Compost heat
A Socratic walk-through of compost heat — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why does a heap of kitchen scraps get hot in the middle?
Push a hand into the middle of a working compost heap and it is unpleasantly warm — warmer than the day, warmer than anything nearby. Nothing was lit, nothing was plugged in, and the same scraps sitting in a caddy on the counter merely go slimy and cold. So where is the heat coming from, and why does the caddy not produce it too?
Reasoning it through
REASONING #Take the first half first. What is actually happening in the heap? Bacteria and fungi are oxidising the carbon in those scraps — breaking sugars, starches and proteins down and combining them with oxygen. That is the same net reaction as burning, just carried out in a great many small enzymatic steps instead of one flame. Some of the energy released is captured as chemical work the microbes can use; the rest leaves as heat, because no energy conversion is complete. So the heap is not being heated. It is a slow, wet fire, and the heat is a by-product of every microbe in it eating.
Now the second half, which is the more interesting one. If the caddy holds the same food and the same microbes, why does it not warm up? Ask where the heat generation happens and where the heat loss happens. Generation happens wherever there is material — so it scales with the volume of the heap. Loss happens at the boundary with the cold air — so it scales with the surface. Double the width of a cube and its volume goes up eightfold while its surface only goes up fourfold. The bigger heap makes disproportionately more heat than it can shed.
Follow that to its consequence. There is a size below which any heap, however active, leaks heat as fast as it makes it and simply sits at air temperature; gardeners find that threshold empirically at somewhere around a cubic metre. Your kitchen caddy is two orders of magnitude below it. Nothing is wrong with the caddy's microbes — they are working, and the heat is real. It is escaping.
Once a heap does cross that threshold, something less obvious follows. As the core warms past roughly 40 degrees Celsius, the ordinary moderate-temperature organisms are outcompeted by heat-loving ones, which work faster still and heat the pile further. A core typically settles between 55 and 65 degrees. Why does it stop there rather than running away? Two brakes. Above about 70 degrees the organisms generating the heat are themselves damaged, so the fire puts itself out. And the easily digested carbon is finite — once the sugars are gone, what remains is lignin and other slow material, and the pile cools whether or not it is disturbed.
Which suggests a test: if heat depends on microbes eating fast, then anything limiting their rate should cool the heap. It does. Run out of oxygen in the middle and you get a cold, sour, anaerobic heap. Let it dry out and activity stops. And the ratio of carbon to nitrogen matters, because microbes need nitrogen to build the proteins that do the digesting — roughly thirty parts carbon to one of nitrogen is the practical target. Too much carbon and everything crawls; too much nitrogen and the surplus is lost as ammonia, which is exactly the smell of a heap with too many grass clippings in it.
The analogy
THE ANALOGY #Think of a crowd in a room. Each person gives off a little body heat continuously — nobody is trying to warm the place. A handful of people in a hall lose that heat to the walls as fast as they make it and the hall stays cold. Pack the same room and the temperature climbs noticeably, because the number of people making heat has grown faster than the wall area losing it.
People warm a room at a steady rate and do not multiply because it is warm, whereas the microbes both respond to the rising temperature — one population replacing another as it climbs — and eventually exhaust the food that keeps them going, so a heap's warmth rises, plateaus, and fades on its own.
Clarifying the model
THE MODEL #Three refinements are worth holding onto.
The first is that "compost gets hot" describes a phase, not a state. A heap runs through a sequence: warming, a hot thermophilic stretch, a decline as the easy carbon runs out, and a long cool curing period during which the slower material is worked over. Turning it partway through mixes the cold outer shell into the hot core and restores oxygen, which is why a turned heap heats a second time — not because turning adds energy, but because it hands the microbes new substrate and air.
The second concerns what the heat is for, which is a question the biology does not ask. Sustained temperature does kill many weed seeds and human pathogens, which is why waste-treatment standards specify holding a temperature for a time. But that is a use we make of the by-product: a heap will decompose perfectly well without ever getting hot — cold composting is slower and less sanitising, not broken.
The third is a caution about the size rule. A cubic metre is a rule of thumb, not a constant: an insulated bin, a wet still climate, or a very rich nitrogen source will heat a smaller volume, while a windy, cold, coarse heap may not heat at a larger one. The surface-to-volume argument tells you which way each factor pushes; it does not give you a threshold you can quote.
A picture of it
THE PICTURE #How to readStart at the arrow entering Mesophilic, the cool stage every pile begins in. The heap moves on only when the labelled condition on an arrow holds — so a small heap never leaves the first state at all, because that first condition is exactly the surface-to-volume argument. The back-edge from Stalled to Thermophilic is the effect of turning the pile: it returns the heap to a state it had left. Nothing forces the heap forward; each arrow is a change in the material, not a step in a schedule.
What became clearer
WHAT CLEARED #The heat is not evidence of anything unusual: it is the ordinary waste energy of respiration, present in every rotting apple. What makes a heap hot is not the biology but the geometry — a shape that generates heat throughout its bulk while losing it only at its skin. And the temperature is self-limiting from both ends, capped by the heat tolerance of the organisms producing it and ended by the exhaustion of the food that fed them.
Where to go next
ONWARD #- Why anaerobic decomposition produces methane and much less heat, and what that means for landfills.
- How the same surface-to-volume argument sets the minimum size of a warm-blooded animal.
Key terms
TERMS #| Term | What it means |
|---|---|
| Thermophilic | describing organisms that grow best at high temperatures, roughly 45 degrees Celsius and above; they dominate a compost core once it warms. |
| Mesophilic | describing organisms favouring moderate temperatures; they start a heap and recolonise it during curing. |
| C:N ratio | the mass ratio of carbon to nitrogen in the feedstock, with about 30:1 the practical target for fast decomposition. |
| Curing | the long, cool final stage in which slower compounds such as lignin are broken down and the material stabilises. |
Every term the collection defines is gathered in the glossary.