How do I make compost that actually breaks down quickly?

The reason most bins take two years is not that the owner is doing anything wrong; it is that a bin filled a bucket at a time can never get hot. Thermophilic composting is a mass effect: a big enough volume insulates its own centre so that bacterial heat accumulates instead of escaping. Below roughly a cubic metre it simply cannot happen, however good the mixture is.

Safety

What you'll need

• A bay or bin of about a cubic metre, ideally two
• A large volume of green material collected together
• A large volume of brown material collected together
• Garden fork
• Compost thermometer (optional)
• Old carpet or a tarpaulin as a cover (optional)

Step by step

1. Stockpile before you build.This is the part people miss. Save cardboard, leaves and straw in bags for weeks, and time it to a big grass cut, a hedge trim or an autumn clearance so you have both halves at once.
2. Get the volume right.About a cubic metre is the working minimum for a hot heap — roughly a metre cube. Two adjacent bays are ideal: build in one, turn into the other.
3. Chop everything smaller.Surface area drives the rate. Shredded prunings, torn cardboard and chopped stems heat far faster than the same material whole. A rotary mower run over dry leaves and light prunings is a cheap shredder.
4. Put coarse twiggy material at the base.A hundred millimetres or so of woody prunings lets air enter from underneath, which is where most heaps are starved.
5. Build it in layers and then mix them.Thin layers of green, thicker layers of brown, roughly equal by volume overall. Fork through as you go so it is a mixture rather than a stack — layers left intact compost at their interfaces only.
6. Wet it to a wrung-out sponge as you build.Squeeze a handful: it should feel damp and give up a drop or two at most. Dry material never heats, and sodden material goes anaerobic.
7. Cover it and leave it alone for four or five days.A lid, a tarpaulin or an old carpet holds heat and keeps rain out. Within two or three days the centre should be too hot to hold a hand in.
8. Turn it when the temperature falls.Fork the whole heap into the second bay, putting what was on the outside into the middle. It will heat again, a little less each time. Three or four turns over two or three months is usually enough.
9. Correct as you turn, using smell and feel.Ammonia smell means too much green — add browns. Sour or eggy means too wet and airless — add browns and turn more often. Dry and unchanged means it needs water and greens.
10. Let it mature before use.After the hot phase it needs several weeks cooling, when worms and fungi finish the job. Compost used straight off a hot phase can still tie up nitrogen.
11. Ignore accelerators and activators.A balanced, damp, well-aired heap of adequate size has everything it needs. Nettles, comfrey, grass clippings and urine are all effective free nitrogen sources if the heap is short of green material.

Tips

• Two bays are worth more than any product you can buy for a heap.
• Grass clippings are the strongest free activator most gardens have. Mixed, never layered thickly.
• A hot heap kills most weed seed and many pathogens in the centre, but not at the edges — which is exactly why you turn the outside in.

Common mistakes

• Adding material a bucket at a time and expecting heat — The mass never reaches the volume needed to insulate itself. That heap still makes good compost — it just takes a year or two and stays cold.
• Building a heap of pure grass clippings — It compacts into an airless, slimy, sour mat. Grass needs a comparable volume of coarse brown material forked through it.
• Turning a heap while it is still hot and rising — You interrupt the cycle. Let the temperature peak and start falling, then turn.
• Making a heap too wet 'to help it along' — Water fills the air spaces the bacteria need. Damp, not wet.

If it doesn't work

Questions people ask