What the Heap Is Doing

A compost heap is a colony of bacteria, fungi, and larger organisms eating organic matter. They need four things, and every compost problem is one of the four being wrong: carbon for energy, nitrogen for building proteins, oxygen to respire, and water to live in.

Get all four roughly right and decomposition is fast, hot, and smells of woodland. Get one wrong and the heap either stalls or shifts to anaerobic bacteria, which are the ones that smell.

Greens and Browns

The gardening shorthand for carbon and nitrogen is browns and greens, and it is a decent guide as long as you know it describes chemistry, not colour.

Greens are nitrogen-rich, and they are wet, soft, and fresh. Fruit and vegetable peelings, coffee grounds, tea leaves, fresh grass clippings, soft green prunings, annual weeds before they seed, spent flowers, crushed eggshells.

Browns are carbon-rich, and they are dry, tough, and dead. Dry leaves, straw, small twigs and woody prunings, cardboard, plain paper, egg boxes, sawdust from untreated wood, dried plant stems, wood chip.

Coffee grounds are brown in colour and count firmly as a green. Fresh grass is a green; the same grass dried into hay is a brown.

The Ratio

The frequently quoted figure is a carbon-to-nitrogen ratio around 25 or 30 to 1, which is a laboratory number rather than a garden one.

In practice: aim for roughly two to three parts browns to one part greens by volume. Not by weight - browns are far lighter.

This surprises beginners, because the material a household generates is overwhelmingly green. Peelings, grass, and weeds are all greens, and a bin filled only with them becomes a wet, airless, smelling mass in a fortnight.

The practical consequence is that browns must be stockpiled. Autumn leaves in bags, flattened cardboard in the shed, prunings kept in a pile. Then every time you add a caddy of kitchen waste, add two or three times its volume of browns on top.

If you take one habit from all of this: keep a bag of torn cardboard or dry leaves next to the bin and add a handful every single time you empty the kitchen caddy.

Air

Compost organisms are aerobic and consume oxygen quickly. A heap that compacts - because it is too wet, too green, or too dense - runs out of air, and anaerobic bacteria take over. Those produce the sulphurous, sour, unmistakable smell of a failed heap, and they work far more slowly.

Air enters two ways.

Structure. Coarse woody material, scrunched cardboard, and twigs create voids that persist. This is a large part of what browns are for. Starting a new heap with a layer of coarse twigs at the bottom keeps the base open.

Turning. Moving the heap with a fork - outside to the middle, top to the bottom - reintroduces oxygen and evens out the mix. A hot heap turned weekly can finish in two months; a heap never turned takes a year or more and still works.

Between the two, structure matters more than turning, and it is far less effort.

Water

The heap should feel like a wrung-out sponge. Squeeze a handful: it should hold together and release a drop or two of water at most.

Too wet is the common failure, and it comes from too many greens, rain, or a bin sitting on an impermeable surface. Fix it by adding dry browns and mixing thoroughly, and by covering the heap.

Too dry stops the process entirely, which is why heaps of pure autumn leaves sit unchanged for two years. Fix it by watering as you turn, or by adding greens.

What Not to Compost

Meat, fish, dairy, oil, and cooked food in an open heap or standard bin: they attract rodents and go putrid. Bokashi handles them; ordinary heaps do not.

Dog and cat faeces, and cat litter. These carry pathogens that home composting does not reliably destroy.

Perennial weed roots - bindweed, couch grass, ground elder - and any weed that has set seed. A cold heap will not kill them and you will spread them across the garden with the compost. Drown them in a bucket of water for a few weeks first, or bin them.

Diseased plant material. Home heaps rarely get hot enough for long enough.

Coal ash, treated or painted wood, glossy printed card, and anything synthetic. Compostable plastics generally need industrial temperatures and will still be there next year.

Citrus and onion are fine in a normal heap in moderation; keep them out of a wormery.

Size and Speed

Bigger heaps work faster because they hold their own heat. Roughly a cubic metre is the threshold at which a heap can get properly hot.

Smaller pieces break down faster because there is more surface for organisms to work on. Chopping kitchen waste and shredding cardboard genuinely accelerates things.

A heap built all at once, at the right ratio and the right size, is the only way to get a truly hot heap - it heats within days, peaks, then cools over weeks. Most household heaps are added to gradually and therefore never get hot. That is fine. Cold composting works; it simply takes six months to two years and does not kill weed seeds.

Compost activators and accelerators are almost never necessary. What a stalled heap needs is nitrogen, air, or water, and a handful of grass clippings supplies the first for nothing.