A compost heap turns kitchen scraps and garden waste into a dark, crumbly soil improver for free. The process is simple once you understand what the microbes doing the work actually need: a balance of materials, enough moisture, and enough air.
Greens and browns
Composting materials fall into two groups. Greens are soft, moist and nitrogen-rich; browns are dry, woody and carbon-rich. A heap of only greens turns into a smelly sludge, while a heap of only browns barely breaks down at all. Aim for roughly equal volumes, mixed together.
- Greens: vegetable peelings, grass clippings, coffee grounds, soft prunings, annual weeds before they seed.
- Browns: cardboard and paper, dry leaves, straw, woody prunings (chopped), wood chip, egg boxes.
Avoid cooked food, meat, dairy and dog or cat waste in an ordinary heap — they attract vermin and can carry pathogens. A sealed bokashi system or a dedicated digester handles cooked waste instead.
Moisture and air
The heap should feel like a wrung-out sponge: damp, but not dripping. Too dry and the microbes stall; too wet and they drown, turning the heap anaerobic and sour. If it is too wet, mix in more browns; if too dry, add greens or a watering can of water.
Air is the other half of the equation. Turning the heap every few weeks with a fork introduces oxygen and mixes the edges into the warm centre. If turning is too much effort, build the heap on a bed of coarse twigs and add plenty of scrunched cardboard to keep it open.
Hot and cold composting
A large heap built all at once, with a good greens-and-browns mix, heats up as microbes multiply — sometimes to 60°C — and can be ready in a few months. This is hot composting, and the heat kills many weed seeds and diseases. Most gardeners instead add material gradually, which gives cold composting: slower, cooler, and ready in about a year, but far less work.
Troubleshooting
- Smells of ammonia or rot: too many greens or too wet — add browns and turn it.
- Nothing is happening: too dry, too small, or too many browns — add greens and water.
- Flies or vermin: exposed food waste — bury fresh additions in the centre and cover with browns.
- Slimy and matted: compacted grass clippings — mix with cardboard to open it up.
Knowing when it is ready
Finished compost is dark brown, crumbly and smells of damp woodland, with the original ingredients no longer recognisable. A few twiggy bits are fine — sieve them out and throw them back in to start the next batch. Spread the finished compost on beds as a mulch or soil improver, and the cycle begins again.
Small spaces
No garden? A wormery on a balcony turns kitchen scraps into rich worm castings and a liquid feed, while a bokashi bin ferments waste under the sink. Both prove that composting scales right down to a flat.