Environmental Engineering Reference
In-Depth Information
drawback to this approach is an increase in crop pests that feed on the
residue.
Wind erosion can be reduced by windbreaks, but they reduce the
acreage available for crops. For every 1,300 feet of windbreak, approxi-
mately 1 acre is taken out of crop production.
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Windbreaks also put
crops on the side of them in shade.
The federal government encourages soil conservation through its Con-
servation Compliance Program. Under this program, farmers who grow
crops on highly erodible land must apply an approved soil conservation
system or risk losing eligibility for federal income support, conservation,
and other payments. Eighty-six percent of all cropland and about 83
percent of highly erodible cropland is located on farms that receive farm
program payments.
Soil Organisms
Good agricultural soil contains not only minerals formed from the
decay of underlying rocks, but an amazing array of living creatures as
well. Fertile soil is alive: an acre of healthy topsoil contains about 54
tons of organic matter, of which about 20 tons is composed of living
organisms such as bacteria, fungi, insects, and earthworms, while the
rest is in various stages of decay. The mobile organisms turn over and
continually aerate the soil and maintain soil porosity so nutrients in soil
moisture can move through the soil. Worm excretions contain a bal-
anced selection of minerals and plant nutrients in a form accessible to
plant roots. Earthworm casts are fi ve times richer in available nitrogen,
seven times richer in available phosphates, and eleven times richer in
available potash than the surrounding upper 6 inches of soil. The weight
of worm excretions may exceed 10 pounds per year.
Soil microorganisms play an important role in the cycling of major
plant nutrients as well. Some species of bacteria attach to the roots of
some plants (legumes) and convert gaseous nitrogen that plants cannot
use into dissolved nitrogen in a form accessible to plants. Soil-dwelling
fungi have a symbiotic relationship with the roots of vascular plants in
which the fungi increase the availability of minerals, water, and organic
nutrients to the plant. Healthy crops rely on soil microorganisms for
their survival.
The best way for a crop's uptake of soil nutrients to be returned to
the soil can be determined by seeing what happens in an undisturbed
natural environment. Mother Nature's method has been honed over
hundreds of millions of years and is a good guide to maintaining a
