Organic 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 cells.
Biological 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.
In 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.
When 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.
There 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.
Mesophylic 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.
High 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.
Aerobic 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.
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