Why Does Compost Get Hot? (Microbes at Work)

why does compost get hot

If you’ve dabbled with composting for any length of time, you’ve probably realized something by now.

Your compost is getting hot.

And as it rots, it gets hotter.

In fact, depending on the method used, your pile can reach pretty impressive temperatures!

But where does all that heat come from? Why does compost get so hot?

Why does a compost heap get hot?

The composting process relies on the microbial activity of millions of tiny organisms that recycle organic matter. As these microbes feed on waste materials, they produce different by-products: a humus-rich material we call compost and heat.

There’s a hugely diverse range of bacteria and fungi responsible for decomposition. These microorganisms use organic materials to fuel their growth and reproduction. As they feed, they also release heat which in turn warms up the mass of compost.

As their numbers grow, they spread out in the compost, and the heat increases. 

But not all microorganisms like hot conditions!

As the temperature rises, the type of microbes responsible for starting the rotting process is replaced by another variety.

In aerobic composting, microbial activity goes through three phases of decomposition, with different microbes working at different temperatures:

  • Cool temperature microbes (psychrophiles)
  • Mid-temperature microbes (mesophiles)
  • High-temperature microbes (thermophiles)

Note that we’re talking about aerobic composting, where oxygen and airflow are used as part of the process. Anaerobic decomposition, which occurs in the absence of oxygen, is not something most gardeners aim for. This is because it is slower produces some pretty nasty smells and by-products that can be harmful to plants.

Psychrophiles get the ball rolling at low temperatures. They can still be effective at temperatures as low as 28°F (-2°C) but are most happy at around 55°F (13°C). Beyond these temperatures, they begin to be replaced by mesophiles.

This cool-temperature stage is also the time when lots of creepy crawlies join the feast! But as soon as things get too hot, they move out.

Mesophiles are active at a mid-temperature range between 50-115°F (10-46°C). In fact, most of the decomposition that happens in a home compost pile is mesophilic. At this stage, your compost heap is rotting at a pretty good rate.

If you’re lucky enough to see your compost get even hotter, the subsequent invasion of microbes will be thermophilic. These microorganisms are active at temperatures between 115-160°F (46-71°C). High-temperature thermophilic composting like this is what gardeners call “hot composting.” It is considered the most effective decomposition state for producing compost quickly.

stages of aerobic composting

You can keep track of your compost temperatures using a simple compost thermometer. Get one with a long probe like this one. (Amazon)

Should my compost be hot?

should my compost be hot

Compost doesn’t need to be hot” to be effective. Decomposition occurs at a wide range of temperatures. But the goal of most gardeners is to produce compost rapidly, which generally occurs in hotter conditions.

You may well ask yourself what temperature you should be aiming for?

In reality, not many backyard compost systems reach thermophilic temperatures. Hot composting is often considered the “holy grail” of composting because it’s said to be the most efficient. But this is not always practical to do.

If you manage to push your compost into the thermophilic range, that’s great. But don’t be disappointed when the temperature drops after a short period.

Some biologists argue that mesophilic bacteria are just as efficient as thermophiles. 

How long does compost stay hot?

A well-designed compost system will heat up to mid-temperatures of around 104-122°F (40-50°C) within 2 to 3 days. 

Next, it could remain at this medium temperature range for up to 15 days and continue to decompose thanks to the mesophilic bacteria.

With a “hot” compost method, the temperature will continue to rise after the initial warming process of 2-3 days. Then, it will move into the thermophilic range (115-160°F or 46-71°C). It can remain in this hot state for around another 12 days.

So on average, the heating period lasts only for the first 2 weeks or so. After which, it will lose heat as microbial activity drops. This is the maturation stage of decomposition. 

This phase is essential to the production of finished compost. Mesophilic organisms take over the long, slow curing process and are joined by fungi and actinomycetes (part bacteria, part fungus).

Can compost get too hot?

Thermophiles raise the temperature in compost to about 160°F (71°C), where it usually stabilizes. Above these temperatures, the pile becomes too hot. As a result, microbial activity will slow down, and the compost becomes inert.

If temperatures continue to rise, this can harm the beneficial microbes until they stop performing and eventually die.

High temperatures help destroy pathogens such as plant diseases and kill weed seeds. In fact, this is the principal advantage of hot composting methods.

But even if you carefully monitor your compost pile (turn at the right time, add fresh materials and check moisture levels), the highest temperatures will only last a few days.

Lots of things affect temperature change. For example, when moisture levels are higher (but not too high), temperatures heat up more than in dry piles. The size of the pile also counts, with bigger piles retaining heat better than smaller heaps. Turning the pile will dissipate heat.

And even the outside temperature can influence microbial activity and, therefore, how hot a pile becomes.

Effect of temperature on composting

effect of temperature on compost

Microbial activity generates heat, which warms the compost. But ambient temperatures around the compost also impact how hot it gets and consequently the decomposition rate. Warmer temperatures will favor mesophilic or even thermophilic action.

It can be helpful to employ methods to increase ambient temperatures for improved composting conditions in certain circumstances. 

For example, compost needs sun to help warm the composting materials in the winter. This is why many bins and tumblers have a dark color to encourage the absorption of solar energy.

If you live in a climate where ambient temperatures are often cool, you might get some benefit from insulating your compost. Or you could use a specially designed insulated tumbler like the Mantis back porch tumbler ​(Amazon).

How to cool down compost

In rare circumstances, you might consider that compost is heating up too much. In that case, there are a couple of steps you can take to cool down compost and control the temperature.

The first and most effective method is to turn the pile. Turning aerates the materials. By increasing airflow, you can also help heat dissipate and cool down a pile’s internal temperature.

Temperatures can rise again after a few hours, so leave the compost to settle and check the temperature using a long probed compost thermometer. (Amazon)

As well as turning the pile, you should monitor the ratio of nitrogen to carbon materials. Adding more dry carbon matter can help slow down activity and lower temperatures.

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  1. Hi Anthony. I share your passion and thank you for your interesting explanation.
    I am wondering what I can do with all the heat from my 1m3 compost bin?

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