Why a Heap Starts to Pong

Compost does not have to smell. A well-managed heap smells of woodland floor after rain — earthy, sweet, faintly mushroomy. If yours is making you hold your nose, the heap is telling you something specific, and the fix is usually quick.

Learn to read the whiff:

• Sharp, ammonia-like or urine smell: too much nitrogen-rich green material. Grass clippings, vegetable peelings and fresh manure have tipped the balance.
• Sour, vinegary, or a whiff of sweaty socks: the heap has gone anaerobic because it is packed too tightly and starved of air.
• Rotting eggs or drains: waterlogged. Too wet, no oxygen, and the wrong sort of bacteria doing the work.

All three come back to the same three levers: the balance of materials, the amount of air, and the moisture level. Get those right and the smell disappears within a week or two.

The Green and Brown Balance

Composting is a partnership between two kinds of waste. Greens are soft, damp and rich in nitrogen: grass clippings, kitchen vegetable trimmings, tea leaves and coffee grounds, annual weeds before they set seed, spent bedding plants, comfrey and nettle tops, and well-rotted manure from a reputable source.

Browns are dry, tough and rich in carbon: torn-up cardboard and egg boxes, newspaper, toilet roll tubes, straw, autumn leaves, woodchip, shredded hedge trimmings, sawdust and the contents of your paper shredder.

The rough rule of thumb for an allotment heap is two to three parts brown to one part green by volume. You are not running a laboratory, so judge it by eye and by feel rather than weighing anything. Layer them roughly; do not blend them to a pulp.

Grass clippings are the classic culprit on British plots. A thick layer of clippings mats down into an airless blanket and stinks within days. Add them in thin layers mixed through browns, or spread them thinly on the surface as a mulch and let the worms deal with them.

Building the Heap So It Breathes

Site the heap on bare soil if you can, in partial shade, so worms and microbes can move in from below. A volume of about one cubic metre — a metre wide, a metre high, a metre deep — is the sweet spot. Anything much smaller struggles to heat up; anything much bigger gets airless in the middle.

Build in alternate layers of roughly 15cm, and scrunch or shred anything flat and shiny so it does not form impermeable sheets. Sprinkle a spadeful of soil or finished compost between layers every so often to introduce a fresh population of microbes. If you want to speed things up, add a handful of nettle or comfrey leaves as an activator.

Keep these out of a domestic heap:

• Cooked food, meat, fish, dairy and oils — these attract rats and foxes, especially on a site with an existing rodent problem
• Dog and cat faeces, and used cat litter
• Perennial weed roots such as couch grass, bindweed and ground elder, unless you are happy to dry them out first
• Diseased plant material, especially anything with clubroot or white rot
• Glossy magazines, treated timber and nappies

Turning: The Easiest Win There Is

Turning the heap is the single most effective way to banish smells. It mixes greens with browns, breaks up matted pockets and drives air into the middle where the work happens.

If you are actively adding material, turn every two to four weeks. With a two-bay pallet set-up, fork everything from bay one into bay two, working from the outside edges inwards so the cooler outer material ends up in the hot core. If you only have a plastic dalek bin, you cannot empty it easily, so make holes down through the contents with a fork or a broom handle instead, and give it a shake.

Turning is not compulsory — a cold heap will still rot down, just more slowly — but it is what separates a sweet-smelling heap from a sour one. If a heap has already gone bad, fork it out, mix in a generous quantity of browns, and rebuild it. The smell will be gone in days.

Moisture: Damp as a Wrung-Out Sponge

The target is the moisture of a wrung-out sponge. Grab a handful from the middle and squeeze. A few drops should appear on your fingers. A trickle means too wet; nothing at all means too dry, and the process stalls.

British weather usually supplies too much rather than too little. In winter, cover the heap with a tarpaulin, a piece of old carpet or the lid of your bin, and make sure it is not standing in a puddle — a pallet base or a good layer of twiggy material at the bottom helps drainage no end. In a dry summer, water each layer as you build it with a watering can rather than soaking the finished heap, which just runs off the surface.

If the heap is sodden, the cure is straightforward: turn it and mix in plenty of dry browns — shredded cardboard is ideal — then cover it and leave it alone.

When It Is Ready, and What to Do Next

Depending on the season and how often you turn it, expect usable compost in six to twelve months. It is ready when it is dark brown, crumbly, smells earthy, and you can no longer recognise what went in. Sieve out the lumps and return them to the next heap to finish off.

Spread it as a mulch around fruit bushes and perennials, dig it into the vegetable bed in autumn, or sieve it finely for potting mixes and seed sowing. And keep the cycle going: a heap that is built in layers, turned now and then, and kept as damp as a wrung-out sponge will reward you with barrowloads of crumbly black gold and not a hint of a smell.