Composting

Composting is the deliberate process of letting waste from your kitchen and garden rot down into dark, crumbly, nutrient-rich material called compost. Compost is one of the best things you can add to garden soil. The whole process is just nature's normal decay, sped up by piling material together in a way that makes it easy for the bacteria and fungi (and worms and beetles) that do the work. Composting also keeps food and garden waste out of landfill, where it would produce harmful greenhouse gases.

• Time for full compost3 to 12 monthsFaster in summer than winter
• Worms in a wormeryApprox. 1,000 per kgEat half their body weight per day
• % reduction in volumeApprox. 50 to 70%Compost is more concentrated than scraps
• Two main ingredient typesGreens and brownsRoughly 50/50 mix is best
• UK household food waste per yearApprox. 4.5 million tonnesMost still goes to landfill
• Compost temperature in summerUp to 60 °CHot enough to kill weed seeds

How composting works

Composting is just controlled rotting. When you pile kitchen and garden waste together in a heap or bin, hundreds of types of bacteria, fungi, worms, woodlice and beetles get to work breaking it down. They eat the material, digest it, and produce dark soil-like compost as their waste. With the right mix of ingredients, air and water, the process can be remarkably quick.

In a well-kept compost heap, the inside can get surprisingly hot. Bacteria break down the material so fast that the heat they produce can raise the temperature to 50 to 60 °C. This is hot enough to kill weed seeds and most disease-causing germs. After a few weeks the heat dies down and worms and other invertebrates move in to finish the job.

What to add

The trick to good compost is mixing two types of material in roughly equal amounts.

• Greens (nitrogen-rich): fresh and soft. Examples: vegetable peelings, fruit scraps, grass clippings, coffee grounds, tea bags (paper ones only), young weeds.
• Browns (carbon-rich): dry and tough. Examples: dead leaves, twigs, shredded cardboard, paper towels, eggshells, wood ash.

Greens alone make a slimy smelly mess; browns alone rot very slowly. A roughly 50/50 mix by volume is ideal. Stir or turn the heap every few weeks if you can, to add air.

What NOT to add

• Meat, fish or bones: attract rats and smell bad.
• Dairy products: same problems.
• Cooked food in large amounts: same.
• Pet waste (dog or cat): can spread disease.
• Glossy magazines and printed paper with shiny ink: chemicals may pollute the compost.
• Plants that have gone to seed: in a cool heap, weed seeds may survive and grow back.
• Diseased plants: may spread the disease.

How long it takes

How fast compost forms depends on the conditions.

• Hot composting (well-mixed, regularly turned, balanced greens and browns): 3 to 6 months.
• Cold composting (just dumped in a heap, left to rot): 9 to 18 months.
• Wormeries (compost made by worms in a special container): 2 to 4 months.

Fact The UK throws away approx. 4.5 million tonnes of food from homes every year. Almost all of it could be composted. Food rotting in landfill releases methane, a greenhouse gas about 30 times more powerful than carbon dioxide. Composting at home (or in council garden waste bins) keeps the carbon in the soil instead of in the atmosphere.

Wormeries: composting with worms

A wormery is a special compost bin full of tiger worms (a kind of compost worm). The worms eat kitchen scraps fast and produce some of the richest compost there is. Wormeries fit in a small garden, garage or even a balcony, and they are great for people who do not have room for a full compost heap. A typical small wormery can deal with the food waste of one or two people.

Did you know? Compost has been called "black gold" by gardeners. A 2 cm layer of compost spread on a vegetable bed each year can dramatically improve the soil: more nutrients, better water retention, more soil life. Some studies have shown that adding compost to depleted farmland can restore good harvests within 3 to 5 years. It is one of the easiest, cheapest things you can do for any garden.

Deeper dive: why composting helps fight climate change

Composting at home looks like a small thing, but it adds up to a real climate impact when done at scale. There are two main reasons.

1. It keeps food out of landfill. When food rots underground in a landfill, it does so without oxygen (anaerobic conditions). Anaerobic decay produces methane, a greenhouse gas roughly 30 times more powerful than carbon dioxide over a 100-year period. Around 8% of all human-caused methane emissions come from landfilled food waste. Composting (which happens with oxygen, called aerobic decay) produces almost no methane at all.

2. It builds carbon-rich soil. The compost you spread on a garden does not just feed plants; it also feeds the soil. As the compost is incorporated, much of its carbon stays in the soil for years or decades, stored as organic matter. This is the basis of the "4 per 1000" initiative, which calculates that increasing soil organic matter by just 0.4% per year worldwide would offset much of the world's annual fossil fuel emissions.

Of course, individual back-garden compost heaps are too small to single-handedly fix climate change. But added up across millions of homes, plus large-scale composting at the council and farm level, plus better food storage to waste less in the first place, the contribution is genuinely meaningful. And every bit of compost you make is one fewer plastic bag of bin liner you have to use, one fewer bag of garden centre soil improver you have to buy, and one slightly better garden for the wildlife that depends on it.

For more, see what is soil and soil erosion.