Making Compost That Supports Soil Life

Compost is a habitat, not a bin

A single teaspoon of healthy garden soil can hold billions of bacteria and several metres of fungal thread, and your compost heap is where much of that life is either fed or starved. Most of us were taught to think of compost as a way of getting rid of waste and feeding plants. That is only half the story. The real prize is a heap teeming with worms, springtails, mites and fungi, which together turn kitchen scraps and prunings into a crumbly, sweet-smelling material that keeps soil alive for years.

Getting there is not complicated. It comes down to three habits: balancing green and brown, turning the heap now and then, and being patient enough to let the slow workers finish the job.

Balance green leafy waste with brown cardboard and twigs

Think of greens as the nitrogen-rich, wet, sappy stuff that heats a heap quickly, and browns as the carbon-rich, dry, structural material that keeps it open and airy. Aim for roughly two to three parts brown to one part green by volume. In a British garden that means:

• Greens: grass clippings, vegetable peelings, tea leaves and coffee grounds, comfrey leaves, nettles before they set seed, spent annuals, soft hedge trimmings.
• Browns: torn cardboard with the tape and staples removed, egg boxes, scrunched paper, autumn leaves, woodchip, twiggy prunings, straw, and the dead stems of last year's perennials.

Chop things up as you go. A heap of whole cabbage stalks and unbroken twigs will sit for months looking exactly the same. Layer rather than dump: a bucket of greens, a few handfuls of torn card, then a scatter of twigs. Those twigs are especially valuable, because they stop the heap slumping into a soggy mat and create the air pockets that fungal threads love to run through.

Add a woodland layer

British woodland plants such as wood anemone, bluebell, ramsons, wood sorrel, dog's mercury and ferns evolved under a slow rain of leaves and decaying branches, on soils dominated by fungi rather than bacteria. You can mimic that in your compost and in your mulch.

Keep a separate wire cage for autumn leaves. Damp them, tread them down, and leave them for a year to make leaf mould; two years gives something finer. Rake it out in spring as a mulch around shade-loving plants, ferns and hellebores. Sifted leaf mould mixed with finished compost makes a gentle seed-sowing mix for woodland species. If you have space, add a few handfuls of chopped bracken, dead fern fronds or leafy perennial stems to the main heap. They are slower to break down than grass, and that is exactly the point: they feed the fungi.

Turn little and often, and keep it damp

Air and moisture matter more than any additive. Turn the heap every two to four weeks through spring and summer, moving the outside to the middle and the middle to the outside. In winter, leave it alone; the worms will keep working at the edges even when the core cools.

Check moisture by squeezing a handful. It should feel like a wrung-out sponge, damp but not dripping. Too wet and slimy? Fork in torn cardboard and twiggy prunings. Too dry and dusty? Water it and add a layer of grass clippings or kitchen waste. Size helps: a heap of at least a cubic metre will hold heat, but a small bin can still make good compost. It will simply take longer and lean more on worms and fungi than on heat-loving bacteria.

Let worms and fungi do the slow work

Once the heap has cooled, brandling worms and later common garden earthworms move in. They drag material down, mix it with soil in their guts, and leave behind casts packed with plant-available nutrients. Fungal threads spread through the woody material, breaking down lignin that bacteria cannot touch.

Resist the urge to speed things up with powders and boosters. A well-built heap with a bit of garden soil or finished compost mixed in already contains everything it needs. If you want to give it a nudge, a spadeful of old compost or a handful of soil from under a hedge is the most useful thing you can add.

Reading the signs, and using what you make

Finished compost smells sweet and earthy, like a woodland floor after rain. It should be dark, crumbly, cool, and you should struggle to recognise what went in.

• Still warm and smelly: too green and wet, so add browns and turn it.
• Dry with unrotted twigs: too brown, so add grass clippings or kitchen waste and water it.
• Flies and fruit flies: bury kitchen waste in the middle and cover it with browns.
• Barely changed after months: turn it, dampen it, and add a spadeful of soil.

Use the finished material as a mulch under shrubs and woodland plants, fork it into vegetable beds, or blend it with leaf mould for potting. Even a thin layer feeds the worms and fungi, which in turn feed your plants. In a small way, you are rebuilding a piece of the woodland floor that British plants have always known.