what’s the perfect temp for compost heaps

  • By: composthq
  • Date: November 5, 2022
  • Time to read: 5 min.

Introduction

Temperature matters a surprising amount when it comes to getting your compost heap operating efficiently, but a lot of people aren’t sure how hot is too hot, or whether their heap still breaks down when it’s cold. In this article, we’re going to look at the different temperatures, and talk about finding the perfect temperature.

To operate at maximum efficiency, a compost heap should be between 135 and 160 degrees F. This is the best temperature for keeping the bacteria in the heap active and making sure that the compost breaks down quickly. It will also help to kill pathogens and weed seeds, making your compost more useful when you put it on the garden.

In this article, we’ll be checking out how warm a compost heap should be, and how the temperature affects the heap’s behavior. We’ll also find out what happens if your compost heap gets too hot, and how to cool it down – as well as how to raise the temperature of a cold heap.

How Hot Should A Compost Heap Be?

Ideally, your compost heap should be somewhere between 135 and 160 degrees F. At this temperature, it will be filled with the most effective kind of bacteria, known as thermophilic bacteria. These will break the organic waste down particularly quickly to make compost that’s garden-ready in just a few weeks.

If your compost heap is cooler than this, between 50 and 115 degrees F, it will be filled with mesophilic bacteria. These still break down waste, but much more slowly, so you’ll have to wait a lot longer for your compost to be ready. Most heaps are pretty efficient at around 104 degrees F, but they’ll get much more efficient at 135 degrees F.

If your compost heap is significantly colder, it will still have beneficial bacteria in it – most likely psychrophiles. These are active to as low as 28 degrees F, but they tend to be much slower. They’ll be far more active at up to 50 degrees F, but cannot survive beyond this temperature, and are still less efficient at decomposing waste than mesophilic bacteria.

As you can see, your compost will break down if it’s at around 28 degrees F or higher, but it will decompose significantly more quickly as the temperature rises. You should therefore aim for higher temperatures, ideally 135 degrees F or more.

What Happens If Your Compost Heap Gets Too Hot?

You do not want your compost heap to go above 160 degrees F at the most, as the bacteria within the heap will start to die. You need those bacteria to break down the organic waste, and if they all die, you’ll have to wait for the heap to cool and new bacteria to form before the decaying process will continue.

There is also a small risk that your compost heap could catch fire if you let it get really hot, especially if it contains a lot of dry materials. You should therefore monitor the temperature of a large compost heap, particularly in the summer.

How Can You Cool A Compost Heap Down?

The best way to cool your compost heap down is to spread the pile out so the heat can escape. Compost traps heat very effectively, so reducing its size and height will release the warmth.

Be aware that just stirring your compost heap could increase the temperature, as being exposed to oxygen makes the microbes more active. However, if you take the top off the heap, some of the warmth will be able to escape, and the temperature will drop.

You can also soak the heap if necessary, but make sure you soak it thoroughly. Just sprinkling it with water will not significantly change the temperature, and could even make it hotter (again, it may increase the activity of the microbes).

What Happens If Your Compost Heap Gets Too Cold?

Being too cold can also be a major issue for your compost heap, and it’s a commoner one than excess heat. If your compost heap gets cold, the decomposition process will slow right down, or even stop. As it drops between the different temperature bands, the different kinds of bacteria will die.

For example, if your heap was at 140 degrees F but drops to 100 degrees F, the thermophilic bacteria will die, and be replaced by mesophilic bacteria. This will slow down the decomposition process. If it drops further, to below 50 degrees F, the mesophilic bacteria will be replaced by psychrophiles, which are even less effective.

Psychrophiles will stop operating at around 28 degrees F, and your compost heap will become inactive or almost inactive. Until it warms up, the waste in it will not decompose efficiently, so be aware of this if you need your compost quickly.

How Can You Warm A Compost Heap Up?

Having the right ratio of greens and browns is critical for temperature maintenance. Cold heaps tend to have too many browns (carbon-based ingredients such as straw, sticks, egg cartons, etc.) and not enough greens (nitrogen-based ingredients such as grass, food waste, etc.).

Add some greens to your compost heap and make sure it is damp enough. You should see a few drops of moisture if you squeeze a handful of it.

Next, start turning the compost heap regularly. This adds oxygen to the inside of the heap, and this will encourage more activity in the microbes, which will produce more heat as they break the food down. You can use a garden fork or even just a stick to stir and lift the material as often as possible.

You should also make sure your compost heap is big enough to hold some of the heat in. Piles that are smaller than 4 to 6 feet will often struggle to stay warm enough.

Conclusion

The perfect temperature for a compost heap is between 135 and 160 degrees F, as this will ensure you have plenty of active and efficient bacteria. Other temperatures will work, but your compost will decompose much more quickly if it’s warm!

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