Worm Bin

A worm bin is a shallow covered container of damp bedding kept alive with composting worms. They eat kitchen scraps and the bedding itself and leave their droppings behind; those droppings are called castings, and the harvest is known as vermicompost. Dug into a bed it holds water, opens heavy soil and slows erosion, and it carries on the order of 4 per cent more nitrogen than compost from an outdoor heap. Kept in a room that stays warm it goes on working through the cold months, when a heap outdoors stalls.

The worms set the shape of the bin. They feed at the surface rather than burrowing, so the bed is made wide and shallow - about 15 cm (6 in) of bedding is enough, and a deep bucket is the wrong vessel. Bedding is dry carbon torn small - newspaper, cardboard, dry leaves or chopped straw. Two conditions keep the colony alive. Temperature has to stay in a narrow band, roughly 13 to 27 degrees C (55 to 80 F) and out of direct sun: worms slow as it cools and die near freezing, which is why the bin belongs indoors or in deep shade. Moisture is held at the dampness of a wrung-out sponge - too wet and air cannot reach worms that breathe through their skin, too dry and that skin stops working.

The usual mistake is generosity. Fed faster than the worms eat, the bed ferments and turns acid; the worms shrink, crawl out or die, and mites move in on the food. Feed every three to five days against how much of the last lot has gone, and bury it under bedding. A smell means overfed or too wet. Meat, dairy and oily food stay out. Capacity scales with the worms - about 450 g (1 lb) of them clears 0.2 to 0.5 kg (0.5 to 1 lb) of scraps a week - so an outdoor heap takes the surplus. Composting worms are not native to most ground, so the vermicompost is often frozen for a few weeks before use to kill any eggs in it. The work is minutes a week and the container can last two decades.