Why a Heap Goes Wrong

If your compost heap has stalled, gone slimy or smells like something has died, the chances are that one of three things has slipped: the balance of materials, the moisture level, or the amount of air reaching the middle. A working heap is a living thing — bacteria, fungi, worms, woodlice and a host of invisible helpers all doing their bit. Give them the right conditions and a cubic metre of mixed waste will reduce to a barrowful of crumbly dark compost in a few months. Get it wrong and you end up with a wet, sour mass that never heats up and attracts more flies than you would like.

The good news is that compost heaps are forgiving. Almost every problem can be fixed with a fork, a few barrowloads of the right material and a bit of patience.

Get the Greens and Browns Right

The single biggest cause of trouble is too much green material and not enough brown. "Greens" are the soft, wet, nitrogen-rich things: grass clippings, kitchen peelings, spent annuals, comfrey, nettles, chicken muck. "Browns" are the dry, woody, carbon-rich things: shredded cardboard, newspaper, egg boxes, straw, woodchip, dead leaves, twiggy prunings.

Aim for roughly two to three parts browns to one part greens by volume. In practice that means every bucket of peelings should be followed by a bucket or two of scrunched-up cardboard, or a good handful of straw.

• Too many greens: the heap heats up fast, goes slimy and collapses into a smelly mat.
• Too many browns: nothing much happens, and the pile sits there dry and unbothered for a year.
• The fix: if it stinks, add browns and a coarse handful of twiggy material; if it is inert, add grass clippings, comfrey or a bucket of kitchen waste.

Keep a bag of shredded cardboard next to the bin and you will never be caught short.

Fix the Moisture Before Anything Else

Compost needs to be as damp as a wrung-out sponge. Squeeze a handful: a few drops of water should appear on your fingers, but it should not be dripping. Too dry and the bacteria shut down; too wet and the air spaces fill with water and the heap turns anaerobic, which is where the really unpleasant smells come from.

• Too dry: water with a can between layers, or add fresh grass clippings and leafy greens. Avoid soaking the surface only — the middle is what matters.
• Too wet: turn the heap and mix in dry browns: cardboard, straw, sawdust from untreated wood. If it is sitting in a puddle, lift it onto a base of twigs or pallets.

In a wet British winter, cover the top with an old tarpaulin, a piece of carpet or a lid. In a hot summer, the reverse — you may need to water a heap that has dried out completely.

Turn It When It Cools

A well-built heap heats up within a few days, often reaching 55–65°C in the core, hot enough to steam on a cold morning. When that heat drops and the pile no longer feels warm to the touch, the microbes have used up the easy food and the oxygen in that zone. That is your signal to turn.

Use a fork or a compost aerator and move the outside material into the middle of the new heap, and the middle to the outside. This re-introduces oxygen, mixes in the half-rotted outer edges and gets everything cooking again. Two or three turns over a season is usually enough; more than that and you are simply burning through the energy the heap needs to finish the job. In the depths of winter, don't worry if the heap goes quiet — it will pick up again in spring.

Rebuild in Layers and Add Coarse Material

If a heap has been neglected, the fastest fix is to take it apart and rebuild it properly. Start with a 10–15cm layer of coarse prunings, twiggy hedge clippings or broken-up stalks across the bottom. This creates a drainage and airflow channel and stops the whole thing turning into a bog.

Then alternate layers: a spadeful of greens, a layer of browns, a sprinkle of soil or old compost to introduce more microbes, and repeat. Coarse material stirred through the middle — sunflower stems chopped small, brassica stalks, woodchip — keeps the pile open so air can move.

• Turn or rebuild roughly once a month in summer, less in winter.
• Keep pieces under 5cm where you can; big lumps rot very slowly.
• Leave out meat, cooked food, dairy, diseased plants and perennial weed roots.

Reading the Smell: A Quick Diagnosis

Your nose is a good guide. Each smell points to a specific fault.

• Ammonia or a sharp chemical whiff: too much nitrogen. Add browns and turn.
• Rotten eggs or boiled cabbage: anaerobic, compacted and too wet. Turn, add coarse browns, cover against rain.
• Sour, vinegary or yeasty: the heap is fermenting rather than composting. Usually too wet and too tightly packed; turn and mix in dry material.
• Nothing at all, plus flies: the heap is too dry, or the greens are sitting exposed on top. Bury kitchen waste in the middle and water the pile.

Keep a fork, a bag of shredded cardboard and a tarpaulin near the bin. A heap that is turned when it cools, kept damp and fed a decent mix of greens and browns will reward you with dark, sweet-smelling compost within a few months — and considerably fewer flies and complaints from the neighbours.