What Actually Happens in a Compost Heap

A compost heap is a busy place. Bacteria and fungi break down kitchen scraps and garden waste first; later, worms, mites and beetles move in and carry on. What’s left is compost: dark, crumbly and full of nutrients for the soil.

What it needs

• GreensFresh, moist, nitrogen-rich material: fruit and vegetable scraps, coffee grounds, grass clippings.
• BrownsDry, carbon-rich material: dry leaves, straw, shredded cardboard and paper. Most heaps work best with more browns than greens.
• AirThe organisms doing the work need oxygen. Turning the heap now and then lets it in.
• WaterDamp, like a wrung-out sponge. Too dry and everything slows down; too wet and it turns slimy and smells.

Hot and cold

A large heap with the right mix can heat up to around 55–65°C in the middle as the microbes work. Hot heaps break down faster, in a couple of months, and the heat kills many weed seeds. A small heap that’s added to bit by bit stays cool and takes longer, often six months to a year, but gets there.

If something’s wrong

• Smells of ammoniaToo many greens. Mix in more browns.
• Wet and slimyToo wet or too compacted. Add browns and turn it.
• Nothing happeningProbably too dry, or too much brown material. Add water and some fresh greens.
• PestsLeave out meat, fish, dairy and oily food in a home heap.