Why your compost smells (and the fix for each smell)
If your heap smells bad, the smell itself is telling you what’s wrong. You don’t need to guess. Stick your nose near the pile (not right in it) and notice which of the two smells below is closer to what 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 a bit, 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 extra nitrogen breaks down into ammonia gas, and that’s what you’re smelling. You’re literally losing nitrogen into the air that should have stayed in the compost feeding your plants.
The fix is straightforward: add browns. Torn-up cardboard, dry autumn leaves, straw, or shredded paper all work. As a rough guide, add roughly two to three buckets of browns for every bucket of fresh greens you put in. Mix it in rather than just laying it on top, since the smell is coming from inside the pile, not off the surface. If you’ve been dumping a lot of grass clippings in one go — a common cause — spread future clippings in thin layers instead of piling them on wet and thick, because a thick mat of grass clippings 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 getting to the middle.” The organisms that break material down properly need air to breathe. When they don’t get it, a different set of organisms takes over, and those produce smelly, sulphur-based gases as a byproduct.
Two things cause this, usually together: too much water, and too little structure. If the pile has been compacted down, or if it’s been sitting through a lot of rain uncovered, the gaps between materials fill with water instead of air. Turning the pile with a fork breaks it open and lets fresh air back in — do this now, and the smell usually eases within a day or two. Turning weekly, or at least whenever you add a big batch of material, helps prevent it recurring.
Rain is often the hidden cause. An open heap that isn’t under any kind of cover will act like a sponge in wet weather. A simple tarp, an old bit of carpet, or a lid if you’re using a bin, keeps the top layer from turning into a soggy cap that seals air out of everything underneath. You don’t need it airtight — you actually want some airflow — just enough to stop direct rain soaking in.
When it’s both
Sometimes you’ll get both smells at once, which usually means a heap that’s wet, compacted, and nitrogen-heavy all together — often from a big dump of fresh grass clippings with no browns mixed in at all. In that case, 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 if you’ve got scraps piling up in the kitchen; a small bucket or covered container to hold scraps for a few days won’t hurt anything.
One more thing worth knowing: an occasional faint smell when you turn the pile is normal, especially in the middle of a hot, active heap. What you’re looking for is a smell so strong you can pick it up standing several feet away. That’s the one worth acting on.