In general, the composting process goes through several distinct heat phases. When a compost pile is built well, it can reach high temperatures around 100 F in as little as a few days. This is called the mesophilic stage where the pile heats up rapidly and there is an explosive growth of bacterial life that is beneficial to the composting process. Ideally, the temperature should peak and stabilize (120-140 F)  for a period of time, usually 1-2 weeks. This is called the thermophilic phase or active composting phase. These high temps destroy pathogens, weed seeds, and fly larvae. After a couple of weeks, the temperature will start to drop off below 100F (back to a mesophilic range) leaving behind more resistant woody biomass and resistant wood-compounds which are worked on by fungi and other soil microbes that like cooler temperatures. Turning/mixing the pile during this point will add air and can help bring the temperature back up to the thermophilic range to reinvigorate the process. After about a month or two, temperatures fall to a low that is not likely to peak again, and the compost pile may transition into the curing and maturation phases.

So, is your pile cooking?

Heat is a great process indicator for how well the pile is decomposing. Residents can purchase a special composting thermometer by searching the web for products like ReoTemp. While this is interesting, it’s not absolutely necessary to be successful at hot composting. Heat production depends on the size of the pile (the ideal pile size is 3 feet x 3 feet x 3 feet), its moisture content (see FAQ about the Squeeze Test), and the materials being composted (hint: small pieces break down faster). By following basic management tips like these, you can ensure your compost pile reaches high temps.