My bag of compost smells sour or looks mouldy — can I still use it?

Three different things people report from a bag — sour smell, white threads and green growth — and only one of them means the compost is actually a problem.

Difficulty: beginner
Time: 20 min
Read: 4 min
Safety: caution

Short answer

White fluffy threads are harmless fungal mycelium and often a sign of good compost; mix it in and use it. Green growth on the surface is algae or liverwort from a wet bag stored in light, and it is cosmetic. A sour, ammonia or rotten-egg smell means the bag went anaerobic in storage — tip it out, spread it and leave it a day or two to air, and it will usually recover completely.

Bagged compost spends months sealed in plastic, sometimes outdoors, sometimes in a heap in full sun. What comes out of the bag can look and smell alarming and is almost always fine after twenty minutes of air. The one case worth taking seriously is compost that stays sour after airing, which means it is still actively decomposing and will damage plants.

Safety

Compost can carry Legionella, which causes a serious lung infection, and the risk is higher with dusty or long-stored material. Open bags away from your face, dampen the compost before tipping or breaking it up, do not breathe the dust, wear gloves and wash your hands afterwards. People with weakened immune systems or lung conditions should be especially careful and may prefer to wear a mask.

What you'll need

A tarpaulin or a spare bag to tip it onto
A fork
Gloves

Step by step

Identify white fluffy threads correctly.A web of fine white filaments through the compost, sometimes with a mushroomy smell, is fungal mycelium. It is decomposing organic matter and it is harmless — in fact its presence generally indicates a biologically active compost. Break it up as you fill pots and use it normally.
Identify green surface growth correctly.Green algae, moss or liverwort on the surface of an opened bag or a stored pot means light plus persistent moisture. It competes with seedlings for a surface but does not harm established plants. Scrape it off, or mix it in on a bed.
Identify the sour smell correctly.A sharp sour, vinegary, silage or rotten-egg smell as you open the bag means anaerobic conditions — the material has been wet and airless and has fermented rather than composted. Ammonia means it is still actively breaking down and has free nitrogen in it.
Air it before writing it off.Tip the whole bag out onto a tarpaulin, break it up with a fork, and leave it a day or two in a dry, airy place, turning it once. The great majority of sour-smelling compost recovers completely — the volatile compounds gas off and the material re-oxygenates.
Smell it again after airing.Sweet and earthy: use it for anything. Still sour or still ammoniacal after two days: it has not finished composting. Do not put it in pots or near seedlings. Spread it on a border as a mulch where it can finish rotting in contact with soil.
Test it before sowing into it, if you have doubts.Sow a few cress or bean seeds in a pot of it alongside a control pot of compost you trust. Poor germination or stunted, distorted growth in the test pot tells you not to use it for anything precious. This also picks up herbicide contamination.
Check for fungus gnats before you take it indoors.Small black flies rising from the bag mean the compost has been damp long enough for them to breed. They are harmless to established plants but their larvae eat seedling roots. Let it dry a little, and use a fresh bag for sowing.
Store what is left properly.Fold the bag over and keep it under cover, off the ground and out of direct sun. Most sour compost is a storage failure — a bag left open in the rain or sealed wet in a hot shed for a year.

Tips

Compost that has been sitting in a garden centre yard through a wet winter is the classic case. Buy from a stack under cover where you can, and buy for the season rather than stockpiling.
Peat-free composts go off faster in storage than peat did, because the organic components continue to break down in the bag. A year-old bag of peat-free is a genuinely worse product than a fresh one.
Compost that has become water-repellent in storage — dry, dusty, water runs off it — is not spoiled. Add water gradually and mix it thoroughly on a tarpaulin, and it will take up water again.
The mushroomy smell of active mycelium is completely different from the sharp acidity of anaerobic fermentation. If you are unsure, the sour one is the one that makes you pull your head back.
Small white or cream lumps that look like polystyrene beads are perlite. They are part of the mix, not a fault.

If it doesn't work

Sharp sour smell on opening, gone after a day spread out

Cause: Anaerobic storage in a sealed wet bag — Fix: It is fine. Use it normally once the smell has gone and it looks and feels like ordinary compost.

Ammonia smell that persists after airing

Cause: Immature compost still actively decomposing, with free nitrogen — Fix: Do not use it in containers or near seedlings — it will scorch. Spread it on an empty bed as a mulch and let it finish there over a couple of months.

Seedlings sown into it germinate then die

Cause: Immature compost, salt content, fungus gnat larvae, or herbicide residue — Fix: Run a bean test against a control pot to distinguish. Immature compost stunts evenly; herbicide residue distorts and twists.

White crust on the surface of pots filled from the bag

Cause: Salts from a high-fertiliser compost, or hard water deposits — Fix: Scrape it off and water thoroughly to flush. If it recurs, the compost is over-fertilised for what you are growing — dilute it with a lower-nutrient mix.