Why a Cold Heap Is a Disappointing Heap

Every smallholder has done it. You build a tidy heap out of grass clippings and a few weeds, wait three months, and pull back the covering to find a slimy, sour-smelling mess that looks barely different from the day you built it. Meanwhile, a neighbouring plot seems to produce dark, crumbly compost out of nothing, apparently without effort. The difference is rarely luck. A compost heap that heats up is simply a heap that has been given the right conditions, and those conditions are not complicated once you understand them.

The heat you feel when you plunge a fork into the middle of a working heap is the by-product of millions of bacteria and fungi digesting organic matter. They need four things: a balanced diet, moisture, air, and a reasonable volume of material to insulate themselves. Get those four roughly right and the heap will do the rest. Get one badly wrong and the whole process stalls.

The Green and Brown Balance

The single most common mistake is piling on too much soft, wet, nitrogen-rich material. Grass cuttings, vegetable peelings, comfrey, nettles and fresh manure are all "greens". They are what fuels the bacteria, but on their own they collapse into an airless sludge within days. The answer is to pair them with "browns": carbon-rich, dry, fibrous materials such as cardboard, straw, shredded paper, dry leaves, woodchip and the stalks of spent brassicas.

A useful rule of thumb is roughly two to three parts browns to one part greens by volume. When in doubt, err on the side of more browns. A heap that is slightly too dry will still work, slowly. A heap that is too wet and airless will rot anaerobically and stink. If you can squeeze a handful and get a few drops of water out, the moisture is about right. If it drips freely, add cardboard. If it crumbles and feels dusty, water it as you turn.

• Greens: grass clippings, kitchen scraps, fresh manure, coffee grounds, green weeds (without seed heads)
• Browns: torn cardboard, straw, dry leaves, shredded paper, chopped stems, sawdust in small quantities
• Keep out: cooked food if you have rats, perennial weed roots such as couch grass or bindweed, diseased plant material, and anything glossy or plastic-coated

Size, Air and the Art of the Turn

A heap needs bulk to hold its heat. Aim for a bin or bay at least a cubic metre, roughly a metre in each direction. Smaller heaps will still break down, but they rarely reach the temperatures that kill weed seeds and speed the process along. Build in layers rather than dumping everything in one go, mixing as you go if you have the patience.

Air is the ingredient most people forget. Anaerobic bacteria work without oxygen, but they are slow and they smell. Aerobic bacteria, the ones that generate real heat, need a steady supply of oxygen throughout the pile, not just at the surface. The easiest way to provide it is to turn the heap. Fork the outside to the middle and the middle to the outside, breaking up any clumps as you go. Every two to three weeks is plenty in summer. In winter, once a month is fine.

If turning feels like too much work, you can build air in from the start. Add a layer of twiggy prunings at the base for drainage, and push a few broom handles or lengths of pipe vertically into the finished heap, pulling them out to leave channels. It is not as effective as turning, but it keeps things moving.

Reading the Signs

A well-built heap will heat up within two or three days. Push a metal rod or a fork into the centre and leave it for ten minutes; it should feel warm to the touch, sometimes uncomfortably so. Temperatures of 55 to 65 degrees Celsius are ideal, hot enough to kill most weed seeds and pathogens, but not so hot that the heap burns itself out.

If nothing happens after a week, diagnose the problem rather than starting again. No heat, dry contents and pale material usually mean the heap needs water and more greens. No heat, a slimy texture and a sour smell mean too much water and no air: turn it thoroughly and mix in plenty of cardboard or straw. A heap that heats up and then cools is simply running out of fuel, so add more greens and turn it to restart the process.

Practical Timing Through the Year

Composting slows in cold weather, and that is perfectly normal. Keep a heap going through winter by banking it up, covering it with a tarpaulin or old carpet to shed rain, and accepting that it will sit largely dormant until spring. Save your kitchen scraps in a sealed container and add them in batches when you turn, rather than trickling them in daily where they may attract vermin.

From spring onwards, everything speeds up. Most smallholders find they can fill one bay, turn it into a second bay after a few weeks, and have finished compost within three to six months in warm weather. Sieve out the lumps at the end and return them to the next heap as a starter, which introduces a fresh population of bacteria and gets the new pile off to a flying start.

There is no great secret to a hot heap. Feed it a balanced diet, keep it as damp as a wrung-out sponge, give it air, and make it big enough to keep itself warm. Do that, and you will have the satisfaction of watching steam rise off your own compost on a frosty morning.