Agriculture Reference
In-Depth Information
plant growth. In addition to being a source of plant nutrients, it increases soil water-
holding capacity and improves soil structure. It does this by helping to bind soil par-
ticles together to form peds. All benefits of organic matter in soil are hard to elucidate,
and even small increases of organic matter are beneficial to plant growth and
crop production.
One general way in which crop production throughout the world can be increased
is by increasing the organic matter in soil. This can be done in a number of different
ways. Plowing all plant residues into the soil, applying animal manure, composting,
and green manure crops are the four most common. In many places in the world and
many traditional cropping systems involve burning crop residue before plowing or
otherwise cultivating a field. Burning crop residue certainly reduces the residue to a
form (ash) that is easier to handle. However, at the same time, it also results in the
loss
of
most
nitrogen
and
sulfur
and
some
phosphorus
and
potassium
plus
other nutrients.
The addition of animal manures to soil is an excellent way to increase both the
organic matter of soil and also its nutrient content. Indeed in organic farming,
manure, from any type of animal, often plays a central role in maintaining soil fertility.
The exception to this would be untreated human waste because disease can be spread in
this way. In all other cases the manure should be maintained in good condition before
addition to soil. For instance, exposure to rain will lead to leaching and loss of nutrients
and thus degrade the usefulness of the manure as a soil amendment.
Manures contain easily decomposed organic matter and so should be composted
for some time before plowing in soil. Fresh organic matter will decompose aerobically,
and this process, which requires oxygen, will lead to an anaerobic (without oxygen)
state in soil. Because plant roots need oxygen, such a condition will be detrimental
to plants if too much undecomposed organic matter is incorporated into the root zone.
Composting involves taking organic matter from any source and placing in a pile to
decompose. Frequently, this pile would consist of organic kitchen waste, weeds pulled
from the garden or other sources, and any other organic waste. The compost pile
(Fig. 9.9) may be bounded with wooden or wire fence and organic matter added as it
becomes available. Once decomposed the organic matter, which is now composted,
is removed from the bottom and put on the garden or field.
Care must be taken during composting to keep pest animals—mice, rats, raccoons,
and the like—out of the compost pile. However, inclusion of worms will hasten com-
posting and improve the quality of the final compost. In this case both compost and
worms are then added as a soil amendment.
Green manures are also an excellent way to improve a soil's organic matter
content. In the strictest since this involves growing a crop—wheat, rye, rye grass, soy-
beans, or sweet clover, for example—and plowing it under before it matures; usually
during the flowering stage. In many respects a field allowed to grow weeds that,
once tall and near maturing, are plowed under is also like a green manure crop and
will add organic matter to soil. However, it is not a green manure crop if the grain is
harvested and then the plant residues are plowed under.
Some organic matter, plant residues such as wheat straw and other high-carbon
plant materials such as sawdust, has little nitrogen. When these are incorporated into
the soil, microorganisms take nitrogen from the soil to carry out the decomposition
