Composting is the process of turning garden waste and kitchen scraps into dark, crumbly, earthy-smelling material that feeds the soil. It is the single most useful thing a gardener can do for their soil, and it costs nothing but a little space and attention. A good compost heap takes in weeds, peelings, leaves and grass clippings, and gives back the best soil improver there is — one that adds nutrients, holds moisture, feeds soil life and improves structure all at once. This guide explains how composting works, how to build a heap, what to put in it, and how to know when it is ready.
Compost is not fertiliser in the ordinary sense: it is a soil conditioner that also feeds. A 5–8 cm layer of compost worked into a bed each year does more for the soil than any bag of feed, because it improves the soil itself rather than just adding nutrients. And composting closes a loop: the garden's waste becomes the garden's food, instead of going to landfill.
How composting works
Composting is rotting, managed well. Bacteria, fungi, worms and other organisms break down organic matter, and in a good heap they do it fast enough to generate heat — a hot heap can reach 55–65°C in the middle, which kills most weed seeds and disease organisms. The organisms need four things: food (a mix of nitrogen-rich greens and carbon-rich browns), air, water and warmth. Give them all four and the heap works quickly; miss one and it slows down or starts to smell.
The balance of greens and browns is the most important part. Greens are nitrogen-rich and wet: grass clippings, vegetable peelings, fresh weeds, manure. Browns are carbon-rich and dry: leaves, straw, shredded cardboard, twigs, sawdust. The standard ratio is roughly one part green to two or three parts brown by volume — our compost ratio guide covers the numbers in detail, and the compost ratio calculator works them out for you. Too many greens and the heap goes slimy and smelly; too many browns and it sits dry and does nothing. Air matters just as much: organisms need oxygen, so a heap that is never turned goes anaerobic and smells. Water matters too: the heap should be damp like a wrung-out sponge, never wet and never dry.
How to build a compost heap
1. Choose a spot. Pick a level spot on bare soil in a sheltered corner, where worms can enter from below and excess water can drain away. Avoid a spot hard against a fence or wall, which restricts air, and avoid a spot where water pools. A heap needs to be at least a cubic metre — roughly 1 m × 1 m × 1 m — to heat up properly; smaller heaps work, but slowly.
2. Choose a container. A simple bin made from wooden pallets, wire mesh or recycled plastic boards works well, as do bought compost bins. Whatever you use, it needs air holes in the sides and an open base on the soil. Two bins are better than one: one filling, one maturing. A lid or cover keeps rain off and warmth in.
3. Start with browns. Put a 15 cm layer of coarse browns — twigs, straw, shredded cardboard — at the bottom for drainage and air. Then add alternating layers of greens and browns, roughly one part green to two or three parts brown by volume, watering each layer lightly as you go.
4. Add an activator. A few spadefuls of garden soil or finished compost introduce the organisms that start the process. Well-rotted manure works too. Do not buy a compost activator — soil and compost do the job for free.
5. Turn and tend. Turn the heap every few weeks with a fork, moving the outside material into the middle, which reintroduces air and mixes the contents. Check moisture each time: if it is dry, water it; if it is wet and smelly, add dry browns and turn it. A hot heap will steam on a cool morning and feel warm when you push your hand into the middle.
What to compost — and what to avoid
Good for the heap: vegetable and fruit peelings, tea bags (plastic-free only), coffee grounds, eggshells (crushed), grass clippings in thin layers, leaves, straw, shredded cardboard and paper, prunings chopped small, spent bedding from herbivore pets, and well-rotted manure. Chop or shred large material first so it breaks down faster, and add grass clippings in thin layers mixed with browns — a thick layer of grass alone goes slimy and smelly.
Do not compost: meat, fish, dairy, fats and oils, cooked food in large amounts, pet waste from dogs or cats, diseased plants, weeds with mature seeds, invasive weeds such as bindweed or couch grass roots, treated timber, glossy paper, or anything with chemicals on it. Meat and dairy attract rats and smell; pet waste can carry pathogens; diseased plants and seeding weeds can survive a cool heap and spread when the compost is used. If your heap runs hot — above 55°C for several days — it will kill most weed seeds and disease organisms, but a cool heap will not, so keep risky material out unless you are sure the heap is hot. Our eggshells in garden and coffee grounds in garden guides cover two common additions in detail.
How to know when compost is ready
Finished compost is dark brown or black, crumbly, and smells like earth — sweet and fresh, not sour or ammonia-like. You should not be able to recognise the original materials in it, apart from the odd twig or eggshell. A hot heap can produce finished compost in two to three months in summer; a cool heap takes six to twelve months. To test readiness, put a handful in a sealed plastic bag for a few days: if it smells sour or like ammonia when you open the bag, it is not ready yet. Or sow a few fast seeds such as cress in a pot of the compost: if they germinate and grow well, the compost is ready; if they fail or look burnt, it needs more time. Immature compost can harm plants, because it continues breaking down in the soil and temporarily uses nitrogen — so do not rush it.
How to use finished compost
Use compost as a soil improver, a mulch, or an ingredient in mixes. Work a 5–8 cm layer into the top 20–30 cm of a bed each year, or spread it as a mulch on the surface and let worms pull it down. Mix it into potting mix at about a third of the volume, or into raised bed soil mix at about 30 per cent. Use it to top-dress lawns, to feed hungry plants such as tomatoes and cabbage, and to brew compost tea. Do not use it as the whole growing medium in a pot — pure compost is too rich, holds too much water and shrinks as it breaks down. And do not use unfinished compost on seedlings or in seed trays; use our seed starting mix instead.
Common mistakes
The most common mistake is too many greens and not enough browns, which makes a slimy, smelly heap — add dry browns and turn it. The second is a heap that is too small to heat up; build it at least a cubic metre. The third is never turning the heap, so the middle goes airless. The fourth is letting it dry out, so nothing happens for months — check moisture and water it. The fifth is adding meat, dairy or pet waste, which attracts rats and can carry disease. The sixth is using unfinished compost, which robs nitrogen from plants. And the seventh is putting weed seeds and diseased plants in a cool heap and then spreading the result around the garden — keep risky material out unless the heap runs genuinely hot.
Safety notes
Wear gloves when handling compost and manure, and wash your hands before eating. Compost dust can irritate lungs, so wear a mask when turning a dry heap, and wet the heap down first if it is dusty. Do not inhale compost dust deliberately — it can carry organisms that cause illness in rare cases, especially for people with weakened immune systems. Use only mature compost on food beds, and never use fresh manure directly on edible crops — compost it for several months first. If you are unsure about safe composting practice, check your local extension service for advice.
Frequently asked questions
How long does it take to make compost?
A hot, well-managed heap can produce finished compost in two to three months in summer. A cool heap takes six to twelve months. Turning regularly, keeping the heap damp, and getting the green-to-brown ratio right all speed it up.
What is the right green to brown ratio for compost?
Roughly one part green to two or three parts brown by volume. Greens are nitrogen-rich and wet — grass, peelings, fresh weeds. Browns are carbon-rich and dry — leaves, straw, cardboard. Too many greens makes a smelly heap; too many browns makes a heap that does nothing. See our compost ratio guide for detail.
Why does my compost heap smell?
Smell usually means too many greens, too much water, or not enough air. Add dry browns such as shredded cardboard or straw, turn the heap to let air in, and check the drainage. A healthy heap smells like damp earth, not like rotting food.
Can I compost weeds?
Yes, if the heap runs hot — above 55°C for several days — which kills most weed seeds. In a cool heap, do not add weeds with mature seeds or invasive roots such as bindweed, or they will spread when you use the compost. When in doubt, leave seeding weeds out.
Do I need a compost bin?
No, a heap works, but a bin keeps it tidy, holds heat and moisture better, and keeps rats out more effectively. A simple bin from pallets or wire mesh is enough. Two bins are ideal — one filling, one maturing.