The compost pile or heap is the time-honored place for all organic refuse, the real gardener's source of humus for incorporation into the soil. The important thing to remember about composting is that it is a biological process, one that involves bacteria, fungi and other soil organisms. These organisms require food to do their work, which means you must supply the same elements that are needed by higher plants. The one difference is that these organisms do not have chlorophyll and are not able to manufacture their own energy foods such as sugars and starches. They draw upon the organic matter for these foods, but in doing so use up large amounts of nitrogen, some phosphorus and potash and small amounts of other elements. For this reason, the application of fertilizers to the compost pile will allow the organisms to work at their best, provided temperatures are favorable for their growth and the pile is moist, without being so wet that air is excluded.
Any form of organic matter that will decay can be composted. Some materials, such as peat moss, spent mushroom manure, spent hops and well-rotted manure, are already partially broken down. These can be applied directly to the soil. However, if a substance contains no cellulose, fiber or lignin, it will not produce humus. Dried blood, perhaps the most valuable of organic fertilizers, is all but worthless as a source of humus, since it contains practically no fibrous material. Urine, a valuable source of nitrogen, urea and other fertilizer elements, is another organic substance which produces little or no humus. Fish emulsion fertilizer is another non-fibrous organic material that leaves very little residue for humus formation. This does not mean they are worthless: on the contrary these three materials are among the most valuable foods for the bacteria that work on compost. A little of any one of these will start the pile or get it working again whenever it begins to slow up.
WHERE AND HOW
The compost heap should be in light shade, on level and well-drained ground. If in full sun, the pile may heat up enough to kill bacteria near the surface. Considerable heat is developed in the composting process itself. In dry regions, the pile might well be made in a shallow depression to catch rainfall, but this basin should not be so deep as to risk "drowning" the lower layer of compost.
Unless the soil in the pile site is naturally high in lime, sprinkle the area with ground limestone before applying the first layer. Each successive added layer should also be sprinkled with limestone: the processes of decay generate acids which will slow up bacteria while favoring fungi.