Organic Material
Any organic material can be added to the compost.
For our purposes organic material is everything growing in the backyard
and most food waste, especially fruit and vegetable
scraps. Weeds, leaves, plant trimmings, apple cores, tea bags and potato
peels are organic. Wood and large twigs are organic,
but may take too long to decompose to be of practical use in composting.
Decomposing Organisms
Decomposing organisms are all the micro-organisms
and larger organisms involved in breaking down organic material. Bacteria
are the primary decomposing micro-organism. They
arrive with the organic material, and start the process by breaking down
the organic material for their own food. Bacteria
grow and multiply while conditions are right for them, and die off as they
create conditions more favorable for others.
Bacteria, actinomycetes and fungi all consume waste directly and are known
as
first level decomposers (see Food Web of the
Compost Pile diagram). They are assisted by larger organisms - earthworms,
beetle mites, sowbugs, whiteworms, and flies
- which also consume waste directly.
First-level decomposing micro-organisms are eaten
by second-level decomposers such as springtails, mold mites,
feather-winged beetles, protozoa and rotifers.
Third- level decomposers eat both first and second-level decomposers and
include centipedes, rove beetles, ants and predatory
mites. Organisms at each level of the food web help keep populations of
the lower levels in check.
Micro-Organisms - Chemical Decomposers
Bacteria
Bacteria are abundant. There may be millions in
a gram of soil; you would need 25,000 laid end to end on a ruler to make
2.54
cm (an inch). They exist on every piece of organic
matter even though you can't see them. When exposed to organic tissue,
bacteria "invade" - eating and digesting the
tissue, breaking it down into simpler forms for other bacteria and organisms
to
consume. As a group, bacteria are considered
to be nutritionally diverse, which means that they can eat almost anything,
living
or dead.
Bacteria require both nitrogen and carbon that
come from organic materials. The more variety, the greater likelihood they
will
find a blend of essential nutrients. Bacteria
use carbon (C) as a source of energy and, by oxidizing carbon, generate
heat and
carbon dioxide (CO2). Nitrogen (N) is their main
source of protein, which is needed for body building and population growth.
There are two types of composting processes: aerobic
and anaerobic - and different species of bacteria occur in each process.
Aerobic composting uses oxygen. Rapid, high-temperature
composting is mostly aerobic. This is the recommended method for
residential composting.
When there is little air and high moisture, anaerobic
composting is likely to result. Anaerobic means without oxygen. In an
anaerobic process fermentation results in the
formation of ammonia-like substances and hydrogen sulfide, which smells
like
rotting eggs. The anaerobic process is not recommended
for backyard composting.
Actinomycetes
Actinomycetes are a higher form of bacteria,
similar to fungi, and second in number to bacteria. They are especially
important in the formation of humus. They liberate carbon (C), nitrate
nitrogen (NO3) and ammonium nitrate (NH4), making nutrients available to
plants.
Fungi
Fungi are smaller in number than bacteria or
actinomycetes, but larger in body mass. Fungi live on dead or dying
material and obtain energy by breaking down organic material.
Food Web of the Compost Pile
Key Factors That Affect Composting
Compost variables are the factors affecting the
speed of composting. The organisms that make compost need food, air and
water. Provide them with a favourable balance
of food, air and water and they will make compost quickly. Other variables
affecting the speed of composting include temperature,
surface area and volume.
Food
Organic material is food for bacteria and other
organisms. This organic material or food contains carbon and nitrogen.
Bacteria
use carbon (C) for energy and nitrogen (N) for
protein to grow and reproduce.
Carbon and nitrogen levels vary with each organic
material. Carbon-rich materials tend to be brown and dry like leaves or
straw. Nitrogen-rich materials tend to be green
and wet like fresh grass clippings and food waste. Another way to think
about
organic material's carbon/nitrogen content is
to remember that fresh, juicy materials are usually high in N and will
decompose
more quickly than older, drier and woodier tissues
which are high in C.
All organic material is composed of carbon and
nitrogen. For most home composting a ratio of between 20 and 30:1 is
recommended - that is 20 to 30 parts carbon to
one part nitrogen. When the ratio is between 20 and 30:1, composting
proceeds most efficiently. When C content rises
above 30, heat production drops and the rate of composting slows. When
the
ratio drops below 20:1, excess nitrogen is lost
to the air as ammonia and there is a rise in pH level, which may be toxic
to some
micro- organisms. A blend of one part carbon-rich
material and one part nitrogen-rich material is a general rule in composting.
Air
Proper aeration is a key environmental factor.
Oxygen is required by many micro-organisms, especially aerobic bacteria.
With
sufficient oxygen they produce energy, grow quickly,
consume more material and make nutrients available for plant growth.
When oxygen is not available, aerobic bacteria
die off and anaerobic bacteria take over. They will break down the material,
but
more slowly, and they produce an unpleasant odor.
Moisture
Aerobic bacteria require water to live. A sufficient
quantity of water must be provided that will lightly coat the materials
to be
composted. Less than 40 per cent moisture in
the compost and bacterial activity slows down. More than 60 per cent and
water
will replace air in the pockets amidst the organic
material causing aerobic bacteria to drown and anaerobic bacteria to take
over. The recommended water level is between
40 and 60 per cent but the ideal percentage will also depend on the organic
material's structure. Straw and wood will need
more moisture than leaves, while food wastes or lawn clippings may not
need
any added moisture. Water forms a film of moisture
on the materials, allowing the bacteria to do their work. Simply, the
compost should feel as moist as a wrung-out sponge.
Temperature
As temperatures rise in the compost, decomposition
speeds up. As temperatures drop, composting slows down. Outside
temperatures also play a role. Warmer outside
temperatures in the summer months stimulate bacteria and speed up composting.
Colder temperatures in the winter slow it down.
Surface Area and Particle Size
Smaller particles of organic material provide
more surface area for microbes to attack and speed up composting. Material
that
is reduced to two inch pieces are ideal, exposing
lots of area for the critters to work and allowing air spaces.
Volume
Volume is a factor in retaining heat in the backyard
compost bin. The more volume of material in the bin, the more
self-insulating it will become in retaining the
heat. For backyard composting in the Region one cubic meter is the maximum
recommended volume for a compost bin. One cubic
meter easily retains heat and moisture, but it is not so large that material
will become too heavy and compacted, or too unwieldy
for turning.