Why is my compost not breaking down?

Composting needs four things in balance: nitrogen-rich green material, carbon-rich brown material, moisture and air. A failing heap is missing one of them, and the symptoms tell you which.

What you'll need

• Fork
• Cardboard or brown material
• Water (optional)

Step by step

1. Smell it and look at it before doing anything.Slimy, smelly and wet means too much green and not enough air. Dry, unchanged and full of recognisable material means too little moisture and nitrogen.
2. For a wet slimy heap, add brown material and turn it.Torn cardboard, shredded paper, dry leaves, straw. Mix it thoroughly through — this adds both carbon and air pockets.
3. For a dry inactive heap, water it and add greens.It should feel like a wrung-out sponge. Grass clippings, vegetable peelings or a handful of used coffee grounds provide the nitrogen.
4. Aim for roughly equal greens and browns by volume.Greens are wet and nitrogen-rich: peelings, grass, coffee grounds. Browns are dry and carbon-rich: cardboard, paper, dry leaves, straw.
5. Turn it every few weeks.It introduces air, which the bacteria need. An unturned heap composts eventually but far more slowly and often anaerobically.
6. Chop material smaller.Smaller pieces break down far faster. Woody stems and whole vegetables can sit unchanged for a year.
7. Keep it covered but not sealed.A lid or a sheet keeps rain from waterlogging it while letting air in. An open heap in a wet climate becomes saturated.
8. Leave out what does not belong.Cooked food, meat and dairy attract vermin. Perennial weed roots and diseased material survive home composting and spread when you use it.

Tips

• A heap needs a reasonable volume to heat up — roughly a cubic metre. Very small bins compost slowly and never get hot.
• Cardboard is the most useful and most available brown material. Tear it up and remove tape and plastic labels.
• If it is full of woodlice and worms rather than heating up, it is composting slowly but perfectly well. It just takes longer.

Common mistakes

• Adding only grass clippings and kitchen waste — It is all green material, which goes wet, slimy and anaerobic. Browns provide structure, air and carbon.
• Never turning the heap — Without air the process goes anaerobic, which is slow and smells unpleasant. Turning every few weeks transforms the speed.
• Composting perennial weed roots and diseased plants — Home compost does not get hot enough to kill them, so you spread both back across the garden.

Questions people ask