Two processes run inside a compost heap, and you only get to choose one of them. The first uses oxygen and throws off heat. The second runs without oxygen and throws off a smell. Almost everything written about good composting is really about staying on the first path.
I have kept heaps for twenty-two years. Three wooden bays behind the shed, a tumbler by the kitchen door, and a four-bay line I help run on a neighbour’s smallholding. My failures were rarely about the brown-to-green ratio. They were about air.
Two Processes in the Same Heap
Aerobic composting is what the word describes. Oxygen-requiring bacteria and fungi digest the material, produce carbon dioxide, water and heat, and leave a stable dark crumb behind. The EPA defines composting as the managed, aerobic breakdown of organic material, and “managed” is doing a lot of work in that sentence. The EPA definition of composting makes that clear.
Anaerobic composting is what happens when the management stops. Same heap, same ingredients, different microbes. Without oxygen, bacteria that thrive in airless conditions take over. They still digest your waste. They just do it slowly, and they leave behind compounds your nose can detect from the far end of the garden.
No home heap is purely one or the other. This is the part most guides skip. A pile has an aerobic shell and can hold anaerobic pockets an inch inside it. A wet, compacted core goes sour while the outer six inches stays sweet. The practical aim is not purity. It is to keep the balance tipped toward air.
What the Temperature Bands Are Telling You
Temperature is the cheapest instrument you own for this job. Two bands matter.
The mesophilic band runs from about 10C to 40C. This is where a heap sits when it is ticking over on a mild day, and where it returns once the easy sugars are gone. Work is happening. It is just steady rather than fast.
The thermophilic band runs from 40C to 65C. Here the heat-loving bacteria take charge, and the pile climbs fast. A well-built heap reaches 40 to 50C within two or three days, which is exactly where Cornell’s temperature monitoring notes put it.
I keep a 60cm probe in the shed and take readings at three depths. In August 2022 I built a heap from horse muck, shredded cardboard and spent hops, then watched the core climb from 34C to 61C over four days. That is a heap doing aerobic work at full stretch. The reading two feet down mattered more than the one near the surface.
Above 65C the useful organisms start to die off, and the pile can go quiet in the wrong way. Below 40C, decomposition slows and the kill that hot composting gives you never happens.
Time to finish follows temperature. My turned heaps are usually ready in six to ten weeks. A heap I leave alone but keep aerated runs three to five months, and the EPA’s composting at home guide notes that an untended pile may take a year. A cold, wet heap that never heats can sit for a year and still not be finished.
Holding 55C Long Enough to Matter
Fifty-five degrees Celsius is the number worth memorising. It is 131F, and it is the line where pathogen reduction gets serious. EPA guidance states that pile temperatures must stay at 131F, or 55C, for several days to meet standards for reducing pathogens. US EPA, approaches to composting
In practice most regulated programs hold a windrow at 55C for three consecutive days. That three-day mark is a fair target for any gardener worried about dog waste, poultry litter or diseased plant material.
Note what the requirement actually is. Not a peak. A hold. A heap that spikes to 58C for an afternoon and then drops to 35C has not done the job that a heap sitting at 56C for three days does.
The same heat handles weed seed. NC State’s extension handbook says that at 140F to 150F, which is 60C to 65C, most weed seeds and pathogens are killed, according to the NC State Extension Gardener Handbook. That sits at the top of the thermophilic band, and it is why hot composting is the route to take if you are bringing in manure or spoiled hay.
Oxygen Below the Surface
You cannot see oxygen, so you read it indirectly. Aerobes need a steady supply. Below roughly 5% oxygen in the pore spaces, anaerobic bacteria take over and the whole character of the heap changes.
NC State’s large-scale composting guide puts the working minimum for an active pile at about 6%, with 16% to 18.5% as the comfortable range. NC State Extension, large-scale organic materials composting
Two things push a heap below that line. Water filling the pore spaces, and compaction. Cornell lists four common causes of anaerobic conditions inside a pile: excess moisture, inadequate porosity, a rapidly degrading substrate and an oversized pile. Cornell Composting, factors leading to anaerobic conditions Every one of those is a gardener’s mistake rather than a mystery, and every one is fixable.
The Cornell Waste Management Institute keeps a wider library of these operator sheets for anyone who wants to go deeper into oxygen transport and pore structure. Cornell Waste Management Institute
Moisture, Porosity and Airless Pockets
The target is 40% to 60% moisture by weight. Squeeze a handful. It should feel like a wrung-out sponge and leave a few drops on your palm, not a stream. NC State gives 40% to 65% as the workable band and 50% to 60% as optimal, and warns that past 65% the water displaces air from the pore spaces, which starts anaerobic conditions and odour. NC State Extension
Porosity is the partner to moisture. Shredded cardboard, twiggy prunings, woodchip and straw keep channels open for air. A heap of nothing but grass clippings collapses into a mat, and a mat has no channels. RHS makes the point bluntly: clippings on their own become a slimy, smelly mess, as the RHS composting guide notes. Mix them with browns.
Wilting grass is worth a word of its own. I let clippings lie in the sun for a day before they go in, and I add them thin. Fresh clippings straight from the mower, packed six inches deep, go anaerobic within forty-eight hours. I have tipped a bin on the third morning and found the bottom layer already black and slick.
A Smelly Bin Is Working, Just Not Aerobically
This is the assumption I most want to correct. A stinking bin is not broken. It is working hard. It is working anaerobically, and the smell is the signal rather than a side effect.
People catch that sourness and decide the bin is “cooking”. It is. It is cooking the wrong way. The compounds coming off an airless heap, including hydrogen sulfide, methanethiol, volatile fatty acids and amines, are products of oxygen-starved metabolism. Cornell’s odour pages list them directly. Cornell Composting, odour management A heap doing that is not digesting material faster than an aerobic one. It is digesting it slower and losing part of its value as gas.
The fix is nearly always the same. Turn it, add dry brown material, and let air back in. Illinois Extension’s troubleshooting page puts it in one line: a bad odour means not enough air, per the Illinois Extension troubleshooting page.
What the Smell Is Telling You
Learn to read the nose. Each smell carries a different message.
Earthy, like a damp forest floor, means a working aerobic heap near the end of its run. RHS describes finished compost as smelling like damp woodland. RHS That is the smell you want.
Sour, sharp or vinegary means acid anaerobic conditions, usually from too much wet green material packed tight. The volatile fatty acids responsible are the same family that gives silage its tang.
Rotting eggs or drains means hydrogen sulfide. That heap is badly starved of air and probably sitting in standing water.
Ammonia is the one people misread. A sharp, urine-like smell is not anaerobic by default. Cornell notes that ammonia forms aerobically as well as anaerobically, and that it signals excess nitrogen. Cornell Composting, ammonia odours Losing nitrogen as gas means losing the fertiliser value of your heap.
If the smell changes after you turn the pile, that tells you something too. A heap that smells clean by the next morning was short of air and is now fine.
Methane, Ammonia and Lost Nitrogen
Anaerobic decomposition carries a second cost: gases you cannot use. Landfill gas runs at roughly half methane and half carbon dioxide, and methane traps heat at least 28 times more effectively than carbon dioxide over a century. US EPA, basic information about landfill gas A home heap is a rounding error beside a landfill, but the chemistry is identical. Carbon leaves the pile as gas instead of staying in the compost.
Nitrogen is the other leak. Airless heaps shed it as ammonia, which is why they smell of urine and why the finished compost is often poorer in nitrogen than the material that went in. That is a real loss for anyone growing hungry crops.
You lose heat as well. A cold heap stays cold, so the temperature band that kills weed seed never arrives.
Leachate Needs Catching
Wet anaerobic heaps drip. The liquid that comes out is leachate, defined as liquid that has passed through unprocessed organic material. NC State’s handbook is careful about it: leachate may carry pathogens, phytotoxins and anaerobic microorganisms that can harm plants. NC State Extension Gardener Handbook
I keep a tray under the front of the wet bay. After a fortnight of rain one October it held nine litres of dark liquid that smelled of vinegar and drains. That is not compost tea. Compost tea is brewed from finished compost under aerobic conditions. Leachate is what drains out of raw, airless material.
Catch it, dilute it well away from edible beds, or send it to a drain. Do not tip it on the tomatoes.
Weed Seeds Survive a Cold Heap
Weed seeds are tough, and they need heat to break. NC State’s handbook repeats the figure: 140F to 150F kills most weed seeds and pathogens, while a slow or cold process leaves them intact. EPA guidance says the same, that below the optimum range weed seeds and pathogens may remain in the finished compost. US EPA
That has a practical consequence. An anaerobic bin is a seed bank being stored, not destroyed. I once spread compost from a cold, neglected bay across a bed and got a carpet of fat hen and chickweed by June. The same material, run through a hot heap, gave me almost none.
If you are dealing with perennial roots or seeding weeds, hot composting is your route. There is no shortcut around it.
Keeping It Aerobic Without Daily Turning
Nobody turns a heap every day, and you do not need to. Air does most of the work on its own if you build for it.
Start with volume. A cubic metre, roughly a one-metre cube, will hold heat. Smaller heaps lose it too fast to reach 55C. Build the pile in layers of two to four inches, and dampen each layer as you go, and keep coarse and fine material mixed through the whole thing.
Turn on temperature rather than a fixed schedule. When the core falls below about 45C, turn it. On a heap I manage closely that means twice a week for the first fortnight. After the first month, once a week is plenty. Cornell notes that the best turning frequency depends on porosity and on how thoroughly the material was mixed at the start, so let the thermometer decide rather than the calendar.
Keep a dry brown stash beside the bin. Every time wet kitchen waste goes in, cap it with a handful of shredded cardboard or leaves. That single habit prevents more smells than any gadget I have bought.
The rest is simple diagnosis. Too wet means add browns and turn. Too dry means water and turn. Smelly means it needs air. Never heats means it needs nitrogen and more volume.
One last note. Bokashi is not a composting method, and it does not replace either route here. It is a fermentation pre-treatment, an anaerobic pickle of kitchen waste in a sealed bucket, and the contents still need to be composted or buried afterwards. Do not treat it as a shortcut past the air question.
Composting well comes down to air. Aerobic heaps heat, finish fast, kill weed seed and smell like a forest. Anaerobic heaps work too, just slowly, wetly and at a cost. Both are composting. Only one of them is the one you actually want.