It Smells of Rotten Eggs or Sewage

Cause: no oxygen. The heap is too wet, too green, and compacted, so anaerobic bacteria have taken over.

Fix: turn it, and as you turn, mix in a large quantity of dry browns - torn cardboard, dry leaves, straw, shredded paper. More than feels reasonable. Then cover it to keep rain off, and if it sits in a puddle, move it or improve the drainage beneath.

It will smell for a day or two after turning and then stop.

Prevention: browns with every addition of kitchen waste, and never dump a thick layer of grass clippings on its own. Grass mats into an airless green sludge within days; mix it with browns before it goes in.

It Smells of Ammonia

Cause: too much nitrogen. Excess greens with nowhere for the nitrogen to go, so it escapes as ammonia gas - which is nutrient you are losing.

Fix: the same as above. Add browns, mix thoroughly.

Nothing Is Happening

A heap that looks the same as it did three months ago.

Check moisture first. Dig into the middle and squeeze a handful. Dry material does not decompose - this is why a bag of autumn leaves can sit intact for years. Water it thoroughly while turning.

Then check nitrogen. A heap of pure cardboard, leaves, and woody prunings has energy but no building material. Add greens: grass clippings, kitchen waste, coffee grounds.

Then check size and season. A small heap in winter is simply cold and will restart in spring. This is not a fault.

Then check particle size. Whole branches and thick stems take years. Chop or shred anything substantial.

Flies

Small fruit flies around the bin are harmless and normal, and they appear when fresh food waste is exposed at the surface. Bury each addition under browns or a layer of existing compost and they mostly disappear.

Large numbers of white maggots are usually the larvae of soldier flies or houseflies, which means exposed food waste. Unpleasant to look at and, in fairness, extremely efficient decomposers. Bury the food, add browns.

Rats and Mice

The uncomfortable truth is that a compost heap is a dry, warm, food-scented shelter, and if there are rats in the area some will investigate.

What attracts them beyond casual interest is cooked food, meat, dairy, and bread. Keep those out entirely and the risk drops sharply.

Reduce it further by keeping the heap moist - rats want dry nesting material - by turning regularly, since disturbance discourages nesting, by using a closed bin with a lid, and by fitting fine wire mesh under an open-bottomed bin, which still admits worms and water but not rodents.

If you find a nest, stop composting food waste for a while, turn the heap frequently, and check your council’s guidance. Rodents near a home are a public health matter rather than a gardening one.

Ants, Woodlice, and Everything Else

Ants indicate a dry heap; water it. Woodlice, worms, beetles, and centipedes are part of the system and want no intervention. Fungal threads and white mould through the material mean it is working properly.

When Is It Ready

Finished compost is dark brown to nearly black, crumbly, uniform, and smells of forest floor. You should not be able to identify what any of it used to be, with the usual exceptions of eggshells, avocado stones, and twigs, which persist for a long time and can simply be sieved out and returned to the next heap.

It will be a fraction of the volume you put in - typically a quarter or less, which is normal and startling the first time.

The cress test settles doubt: sow some cress or other fast seed in a pot of the compost and a pot of ordinary soil. If it germinates and grows equally in both, the compost is mature. If it fails or grows poorly, the material is still actively decomposing and is temporarily locking up nitrogen and producing acids. Give it another month.

Unfinished compost dug into a bed will stall the plants growing there, because the decomposers outcompete the roots for nitrogen until they are done. This is the most common way good compost gets a bad reputation.

Typical timings: a well-managed hot heap, two to three months. An ordinary bin added to gradually, six to twelve. A neglected pile, one to two years. All three eventually produce compost.

Using It

As a mulch, spread 3 to 5cm on the surface around established plants, and let the worms take it down. This is the least work and arguably the most benefit - it suppresses weeds, retains moisture, and feeds soil life.

Dug into beds before planting, a bucket or two per square metre, to improve both structure and water-holding.

As a soil improver in a new bed, mixed into the top 15 to 20cm.

In potting mixes only when sieved and only as a component - typically no more than a third, mixed with soil and something for drainage. Home compost on its own is too rich and too variable for seedlings, which want a low-nutrient medium.

Rough or partly finished compost is perfectly good spread on empty beds over winter, or on the bottom of a planting hole for shrubs, or returned to the next heap as an inoculant.

A note on expectations: home compost is a soil conditioner, not a fertiliser. Its nutrient content is modest and slow-release, and its real value is what it does to soil structure, water retention, and the population of organisms living in it. Hungry crops will still want feeding.