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.