If you drink coffee at home, you're throwing away a free, steady supply of compost-friendly material every morning. Used grounds are one of the easiest things to add to a heap — but they're also one of the easiest to add badly. Get the balance right and they'll help keep the pile ticking over nicely; get it wrong and you'll end up with a sour, matted, smelly mess. The good news is that the fix is simple: thin layers, plenty of dry material, and a bit of attention.

What coffee grounds actually bring to a heap

Used grounds are roughly two per cent nitrogen by weight, which puts them firmly in the "green" camp alongside grass clippings and vegetable peelings. They also carry a little phosphorus, potassium and a range of trace minerals, plus a texture that helps hold moisture without turning to sludge — provided they're spread out rather than dumped in a lump.

A common worry is that they'll make the heap too acidic. In practice, most of the acidity has already gone into your cup. Once they're mixed into a working compost heap, grounds break down to something close to neutral, and the finished compost will be perfectly comfortable around most garden plants. If you're spreading them fresh as a mulch around blueberries, camellias or rhododendrons, that slight acidity is a bonus rather than a problem.

Which side of the ledger do they sit on?

Treat them as a green. That's the single most useful thing to remember. Because they look dark and dry, it's tempting to think of grounds as a brown, and heaps get overloaded as a result. A pile fed mostly on grounds and kitchen scraps will turn wet, compacted and airless, which is where the rotten-egg smell and the slimy texture come from.

As a rough guide, aim for something like three or four parts brown material to one part green by volume. A mug of grounds counts as a green, so it needs several mugs of shredded cardboard, dry leaves or torn egg boxes to balance it out. If in doubt, add more browns.

Layer them thin and mix them through

Never tip a solid clump of grounds into the middle of the heap. Spread them in a layer no more than a couple of centimetres deep, then cover with a generous handful or two of dry brown material before adding anything else. If you're using a bin or a tumbler, sprinkle and turn as you go.

• Scatter the grounds across the whole surface rather than in one spot.
• Follow immediately with dry leaves, shredded cardboard, torn paper or chopped straw.
• Give the heap a turn or a stir so the grounds disappear into the mix.
• If the pile is on the dry side, water lightly after adding.
• Keep a lidded tub by the kettle so the grounds don't go mouldy before they reach the heap.

Good partners for coffee grounds

Autumn leaves are the classic match — crumbly, carbon-rich and usually in surplus exactly when you need them. Shredded cardboard and the insides of toilet roll tubes work brilliantly, as do scrunched-up paper, sawdust from untreated wood, and chopped woody prunings.

Keep an eye on the very wet greens as well. If you're adding grass clippings in summer, add grounds in smaller amounts and bulk up with cardboard, otherwise you'll create a dense, airless layer that takes months to break down. Worms adore coffee grounds, so if you keep a wormery the same rules apply: small amounts, mixed in, never heaped on top of the bedding.

Keeping the heap sweet and active

A well-balanced heap should smell earthy, not sour, and should warm up slightly in the middle within a few days of being turned. If your grounds are disappearing quickly and the pile is shrinking, you've got the ratio about right.

If you drink a lot of coffee, don't feel obliged to put every batch in the compost. Grounds also work well scattered thinly as a mulch around established shrubs, or dug lightly into the top few centimetres of soil. A little at a time is the watchword wherever they go.

If things start to go wrong

Sour smell, grey mould and a slimy, clumped texture all point to the same problem: too many wet greens, not enough air and carbon. The remedy is always the same — turn the heap, add a good layer of dry browns, and leave it alone for a week or two. If the pile has gone completely compacted, break it up with a fork and remix rather than adding anything new on top.

Fruit flies hovering around the bin are usually a sign of uncovered kitchen waste rather than the grounds themselves. Bury each addition under a layer of browns and they'll soon lose interest.

Used grounds are a quiet, generous resource — free, constant and genuinely useful. Handle them as a green, layer them thinly, and always give them dry material to work against, and your heap will stay warm, crumbly and sweet-smelling right through the year.