Why your compost smells (and the fix for each smell)
If your heap smells bad, the smell itself is the diagnosis. You don’t need to guess. Get your nose near the pile (not right in it) and work out which of the two smells below you’re getting.
Quick fix
- Sharp, ammonia smell → mix in a bucket of dry browns (cardboard, dry leaves, straw) for every bucket of fresh scraps you’ve added recently.
- Rotten-egg or sour smell → turn the pile to get air into the middle, and cover it so rain stops getting in.
- Either way, stop adding fresh kitchen scraps for a few days and let it settle.
The ammonia smell: too much nitrogen
If the smell is sharp and stings your nose, like the smell under a cat litter tray, you’ve got too many greens relative to browns. Greens are the wet, nitrogen-rich stuff: grass clippings, fresh kitchen scraps, coffee grounds. When there’s too much of that and not enough dry, carbon-rich material to balance it, the surplus nitrogen breaks down into ammonia gas. That sting in your nose is fertility escaping — nitrogen that should have stayed in the heap to feed your plants.
The fix is browns. Torn-up cardboard, dry autumn leaves, straw, and shredded paper all work; as a rough guide, add two to three buckets of browns for every bucket of fresh greens. Mix them in rather than laying them on top, because the smell is coming from inside the pile, not off the surface. And if the trigger was a big dump of grass clippings — a common one — spread future clippings in thin layers. A thick wet mat of grass turns slimy and airless almost immediately.
The rotten-egg smell: not enough air
A sour, sulphurous smell, like rotten eggs or a stagnant pond, means the pile has gone anaerobic. That’s compost-speak for “no oxygen is reaching the middle.” The organisms that break material down properly need air to breathe. Starve them of it and a different set takes over, one that gives off sulphurous gases as it works.
Two things cause this, usually together: too much water and too little structure. When a pile compacts, or sits uncovered through a stretch of rain, the gaps between materials fill with water instead of air. Break it open with a fork and let air back in — the smell usually eases within a day or two. Turning weekly, or at least whenever you add a big batch of material, keeps it from coming back.
Rain is often the hidden cause. An open, uncovered heap soaks it up like a sponge. A tarp, an old bit of carpet, or a bin lid stops the top layer becoming a soggy cap that seals air out of everything underneath. You don’t want it airtight, since the pile needs airflow; just keep direct rain off.
When it’s both
Sometimes you’ll get both smells at once. That usually means a heap that is wet, compacted, and nitrogen-heavy all together, classically from a big dump of fresh grass clippings with no browns mixed in at all. Turn the pile first to get air moving, then work in a generous amount of dry browns as you go. Don’t add more greens until the smell has settled, even with scraps piling up in the kitchen — a small covered container will hold them for a few days without any harm.
An occasional faint whiff when you turn the pile is normal, especially from the middle of a hot, active heap. The smell worth acting on is the one you can pick up standing several feet away.