On the one hand, building a compost pile is like building a fire; on the other hand, it is like baking bread. A fire needs dry fuel, air, and a source of ignition heat to start, and depends for its success on placing the fuel and air in the right relation to each other—compactly enough so that the fire feeds on itself to build its heat, but loosely enough so that it can get enough air. A compost pile needs moist fuel, air, and a source of bacteria to start, and it, too, depends on the proper placement of the ingredients. And like making bread it depends on both bacteria and a warm protected place for them to do their work.
There are relatively fast methods of making compost which require a fair, but not unreasonable, amount of effort and attention, and then there are slower but less arduous methods. In both cases the process by which garden refuse, kitchen garbage, manure, yard trimmings, and decomposable trash are transmuted into the black gold of compost is similar. Whether a freestanding compost pile is built, or the materials are kept in a bin or other enclosure during the composting process, is immaterial; only the appearance is different.
The fast method is like building a fire, and it differs from the slow method solely in that air is actively incorporated into the pile. The fuel is carbon: dry plant matter like leaves, straw, hay, or dry weeds and yard trimmings. The heat comes from nitrogen. The surest form of nitrogen to fuel the compost pile is manure. Where it is available it should be used (with the exception of manure from pets or people which, unless specially treated, may contain pathogens). Where manure is not available the readiest sources of nitrogen are freshly cut grass clippings, freshly pulled weeds, and kitchen garbage, that is, vegetable and fruit trimmings, coffee grounds, spoiled leftovers, and the like. If none of these is available, purchase supplements like blood mean, available at garden centers. It also makes sense to include a bit of soil or compost from an earlier pile, to serve as a “starter” by providing a population of decomposing organisms early-on (though they would eventually find the pile anyway). Such compost starters can also be bought ready-made if a starter source isn’t available.
Two last elements that are important are moisture and the size and shape of the pile. The microorganisms that drive the composting process forward require moisture. If the materials used are not succulent, the pile should be watered as it is built, and periodically afterward to keep it uniformly moist. If the materials are just glistening and damp to the touch, but not soaking wet, conditions are likely to be ideal. Size and shape of the pile become important as it heats up—which increases the biological activity within the speeds along the composting—because there must be enough bulk in relation to its surface area so that it generates more heat than it loses. Many experienced composters actually insulate their piles with a layer of straw or manure to conserve its natural heat. An effective minimum size for freestanding piles seems to be about 4 feet on a side (or longer if you need more compost) and about 4 fee tall. Some commercially manufactured compost bins are insulated, and so make it possible for a smaller volume of material to attain the critical heat levels needed for fast composting.
The ingredients should be kept in proper proportion when assembling a compost pile. A common rule-of-thumb is that you’ll want, by volume, four to five times as much carbon-rich material as there is nitrogen-rich refuse. They are kept well mixed by layering them on the pile as they are collected. Two of the most common products of the American yard, however, require special attention. Autumn leaves, unless shredded before piling, them to pack down and exclude air, and lawn clippings are so rich and succulent that unless dried first or mixed thoroughly with drier material, they will rot into a slimy mess instead of composting. Both are excellent sources of nutrients, though, and composting them saves money and landfill space at the same time it provides nutrients for your garden.
One of the more difficult tasks is to make sure that air can get to all parts of the pile throughout the composting process. The first layer should be of shrub prunings, twigs, or other light materials. As the pile increases in size, larger branches or even poles can be laid horizontally on top, and then later, during its decomposition, withdrawn to allow air to enter. Some other methods of getting air into a compost pile to speed its decomposition are to stick vertical lengths of drainage tile into the first layers of the pile and then build it right up around them; poke holes with a bar or piece of iron pipe once the pile is finished; or, as many gardeners do, turn the pile with a manure fork each time it starts to cool down, placing the materials from the outside of the pile on the inside and vice versa. For gardeners with limited space and fairly formal yards, there are manufactured compost tumblers that will accomplish this process of re-aeration without the heavy lifting.
The basic proportion between the dry, fuel-type materials in the pile and the moister, heat-supplying materials should be four or five to one. The soil or compost fraction usually covers lightly each layer of the nitrogenous material to hold its moisture and discourage scavenging animals (not a problem if a bin is used for the pile). In our hot piles we alternate six-to-eight-inch layers of dry matter with one-to-two-inch layers of manure and half0to-one-inch layers of compost from the previous pile. Many composters, including us, like to add a bit of phosphorus to the pile, as it is proportionately low in both compost and manure. We use rock phosphate, and sprinkle three or four handfuls over the compost layer before starting again with the dry layer. Piles that use no manure tend to be a bit more acidic, and wood ash or bone meal would make a good addition, as they would more effectively raise the pH a bit.
This layering process should continue until the pile is 4 feet tall. Water it if necessary to assure that there is sufficient moisture for decomposition to proceed. Within a few days the pile should begin to heat noticeably. During cool weather it may actually steam; but during the spring and summer months (when composting proceeds most quickly, anyway) you can check it simply by thrusting your fist into the pile. In the center it may well be too hot to touch, between 120º and 140º Fahrenheit. Once it begins to cool down—if you want to keep the process moving as quickly as possible—consider supplying more air, either by turning the pile, or by the other methods mentioned above. If you are of a precise mind, you can check its progress with a thermometer. We use a small, inexpensive meat thermometer; after thrusting my fist into the pile, I stock the thermometer into the end of the passage so that it is within the heart of the pile. As soon as it drops below 100ºF, I consider the pile ready for turning.
A properly proportioned pile, made with the right ingredients and closely managed, can be ready for use in as little as two weeks, having been turned twice or three times over that period. This is particularly true if the materials from which the pile is built are shredded before composting; the smaller particle size makes the work of decomposition easier for the microorganisms in the pile. If you have a garden spot that you want to improve as quickly as possible, simply scour the neighborhood for leaves, hay, stable cleanings, sawdust, and manure, rent a shredder, and make a whole series of piles. Even without turning—if you build the piles properly, with provision for getting air to the center—you should be able to produce large amounts of ripe compost in a month.