Making Compost: Steering the Thermal Biology of the Heap

A Compost Heap Does Not Rot, It Breathes

What happens inside a heap is nothing like passive decay. Bacteria, fungi and actinobacteria consume the material the way any living thing does: they oxidise carbon to draw energy from it, and take up nitrogen to build their proteins and enzymes. Only part of the energy released goes into their growth, the rest leaves as heat. It is that waste heat, produced by billions of cells in a confined volume, that pushes a heap to temperatures nothing outside it supplied. A composter is a biological reactor, and its temperature is the dial that tells you whether the population is working.

The condition for all of this is oxygen. Composting micro-organisms are aerobic, and the working figure is that they need more than 10 percent oxygen in the air held in the pores to do the job properly. As soon as oxygen runs short, the material does not stop: another flora takes over, anaerobic, which ferments instead of oxidising. It produces organic acids, sulphur compounds and methane, gives off very little heat, and works far more slowly. That is exactly what we call rotting. Composting and putrefaction are not two degrees of the same phenomenon, they are two metabolisms competing for the same material, and oxygen availability is what decides between them.

One practical consequence follows immediately: the products sold as compost activators or accelerators are useless. The micro-organisms involved are already present in astronomical numbers on every piece of organic matter, on peelings as much as on fallen leaves, and a properly built heap has no need of any more. What starts a heap is its physical and chemical conditions, not its inoculum. If you really want to seed it, a shovelful of finished compost or garden soil does the same job and costs nothing.

The Carbon/Nitrogen Ratio: The Only Setting That Really Decides

The carbon/nitrogen ratio, or C/N, is the amount of carbon available for each unit of nitrogen in the mix. Aim for a ratio of about 30 to 1 by mass, with a working range of roughly 25 to 35. The mechanism is simple: microbes consume far more carbon than nitrogen, and that proportion is theirs, not ours. Below 25 to 1, nitrogen is in excess of what they can take up, and the surplus leaves into the air as ammonia: the heap stings the nose, and the nitrogen you thought you were bringing to the garden evaporates. Above 35 to 1, nitrogen becomes the limiting factor, the population plateaus, the temperature does not climb and breakdown drags on for months.

Sorting materials into greens and browns is a popular translation of that ratio, and it is a translation that betrays you exactly where it most needs to be right, because it rests on colour. Coffee grounds are the textbook case: brown, dry to the touch, they look like everything we file under carbon, when their C/N sits around 20 to 1, with about 2 percent nitrogen. They are a nitrogen input, to be treated like peelings. Nor are they acidic, by the way: the grounds, once the coffee has been brewed, come out between pH 6.5 and 6.8, close to neutral, the acidity having gone into the cup. Fresh grass clippings are the other badly judged material, in the opposite direction: they are taken for harmless filler when they sit between 9 and 25 to 1, around 17 on average, which makes them one of the most nitrogen-rich and most unbalancing materials in the garden.

In practice nobody weighs anything. You think in volumes, and the common marker of two to three volumes of carbon-rich material to one volume of nitrogen-rich material works, on one condition: that the carbon material is fallen leaves, straw or fresh branch chippings, which sit broadly between 40 and 150 to 1. With corrugated cardboard, at close to 560 to 1, that marker becomes wrong and leads to a heap frozen for months: a far smaller volume is enough to correct the same nitrogen excess. The heap then tells you itself where it stands, and it says so above all through smell and temperature.

Two inputs deserve a note of their own. Eggshells supply neither usable carbon nor usable nitrogen and do not break down on the timescale of a compost heap: they come out whole, and they have to be ground fine for their calcium to be of any use one day. Wood ash is a pH corrector, not a carbon material: beyond about 5 percent of the heap volume, it drives the pH up and speeds the loss of nitrogen as ammonia, which is exactly the opposite of the intended effect.

The Four Thermal Phases, and What Each One Destroys

A properly built heap follows a reproducible temperature curve, and each segment of that curve matches a different microbial population and a different job.

The mesophilic phase covers the first hours to the first days. Micro-organisms that live at ordinary temperatures attack whatever is immediately available, soluble sugars, starches, simple proteins. They produce heat, and that heat gradually eliminates them. It is also the phase where the pH falls, often to around 5 to 6, under the effect of the organic acids released: a young heap is acidic, and that is normal.

Beyond around 40 °C, a thermophilic flora takes over and the most spectacular phase begins. The useful plateau lies between 50 and 65 °C. This is where cellulose, hemicelluloses, fats and complex proteins are attacked, which is to say the bulk of plant structure. It is also the only phase that sanitises: from 55 °C held onward, a good share of human and plant pathogens is destroyed. Above about 65 °C, the process limits itself in a counter-intuitive way: the heat kills the micro-organisms producing it, the population collapses and breakdown slows. A very hot heap is not a high-performing heap, it is a heap sterilising itself of its own workers. During this phase the pH climbs sharply, up towards 7 to 8.5, and beyond 8 nitrogen starts escaping as ammonia.

Then comes cooling, once the easy substrates are exhausted. The temperature falls back, and mesophiles recolonise the heap from the outer edge, which never heated and served them as a refuge. Then curing, the long, silent phase almost everyone skips: several weeks to several months at ambient temperature, during which fungi attack the lignin, worms, woodlice and springtails mix and break up the material, the C/N falls, humic substances form and phytotoxicity disappears. This phase cannot be sped up with heat: it is time, and nothing else.

Hence the most useful tool in the whole setup, and the least common: a long-probe thermometer, pushed into the core of the heap. It turns an opaque pile into a readable process, and it tells you when to turn far better than any calendar.

Why a Small Bin Never Heats Up

Heat is produced throughout the volume of the heap and lost through its surface. Those two quantities do not scale at the same rate: halve the dimensions and the volume is divided by eight while the surface is divided only by four. A small heap therefore offers proportionally far more exchange surface for the same density of heat production, and it cools faster than it warms. This is not a matter of adjustment, it is a matter of geometry, and no mix, no activator, no turning makes up for it.

The practical threshold used everywhere is around one cubic metre, that is a heap one metre on each side, to reach and above all to hold thermophilic temperatures. Below that you may get a spike of warmth, never a plateau. You also need to understand where the reactor actually is: the outer 15 to 25 cm of the heap barely work at all, they act as thermal insulation and stay close to air temperature. A 300 litre bin filled to a third therefore does not have a small hot core, it has nothing but insulation. And a balcony composter of a few dozen litres is physically incapable of heating up, whatever care goes into the mix.

The opposite excess exists too. Commercial windrows are kept between 1 and 3 m high and 3 to 4 m wide: beyond that, the material compacts under its own weight, air no longer moves through the lower layers, and the base of the heap turns anaerobic while the core overheats.

The last factor is the feeding pattern, and it is the one that condemns the most home composters. A bin fed a handful a day never gathers enough fresh substrate at one moment to trigger a temperature rise: each addition is diluted into a mass already digested. The same annual quantity, stockpiled separately then built in one go, heats up. That is the whole principle behind the fast methods, including the one developed at the University of California, Berkeley: a heap of at least one cubic metre built in a single session, a C/N close to 30, wrung-out-sponge moisture, a first turn on the fourth day then a turn every two days, for compost in about eighteen days. Nothing magic about it, only the four variables held at the same time.

Moisture, Oxygen, and What Turning Actually Does

Water is the living medium of the micro-organisms, and it is also what deprives them of air. The optimum range lies between 50 and 60 percent moisture. Below 30 percent, bacterial activity stops: a dry heap is not dead, it is paused, and it will restart with the first rain. Beyond 65 percent, water fills the pores, oxygen no longer diffuses and the heap goes anaerobic, with the smells and the slowness that come with it, plus a loss of nutrients through leaching.

The squeeze test replaces any measurement. Take a handful of material from the core of the heap and squeeze hard. It should hold together in a ball and glisten slightly, releasing at most a drop or two. If water runs between your fingers, it is too wet. If the handful crumbles as soon as you open your hand, it is too dry. Correcting a heap that is too dry is not done by watering it from the top: the water carves preferential channels and comes out at the bottom without wetting the mass. You moisten layer by layer, while building or while turning.

Oxygen, for its part, is managed through structure, not with a fork. What keeps air channels open is coarse, rigid material: twigs, hollow stems, chippings, cardboard crumpled rather than laid flat. Particles that are too fine increase the surface offered to microbes, but they pack down and close off the air passage, and the gain is quickly cancelled out.

Which brings us to turning, whose point is almost always badly explained. The oxygen introduced by a pass with the fork is used up within hours: this is not aeration in any lasting sense of the word. Turning earns its keep through three other effects, and they are considerable. It restores the porosity lost to settling and breaks up compacted pockets. It exposes material still intact, which usually triggers a fresh temperature peak. And above all, it brings the outer shell that never heated into the hot core: that is the only way to run the whole mass through the sanitising phase, and it is why the American regulation requires a minimum number of turns on top of a temperature target, where its European counterpart prefers to extend the required duration. Turning is not stirring the top, it is moving the heap and putting inside what was outside.

One safety point the guides almost always leave out: turning a heap puts spores into the air. The thermophilic moulds that do the work, Aspergillus fumigatus foremost among them, and the actinobacteria responsible for that damp-earth smell disperse in a cloud as soon as you disturb dry, dusty material. In most people this causes nothing at all, but respiratory irritation and hypersensitivity pneumonitis have been documented after nothing more than a spell of gardening, and the risk becomes serious for someone with severe asthma or a weakened immune system. The move that settles almost all of it is to moisten the heap before turning rather than after, and to stand with your back to the wind. An FFP2 mask is justified for those whose health puts them at risk.

Climate shifts the moisture setting, in both directions. In a very wet maritime climate, a heap left uncovered soaks up water from above, packs down and leaches: cover it with a breathable tarpaulin or a thick layer of straw, without ever sealing it. In a dry climate, or one with continental summers, the problem is exactly the reverse, an open heap dries out until it stops: it needs watering at every turn and a shaded spot. No fixed rule holds from one climate to another, only the squeeze test settles it.

Hot Compost Against Cold Compost: What Survives

Both approaches produce compost. The difference is not in the quality of the material you end up with, it is in the fate of whatever problematic thing went in, and incidentally in the time it takes.

For weed seeds, everything depends on heat and moisture. The reference work on thermal seed mortality gives clear orders of magnitude: at 60 °C, the seeds of all six species tested were dead within three hours or less, and at 70 °C within forty minutes at most. Those figures come from laboratory trials on batches of seed brought to temperature in a thermostatic bath, not in a heap: they give the order of magnitude, not a guarantee in the field. Against that, a dry seed holds out far longer than a moist one, and the dry pockets of a heap serve as its refuge even in hot composting. In a cold heap, which never exceeds ambient temperature, no seed is destroyed: composting weeds that have gone to seed then amounts to sowing them along with the compost.

For disease agents, the commonly accepted threshold is 55 °C held. The regulatory standards give the measure of what held means, and they do not go about it the same way on either side of the Atlantic. The American standard, applied to sewage sludge as well as to organic certification, calls for 55 °C held for three days in a closed or aerated system, or 55 °C held for fifteen days in a windrow with at least five turns during that period. The European regulation imposes no number of turns but demands more duration, and offers a choice of four pairings: 70 °C for three days, 65 °C for five days, 60 °C for seven days, or 55 °C for fourteen days, with the material regularly mixed or ventilated. The figure to remember is therefore not the 55, which plenty of heaps reach for an afternoon, it is the pairing of temperature and duration and the fact of running the whole mass through it.

What survives anyway deserves naming, because that is where the bad decisions get made. The resistant structures of fungi and protists, sclerotia and resting spores, are of another order of toughness than ordinary spores. A six-month home composting trial completely eliminated tobacco mosaic virus but did not eliminate clubroot, whose resting spores came through the process; in the laboratory it takes about 96 hours at 54 °C, or 24 hours at 65 to 75 °C, before they can no longer be detected, conditions a garden heap rarely holds everywhere at once. The other resistant category is the eggs of parasites present in carnivore faeces.

One last consequence, often ignored, shows clearly what heat buys you. Under the American organic farming standards, a compost that meets those temperature and turning conditions can be used with no waiting period before harvest at all, while raw manure has to be incorporated at least 120 days before harvesting a crop whose edible part touches the soil, and 90 days in other cases. It is the clearest statement there is of what the thermophilic phase actually delivers.

Finally, caution is in order with raw kitchen waste, which can carry foodborne bacteria such as Salmonella and Escherichia coli. A heap that never heated offers no guarantee of destruction, and regrowth after cooling has been documented even in composts that did heat. One more organism is worth knowing about, because it comes not from the waste but from the compost itself: Legionella longbeachae lives naturally in potting mixes and composts, and it is the leading environmental cause of legionellosis in Australia and New Zealand. Contamination happens by breathing in the dust or by putting hands to mouth. In a counter-intuitive finding established by the New Zealand investigations, neither masks nor gloves showed any protective effect, whereas handwashing did. The sensible approach does not mean giving up on peelings: wash your hands after handling the heap, do not spread cold or young compost on the surface around lettuces and leafy vegetables close to harvest, and work it into the soil well before planting rather than laying it around crops already growing.

What Does Not Go In, and the Real Reason

The list of forbidden items circulates from site to site without anyone asking why again, and it mixes copied legends with real problems, each with its own cause.

Start with the legends. Citrus carries two accusations: its acidity, and the limonene in its peel, an antimicrobial. The acidity is neutralised during the process, which moves precisely from an acidic pH to a neutral or slightly alkaline one; limonene is genuinely active at high concentration, but one peel is a negligible fraction of the volume of a heap and the compound evaporates quickly once the skin is opened. The only well-founded objection is mechanical: the waxy cuticle slows breakdown in a cold heap, which cutting it up solves. Onion and garlic are nothing special either, the reservation comes from worm composting, where worms avoid strongly scented zones in a confined space. Bread breaks down perfectly well; the problem is not one of composting but of easy food left at snout height, so a question of burying it, not of banning it.

Then come the real problems, and they are not alike. Dog and cat faeces are a health issue, not a smell issue: they can contain parasite eggs, notably Toxocara and the oocysts of Toxoplasma gondii, known to be resistant and not reliably destroyed by the conditions of a garden heap. They have no place in compost destined for the vegetable garden. Meat, fish, dairy and fats pose two distinct and compounding problems: they attract rodents and flies, and their density in protein and lipids tips the zone locally into anaerobic putrefaction, with the fats also coating the particles and blocking the exchange of water and air.

Diseased plants call for finer reasoning than yes or no. What counts is not the disease but its survival structure. A leaf fungus with fragile spores is destroyed even in a lukewarm heap. A resistant structure is not: sclerotia and resting spores come through home composting, as the clubroot case shows, which means the roots of affected cabbage, cauliflower, broccoli or Brussels sprouts have to leave the circuit. Weeds that have gone to seed follow yet another logic: it is not the plant that is the problem, it is the seed, and it only dies to heat. The roots of creeping perennials, couch grass, bindweed, horsetail, follow a third logic: every fragment is capable of regrowing, and a cold heap stores them intact until you redistribute them around the garden. Gardeners deal with them beforehand: several weeks drowning in a closed bucket of water, or smothering in an opaque bag until they are unrecognisable, before they join the heap.

Recognising Finished Compost, and the Cress Test

Two distinct ideas hide behind the word maturity, and confusing them costs crops. Stability means the end of intense microbial activity: the material no longer consumes oxygen in quantity and no longer gives off heat. Maturity means the absence of phytotoxicity, that is the product’s fitness to sit alongside living roots without hindering them. A compost can be stable without being mature.

The field signs are well known: an even dark brown colour, a crumbly structure that breaks apart between the fingers, a smell of woodland floor rather than of ammonia or sourness, original materials become unrecognisable apart from the resistant ones, shells, stones, twigs. The volume has usually shrunk by half or more. The forest-soil smell comes from the geosmin produced by actinobacteria, and it is a good signal: a sharp or sour smell says the opposite.

Two tests go further than a glance, and both can be done at home. The self-heating test consists of filling a well-insulated container, a vacuum flask or a lined bucket, with compost brought back to the right moisture, and following its temperature over a few days. The standardised protocol counts a compost as stabilised when it rises no more than 20 °C above ambient temperature. Watch out for a documented trap: compost that has been dried, refrigerated or frozen heats up artificially in the first days after rewetting, which leads to a wrong verdict of instability. Let it come back to equilibrium before testing.

The cress test is the second, and it is the one that detects phytotoxicity, which no visual inspection does. It is a bioassay described by Zucconi and colleagues in the early 1980s, since become the reference. You sow a set number of garden cress seeds, Lepidium sativum, on the compost or on its water extract, and the same number on an identical support moistened with clean water, which serves as the control. After a few days you count the germinations and measure the length of the rootlets in both series. The germination index is the product of the relative germination rate and the relative root elongation. Below 50 percent of the control, the compost is clearly toxic. It is considered mature from 80 to 90 percent. Cress is used because it germinates fast and reacts strongly, which makes the verdict readable within a few days.

Using a compost that is too young does two different kinds of damage. The first is the ammonium nitrogen still present, phytotoxic at high concentration, which burns root tips and the edges of young foliage. The second is more insidious: if the residual C/N stays high, soil microbes carry on the digestion in place and draw on the soil’s mineral nitrogen to do it, the very nitrogen the crops were expecting. The plants yellow and stall, and the gardener concludes there is not enough compost. The risk of immobilisation rises as soon as the C/N goes above 20 to 30 to 1.

A mature compost, finally, is a slow-release soil improver, not a fertiliser: only something like 10 to 25 percent of its nitrogen becomes available in the first year, then a few percent a year in the years that follow. It is worked into the top few centimetres of the soil or laid down as a mulch. Hungry crops make the best use of it, tomato, courgette, pumpkin, cucumber, aubergine, pepper, artichoke, rhubarb, leek, celery, chard, spinach and the brassicas. Root vegetables such as carrot, radish and beetroot are better sown on a bed improved the previous season. Bean and broad bean fix their own nitrogen and ask for none. Herbs from dry climates, thyme, rosemary, lavender, sage, dislike soil that is too rich and too moist. And since mature compost usually comes out neutral to slightly alkaline, it is not the amendment to reserve for acid-lovers such as blueberry or hydrangea. In the orchard and among the soft fruit, raspberry, redcurrant, blackcurrant, strawberry, grapevine and rose take readily to a surface mulch of mature compost.

The Failures, and What They Say About the Heap

Every classic failure is one of the four variables outside its range, and identifying which one avoids the corrections that make things worse.

It stinks, first case: a sharp, aggressive smell of ammonia. That is an excess of nitrogen, a C/N too low, often together with a pH above 8. The smell is literally the escape of the nitrogen you wanted to keep. Work in carbon-rich, structural material straight away, torn cardboard, fallen leaves, chippings, and hold off on grass clippings and peelings for a while. Above all do not lime it: lime would push the pH up and speed the loss.

It stinks, second case: a smell of rotten egg, of pond mud or of sewer. The cause is exactly the reverse, it is anaerobic conditions. The heap is too wet, too packed, or both. Rebuild it with coarse, dry material, breaking up the compacted layers. Gardeners report these two smells in the same way and they call for opposite corrections: it is the single most rewarding distinction in the whole diagnosis.

It does not heat up: five checks, in this order. Volume, below one cubic metre it is lost from the start. Moisture, below 30 percent everything stops. Nitrogen, a heap of leaves alone at 40 to 80 to 1 will stay cold for months. The feeding pattern, a handful a day never triggers anything while the same mass built in one go heats up. And finally the most ordinary possibility of all, a heap that heated three weeks ago is not broken, it is in its cooling phase, and that is the normal course of events. None of these five causes is fixed by a shop-bought activator.

It is soaked: water is coming in faster than it leaves, or the heap has no drainage. Rebuild it on a bed of coarse woody material that lets the water run through, cover it without sealing it, and work in absorbent carbon material, torn cardboard or dry leaves. In a very wet climate this is not an incident but a permanent constraint to handle from the design of the heap onward.

Rodents: they come looking for two things, concentrated food and dry, warm, undisturbed shelter. The two levers match. Never leave meat, fish, dairy, fats, bread or cooked leftovers on the surface, and systematically bury each fresh addition under a layer of brown material. And remove the shelter value: a heap turned regularly and kept properly moist does not make a good nest. A metal mesh with openings under a centimetre placed beneath the composter blocks access from below. The presence of rodents is also an indicator in itself, it usually signals a heap that is too dry and never disturbed.

Clouds of small flies: these are fruit flies and fungus gnats, which lay on sugary, moist material left out in the open. They are harmless but multiply very fast. The correction is purely mechanical: bury each fresh addition under a good layer of dry brown material, and to make that gesture a reflex, keep a stock of fallen leaves or torn cardboard next to the composter. A cider vinegar trap with a drop of washing-up liquid brings the adult population down while the cause is being fixed.

An ant nest set up in the bin: that is a signal of dryness. Ants colonise dry, stable material, not an active heap. Moisten it and turn it, and they leave of their own accord.

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