How do I know when my compost is ready to use?

The look, feel and smell of finished compost, the cress test that settles it, and what immature compost does to plants if you use it too early.

Difficulty: beginner
Time: 20 min
Read: 4 min
Safety: caution

Short answer

Ready compost is dark, crumbly, cool to the hand and smells of woodland floor, with nothing recognisable left except the odd woody fragment. If it is still warm, still smells sharp, or you can still identify what went in, it is not finished — and used now it robs nitrogen from the soil and checks young plants. Ten days of cress on a tray of it against a tray of bought compost tells you for certain.

Compost that looks finished and compost that is finished are different things, and the difference matters most in a pot or a seed bed. Material that is still actively decomposing goes on taking nitrogen out of its surroundings to do it. Spread on the surface of a border that hardly matters; mixed into a pot or a seed drill, it can stall a plant for weeks.

Safety

Handle finished and unfinished compost with gloves, avoid breathing the dust when sieving, and dampen it first. Compost heaps carry Legionella and other bacteria, and garden soil carries tetanus — keep your cover current and wash your hands afterwards.

Telling finished compost from compost that only looks finished

Step by step

Look for recognisable material.Eggshell, cardboard, peel, stems. If you can still name it, the heap is not finished. A few woody fragments are normal and are sieved out.
Feel the temperature.Finished compost is at air temperature. Warmth at the centre means the bacteria are still working, whatever the surface looks like.
Smell it.Woodland floor and damp earth means ready. Sharp, sour, ammoniacal or vinegary means it is still active or has gone anaerobic.
Squeeze a handful.It should hold together loosely and fall apart when you prod it. A ball that stays as a ball is too wet, and dust that will not cohere is too dry.
Run the cress test if it matters.Sow a pinch of cress on a tray of your compost and a pinch on a tray of bought multipurpose, water both, and compare after ten days. Equal growth means mature. Poor, yellow or failed germination means it is still active.
Sieve it and return the coarse fraction.A garden riddle takes out the twigs and unrotted lumps. Those go back into the new heap, where they carry on the process and act as a starter.
Match the compost to the job.Half-finished compost is a perfectly good surface mulch under established plants. Finished, sieved compost is what goes into potting mixes, over seed beds and around new plantings.
Expect weed seed and plan for it.Home heaps rarely get hot enough throughout to kill seed. Use unsieved compost as a mulch where seedlings can be hoed off, not on a fine seed bed you are about to sow into.
Let it mature after the hot phase.A hot heap needs several weeks cool and undisturbed afterwards, when the worms and fungi move back in and finish the job. That maturing stage is what makes it safe for pots.

Tips

Two bins, or a heap and a maturing pile, removes the question entirely — you always have finished compost at one end and raw material at the other.
The cress test is worth doing once, just to calibrate your judgement. After that you can tell by smell.
Woody bits sieved out and returned are the best compost starter there is, and free.

Common mistakes

Potting up into unsieved, unfinished compost — It carries on decomposing in the pot, tying up nitrogen and starving the plant. Growth goes pale and stops and the cause is invisible.
Sowing seed into homemade compost — Weed seed, uneven nutrients and coarse texture. Homemade compost is a soil improver and a mulch; seed compost is a specific product for a reason.
Judging by the top of the heap — The surface layer is always the least decomposed. Dig into the middle and the base, which is where the finished material is.

If it doesn't work

Looks fine but plants stall after using it

Cause: Immature compost tying up nitrogen — Fix: Give a nitrogen-rich liquid feed and use the rest as a surface mulch rather than mixing it in.

Still warm after a year

Cause: Fresh material keeps being added to the same bin — Fix: Stop adding, cover it, and start a second bin for new material.

Full of weed seedlings after spreading

Cause: Heap never got hot enough to kill seed — Fix: Hoe them off while small, and apply future batches under existing plants rather than on bare beds.

Sour smell in what should be finished compost

Cause: Waterlogged and anaerobic at the base — Fix: Turn it, mix in dry brown material, and cover the heap from rain.

Questions people ask

How long does compost take?

A hot, turned heap of adequate volume: two to four months plus maturing. A cold, trickle-fed bin: twelve to twenty-four months. Both make good compost; only one of them is quick.

Can I use compost that still has twigs in it?

Yes, as a mulch on the surface of a border. Sieve it before it goes anywhere near a pot or a seed bed.

Is it safe to use compost from a heap rats have been in?

Use it on ornamental borders rather than on salad, fruit or anything eaten raw, and handle it with gloves. A cold heap does not reach temperatures that deal with what rats leave behind.