How to make compost: greens, browns, moisture and air

Good compost comes from four ingredients in balance: nitrogen-rich "greens", carbon-rich "browns", water and air. Get the mix to roughly 25 to 30 parts carbon for every part nitrogen, keep it as damp as a wrung-out sponge, and turn it now and then, and microbes will turn garden and kitchen waste into dark, crumbly soil improver. The compost calculator estimates the ratio of the mix you have, and this guide covers the rest.

What compost needs

Composting is done mostly by bacteria and fungi, helped by worms, woodlice and other small creatures. The microbes use carbon for energy and nitrogen to build protein, so they need both. Greens supply nitrogen: grass clippings, fruit and vegetable scraps, coffee grounds, fresh plant trimmings and manure from plant-eating animals. Browns supply carbon and structure: dry leaves, straw, woody prunings, shredded paper and cardboard. Water lets the microbes live and move, and air keeps the process aerobic, which is what stops it from smelling.

Getting the carbon-to-nitrogen ratio right

Composting works fastest at a carbon-to-nitrogen (C:N) ratio of about 25 to 30:1. Greens on their own are around 10 to 25:1; browns run from about 60:1 for dry leaves to several hundred to one for sawdust and cardboard. Some worked examples, by weight, using the typical values in the calculator:

• Grass clippings alone come to about 17:1, which is why a pile of fresh clippings turns slimy and smells of ammonia.
• Equal weights of vegetable scraps and dry leaves give about 26:1, right in the ideal range.
• Two parts grass, one part kitchen scraps and two parts dry leaves give about 24:1.
• Dry leaves alone are about 60:1 and will sit for a long time without breaking down.

Weight and volume differ a lot: a bucket of cardboard weighs far less than a bucket of scraps. For volume, the RHS suggests roughly 25 to 50% soft green material and the rest woody brown material. Real materials vary with moisture and source, so treat any ratio as a guide and adjust by what the heap does.

Building the heap

Choose a level spot on bare soil so worms can move in, and a bin or bay big enough to hold heat; a heap of around a cubic metre stays warmer than a small pile. Chop or shred large material, because smaller pieces break down faster. Add greens and browns in thin layers or mix them as you go, and keep a bag of dry leaves or shredded cardboard beside the bin so every caddy of kitchen scraps gets a handful of browns. Keep it moist: squeeze a handful and it should feel like a wrung-out sponge, damp but not dripping. Turn it every few weeks with a fork to add air; a well-balanced, well-turned heap can reach 54 to 71 °C (130 to 160 °F), which speeds things up and kills many weed seeds.

What not to compost

• Meat, fish, bones, dairy, fats and oils, which attract rats and smell.
• Dog and cat waste, which can carry parasites and disease.
• Diseased plants and the roots of persistent weeds, which a cool heap may not kill.
• Coal ash, glossy or coated paper and anything treated with chemicals.

Cooked food scraps are best kept out of an open heap for the same reason as meat. Local councils and enclosed food digesters handle what a garden heap cannot.

Fixing problems and knowing when it is ready

A heap tells you what it needs. If it smells of ammonia or turns slimy, it has too much nitrogen and too little air: mix in browns and turn it. If it is dry and nothing is happening, add water and some greens. If it stays cold, it may be too small, too dry or too low in nitrogen. Finished compost is dark, crumbly and smells earthy, and you can no longer recognise what went in. A hot, well-managed heap can be ready in a few months; a cold heap that is simply added to can take up to a year. Spread it as a mulch about 5 cm deep or fork it into beds; the gravel, mulch and soil calculator works out the volume for an area, and the plant spacing calculator plans what goes in next. Compost improves soil structure but is low in nutrients, so if plants still need feeding, NPK fertilizer numbers: what the three figures mean and how much to apply explains how to read a fertiliser bag, and the fertilizer calculator sets the amount.