In many a local gardener's backyard, the black plastic of an "Earth Machine" compost bin is easy enough to spot. These 3' wide composting bins, similar in color and shape to Darth Vader's helmet, have a slightly less sinister purpose. They help gardeners and other green citizens turn organic waste into a high-quality soil amendment, to be used in the home, the greenhouse, the garden, or the farmer's fields. Composting makes productive use of our food waste, lightens the load on our landfills, reduces dependence on synthetic chemical fertilizer, and returns natural nutrients to soil that is often depleted by erosive runoff and intensive harvest. It is a fairly simple and effective way to help the environment, and the soil it produces is known among organic growers as "black gold" for its excellent fertility.
Composting is something of a ritual in certain circles, and like any ritual it demands a certain authenticity. The "dump and run" style is easy and helpful enough, but a more sophisticated method is fairly easy to implement with a little bit of know-how. The key is to encourage the decomposing organisms that return organic matter to the bottom of the food chain as they return it to the soil. The four ingredients of a successful compost bin are nitrogen, carbon, water and air. These ingredients create optimal conditions for decomposing microbes and worms and other organisms, which break down the organic waste into its component nutrients and create soil that is nearly perfect for new plant growth.
Nitrogenous wastes, also known as "greens", are the first ingredient in a successful compost pile. Kitchen wastes, fresh grass, and manure all supply a wealth of nutrients to the soil as they decompose, and some gardeners will even add crushed eggshells for calcum or kelp for micronutrients. Nitrogen is especially important because it provides a food source for the decomposing bacteria. In this role it is complemented by another nutrient, and one criticism of the "dump and run" style is that its nitrogen inputs ought to be balanced with greater amounts of carbon.
Carbonaceous materials or "browns" are the second ingredient of a healthy compost. Savvy composters will keep a pile of dried grass or autumn leaves or woody debris beside their compost bin, and bury each deposit of greens with a plentiful handful of browns. Equal volumes of the two materials will approximate the bacteria's ideal ratio of carbon to nitrogen, which is around 30 to 1 by mass. Burying the green material cuts down on the attraction of pests, and the brown materials also tend to settle less densely, which helps keep the pile well aerated.
Air and water are also very important to successful composting. Without a good supply of oxygen, decomposition will be anaerobic rather than aerobic, and this will result in an unpleasant-smelling pile that is not very good for plants. A well-designed compost bin, which can be purchased or home-made, will allow air to flow through its sides and up from the bottom. In the latter location, a layer of larger sticks laid any way but parallel will allow for greater air flow. Moisture is important to encourage bacterial activity, and the compost will ideally be as wet as a well wrung-out sponge. The occasional watering during times of drought will be happily welcomed by the entire compost ecosystem.
A few handfuls of nearby soil will innoculate a new bin with composting microbes. When these microbes are pampered with good amounts of carbon, nitrogen, water and air, they will process waste more quickly and their activity will raise the internal temperature closer to the ideal of 60°C. At this temperature faster-acting thermophilic bacteria get to work, creating heat that destroys most weed seeds and pathogens. In theory, this makes it safe to include meat, dairy products and eggs in a professionally managed pile. But given the potential these items have to attract vermin, and the amount of attention a pile needs to maintain this temperature, most backyard compost bins are better left vegan. Diseased plants and weeds with seeds should also be left out of the pile, and full-size fruits and vegetables should be chopped up to offer plenty of surface area to the microbes.
The ideal size for a backyard compost bin is about 3' in diameter, small enough for air flow but large enough to retain heat. One can monitor the temperature at the core of the pile with a long thermometer, and note when it passes its peak; this indicates that the waste at the core is mainly decomposed and the bacteria are starting to relax. Mixing or "turning" the compost pile at this point will bring fresher waste into the more active core section, providing new food to the microbes and raising activity levels once again. This is an optional way to speed up the composting process, and turning is also used in larger piles to provide aeration. When the temperature rises little after turning, and the soil becomes a sweet-smelling dark brown or black, the pile is ready for use. A miniature door at the bottom of the bin will make it easy enough to collect.
Once the decomposers have done their duty, the nutrient-rich compost can give your plants a major boost. For potted plants and greenhouses, mix one part compost to two parts potting soil or one part sand; the latter will improve drainage, provide trace minerals, help anchor roots, and keep the soil mix from shrinking with further decomposition. For gardens, a five-inch layer of compost mixed into the topsoil each year will maintain soil structure and fertility very well. For lawns, one part finely sifted compost to one part sand can be used as a leveler or sprinkled evenly as a top dressing.
For pots, greenhouses, gardens and lawns alike, compost tea can make an excellent organic fertilizer. Brew the tea by tying up two handfuls of compost in a cheesecloth "teabag", submerging this bag in a bucket of water, and letting it brew for two days prior to application. You'll need a small aquarium pump, which sends air through plastic tubing to two or three aquarium "bubblers" or air stones, which are secured at the bottom of the brewing bucket. Aeration prevents the anaerobic conditions that can harm your plants, and will also bubble away the chlorine in city water if it's done for two hours before the compost is added. Use the tea quickly after unhooking the aeration apparatus, and re-aerate if it begins to smell unpleasant. Apply the tea like a normal watering to the soil, or use it as a foliar spray when evaporative rates are low and plants have the time to absorb the nutrients in water.
Successful composters will be happy knowing that they've kept waste out of the landfill, kept nutrients in the soil, avoided chemical fertilizers, and invested a "black gold" into their soil that will pay off as healthy and delicious plants and produce. With a good compost bin, a source of brown materials, a spot in the backyard, and some time and knowledge and energy, anyone can enjoy this rewarding and environmentally helpful practice. Your planet and your plants will love you for it.