Getting hot with compost

My friend Cyrus asked me this question months ago.

It was only after I drafted a whole, essay-length response to him about different temperature ranges in compost, however, did I realize I had misunderstood his curiosity. He was not asking me why compost gets hot. He was asking why hot compost decomposes faster than cool.

Luckily, the answer to his actual question is quite simple:

Microbes, which are responsible for breaking stuff down in your pile, move at slower rates when it’s cold. The more slowly they move, the longer decomposition takes.

That doesn’t change the fact that the temperature of your compost is a fascinating phenomenon. The heat of a pile remains one of the biggest mysteries to beginning composters and, occasionally, a frustratingly unreachable goal. People do want their compost to get hot. They also often misunderstand how that heat is created and why—or why not—it might be necessary to achieve.

Firstly, your compost gets hot because of microbial activity.

It is a common misconception that this heat is a result of exposure to the sun or some other external factor. In fact, the heat is generated from activity within the pile itself. As microbes break down organic matter, they turn it into fuel for their own cellular processes, which creates heat energy. In a decently-sized pile, that heat builds up. You’ll notice that the pile becomes hot to the touch.

You might even see steam rising.

Different temperature ranges indicate different things about the state of your pile and, in combination with other visible factors, can tell you a lot about what type of microbes are at work in your compost and where you are on the timeline of overall decomposition. To start breaking this down for you, it’s helpful to first know that there are two main temperature ranges in decomposition: mesophilic and thermophilic.

A pile is considered mesophilic when it is between 50°F and 113°F. Within this range, mesophilic microbes will flourish. These include aerobic bacteria, actinomycetes (pictured above), and fungi. Many pathogens are also able to thrive at these temperatures, although they have to compete with a great diversity of other microbial life and often don’t survive, as a result. Within the mesophilic universe, you have the greatest diversity of microbial life. Diversity makes for resilience and a highly-nutritious final product. Many gardeners will deliberately maintain low temperatures in their compost for this reason.

A pile is considered thermophilic between 113°F and 158°F. At these temperatures, aerobic thermophilic bacteria will flourish. Pathogens die off, which is good, but the overall microbial life is also reduced in diversity. There’s a trade-off. Thermophilic decomposition is very speedy and the high temperature reduces any risk of contamination in your finished compost. Many gardeners will deliberately engineer high temperatures in their compost for this reason.

You have choices!

Composts will only stay in the thermophilic range for a limited amount of time, of course. Once the material in your pile cannot be broken down any further, the thermophilic microbes will simply die off. The heat energy they were creating will begin to ebb. As a result, the temperature of the pile will begin to drop. As it does, mesophilic microbes will begin to repopulate. Microbial diversity will begin to rebuild.

This is why some gardeners who attain thermophilic temperatures will let their compost rest for long periods after it looks finished and the temperature has cooled. It allows for the microbes in their pile to “mature,” as in grow in numbers and diversity, which enriches the nutrient-delivering capacity of the final product.

If you’ve attained very high temperatures in your pile and you start to see a drop off, it can often be a signal that your compost is nearing completion. You’ll want to use both temperature and visual cues to determine this with certainty, though. A finished pile should look and smell like good, rich earth. There should be no big chunks of material remaining. If your temperature is dropping and you still have obvious chunks of undigested matter, you might be missing the correct ratio of inputs to support strong microbial activity. In those cases, adding a kick of high-nitrogen material can help give things a boost.

Think: manures, peeing on it, coffee grounds, juice pulp.

Most backyard composts will fluctuate between the mesophilic and thermophilic ranges as new material is added, the pile is turned, and the overall size of the heap grows and shrinks. This is normal and to be expected. And while piles can get hotter than 158°F, “hot” is not necessarily an ultimate good. At very high temperatures, you have less and less microbial diversity, and less diversity equates to a less nutritious final compost. 

If you want to try your hand at reaching thermophilic temperatures, you can do it by making sure you have the correct ratio of high-carbon to high-nitrogen materials and by building the pile all at once versus adding bits of stuff slowly, over time. You will also want to make sure your compost heap is the right size. Too small of a pile won’t retain the heat it creates. Too large of a pile may compact under its own weight and eliminate oxygen flow, which will also slow things down. You want to aim for 3x3x3 feet.

That’ll help you get things cookin’. Now send me your temps!

Love, Cass