Using finished compost
Finished compost is the reward for all the turning and balancing. It improves the structure of almost any soil, helps sandy soil hold water and heavy clay drain better, and feeds the organisms that keep soil healthy. It is not a fertiliser in the strict sense, because its nutrients are released slowly and in modest amounts, but used steadily over several seasons it changes how a garden grows. This page covers how to tell when compost is ready and the most useful ways to put it to work.
Is it ready?
Finished compost is dark brown, crumbly and moist, and it smells like the floor of a forest. Most of the original ingredients should be impossible to recognise, though a few woody pieces, eggshells or stones are normal. If it still feels warm, smells sour or contains recognisable scraps, it needs more time.
A simple test helps when you are unsure. Fill a small pot with the compost, sow a few quick seeds such as cress or radish, and keep it watered. If the seeds come up and grow normally within a week or so, the compost is ready for most uses. If they struggle, give the compost a few more weeks to cure. Compost that has not finished can hold back young plants while it goes on breaking down.
Sieving
Most gardeners sieve compost before using it in pots or for sowing, and many do not bother for beds. A sieve with holes about the width of a finger takes out sticks and lumps, which go back into the heap. Sieving is easier when the compost is slightly dry rather than wet.
On vegetable and flower beds
The most common use is to spread compost over beds as a layer a few centimetres deep and either fork it lightly into the top of the soil or simply leave it on the surface for worms to pull down. Spreading it in autumn or early spring gives it time to settle before planting. Many gardeners add compost to each bed once a year. Hungry crops such as squash, tomatoes and sweetcorn benefit from a little extra worked into their planting holes.
As a mulch
A layer of compost on the surface of the soil around plants works as a mulch. It keeps the soil cooler and moister in summer, reduces weeds a little and feeds the soil as it breaks down. Keep it a short distance away from the stems of plants and the trunks of trees and shrubs, since compost piled against bark can encourage rot.
Around trees and shrubs
Established trees and shrubs benefit from a ring of compost spread over the soil above their roots, which reach well beyond the trunk. Spread it thinly over a wide area rather than in a thick pile close to the trunk. For new plantings, mix some compost with the soil you put back in the planting hole, but do not fill the hole with compost alone, since the plant needs to grow its roots into the surrounding soil.
On lawns
Compost can be used as a top dressing for lawns. Sieve it finely, spread a thin layer over the grass, and brush or rake it down into the turf so the grass is not smothered. This is usually done in spring or autumn when the grass is growing. It improves the soil underneath over time and can help with patches that struggle.
In pots and containers
Garden compost on its own is usually too rich and too variable for pots, and it can contain weed seeds. Mixing it with other materials gives better results. Many gardeners blend sieved compost with garden soil, or with a bought potting mix, rather than using it neat. For sowing seeds, a finer and more consistent mix is best, so sieve the compost well and use it as one part of the mix.
Storing compost
If you have more compost than you can use, keep it in a covered heap or bags so rain does not wash its nutrients away. It keeps for many months. Compost left uncovered in the open will slowly lose some of its goodness and gather weed seeds blown in from elsewhere.
Compost tea and other extracts
You may read about steeping compost in water to make a liquid feed. Gardeners use these, but the evidence for their benefits is mixed, and spreading the compost itself is simpler and more reliable for most home gardens. If you try one, use it on soil rather than on the leaves of food crops.
How much you need
There is no single right amount. A few centimetres over your beds each year is a sensible routine for most gardens. If you have less than that, put it where it does the most good: beds for hungry crops, new plantings and any patch of poor soil.