Building a Compost Heap That Actually Works

Start With a Heap You Can Actually Manage

A compost heap is not a bin you fill and forget. It is a living thing — a slow-burning engine of bacteria, fungi, worms and woodlice — and like any engine it needs the right fuel, air and water. Get those three roughly right and the heap does the work for you. Get them wrong and you end up with a slimy, sour sack of grass clippings that puts you off the whole idea for a year.

Choose the spot before you build anything. Heaps work best sitting directly on soil, so worms and microbes can move up from below. Somewhere that stays reasonably damp, catches a bit of sun but not all-day baking, and sits within easy barrow distance of both the kitchen door and the veg patch is ideal. If you have the space, build two or three bays side by side from pallets, scaffold boards or straw bales. You fill one, turn it into the next, and always have somewhere to put the fresh material.

Size matters more than most people expect. A heap needs to be at least a cubic metre — roughly a metre wide, a metre high and a metre deep — to hold enough heat to compost properly. Anything smaller dries out and sulks. Anything much bigger is hard work to turn.

Greens, Browns and the Balance Between Them

Everything that goes on the heap is either a green or a brown. Greens are wet and nitrogen-rich; they are what makes the heap heat up. Browns are dry and carbon-rich; they keep it open, airy and sweet-smelling. The whole craft of composting comes down to balancing the two.

• Greens: grass clippings, kitchen peelings, tea leaves and coffee grounds, nettles, comfrey, annual weeds pulled before they seed, fresh muck from horses, chickens or rabbits.
• Browns: torn cardboard, scrunched paper, straw, spent hen bedding, autumn leaves, hedge trimmings, a little woodchip, and the contents of the vacuum cleaner if you have wool carpets.

Aim for roughly two parts browns to one part greens by volume. That single ratio solves most heap problems before they start. Grass clippings are the usual culprit — they slump into an airtight mat within hours. Mix them with shredded cardboard or last autumn's leaves as you go, and you will never have a stinking heap.

Keep a sack of torn-up cardboard by the compost caddy and a bag of straw or leaves by the heap. Layering a green and a brown in together as you build is far easier than trying to fix an imbalance later.

Build It in Mixtures, Not Neat Layers

The old advice about alternating neat layers of green and brown looks tidy in books and works poorly in practice. Better to chop everything roughly and mix it as you go. The smaller the pieces, the faster the compost — run a spade through nettles, split brassica stalks, tear cardboard into palm-sized scraps.

Every so often, scatter a spadeful of garden soil or finished compost over the top. That is not feeding the heap so much as inoculating it, introducing the microbes that will do the digesting. A handful of lime now and then helps if you are adding a lot of acidic material, though most heaps manage perfectly well without it.

Air, Water and the Wrung-Out Sponge Test

Composting is an aerobic process, which is a polite way of saying the heap needs oxygen. If it goes anaerobic, it turns black, smells of rotten eggs and takes forever. Turning is how you get air in, and how you move the cooler outer material into the hot middle.

Turn the heap every four to six weeks through the growing season, or whenever it has cooled down noticeably. Fork it from one bay into the next, keeping the outside on the inside. Two or three turns across the life of a heap is usually plenty.

Moisture matters just as much. The test is simple: squeeze a handful of the contents and it should feel like a wrung-out sponge — damp, but not dripping. Dryer than that and the process stalls, so water it as you turn. Wetter and it will go sour and slimy, so fork in more cardboard and straw. In a dry summer, leave the lid off a heap that needs rain; in a wet winter, cover it with a piece of old carpet or a tarpaulin weighed down with bricks, leaving the sides open for air.

What to Leave Out

A well-run heap handles most things, but some material causes more trouble than it is worth.

• Perennial weeds such as bindweed, ground elder and dock roots, unless you are certain the heap gets properly hot.
• Weeds in seed, and any diseased foliage from potatoes, tomatoes or roses.
• Cooked food, meat, fish and dairy, which draw rats and foxes.
• Dog and cat faeces, and cat litter.
• Glossy magazines, plasticised paper and sticky tape.
• Treated or painted timber, and coal ash — wood ash is fine in small amounts.

Reading the Heap, and Giving It Time

A working heap tells you what it needs. Warm to the hand in the middle, with a faint earthy smell, means it is doing nicely. Cold and dry means add water and more greens. Cold and wet means turn it and add browns. Slimy and smelly means too much green and not enough air. A heap that never heats up at all usually just needs more nitrogen — a bucket of nettles or a splash of diluted urine works wonders.

How long? Six months to a year is realistic for a domestic heap, longer over winter, faster in a warm, damp summer. The good stuff is ready when it is dark, crumbly, smells of woodland floor, and you cannot recognise what went in. Sieve out the lumpy bits and return them to the next heap. Then spread it on the beds, and start again — because the finest thing about compost is that it never lets you finish.