Composting turns food scraps and garden waste into a dark, crumbly material that improves soil. It also reduces the amount of rubbish sent to landfill, where food waste can produce methane. You can compost at home with a small outdoor bin, a heap or a worm bin.
Key takeaways
- Compost is made by microbes breaking down organic material with air and moisture.
- Balance nitrogen-rich "greens" with carbon-rich "browns."
- Keep the pile damp, like a wrung-out sponge, and turn it occasionally.
- Avoid meat, dairy and oily food in simple home piles, and pet waste.
- Finished compost smells earthy and looks like dark soil.
How composting works
Bacteria, fungi and other organisms feed on organic matter and break it down. They need four things: food, water, air and warmth. A good mix and conditions speed the process, which can take from a few months to a year. As the microbes work, the pile can heat up. Eventually it settles into humus-like compost.
Choose a method
- Compost bin or tumbler: a closed container that is tidy and can deter pests.
- Open heap: a simple pile in a corner of the garden, which works if you have space.
- Worm bin: red worms in a container digest kitchen scraps, suited to apartments.
- Council or community schemes: some areas collect food waste or run shared composting sites.
Place an outdoor bin on bare soil in a spot with partial sun, since contact with soil lets worms and organisms enter.
Greens and browns
Greens are moist, nitrogen-rich materials: vegetable and fruit scraps, coffee grounds, tea leaves, grass clippings and fresh plant trimmings. Browns are dry, carbon-rich materials: dry leaves, straw, cardboard, paper, sawdust and small twigs. A rough guide is about two to three parts brown to one part green by volume, but adjust by observation. Too many greens makes the pile smelly and slimy. Too many browns slows decomposition.
What to avoid
In a simple home system, avoid meat, fish, dairy and greasy food, which can attract pests and smell. Do not add dog or cat waste, which may contain pathogens. Avoid diseased plants and persistent weeds with seeds, unless your pile gets very hot. Do not add glossy printed paper, treated wood or anything containing chemicals. Check which materials your own local guidance suggests.
Maintain it
- Moisture: it should feel like a wrung-out sponge. Add water if it is dry, or browns if it is wet.
- Air: turn or mix the pile every few weeks, or poke holes, to add oxygen.
- Size: chop large items into smaller pieces. A pile about one metre cube heats best, but smaller piles still work.
- Cover: a lid or cover helps with moisture and pests.
Troubleshooting
- Smells like rotten eggs or ammonia: too wet or too many greens. Add browns and turn.
- Not breaking down: too dry, too many browns or too cold. Add water and greens and mix.
- Flies: bury food scraps under browns or use a lid.
- Rodents: avoid cooked food and meat, use a bin with a secure base and keep the pile tidy.
Using compost
Compost is ready when it is dark, crumbly and smells like forest soil, and you can no longer recognise the original materials. Sieve out large pieces and return them to the bin. Spread it on garden beds, mix it into potting soil or use it as mulch. Do not use it neat as potting mix for seedlings, as it can be too rich.
Be patient
Composting is forgiving. If you are not ready for the effort, a small start with kitchen scraps and dry leaves still helps. Learn by observing and adjusting.
The science of heat
A healthy compost pile goes through stages. In the first days, microbes that thrive at moderate temperatures feed on the easily digested material and the pile warms up. If the pile is large enough and well mixed, it can enter a hot phase, reaching around 55 to 65 degrees Celsius. At these temperatures, many weed seeds and disease-causing organisms are killed, and decomposition speeds up. After the hot phase, the temperature falls and a slower stage follows in which fungi and larger organisms such as worms and beetles finish the job, leaving a stable, earthy material. Turning the pile reintroduces oxygen and can restart heating.
Composting in small spaces
You do not need a garden. A worm bin uses red worms, usually the species Eisenia fetida, which eat kitchen scraps and produce a rich material called worm castings. Keep it moist but not soggy, avoid too many citrus peels and onions, and keep it out of extremes of heat and cold. Bokashi is another system for apartments: it ferments food waste, including small amounts of cooked food, in a sealed bucket with a special bran, and the pre-compost is then buried or added to a compost heap. Some cities offer collection schemes for food waste, which are another good option.
Using finished compost
Finished compost improves soil in several ways. It adds organic matter that helps sandy soils hold water and heavy clay drain better, and it provides nutrients slowly. Spread a layer of a few centimetres on garden beds as a mulch or dig it lightly into the top soil. For container plants, mix compost with other ingredients such as coarse sand or bark, because pure compost can hold too much water. If you want a finer material, push it through a sieve and return the large pieces to the bin.
Why composting matters
When food waste is buried in landfill, it breaks down without oxygen and produces methane, a potent greenhouse gas. Composting with air produces mostly carbon dioxide and returns nutrients to the soil. Compost also supports soil life and can help store carbon in the ground. It is not a solution to climate change on its own, but it is a useful habit that reduces waste.
Check local rules and schemes
Some local authorities collect food scraps along with garden waste or provide subsidised bins. Rules about what can go in the collection differ, with some accepting cooked food and meat and others not. Community gardens and allotments may welcome contributions or run shared compost heaps. Check your local council or municipality website for guidance.
Frequently asked questions
How long does it take?
From two months to a year, depending on conditions and how often you turn it.
Will it smell?
A balanced, aerated pile smells earthy. Bad odours signal an imbalance.
Can I compost in an apartment?
A worm bin or a bokashi system can work indoors, or use local food scrap collections.
Can I compost citrus peels and onions?
In a normal outdoor compost heap, small amounts are fine, and they will break down. In worm bins, large amounts can upset the worms, so add them sparingly or leave them out.
Can I compost cooked food?
In a simple home heap, cooked food, meat and dairy can attract pests and cause smells, so it is usually best avoided. Hot, enclosed systems, bokashi buckets and some municipal collection schemes can handle cooked food.
How do I know when compost is safe to use on vegetables?
Finished compost looks dark and crumbly, smells earthy and has no recognisable food scraps. Let it mature for a few weeks after the active phase. Avoid using compost made with pet waste or diseased material on edible plants.
Sources and further reading
Written by
FactNest Editorial Team
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