Compost

What is compost?

Compost is what remains when bacteria, fungi and small soil animals have digested organic residues with the help of air and water. Kitchen scraps, prunings, straw, leaves and manure go in; weeks or months later a dark, crumbly material comes out that smells of woodland soil. It releases its nutrients slowly, and it is itself alive, carrying the soil food web that goes on working in the ground.

The process copies the forest floor in concentrated form. In a forest, leaves decay thinly where they fall; in a heap the same material is gathered in one place, so the microbes work faster. Compost is therefore a concentrate, made in one spot and carried to another. It is not yet humus, the stable fraction of organic matter in soil: compost feeds the process that builds humus, but does not replace it.

The four levers: carbon, nitrogen, water and air

Carbon and nitrogen. Microbes use carbon as energy and nitrogen to build their bodies. The FAO guide on on-farm composting puts the best starting ratio at about 25 to 30 parts carbon to one part nitrogen, with 20 to 40 still workable. Straw, dry leaves and wood chips are rich in carbon; fresh cuttings, kitchen scraps and manure in nitrogen. Too much carbon and the heap stays cold; too much nitrogen and it smells of ammonia, because the surplus escapes into the air. By the end the ratio falls to around 10 to 15, as much of the carbon leaves as carbon dioxide.

Water. Microbes live in the film of water around each particle. The FAO gives 40 to 65 percent moisture as the working range, ideally starting at 50 to 60. A handful squeezed hard should feel like a wrung-out sponge. Above about 65 percent air is pushed out and the heap begins to rot and stink.

Air. Composting needs oxygen. Coarse material keeps the heap open, turning brings air into the core. A heap that is too dense suffocates in the middle, one that is too small loses its heat.

Hot and cold rotting, and when compost is ripe

A well built heap follows a clear temperature curve. First, organisms that like moderate warmth, between about 20 and 45 degrees Celsius, break down sugars and starches. Their activity heats the heap, and heat-loving organisms take over at 50 to 70 degrees. According to the FAO, pathogens are normally destroyed from 55 degrees, weed seeds need around 62. After days or a few weeks the easy food is used up and the temperature falls; turning can rekindle it. Once the heap no longer warms after turning, curing begins, and fungi, worms and other soil animals let the material mature.

This is hot composting: the heap is built at once from enough mixed material. Cold composting means adding a little at a time. The heap stays near outdoor temperature, takes many months and spares fungal life, but cannot be relied on to kill weed seeds or plant diseases. Diseased plants and seeding weeds belong in a hot heap or not in compost at all.

Ripe compost is dark, crumbly and smells of forest floor. A simple test is to sow cress into a pot of it: if the seeds come up evenly and stay green, nothing in it will harm young roots.

Worm compost

Worm compost, or vermicompost, is made by compost worms, mostly Eisenia fetida, which live in decaying organic matter rather than in mineral soil. They work through kitchen scraps and moist bedding and leave fine, crumbly castings. There is no hot phase: the worms prefer roughly 15 to 25 degrees and, according to a review by Clive Edwards, reach their limit around 35. A worm bin needs a cool, shaded place and steady moisture; in a Mediterranean summer, the coolest corner of the site or a box sunk partly into the ground. The page on vermicompost goes into the practice.

Compost in a dry Mediterranean climate

Most compost advice comes from regions with rain all year. In Sardinia the limit is water: an open heap in July dries out within days, and dry compost simply stops. The FAO describes pit methods for areas with scanty rainfall, covered with earth to keep the moisture in, and recommends shady places such as the ground under large trees. Here that means building in autumn and winter, when rain falls and green material is plentiful, wetting every layer, choosing a pit or a narrow windrow covered with straw or soil instead of a loose open heap, and letting it rest under cover through the summer.

Compost on the surface needs protection too. Left bare, it dries out and loses nitrogen to the air; under mulch it stays moist and keeps feeding soil life. Sheet mulching goes one step further and lets material decay where it lies. The heat of rotting can even be used: in Provence around 1970, Jean Pain ran water pipes through heaps of shredded brushwood to warm water.

On a dry slope, compost is a tool for places that need concentrated fertility: planting pits, nursery beds, young trees. The larger work is done by ground cover and by material that decays where it falls, because nothing has to be carried and nothing dries out on the way.