How Hot Is Your (Compost) Heap?

Keep your compost temperature in check.


Your compost pile can get too hot, Ingham cautions. “If the pile starts getting into the range of 160 to 165 degrees, the organisms are growing so fast that they can use up the oxygen in the pile, causing the good-guy aerobic organisms to start to go dormant or die.” Then the anaerobic bacteria and yeasts (the anaerobic forms of fungi) start growing and, sooner or later, alcohol will be produced. “There are hundreds of different kinds of alcohols that these anaerobic organisms can make, and some have a heat of combustion around 180°F,” warns Ingham. “If the pile is anaerobic and temperatures reach 180°F or thereabouts, it is very likely that the pile will spontaneously combust.” To prevent this from happening, turn the pile if its temperature exceeds 165 degrees F.

Conversely, a pile that is short on nitrogen-rich ingredients may never reach the thermal range needed to wipe out pathogens and weedy seeds. If things aren’t heating up, it’s time to mix in some high-nitrogen materials, such as fresh grass clippings, legumes, cottonseed meal, or manure, as you moisten and aerate the pile.

After your first attempt at thermal composting, Ingham says, you figure out how much high-nitrogen “party” food, green bacterial food, and woody, high-carbon food to add to make things heat up. “Monitoring temperature takes mere minutes,” Ingham concludes, “but is essential to letting you know when you need to turn the pile or add party foods to keep things just right for your guests.”

Originally published in Organic Gardening magazine, December 2013/January 2014
Photo: (cc) nancybeetoo/flickr