help you with compost aeration, and of course, those do the trick. But gardeners have used pitchforks to aerate their compost for centuries with success, and other simple tools work just as well, such a plain old shovel.

To aerate easily using a shovel if you’re using the usual composting setup, with a bin that does not have a bottom or base plate, start by picking the bin up and placing it next to the compost pile, leaving a few feet of space between the two. Then, using your shovel, transfer the compost materials back into their bin in its new location. Easy as pie. Another way to aerate your compost is by using a compost tumbler, which is a bin designed to spin so that it aerates the materials inside the barrel when you crank its handle. 

Unless your compost is showing signs that it needs aeration urgently (which we’ll describe for you below), you should turn or mix the compost every few weeks. Although turning the compost materials only buys you a few hours’ worth of air, the looser, broken-up texture of the materials after they’ve been turned allows more oxygen to circulate through the pile, increasing the overall availability of air in your compost.

Some people wonder whether it’s possible to aerate too much or turn the compost too much, and the answer is yes. Your compost ingredients need time to sit and rest so that the microbes can get down to business and work on decomposing them and moving them closer to their final form. 

The microbes that work on decomposing your compost materials can survive without much oxygen—they only need five percent oxygen available to survive. However, if the oxygen level in your compost bin drops lower than 10 percent, parts of the compost heap may actually be at zero. This can happen because the amount of air available is not necessarily uniform throughout your compost pile. 

Whenever any part of your compost becomes anaerobic (in other words, devoid of oxygen), you’ll begin to smell the hydrogen sulfide gas that the microbes produce as a byproduct whenever they break down your compost materials. This gas has an unpleasant aroma similar to that of rotten eggs, so you’re sure to notice it if the amount of air in your compost dips below a healthy level.

If you detect this rotten-egg smell around your compost area, you need to aerate the compost just as you would in a routine session as described above to correct the problem.