Decomposition happens rapidly in a hot compost heap with the main agents of decay being heat-loving microorganisms. Decomposition happens slowly at the soil's surface with the main agents of decay being soil animals. However, if the leaves and forest duff on the floor of a forest or a thick matted sod are tilled into the topsoil, decomposition is greatly accelerated.
For two centuries, frontier American agriculture depended on just such a method. Early pioneers would move into an untouched region, clear the forest, and plow in millennia of accumulated nutrients held as biomass on the forest floor. For a few years, perhaps a decade, or even twenty years if the soil carried a higher level of mineralization than the average, crops from forest soils grew magnificently. Then, unless other methods were introduced to rebuild fertility, yields, crop, animal, and human health all declined. When the less-leached grassy prairies of what we now call the Midwest were reached, even greater bounties were mined out for more years because rich black-soil grasslands contain more mineral nutrients and sod accumulates far more humus than do forests.
Sheet composting mimics this system while saving a great deal of effort. Instead of first heaping organic matter up, turning it several times, carting humus back to the garden, spreading it, and tilling it in, sheet composting conducts the decomposition process with far less effort right in the soil needing enrichment.
Sheet composting is the easiest method of all. However, the method has certain liabilities. Unless the material being spread is pure manure without significant amounts of bedding, or only fresh spring grass clippings, or alfalfa hay, the carbon-nitrogen ratio will almost certainly be well above that of stable humus. As explained earlier, during the initial stages of decay the soil will be thoroughly depleted of nutrients. Only after the surplus carbon has been consumed will the soil ecology and nutrient profile normalize. The time this will take depends on the nature of the materials being composted and on soil conditions.
If the soil is moist, airy, and warm and if it already contained high levels of nutrients, and if the organic materials are not ligninous and tough and have a reasonable C/N, then sheet composting will proceed rapidly. If the soil is cold, dry, clayey (relatively airless) or infertile and/or the organic matter consists of things like grain straw, paper, or the very worst, barkless sawdust, then decomposition will be slowed. Obviously, it is not possible to state with any precision how fast sheet composting would proceed for you.
Autumn leaves usually sheet compost very successfully. These are gathered, spread over all of the garden (except for those areas intended for early spring sowing), and tilled in as shallowly as possible before winter. Even in the North where soil freezes solid for months, some decomposition will occur in autumn and then in spring, as the soil warms, composting instantly resumes and is finished by the time frost danger is over. Sheet composting higher C/N materials in spring is also workable where the land is not scheduled for planting early. If the organic matter has a low C/N, like manure, a tender green manure crop not yet forming seed, alfalfa hay or grass clippings, quite a large volume of material can be decomposed by warm soil in a matter of weeks.
However, rotting large quantities of very resistant material like sawdust can take many months, even in hot, moist soil. Most gardeners cannot afford to give their valuable land over to being a compost factory for months. One way to speed the sheet composting of something with a high C/N is to amend it with a strong nitrogen source like chicken manure or seed meal. If sawdust is the only organic matter you can find, I recommend an exception to avoiding chemical fertilizer. By adding about 80 pounds of urea to each cubic yard of sawdust, its overall C/N is reduced from 500:1 to about 20:1. Urea is perhaps the most benign of all chemical nitrogen sources. It does not acidify the soil, is not toxic to worms or other soil animals or microorganisms, and is actually a synthetic form of the naturally occurring chemical that contains most of the nitrogen in animal urine. In that sense, putting urea in soil is not that different than putting synthetic vitamin C in a human body
Burying kitchen garbage is a traditional form of sheet composting practiced by row-cropping gardeners usually in mild climates where the soil does not freeze in winter. Some people use a post hole digger to make a neat six-to eight-inch diameter hole about eighteen inches deep between well-spaced growing rows of plants. When the hole has been filled to within two or three inches of the surface, it is topped off with soil. Rarely will animals molest buried garbage, it is safe from flies and yet enough air exists in the soil for it to rapidly decompose. The local soil ecology and nutrient balance is temporarily disrupted, but the upset only happens in this one little spot far enough away from growing plants to have no harmful effect.
Another garbage disposal variation has been called "trench composting." Instead of a post hole, a long trench about the width of a combination shovel and a foot deep is gradually dug between row crops spaced about four feet (or more) apart. As bucket after bucket of garbage, manure, and other organic matter are emptied into the trench, it is covered with soil dug from a little further along. Next year, the rows are shifted two feet over so that crops are sown above the composted garbage.Back to Composting