Heat, ratio, moisture and time — and what the thermometer cannot certify
Heat, ratio, moisture and time — and why the thermometer cannot tell you it is safe
Temperature is not something you add to a compost heap. It is the readout that tells you whether the four things you do control — carbon-to-nitrogen ratio, moisture, oxygen and particle size — are set right. This guide gives the measured range for each, and is honest about what a thermometer in the middle of a heap can and cannot certify.
Written by Abdelmounim El Moussaddar · published by TopDigitalKnowledge · 49 pages, 12 figures, 38 sources, every one cited inline · last verified September 2026
A heap makes heat throughout its volume and loses heat through its surface. Whether it can hold what it makes is therefore a question about the ratio between the two — and that is arithmetic, not opinion. Put your own heap in.
Treated as a box, which is close enough: the ratio is what matters, not the exact shape.
Convention
The verdict band uses the usual one-cubic-metre
minimum. That figure appears in every advisory source and in no experiment this
book could locate, so it is craft convention. The arithmetic above it is exact.
Halve the side of a cube and the volume falls to an eighth while the surface falls only to a quarter — every unit of heat-making material now sits behind twice as much heat-losing area. That is why the same ingredients that work in a big heap do nothing in a small one, and why a 220–litre plastic bin was never going to reach 55 °C.
55 °C for three days is in every guide and written into regulation. Salmonella, E. coli O157:H7 and Listeria have been recovered from compost held above it for more than eight days, and Salmonella Typhimurium survived at least nine days at 60–70 °C.
Pathogens warmed for an hour at 47.5 °C first, then held at 55 °C, lost 1.2 log where unwarmed controls lost 4.0. About a thousandfold difference, at exactly the temperature the rule specifies. Every heap has a warm-up, and the longest ones belong to the worst-built heaps — so a heap that struggles to heat spends that time hardening the organisms the heat is meant to kill.
McGauhey and Gotaas tested 20:1 to 78:1 and found 30–35:1 fastest. Later work found the least nitrogen lost at 25:1 — a different number — and that every ratio from 20:1 to 30:1 matured at the same 80 days. The precision buys retention, not speed.
Grinding roughly doubles the decomposition rate, and the same grinding closes the pores that carry the air — narrow pores fill by capillary action, so fine material goes anaerobic where coarse does not. There is no setting that maximises both, which is why “shred everything finely” is wrong for a garden heap.
A heap makes heat in its volume and loses it through its surface. For a cube of side L that ratio is exactly 6/L. Halve the side and every unit of heat-making material sits behind twice as much losing area. Try it on the widget above with your own heap.
A page of the book lists the questions the research could not answer: whether cold compost differs from hot in anything measurable, what a turn actually costs in nitrogen, and where the minimum heap size figure really comes from. Craft convention is labelled as convention throughout, not dressed up as measurement.
Only if you are genuinely running a hot heap with manure in it and intend to turn on what it tells you. Otherwise it measures the one thing you cannot act on directly — temperature is an output, not an input — and it mostly produces anxiety. The four settings you can change are ratio, moisture, air and particle size, and the book is about those.
No, but it is honest about what a small bin can and cannot do. A common plastic bin holds 220–330 litres, which is a fifth to a third of the conventional minimum for a hot process. That is not a defect — it is a cold composting vessel, it still makes perfectly good compost, and the book tells you how to run one rather than pretending it will reach 55 °C.
Yes, as what they actually are. Above about 35 °C the earthworms die, so a worm bin has no hot phase and cannot sanitise; bokashi is anaerobic fermentation, which makes it a pre-treatment rather than composting. Both are useful. Neither is a gentler version of a heap, and treating them as one is where people come unstuck.
Metric first, with the other unit in brackets, and written for an international audience. Where a figure exists only inside one country’s regulation, the country is named — because a rule is not a fact about compost.
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Temperature is not something you add to a heap. It is what the heap gives back when the four things you control are right — and it is the worst possible certificate of safety.
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TopDigitalKnowledge · written by Abdelmounim El Moussaddar · last verified September 2026
Every number in this guide is traced to a named, published source.
Read the free article on why heaps do not heat, or see the rest of the guides.