How Does Composting Work? Managing a Population of Microbes

Almost everything that goes wrong traces to one of four conditions being off:

• •The carbon to nitrogen ratio, ideally around 25 or 30 parts carbon to one of nitrogen. Carbon-rich material, called browns, includes dry leaves, straw, cardboard and woody prunings; nitrogen-rich material, called greens, includes grass clippings, vegetable waste and manure. Too much nitrogen and the surplus is lost as ammonia, which is the source of the smell; too much carbon and the process stalls
• •Oxygen, since the organisms that work fast are aerobic. A compacted or waterlogged heap goes anaerobic, which is far slower and produces hydrogen sulphide, organic acids and methane, meaning a rotten smell rather than an earthy one
• •Moisture, ideally around fifty to sixty percent, which feels like a wrung-out sponge. Too dry and activity stops, too wet and the water displaces air and the heap goes anaerobic
• •Particle size and volume, since smaller pieces decompose faster by offering more surface, and a heap below about a cubic metre loses heat faster than it generates it and never gets hot

A hot heap runs through a sequence of microbial communities and the temperature curve records it. In the first day or two, mesophilic bacteria, which prefer moderate temperatures, multiply rapidly on the easily available sugars and proteins, and their metabolism releases heat that the insulating bulk of the heap traps. As the temperature passes about forty degrees they are replaced by thermophilic bacteria and fungi, which work at fifty to seventy degrees and break down fats, cellulose and other tougher material; this phase lasts days to weeks depending on how much food remains. Above about sixty-five degrees activity drops off because even the thermophiles have limits, which is one reason turning matters. As the accessible material is consumed, the heap cools and the mesophiles return, along with fungi that attack lignin, and finally the curing phase begins, during which the material stabilises slowly at ambient temperature and is colonised by worms, mites, springtails and beetles that fragment what remains.

The thermophilic phase is not necessary to produce compost and it is worth achieving. Sustained temperatures above about fifty-five degrees for several days kill most weed seeds, plant pathogens and human pathogens, which is why regulatory standards for commercial composting specify a minimum temperature held for a minimum period with a minimum number of turns. A cold heap will eventually decompose the same material and will happily return every weed seed and fungal spore to the garden. Heat also speeds everything up dramatically, turning a process of a year into one of two months. The practical method for achieving it is to build the heap all at once rather than adding material gradually, to get the ratio and moisture right at the start, to make it at least a cubic metre, and to turn it when the temperature begins to fall, which reintroduces oxygen and moves the outer material, which never gets hot, into the middle.

Hot aerobic composting is one approach among several, each suited to different conditions. Cold composting simply piles material and waits, taking six months to two years with almost no effort and no pathogen kill. Vermicomposting uses worms, usually tiger or red wiggler species rather than earthworms, in a contained system at room temperature, which handles kitchen waste well in a small space and produces a rich material along with a liquid that drains off. Bokashi is not composting at all but anaerobic fermentation in a sealed bucket using inoculated bran, which pickles the material, handles meat and dairy that other methods cannot, and produces something that must then be buried or composted to finish. Anaerobic digestion at industrial scale does the same thing deliberately in sealed tanks and captures the methane as fuel, which is the treatment used for food waste collected by local authorities in many places, with the solid residue returned to land as fertiliser.

The product improves soil in several distinct ways at once, which is why it outperforms a mineral fertiliser at the same nutrient content. It supplies nutrients slowly as the organic matter mineralises, rather than in a pulse that can be washed out. It improves structure, since the organic matter binds mineral particles into crumbs, which opens up pore space, improving both drainage in heavy soil and water retention in light soil. It feeds the soil microbial community, which mediates nutrient availability to roots. And it raises the cation exchange capacity, meaning the soil's ability to hold positively charged nutrients against leaching. The waste argument is separate and substantial: food and garden waste sent to landfill decomposes anaerobically under the surface and generates methane, a greenhouse gas roughly eighty times more potent than carbon dioxide over twenty years, so diverting it to aerobic composting or to a digester that captures the gas is one of the more straightforward emissions reductions available to a household.

Composting is the management of microbial populations, controlled by four variables: a carbon to nitrogen ratio near thirty to one, adequate oxygen, moisture like a wrung-out sponge, and enough bulk to retain heat. A well-built heap runs through mesophilic, thermophilic and curing phases, and the hot phase matters because temperatures above fifty-five degrees kill weed seeds and pathogens that a cold heap returns to the garden. Worm bins, bokashi fermentation and industrial anaerobic digestion are alternatives, and the product improves soil structure and microbial life as well as supplying nutrients.