Making compost at home

Compost is the most useful thing you can add to any Australian soil. In sand it holds the water and nutrients that would otherwise drain past the root zone. In clay it separates the fine particles and creates the pore space clay does not have. It costs nothing except the material you were going to throw away, and the process is simple — but it fails in the same four ways every time.

What actually breaks material down

A compost heap is a population of bacteria, fungi and larger organisms working through organic material. They need four things: a food supply with the right balance of carbon and nitrogen, air, moisture, and enough bulk to hold heat. Take away any one and the process slows or stops. Everything below is about supplying those four.

Carbon is the energy source and it comes from dry brown material. Nitrogen is the protein source and it comes from fresh green material. The organisms need considerably more of the first than the second, and the mistake almost everyone makes is supplying green material generously — because that is what the kitchen produces — and brown material hardly at all.

What to put in

Brown, carbon-rich: fallen leaves, straw, shredded plain cardboard and paper, small dry prunings, spent stems, untreated sawdust in small quantities, shredded egg cartons.

Green, nitrogen-rich: grass clippings, kitchen vegetable and fruit scraps, coffee grounds, tea leaves, spent annual plants, aged manure from herbivores.

Leave out: meat, fish, dairy and cooked food, which attract vermin in a home heap; large woody prunings over about 20 mm, which will not break down in step with the rest; anything with persistent perennial roots, such as couch runners or bulbous weeds, unless the heap reliably gets hot; diseased plant material, which is better in the bin; treated or painted timber; and plastics of any kind, including tea bags with synthetic mesh.

The nine steps

1. Choose a position

   Put the heap in part shade rather than full sun. A heap in direct summer sun dries out from the outside in and stops working, and in an arid or tropical garden it can dry out in a few days. Deep shade is workable but slower.

   Set it on bare soil rather than on concrete or paving, so soil organisms can move up into it and excess moisture can drain away. Allow enough room alongside to turn the heap into a second space; that adjacent square metre is what makes turning easy rather than a chore.

2. Get the size right

   A heap needs volume to hold heat. About one cubic metre — roughly a metre in each direction — is the practical minimum for a heap that heats up. Much smaller than that and it will still break down, but slowly and cold, and it will not kill weed seed.

   Two bays side by side, or two bins, is the arrangement that works best at home: one being filled, one maturing.

3. Balance carbon and nitrogen

   Compost needs roughly two to three parts carbon-rich material to one part nitrogen-rich material by volume. Carbon-rich material is dry and brown: fallen leaves, straw, shredded plain cardboard and paper, small dry prunings, sawdust in small quantities. Nitrogen-rich material is fresh and green: grass clippings, vegetable and fruit scraps from the kitchen, spent plants, aged manure.

   Almost every failed home heap is failed for the same reason: too much green material and not enough brown. A heap that is slimy, dense and smells sour has too much nitrogen and too little air. Keep a bag of dry leaves or straw beside the heap and add a couple of handfuls every time you add kitchen scraps.

4. Chop material down before it goes in

   Particle size controls speed more than almost anything else. Material cut to 50 mm or smaller breaks down several times faster than the same material left whole, because there is far more surface area for organisms to work on.

   Run the mower over leaves and light prunings, cut woody stems with secateurs, and tear cardboard rather than laying it in sheets. Thick woody prunings over about 20 mm are better set aside for a slower heap or a mulch pile; they will not break down in step with everything else.

5. Build the heap in layers

   Start with a coarse layer of small prunings or straw at the base, about 100 mm deep, so air can enter from underneath. Then alternate: a layer of brown material about 100 mm deep, a layer of green material about 50 mm deep, and a scattering of soil or finished compost to introduce organisms.

   Water each layer as you build so the material is damp throughout rather than watering the top at the end, which soaks the surface and leaves the middle dry.

6. Get the moisture right and keep it there

   The target is the moisture of a wrung-out cloth. Squeeze a handful: it should hold together and release a drop or two of water, no more. Too dry and the heap stalls completely; too wet and air is displaced, the process turns sour and the smell changes.

   Cover the heap with hessian, a lid or a sheet of old carpet. This holds moisture in a dry climate and keeps heavy rain out in a wet one. In the tropical wet season, covering the heap is not optional; an uncovered heap becomes waterlogged within a week.

7. Turn it

   Turning moves the outside of the heap to the inside, reintroduces air, and evens out moisture. Fork the whole heap across into the adjacent space, breaking up any matted clumps as you go.

   Turn a hot heap after about a week, then every one to two weeks. Each turn will restart the heating. If you are not in a hurry, turning once a month still works — it is simply slower. Turning is the single most effective intervention available if a heap has slowed down.

8. Watch the temperature

   A well-built heap heats within two or three days and will be too hot to hold a hand in the centre for a week or more. That heat is a sign the process is working, and it is what kills most weed seed.

   When the heap stops reheating after turning, the active phase is over. Leave it to mature for another four to six weeks. Finished compost is dark, crumbly, smells of earth rather than of anything else, and the original ingredients are no longer identifiable.

9. Use it

   Spread 50 mm over garden beds and work it into the top 150 mm, or spread it as a top-dressing under mulch. In vegetable beds, add compost every season; in ornamental beds, once a year is enough.

   If some material has not broken down — woody stems, avocado stones, corn cobs — sieve it out and put it back into the next heap. It will finish there rather than in the garden.

Diagnosing a heap that is not working

Nothing is happening and the heap is cool and dry. Too little moisture, or too little nitrogen. Water it thoroughly while turning, and add a bucket of grass clippings or aged manure.

The heap smells sour and is slimy and dense. Too much nitrogen and too little air. Turn it and mix in a generous quantity of dry brown material — straw, shredded cardboard, dry leaves — to open it up.

The heap is wet through and rainwater is pooling. Cover it, turn it, and mix in dry material. In a wet-season climate, build the heap under cover from the start.

It heats and then stops after a few days. Normal. Turn it and it will heat again. Each cycle is shorter than the last until the material is finished.

Everything has broken down except woody pieces. Sieve them out and return them to the next heap.

The heap has ants through it. Almost always a sign that the heap is too dry. Water and turn.

Leaf mould, separately

Autumn leaves are worth keeping apart from the main heap. Piled in a wire cage or a bag with a few holes punched in it, and kept damp, they break down over about a year into leaf mould: low in nutrients but excellent for soil structure and moisture holding. It needs no turning and no management. In cool and temperate districts with deciduous trees, this is free soil improvement in exchange for raking.

Cold composting, if you would rather not turn

A heap that is never turned still works; it simply takes six to twelve months instead of two to three, runs cool, and will not kill weed seed. If that suits you, keep the carbon and nitrogen balance roughly right, keep it covered and damp, and simply add material as it comes. Use two bins so one can mature while the other fills.

The output is the same material. The only real costs of not turning are time and the fact that weed seed and perennial roots survive, so be more selective about what goes in.