Starting a compost bin without smells

Why a healthy compost bin shouldn't smell

A good compost bin has a smell, but not an unpleasant one. Push a fork into a working heap and you should get a waft of damp woodland — earthy, mushroomy, like the floor of an oak wood after rain. That scent means aerobic bacteria and fungi are doing their job, breaking down scraps and prunings with plenty of oxygen to hand.

If your bin smells sour, vinegary or like a drain, a different process has taken over. Without air, anaerobic bacteria move in. They work slowly, produce acids and sulphur compounds, and leave a slimy mess that takes months longer to rot down. The fix is rarely a magic additive. It comes down to two things: balance and air.

Greens and browns: the balance that matters most

Composting is a recipe, and the ingredients fall into two camps. Greens are wet, soft and nitrogen-rich. Browns are dry, tough and carbon-rich. Too many greens and the heap turns slimy and sour; too many browns and it sits there doing nothing for a year.

Aim for two to three parts browns to one part greens by volume. In practice, every time you tip a caddy of peelings in, add a couple of generous handfuls of torn cardboard or a bucket of dead leaves alongside it.

• Greens: grass clippings, vegetable peelings, fruit scraps, tea leaves, coffee grounds, spent bedding plants, soft prunings, annual weeds without seed heads, comfrey, nettles.
• Browns: torn cardboard with the tape removed, egg boxes, newspaper, toilet roll tubes, dead leaves, straw, woodchip, shredded twiggy prunings, untreated sawdust.

Grass clippings cause more smelly bins than anything else. They're wet, nitrogen-heavy and mat into an airtight layer within days. Never tip a whole mower box in at once — spread them thinly, mix them through with cardboard, or leave them on the lawn as a mulch instead.

What to leave out

Some things don't belong in a domestic bin, usually because they attract pests or carry disease.

• Cooked food, meat, fish, bones, dairy and oils — these pong and bring rats and foxes.
• Dog and cat waste, and used cat litter.
• Diseased material, particularly potato and tomato foliage with blight.
• Perennial weeds such as dock, ground elder, bindweed and couch grass, plus anything with seeds still attached.
• Glossy magazines, treated timber, coal ash and nappies.
• Tea bags with a plastic mesh — tear them open and compost the leaves only.

If you've used weedkiller on the lawn, leave those clippings out for several mowings.

Setting up the bin properly

Position matters. Stand the bin on bare soil or paving with room to drain, not on a concrete slab where it will sit in a puddle. Soil contact lets worms and microbes move in and get things started.

Begin with a 10cm layer of twiggy prunings or scrunched cardboard at the base to keep air moving underneath. Then build in layers: a few inches of greens, a few inches of browns, and a sprinkle of soil or old compost now and then to introduce more microbes. Water each layer if it's dry — aim for the consistency of a wrung-out sponge.

A cubic metre makes the ideal hot heap, but most of us are working with a council dalek bin, which is smaller and cooler. That's fine; it simply works more slowly. Consider running two, filling one while the other matures.

Turning, and the smells that tell you something

Turn your heap with a fork every two to four weeks. It isn't essential, but it speeds things up and heads off a smell before it takes hold.

• Sour or vinegary: too wet and too many greens. Add browns, turn it over, and leave the lid off for a day in dry weather.
• Sharp ammonia smell: too much nitrogen. Same fix — cardboard, newspaper, a good turn.
• Slimy and matted: almost always grass clippings. Break it up and mix in dry browns.
• Bone dry with nothing happening: add water and a handful of greens.
• Fruit flies: bury kitchen waste beneath the surface rather than leaving it on top.

In a wet British winter the main risk is waterlogging. Keep the lid on, add extra cardboard, and don't worry if everything slows down — cold composting carries on quietly until spring.

When it's ready

Expect usable compost in six to twelve months, depending on the season and how often you turn it. It's ready when it's dark, crumbly, smells of woodland soil and you can no longer recognise what went in. Sieve out stubborn lumps and return them to the next heap.

Use it as a mulch around shrubs, fork it into vegetable beds, or blend it with loam and grit for a decent potting mix. Your garden gets fed, your bin stops smelling, and the whole cycle costs nothing but a little attention. A heap that smells like a forest floor is a heap that's working.