Making and Using Composts
Making
and Using Composts
MAKING
COMPOSTS Moisture
Pile Size
Turning the
Pile
OTHER COMPOSTING TECHNIQUES
USING
COMPOSTS
ADVANTAGES OF
COMPOSTING
The decomposition of organic
materials takes place naturally in forests and fields all around us. Composting
is the art and science of combining available organic wastes so that they
decompose to form a uniform and stable finished product. Composts are excellent
organic amendments for soils. Composting reduces bulk, stabilizes soluble
nutrients, and hastens the formation of humus. Most organic materials, such as
manure, crop residues, grass clippings, leaves, sawdust, and many kitchen
wastes, can be composted.
The
microorganisms that do much of the work of rapid composting perform well at
elevated temperatures with plenty of oxygen and moisture. These compost-adapted
organisms cover the entire range of warm,
or mesophilic (up to 110°F),
and hot, or thermophilic (from 110° up to 130°F and even higher), conditions.
Temperatures above 160°F can develop in compost piles, helping kill off weed
seeds and disease organisms, but this overheating usually slows down the
process, since
it may cause extreme drying and triggers a
die-off of all but the most heat-resistant organisms. At temperatures below
110°F, the more prolific mesophilic organisms take over and the rate of
composting again slows down, especially as it drops toward ambient
temperatures, a process known as “curing.” The composting process is slowed by
anything that inhibits good aeration or the maintenance of high enough
temperatures and sufficient moisture.
Composting farm wastes
and organic residues from off the farm has become a widespread practice
Accepting and composting lawn and garden wastes provide some income for farmers near cities and towns. They may charge for accepting the wastes and for selling compost. Some farmers, especially those without ani-mals or perennial forage crops that help increase organic matter, may want to utilize the compost as a source of organic matter for their own soils.
MAKING COMPOSTS Moisture
The amount of moisture in a compost pile is important. If the materials mat and rainwater can’t drain easily through the pile, it may not stay aerobic in a humid climatic zone. On the other hand, if composting is done inside a barn or under dry climatic conditions, the pile may not be moist enough to allow microorganisms to do their job. Moisture is lost during the active phase
of composting, so it may be necessary to add water to a pile. In fact, even in a humid region, it is a good idea to moisten the pile at first, if dry materials are used
, However, if something like liquid manure is used to provide a high-nitrogen material, sufficient moisture will most likely be present to start the composting process. The ideal moisture content of composting material is about 40% to 60%, or about as damp as a wrung-out sponge. If the pile is too dry—35% or less—ammonia is lost as a gas, and beneficial organisms don’t repopulate the compost after the temperature moderates. Very dry, dusty composts become populated by molds instead of the beneficial organisms we want
A Sample Compost Recipe
Start with the following
•grass clippings (77% moisture, 45% C, and 2.4% N
• leaves (35% moisture, 50% C, and 0.75% N
food scraps (80 % moisture, 42% C, and 5.0% N
The ratio of the materials needed to get 60% moisture and a C:N of 30:1 is: 100 lbs of grass, 130 lbs of leaves, and 80 lbs of food scraps
Types of Starting Materials
The combined organic materials used should have lots of carbon and nitrogen available for the microorgan-isms to use. High-nitrogen materials, such as chicken manure, can be mixed with high-carbon materials like hay, straw, leaves, or sawdust. Compost piles are often built by alternating layers of these materials. Turning the pile mixes the materials. Manure mixed with saw-dust or wood chips used for bedding can be composted as is. Composting occurs most easily if the average C:N ratio of the materials is about 25–40 parts carbon for every part nitrogen (see chapter 9 for a discussion of C:N ratios).
There are too many different types of materials t
that you might work with to give
about how much of each to mix to get the moisture con-tent and the C:N into reasonable ranges so the process can get off to a good start. One example is given in the box “A Sample Compost Recipe” on
Cornell University’s website for composting issues (http://cwmi.css.cornell.edu/composting.htm) features formulas to help you estimate the proportions of the specific materials you might want to use in the compost pile. Sometimes it will work out that the pile may be too wet, too low in C:N (that means too high in nitrogen), or too high in C:N (low in nitrogen). To balance your pile, you may need to add other materials or change the ratios used. The problems can be remedied by adding dry sawdust or wood chips in the first two cases or nitro-gen fertilizer in the third. If a pile is too dry, you can add water with a hose or sprinkler system.
One thing to keep in mind is that not all carbon is equally available for microorganisms. Lignin is not eas-ily decomposed (we mentioned this when discussing soil organisms in chapter 4 and again in chapter 9, when we talked about the different effects that various residues have when applied to soil). Although some lignin is decomposed during composting—probably depending on factors such as the type of lignin and the moisture content—high amounts of carbon present as lignin may indicate that not all of the carbon will be available for rapid composting. When residues contain high amounts of lignin, it means that the effective C:N can be quite
a bit lower than indicated by using total carbon in the calculation . For some materials, there is little
Pile Size
compost pile or windrow is a large, natural convective structure—something like many
chimneys all next to each
other. Oxygen moves into the pile as carbon dioxide, moisture, and heat rise
from it. The materials need to fit together in a way that allows oxygen from
the air to flow freely. On the other hand, it is also important that not too
much heat escapes from the center of the pile. If small sizes of organic
materials are used, a “bulking agent” may be needed to make sure that enough
air can enter the pile. Sawdust, dry leaves, hay, and wood shavings are
frequently used as bulking agents. Tree branches need to be “chipped” and hay
chopped so that these ingredients don’t mat and slow composting. Composting
will take longer when large particles are used, especially those resistant to
decay. The pile needs to be
large enough to retain much of the heat that develops during composting, but
not so large and compacted that air can’t easily flow in from the outside.
Compost piles should be 3 to 5 feet tall and about 6 to 10 feet across the base
after the ingredients have settled (see figure 13.2). (You might want it on the
wide side in the winter, to help maintain warm temperatures, while gardeners
can make compost in a 3-foot-tall by 3-foot-wide pile in the summer.) Easily
condensed material should initially be piled higher than 5 feet. It is possible
to have long windrows of composting materials, as long as they are not too tall
or wide
turning the
Pile
Turning the composting
residues exposes all the materials to the high-temperature conditions at the
center
of the pile, and heat convection further
exposes the upper reaches of the pile (figure 13.3). Materials at the lower sides
of the pile often barely compost. Turning the pile rearranges all the materials
and creates a new center. If piles are gently turned every time the interior
reaches and stabilizes for a few days at about 140°F, it is possible to
complete the composting process within months, all other factors of moisture
and aeration being optimal. On the other hand, if you turn the pile only
occasionally, it
may take a year or longer to complete,
especially if it has settled down too densely. Equipment is now available to
quickly turn long compost windrows at large-scale composting facilities (figure
13.3). Tractor-powered compost turners designed for composting on farms are
also available, and some farmers use manure spreaders to remix and throw out
piles.
Although turning
compost frequently speeds up the process, too much turning may dry out the pile
and cause more nitrogen and organic matter loss. If the pile is too dry, you
might consider turning it on a rainy day to help moisten it. If the pile is
very wet, you might want to turn it on a sunny day, or cover it with moisture
protective material like chopped straw or compost fleece, a type of breathing
cover that is now widely available. Very frequent turning may not be
advantageous, because it can cause the physical breakdown of important
structural materials that aid natural aeration. The right amount of turning
depends on a variety of factors, such as
aeration, moisture, and temperature. Turn your compost pile to avoid cold, wet
centers; break up clumps; and make the compost more uniform later in the process
before use or marketing. Use caution turn-ing in cold, windy weather if the
pile is warm, for it may never reheat
The Curing
Stage
Following high-temperature composting, the
pile should be left to cure for about one to three months. Usually, this is
done once pile temperatures cool to 105°F and high temperatures don’t recur the following turning. Curing is especially needed if the active (hot) process is
short or poorly managed. There is a reduced need to turn the pile during curing
because the phase of maximum decomposition is over and there is significantly
less need for rapid oxygen entry into the pile’s center when the decomposition
rate is slow. (However, the pile may still need turn-ing during the curing
stage if it is very large or didn’t really finish composting—determining when
compost is finished is sometimes difficult, but if it reheats, it is not
finished—or is soaked by rain.) Curing the pile furthers aerobic decomposition
of resistant chemicals and larger particles. Common beneficial soil organisms
populate the pile during curing, the pH becomes closer to neutral, ammonium is
converted to nitrate, and soluble salts are leached out if the pile is outside
and sufficient precipitation occurs. Be sure to maintain water content at the
moisture-holding capacity (around 50% or less during curing) to ensure that
active populations of beneficial organisms develop.
It is thought that the processes that occur
during the
early curing process gives
compost some of its disease-suppressing qualities. On the other hand,
beneficial organisms require sources of food to sustain them. Thus, if composts
are allowed to cure for too long—depleting all the available food sources—disease
suppression qualities may decrease and eventually be lost.
OTHER COMPOSTING TECHNIQUES
High-temperature piles
account for most composting in the U.S., but other methods are also used.
Instead of making piles, small farmers in developing countries often dig pits
for composting (figure 13.4), especially in dry and hot climates. The pits can
be covered with soil material to prevent animals from getting into them, and
they retain moisture in the compost material better.
Vermicomposting involves the use of earth-worms—typically
red worms—to perform the decomposition process. The method is, in a way, still
mostly bacteria based, but the process occurs in the gut of the worm. The end
product is worm casts, coated with mucus consisting of polysaccharides that
make them into somewhat stable aggregates. The system requires bedding material—like
newspaper strips, cardboard, hay, and similar materials—that mimics the
decaying dried leaves that worms find in their natural habitat. The process is
fast and efficient—worms can process half their weight in organic material in
one day. The final product has an attractive feel and smell and is appealing to
consumers.
Vermicomposting is often
used to process kitchen scraps and can be done indoors in small bins. Recently,
vermicomposting methods have been developed for large commercial operations.
Two main approaches are used, using windows or raised beds. With wind-rows,
new materials are added on one side of the bed, and the other side is harvested
for compost after about sixty days. With the raised-bed or container system—
preferred for indoor operations in colder climates—the worms are fed at the top
of the beds and the castings
are removed at the
bottom. Some vermicomposting operations are connected with livestock farms to
process manure for export of excess nutrients off the farm as a value-added
product
USING
COMPOSTS
Finished composts generally
provide only low relative amounts of readily available nutrients. During
compost-ing, much of the nitrogen is converted into more stable
organic forms, although potassium and
phosphorus availability remains unchanged. However, it should be kept in mind
that composts can vary significantly and some that have matured well may have
high levels of nitrate. Even though most composts don’t supply a large amount
of available nitrogen per ton, they still supply fair amounts of other
nutrients in available forms and greatly help the fertility of soil by
increasing organic matter and by slowly releasing nutrients. Compost materials
can be tested at selected commercial agricultural and environmental
laboratories, which is especially important if certification is sought.
Composts can be used on turf, in flower gardens, and for vegetable and
agronomic crops. Composts can be spread and left on the surface or incorporated
into the soil by plowing or rototilling. Composts also are used to grow
greenhouse crops and form the basis of some potting soil mixes. Composts should
not be applied annually at high rates. That is a recipe for overloading the
soil with nutrients (see discussion in chapter 7
ADVANTAGES OF
COMPOSTING
Composted material is
less bulky than the original material, and easier and more pleasant to handle.
During the composting process, carbon dioxide and water are lost to the
atmosphere and the size of the pile decreases by 30–60%. In addition, many weed
seeds and disease-causing organisms may be killed by the high temperatures in
the pile. Unpleasant odors are eliminated. Flies, a common problem around
manures and other organic wastes, are much less of a problem with composts.
Composting reduces or eliminates the decline in nitrogen availability that
commonly occurs when organic materials, such as sawdust or straw, are added
directly to the soil. Composting is also very useful for recycling kitchen wastes,
leftover crop residues, weeds, and manures. Many types of local organic waste,
such as apple pumice, lake weeds, leaves, and grass clippings, can be composted
