Troubleshooting a Smelly Compost Bin: 5 Common Causes and Fixes

A compost bin should smell of almost nothing — damp earth, perhaps, or a woodland floor after rain. If yours greets you with something closer to a blocked drain, sour milk or rotten eggs, don't dig the whole lot out and start again just yet. That smell is information. It points to one of a handful of conditions inside the bin, and most of them can be corrected in a single afternoon.

Why Compost Starts to Smell

Composting works best when microorganisms break material down with oxygen present. This is aerobic decomposition, and it is fast, hot and remarkably odour-free. When the oxygen runs out, a different community of microbes takes over and works without it. Their by-products — hydrogen sulphide, ammonia and various organic acids — are the ones your nose objects to.

Three things starve a bin of air: it is too wet, it is too tightly packed, or it is overloaded with soft green material that collapses into a slimy, airless mass. Almost every smelly bin comes down to water, air or balance, with the occasional case of something going in that should never have been added.

Reading the smell

Before you start digging, take note of what you are actually smelling. It is a surprisingly precise diagnostic tool.

• Rotten eggs, drains, sewage — anaerobic conditions, usually too wet and compacted.
• Sharp ammonia or a farmyard tang — too much nitrogen-rich green material, often fresh grass clippings.
• Sour, vinegary, faintly pickled — the contents are fermenting rather than composting, typically because they are wet and airless.
• Rotten meat or fish — something has gone in that should not have.
• Earthy, mushroomy, like leaf litter — nothing is wrong. Leave it alone.

Cause 1: The Bin Has Run Out of Air

A bin filled steadily for months is often a solid block. Material in the middle gets compressed under the weight above, air channels close, and the centre turns to sludge.

The fix is mechanical. Fork the contents out onto a tarpaulin, break up the clumps, and put it back loosely, mixing in something bulky as you go — scrunched cardboard, torn egg boxes, straw, a few handfuls of woodchip. A layer of twiggy material at the bottom helps air move up from below. If you would rather not empty the whole bin, push a broom handle or a length of perforated pipe into the middle and work it around to open some channels. Then turn the heap every week or two while the problem settles.

Cause 2: It Is Far Too Wet

Wet material is the most common reason a bin goes anaerobic, and it is easy to test. Squeeze a handful of the contents. It should feel like a wrung-out sponge: damp, but not dripping. If liquid runs down your wrist, the bin is too wet.

Add browns — cardboard, shredded paper, straw, dry leaves — in generous handfuls and mix them through rather than laying them on top. Leave the lid off for a few hours on a dry day so some moisture can escape. If the bin sits on soil and has no base, check that the drainage holes are not blocked, because standing water at the bottom keeps the whole heap sour. Keep grass clippings out of the mix for a few weeks, or spread them thinly with cardboard instead of tipping in a solid layer.

Cause 3: The Green-to-Brown Balance Has Slipped

Green materials are wet and nitrogen-rich. Browns are dry and carbon-rich. Getting the mix roughly right does most of the work of keeping a bin sweet, because browns soak up moisture and create the structure that lets air in.

• Greens: grass clippings, vegetable peelings and fruit scraps, coffee grounds, tea leaves, fresh plant trimmings, annual weeds before they set seed.
• Browns: cardboard, newspaper and plain paper, straw, autumn leaves, woodchip, shredded twigs, sawdust from untreated wood, kitchen roll tubes.

Aim for roughly two or three parts browns to one part greens by volume. That surprises people, because most kitchen caddies are almost entirely green. If the bin smells or looks wet, don't measure anything — just add more browns. It is very difficult to add too many.

Cause 4: The Wrong Things Are Going In

Some materials smell unpleasant because they are unpleasant, and no amount of turning will fix that. Keep cooked food, meat, fish, dairy, oils and fats out of a standard open compost bin. They attract rats and foxes, and they rot into exactly the smell you are trying to get rid of. Dog and cat faeces, cat litter and disposable nappies should stay out too, for odour and for hygiene.

If you want to compost cooked food, use a sealed system designed for it, such as a bokashi bin or a hot composter, or check whether your council offers a food waste collection. Glossy magazines and treated timber belong elsewhere. Diseased plants and perennial weed roots will not smell, but they rarely break down properly in a domestic bin.

Cause 5: The Bin and Its Position Are Working Against You

A small bin on a sunny patio with a tight-fitting lid will always struggle. Small volumes cannot hold heat, and there is no soil contact to bring in worms and fungi. Where you can, site a bin on bare soil in light shade, out of the strongest midday sun, with drainage underneath and a lid that keeps rain out.

If you use a tumbler, remember that it needs roughly half its volume empty for the contents to tumble properly. Overfilling is as bad as never turning it. And if your bin is regularly full before the contents have finished, a second bin is often the simplest upgrade you can make — one to fill, one to mature.

A Weekend Fix, Step by Step

If the bin is genuinely unpleasant, set aside an hour or two and do the job properly.

1. Empty the bin onto a tarpaulin or a clear patch of ground.
2. Pull out anything that should not be there — cooked food, packaging, meat — and dispose of it.
3. Break up compacted lumps and mix wet, slimy material with dry browns.
4. Rebuild the heap in thin layers, adding a handful of browns between each green addition, and start with a few inches of twiggy material at the base.
5. Check moisture as you go, aiming for the wrung-out sponge test throughout.
6. Leave the lid on, turn the heap weekly for the next month, and add only balanced material in the meantime.

Most bins respond within a fortnight. The smell fades, the contents darken, and you will start to see finished compost at the bottom. If it comes back, check the moisture first, then the greens — those two account for the great majority of problems.

A last word on safety: turning compost releases dust and mould spores. Wear gloves, and if you have a respiratory condition such as asthma, wear a mask and work in the open air. Speak to a pharmacist or your GP if you are unsure whether it is safe for you.

Photo: Magda Ehlers / Pexels