A heap that cooks itself
Why Does a Compost Heap Get Hot?
Nobody heats a compost heap. The heat comes from the things eating it, and there are enough of them to steam.
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Step 1 of 5
Everything that feeds gives off a little warmth.
You do it yourself — that is why a crowded room warms up. A bacterium gives off a vanishingly small amount, but there are unimaginable numbers of them.
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Step 2 of 5
A heap is big enough to keep that warmth in.
A heap smaller than about a cubic metre never gets hot at all. A thin scattering of peelings loses heat as fast as it makes it. A cubic metre of heap has an inside that the cold cannot easily reach.
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Step 3 of 5
So the middle climbs to around sixty degrees, and that is useful.
At that temperature weed seeds and many plant diseases are killed, and heat-loving bacteria take over and work far faster than cool ones.
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Step 4 of 5
Keeping it hot means keeping it fed, damp and breathing.
It needs a mix of soft green material and dry brown, enough water to be like a wrung-out cloth, and air — which is why heaps are turned.
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Step 5 of 5
Without air it stops cooking and starts stinking.
Squash the air out and different bacteria take over, working slowly and giving off sour gases and methane. That is a landfill, not a compost heap.
Worms do the last part of the job. They move in once the heap has cooled, and what they leave behind is the crumbly dark stuff at the bottom.
The short version
A compost heap heats itself because billions of feeding bacteria give off warmth faster than a large heap can lose it — and staying hot needs air.
Try it yourself
Push a stick into the middle of a working compost heap and leave it a minute. It comes out noticeably warm.