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The Biology of Composting
What is Composting?
Composting is the biological degradation or breakdown of organic
material into a dark, soil-like material called humus. This occurs
mainly by the enzymatic digestion of material by soil microorganisms.
A host of other soil organisms form a crucial part of the composting
process too. Put a pile of leaves, an old cotton rag, or a freshly 2x4
board out in the environment, and decomposition is bound to occur.
There are two types of composting processes: aerobic and anaerobic.
The first takes place in the presence of oxygen, the second in the
absence of oxygen. Different organisms do the decomposing work,
depending on whether the conditions in the compost pile are aerobic
or anaerobic. The finished compost, or humus, is exactly the same
whichever process occurs. In backyard or home composting we
concentrate on the aerobic process.
Decomposer Organisms
The compost heap provides a good environment for the
microorganisms and other soil life, which break down organic matter.
The microorganisms help to make nutrients available to plants and
develop soil structure.
As kitchen trimmings, leaves, weeds, manure etc. break down into
simpler forms of proteins and carbohydrates, the nutrients become
available to a wider array of bacterial species that will carry them to a
further stage of decomposition. Bacteria are the primary decomposer
organisms of a compost pile in that they do the majority of the work.
Bacteria
They exist on every single bit of organic matter. When organic tissue is exposed (for example
when an apple is peeled or bruised), the bacteria “invade” and begin to eat and digest the
tissue. During this eating and digesting process the organic matter is broken down into its
basic elements. The result is humus.
Given the proper environmental conditions, bacteria can regenerate at a remarkable rate.
They reproduce by binary fission: simply laying down a dividing wall through the middle of
their bodies and becoming two. Then they do it again and become four, eight, 16, 32, and so
on. This wouldn't be as impressive if it didn't happen so fast. One gram of the common
intestinal bacteria Escherichia coli, would become a pound in three hours and a mass the size
of the earth in one and a half days if sufficient food and proper conditions were available.
Luckily for us, these conditions have never been met!
Does organic material stop composting when it's cold? No, even at temperatures below
freezing bacteria can be at work on organic matter. Psychrophilic bacteria (bacterial species
that work in the lowest temperature range) do their best work at about 13 degrees C (55
degrees F), but they are able to carry on right down to -18 degrees C (0 degrees F).
Psychrophiles give off a small amount of heat as a by-product of their work, and this heat
causes a rise in ambient air temperature in the pile. As the temperature in a compost pile
reaches the medium range, mesophilic bacteria become the active workers. These are the
aerobic bacteria that do most of the decomposition work in the pile. Like us
they thrive at 21-32 degrees C (70-90 degrees F)
and just survive in the 4-21 C (40-70 F)
and 32-43 C (90-110 F) range. The heat
generated as a by-product of the
mesophiles work raises the temperature
in the pile
even more, preparing the way for the
real "hot shots":
thermophiles. These bacteria work best
in temperatures of 40-93 C (104-200 F). They work fast, and last only about
three-five days. But what three-five days! In that time, they turn green, gold, and tan organic
material into a uniform deep brown. If the pile is fed new organic matter or turned at a
strategic time, the thermophilic action can occur again. (Turning brings fresh air to the
microbes. Their numbers multiply quickly again and the pile reheats. However, the pile seldom
heats as much as during the first build-up.)
Conditions of little air and high moisture in the pile promote a different order of bacteria:
anaerobes. They are slower working and in addition to the useful products of their
metabolism, they produce ammonia-like substances and hydrogen sulfide, which smells like
rotting eggs. Anaerobic bacteria are the work force of methane plants, where methane gas is
taken off as a source of energy before the organic materials are applied to the soil. For the
backyard composter anaerobic composting is less desirable than aerobic, but workable in
certain situations.
Actinomycetes
Actinomycetes are a higher form of bacteria similar to fungi and molds. Actinomycetes liberate
carbon, nitrogen and ammonia, making nutrients available for plants. They take over during
the final stages of decomposition, often producing antibiotics; chemical substances that
destroy bacterial growth. Actinomycetes can be recognized by grayish, cobwebby growths that
give a pleasing, earthy smell to compost.
Fungi
Most fungi are classified as saprophytes because they live on dead or dying material and
obtain energy by breaking down organic material. Like actinomycetes, fungi take over during
the final stages of composting when the organic material has been changed to a more
digestible form. The optimal pH range for fungi is lower than the range for bacteria.
The Zoo
Microscopic decomposers are not solely responsible for the complex transition from organic
“waste” to rich humus. Visible organisms of great variety transform organic matter by physical
action such as chewing, sucking and grinding.
Nematodes or roundworms are the most abundant invertebrates in the soil. Typically less than
1 mm in length, they prey upon bacteria, protozoa, fungal spores and each other. Though
there are pest forms of nematodes, most nematodes found in soil and compost are beneficial.
Fermentation mites, also called mold mites, are transparent bodied creatures that feed
primarily on yeasts in fermenting masses of organic debris. This mite is able to withstand
anaerobic conditions for moderate periods of time, and is thus a good indicator of these
conditions in compost.
Springtails, along with nematodes and mites, share the numerical dominance among soil
invertebrates. They are principally fungi-feeders, though they eat nematodes and small bits of
organic detritus. They are a major population-controlling factor on fungi.
Wolf spiders live in soil and leaf litter, and feed on arthropods. They build no webs, and run
freely to hunt their prey.
Centipedes are found frequently in soil microcommunities. They prey on almost any type of
soil invertebrate that is their size or slightly larger.
Sow bugs feed on rotting woody materials and highly durable tissues such as ligneous leaf
veins. The sow bugs that roll up like an armadillo are known as pill bugs. Sow bugs can be
found in great abundance in a compost pile.
Ground beetles of many kinds lurk in litter and soil spaces. Most of them feed on other
organisms, but some feed on seeds and other vegetable matter.
Earthworms: “It is the wise gardener who adjusts his composting methods to take full
advantage of the earthworm's special talents” (Rodale Guide to Composting). Earthworms
digest organic matter, depositing it as worm castings in the soil. In this way they add valuable
nutrients to, and increase the porosity of, the soil.
Most of the information in this section is quoted from the script of Daniel L. Dindal's slide show
“The Decomposer Food Web.”
For a full diagram of the food web in a compost pile, click here (PDF)
Organisms commonly found in compost. Energy flows from organism to organism as one is
eaten by the other in a natural recycling system. Snails, beetles, millipedes, centipedes, and
ants are less likely to find their way to worm bins set up with shredded corrugated and paper
beddings.
This material is being distributed by the Massachusetts Department of Environmental
Protection's Division of Solid Waste Management as part of the Network of Home Composters
Program. It was adapted from the Seattle Tilth Association and Seattle Engineering
Department's Solid Waste Utility' s Master Composter Resource Manual by the Recycling
Council of Ontrario and has been copied by permission.
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