Compost Pile

A compost pile is a heap of dead plant material - kitchen trimmings, leaves, weeds, spent crops - stacked in the open and left to rot. The organisms breaking it down need air, so the heap is built loose rather than packed tight, and their work gives off heat. A heap with enough bulk holds that heat in and runs at 40 to 60 degrees Celsius (104 to 140 Fahrenheit) inside. Three or four months later it has become a dark crumbly material to spread on growing ground, and 40 to 70 percent of the volume has gone.

Three things decide whether a heap heats. The first is the mix: leaves, straw and sawdust are mostly carbon, while kitchen scraps and cut weeds carry the nitrogen the organisms need to multiply, and about thirty parts carbon to one of nitrogen is the working recipe. The second is water, kept at 40 to 60 percent, which in the hand means as damp as a wrung-out sponge; waterlogged, the heap runs out of air and turns sour. The third is bulk: below about 1 m tall and 2 m across the base (3 ft by 6 ft) it cannot hold its own warmth and merely rots slowly. Heat is the point, because the band from 43 to 71 degrees Celsius (110 to 160 Fahrenheit) kills weed seed and plant disease, and a cold heap hands both back to the beds it was meant to feed.

Turning replaces the air the organisms use up, often through the first three or four weeks and once or twice after, with a probe thermometer to say when. It is not free: a heap turned while dry loses nitrogen to the air as ammonia. Meat, bones, fat, dairy and oils are kept out because they draw rodents, dogs, cats and flies. Watch what drains from the base as well - that brown liquid, called leachate, carries nutrients into whatever lies downhill, so a heap belongs on flat ground clear of runoff and well away from a well or a watercourse, never in a low spot that stands water. Poultry given access will turn it and take the grubs.