material decomposing with oxygen is an "aerobic" process.
When living organisms that
use oxygen feed upon organic matter, they develop cell protoplasm
from the nitrogen, phosphorus, some of the carbon, and other required
nutrients. Carbon serves as a source of energy for organisms and
is burned up and respired as carbon dioxide (CO2). Since carbon
serves both as a source of energy and as an element in the cell
protoplasm, much more carbon than nitrogen is needed. Generally,
organisms respire about two-thirds of the carbon they consume as
CO2, while the other third is combined with nitrogen in the living
activity diminishes if the compost mix contains too much carbon
in relation to nitrogen. Several cycles of organisms are required
to burn excess carbon. This is a complex chemical process. When
organisms die, their stored nitrogen and carbon become available
to other organisms. These new organisms form new cells which again
need nitrogen to burn excess carbon and produce CO2. Thus, the
amount of carbon is reduced and the limited amount of nitrogen
is recycled. Finally, when the ratio of available carbon to available
nitrogen is low enough, nitrogen is released as ammonia. Under
favorable conditions, some ammonia may oxidize to nitrates. Phosphorus,
potash, and various micronutrients are also essential for biological
growth. These are normally present in more than adequate amounts
in compostable materials.
nature, the aerobic process is most common in areas such as the
forest floor, where droppings from trees and animals are converted
into relatively stable organic matter. This decomposition doesn’t
smell when adequate oxygen is present. We can try to imitate these
natural systems when we plan and maintain our landscapes. As we
learn more about the biology and chemistry of composting, we can
actually hasten the decomposition process.
carbon is oxidized to CO2, a great deal of energy is released as
heat. For example, if a gram of glucose molecules is dissimilated
under aerobic conditions, 484 to 674 kilogram calories (kcal) of
heat may be released. If organic material is in a large enough
pile or arranged to provide some insulation, temperatures during
decomposition may rise to over 170° F. At temperatures above 160° F, however, the bacterial activity decreases.
are many different kinds of bacteria at work in the compost pile.
Each type needs specific conditions and the right kind of organic
material. Some bacteria can even decompose organic material at
temperatures below freezing. These are called psychrophilic bacteria,
and although they work best at around 55°, they continue to work
down to 0° F. As they work, they give off small amounts of heat.
If conditions are right, this heat will be enough to set the stage
for the next group of bacteria, the “mesophylic,” or
middle range temperature bacteria.
bacteria thrive from 70° to 90° F, but just survive at
temperatures above and below (40° to 70° F, and 90° to
110° F) In many backyard piles, these mid range bacteria do most
of the work. However, if
conditions are right, they produce enough heat to activate the “thermophilic,” or
heat loving bacteria. Thermophilic bacteria work fast. Their optimum
temperature range is from 104° to 160° F.
temperatures destroy pathogenic bacteria and protozoa (microscopic
one celled animals), and weed seeds, which are detrimental to health
and agriculture when the final compost is used on the land.
oxidation does not stink. If odors are present, either the process
is not entirely aerobic or there are materials present, arising
from other sources than the oxidation, which have an odor. Aerobic
decomposition or composting can be accomplished in pits, bins,
stacks, or piles, if adequate oxygen is provided. To maintain aerobic
conditions, it is necessary to add oxygen by turning the pile occasionally
or by some other method.