Q

Michelle Anderson, The Horse digital managing editor
My manure compost pile got extremely hot, to the point that it was smoking and steaming. I broke the pile down with my tractor, and it looked like it had ash inside the pile. How likely is a compost pile to spontaneously combust? How do I ensure it’s not getting too hot?

That’s awesome! Hot and steaming compost means you are doing a good job managing it. Usually it’s steaming due to the difference between the ambient air temperature and the temperature of the pile.

Properly managed compost should have air and water to keep the beneficial microbes and fungi happily breaking things down. Turn it occasionally with a tractor to add air, or if you want an aerobic workout yourself, you can do this by hand. Occasionally hand-water the pile to keep it damp, and keep it covered—especially if you live in a rainy part of the world—to keep it from getting too wet and soggy.

Spontaneous combustion is unlikely with compost, especially with smaller piles such as those on small-acreage horse facilities, for several reasons. Firstly, spontaneous combustion occurs at much higher temperatures then what is typical for compost, like 180° F or higher. Compost should be at about 130° to 150° F; you want this temperature for at least three days to kill weed seeds, pathogenic diseases, and parasites. Measure your compost temperature with a simple garden thermometer purchased from a garden supply store. If you don’t have a thermometer, a simple way to test if it’s too hot is by placing a metal tamping rod into the center of the pile for several hours. Upon removing it, if the end is too hot to touch then you might be getting close to 180° F. Temps this high (and even higher) are not beneficial to the composting process, but still don’t necessarily mean it will spontaneously combust.

The second reason spontaneous combustion is unlikely in a backyard composting situation is that it usually requires very large piles, like the enormous ones some commercial composting facilities have. Your pile needs to be at least as big as a washing machine to begin to get hot and generate heat. For convenience sake you probably don’t want it larger than what you can manage with your tractor.

And third, spontaneous combustion usually occurs when a saturated pile dries out over a very long period.

The bottom line: Unless you have an enormous pile that was completely saturated and dried out over many long weeks, then began composting and reached internal temperatures of 180° or higher, I wouldn’t worry. Just be thrilled that you are creating a wonderful free soil supplement that will be beneficial to your pastures and gardens.

And by the way, what you saw in the center of the pile that looked like ash was most likely actinomycetes, a white bacteria that has a fungus-like look. It’s quite beneficial in the compost process and helps with odor control.