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Hello SANET folks,

The following fact sheet includes a nice discussion of the benefits of
having **both** anaerobic and aerobic microorganisms active in composting


Joel Gruver
Dept of Soil Science
NC State University


by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension
(Adapted from information presented at workshops by Dr. Will Brinton, Woods
End Lab, Mt. Vernon, Maine.)

Anywhere you go, there are waste products that are totally useless in
farming that can be made into a useful product if proper compost recipes are
developed. Some of these materials are more challenging than others, such as
fish or paper mill waste, while others are easier to compost, like dairy or
poultry manure plus bedding. On-farm wastes and yard wastes tend to be
relatively free of contaminants compared to municipal wastes. With off-farm
wastes, total source separation that results in "clean" residues is
essential if the end-product is to be used in farming. Otherwise, a variety
of contaminants may accumulate in the farming system over time. Composting
for waste disposal and composting for production of a soil amendment can be
conflicting approaches.

A book that is helpful with compost recipe development, compost siting, and
compost marketing is On Farm Composting, available from the Northeast
Agricultural Engineering Service, Riley Robb Hall, Ithaca, NY 14853, as well
as from many Extension offices.

To succeed in making the highest quality compost with the least investment
of time and equipment, it is important to understand that there are 4
different groups of  microbes involved in composting. Obligate aerobes are
strict oxygen consumers that die off when oxygen is lacking. Facultative
anaerobes are versatile organisms that can be simultaneously aerobic and
anaerobic - in other words they adapt to the oxygen level of their
environment. Aero-tolerant anaerobes grown anaerobically in the presence of
oxygen - they don't use it but they can tolerate it. Obligate anaerobes are
limited to oxygen-free environments and die off if it is present.

All of these organisms have a contribution to make to the compost process -
there are no "good guys" and "bad guys". Contrary to popular belief, it is
not optimal to manage composting as an entirely aerobic process - obligate
anaerobes do not normally dominate until the end of the compost process
unless extreme measures are take to aerate the pile, such as frequent
turning or forced ventilation, which are expensive and unnecessary. Obligate
anaerobes do generate heat, which is important if the compost ingredients
are unsanitary (ie sewage sludge), and they work fast, which is helpful if
you are in a rush for compost. But the facultative anaerobes are just as
important to on-farm composting, because they function at lower oxygen
levels, minimizing the need for manipulating the pile, and they are also
responsible for generating the disease-suppressing abilities that have been
observed with certain composts.

Some degree of anaerobic respiration during composting is also advantageous
because it results in less loss of carbon than a fully aerobic process,
which in theory converts everything to the end products carbon dioxide and
water, although in practice a small amount of microbial biomass, or humus,
also results. Because anaerobic respiration results in the production of a
variety of end products such as organic acids and alcohol, the rapid loss of
carbon as CO2 is avoided, and there is greater opportunity for conversion of
the original carbon to humus.  Although the products of anaerobic
decomposition can be toxic to plants, as the compost process continues and
oxygen levels rise, aerobes will readily decompose these materials.

In other words, the extra metabolic steps that non-aerobic decomposition
contributes to the compost process leads to more microbial biomass and
byproducts to be generated, and these are the high molecular weight
compounds that comprise finished compost or humus. The result is that
carbon, and plant nutrients such as nitrogen, are more effectively recycled
than with highly aerobic, "clean-burning" composting.

All of the microbes required for a healthy compost process are already
present in most wastes that can be composted, and therefore the utility of
adding compost inoculant is limited. A notable exception can be food wastes,
which are often cooked (ie sterilized). In a study that examined the effect
of inoculants, added microbes could not subsequently be found in dairy
manure compost, apparently because the existing microbes excluded introduced

The natural mix of organisms in compost makes the process very resilient to
varying environment is extremely low in oxygen for extended periods of time,
the result can be strong odors, a slow process, and the accumulation of
phytotoxic compounds. To avoid this situation without having to turn
frequently, compost piles should be built so that they are self-aerating.
This is accomplished by using materials with sufficient porosity, managing
moisture content to avoid soaking, and keeping windrows small enough to
avoid internal compaction. Then, the natural oxygen demand of the compost
process will assure a steady, low level of oxygen as it diffuses into the
windrow to replace the oxygen that is consumed by the microbes.

Although most farmer-composters do not monitor the oxygen levels of their
windrows, it is instructive to understand how these levels vary during the
compost process. Immediately after raw materials are mixed together, the
oxygen levels in compost are just below the ambient (outdoor) level of 22%
but drop off very rapidly as aerobic decomposition takes off. The process
soon shifts to a mixture of aerobic and anaerobic decomposition as oxygen
levels drop down and then fluctuates between about 2% and 5%, which is a
healthy range for facultative anaerobes. This level can be naturally
maintained in a windrow with sufficient porosity. Toward the end of the
compost process, oxygen levels start to climb back up, nearing ambient once
the compost is mature and decomposition slows to a halt. That's because as
internal oxygen demand subsides, diffusion of oxygen into the pile can catch

However, the porosity of a windrow tends to decrease as materials are
decomposed, and for a short period of time, when oxygen demand is still
high, a windrow may go excessively anaerobic until diffusion of oxygen can
catch up. In many cases the best thing is to leave the windrow alone until
it re-equilabrates. But if the pile is not porous enough, or its core is
wet, then diffusion may be too limited, requiring turning to aerate the
pile. The key is not to over-do it, because aggressive turning further
reduces the porosity by breaking down the aggregates or small clumps of
composting materials, increasing the need for subsequent turning. In other
words, the need for frequent turning becomes a self-fulfilling prophecy.

In conclusion, some turning is usually needed to facilitate the compost
process. The use of sufficient bulking material and low-impact turning
methods (like a bucket loader versus a compost turner) can allow a windrow
to remain self-aerating with a minimum of  turning.  At the outset, raw
materials with sufficient porosity must be mixed together to distribute
carbon, nitrogen, moisture, and oxygen evenly through the windrow. Then,
after the initial burst of decomposition and oxygen consumption, a gentle
turning can help keep the pile sufficiently aerated. Finally, as the process
nears completion, turning helps homogenize the finished compost and mixes in
materials on surface that have not been fully decomposed.

Windrow height, after settling, should be no more than 3 to 5 feet unless
large amounts of porous materials like straw are being composted, otherwise
the core will become compacted. Keeping the windrows small also avoids
high-heat conditions which encourage the loss of moisture. The windrows must
be kept moist with irrigation if necessary. If rains threaten to saturate
the windrows, they must be covered.

Under this type of low-tech, "minimum tillage" management system, dairy
manure or similar materials should compost to maturity in 4 to 5 months. The
cost to produce compost using a bucket loader to turn 3 times is about $7
per ton. Obviously this figure will vary depending on material and labor

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