Fixing common compost problems
Almost every compost problem has one of four causes: too much water, too little water, too little air, or the wrong mix of greens and browns. Smells, slime, flies and stalled heaps are all symptoms of those four. The good news is that each one can be fixed with the materials you already have, usually in an afternoon. This page goes through the problems people ask about most and the fix for each.
It smells rotten or sour
A healthy heap smells mild and earthy. A rotten egg or sour smell means the organisms that need oxygen have run out of it, and other organisms that work without air have taken over. The usual reasons are too much wet green material packed together, a heap that has been squashed flat, or a heap that is soaked from rain. The fix is to add air and dry material at the same time. Fork the heap over, breaking up any compacted clumps, and mix in a generous amount of browns such as dry leaves, straw or shredded cardboard. If the heap sits in an open container in a wet spell, cover it.
It smells of ammonia
A sharp smell like cleaning fluid comes from nitrogen escaping as ammonia. It means there is more green material than the organisms can use, often after a big load of grass clippings. Mix in browns until the smell fades, and next time spread clippings thinly or let them wilt for a day before adding them.
It is slimy and heavy
A heap that feels like wet sludge has too much water and too little structure. Wet greens collapse on themselves and shut out air. Turn it, add coarse browns that hold open air spaces, such as small twigs, straw or torn cardboard, and leave the lid off on a dry day if your bin has one. If water pools at the base, check that the bin sits on soil that drains rather than on paving.
It is dry and nothing happens
If a heap stays the same for weeks and the material feels dry and dusty, the organisms do not have the moisture they need. Dry leaves and cardboard on their own will sit unchanged for a very long time. Water the heap as you turn it, so the water reaches the middle rather than running off the top, and add some greens to give the organisms something easy to eat. Aim for damp, like a wrung out sponge.
It is damp but still not breaking down
A heap with the right moisture that still will not get going is usually short of greens, short of air, or too small. Add a layer of fresh greens such as vegetable scraps or grass clippings and mix them in. Turn it to let air in. If the heap is only a few buckets of material, it may never warm up, and that is fine: it will still break down, just slowly. Chopping large pieces into smaller ones also helps, since organisms work from the surface inward.
Fruit flies and other flies
Small flies gather where fruit and vegetable scraps are exposed. They are harmless but annoying, especially in a kitchen caddy. Bury fresh scraps a little way into the heap and cover them with a layer of browns every time. Keep the caddy lid closed and empty it often. In a worm bin, add scraps under the bedding rather than on top.
Rats, mice and other visitors
Rodents and other animals come for food and for a warm, dry place to nest. Keep meat, fish, dairy, cooked food and fats out of an open heap or a bin with an open base. Bury scraps in the middle of the heap. Turning the heap regularly disturbs nesting animals. A bin with a solid lid and a fine wire mesh under its base keeps most of them out. If you want to compost cooked food, a sealed bokashi bucket or a well made tumbler is a better choice than a heap.
Seedlings growing in the heap
Tomato, squash and other seedlings often sprout from a cool heap. They show that seeds survived, which is normal for a heap that never gets hot. Pull them out and turn them back in. If you want weed seeds killed, the heap needs to get hot, or you can keep seeding weeds out altogether.
Lumps and woody bits in the finished compost
Twigs, corn cobs, avocado stones and similar tough material often come out of a heap mostly intact. That is not a failure. Sieve the finished compost through a coarse mesh and put the leftovers back into the heap to go round again. Chopping woody material before adding it shortens the wait.
When to ask for help
Most problems fix themselves once moisture, air and the mix are right. If you want to compost manure from animals you keep, or you are unsure about something specific to your area, your local council or agricultural extension service publishes guidance for local conditions.