Wind
Erosion
Wind erosion may
occur when the wind at one foot above the ground surface blows greater
than 12 mph. It is a serious hazard on millions of acres of land in the
United States, most of which are in the Great Plains. However, larger
fields and fall plowing has spread the potential for winter and spring
wind erosion to a larger part of the country.
There are a number of things an individual can do control wind erosion but
basically they all point to accomplishing one or more of the following
objectives:
-
Reduce the wind
velocity at the soil surface.
This is done with windbreaks, crop residues, cover crops, surface
roughness, and wind strip cropping.
-
Trap soil particles.
This is accomplished by ridging or roughening the soil surface to trap
moving soil particles.
-
Increase size of
soil aggregates. This is
accomplished by using crop rotations that include grasses and legumes,
by growing high-residue crops and returning crop residues to the soil,
and by emergency tillage, which creates clods on the soil surface.
Increasing the size of aggregates means that it takes a stronger wind to
move the soil.
Factors that affect the quantity of wind erosion that takes place include
soil erodibility, soil ridge roughness, climate and weather, the
unsheltered distance across a field along the prevailing wind erosion
direction, and residue or vegetative cover.
Different soils erode at different rates because of their inherent
characteristics. Soil texture and cloudiness are the primary properties in
determining soil erodibility.
Soil erodibility can be altered by growing different crops. Some crops,
such as hay, will normally help increase the size of soil aggregates more
than wheat, corn, or soybeans whereas tillage operations break down the
aggregates. The bigger an aggregate is, the less chance of blowing. Some
types of tillage equipment, such as the disc, reduce aggregate size more
than other types, such as the chisel. Early melting of snow or a lack of
snow cover exposes the soil surface to alternate freezing and thawing: in
the daytime, dark surfaces absorb more heat then the white snow, and at
night the soil surface may again freeze. This can occur daily in the late
winter and early spring, reducing the size of soil aggregates and
increasing the chances of wind erosion. Growing crops that produce a large
amount of crop residues and using conservation tillage can result in
larger soil aggregates than when conventional practices are followed.
Producing a crop in a ridged field will reduce wind erosion. The ratio of
ridge height to distance between the ridges is very important. The most
effective ratio of height of ridge to distance between rows is 1:4. For
example, to be most effective the ridge should be at least 2 inches high
if the distance between the ridges is 8 inches. The ridge is usually
formed with a tillage tool before planting or during cultivation of the
crop in the growing season.
Dry climates are more subject to wind erosion than humid climates. High
wind velocity contributes to increasing wind erosion.
The unsheltered distance across a field or strip along the prevailing wind
erosion direction is an important factor; a factor that can be changed by
man. Soil flow across a field is directly related to the width of the
unprotected area and may be likened to an avalanche moving down a
mountain. Soil flow or erosion increases with distance until the wind
becomes saturated or is carrying its maximum load. The more erodible the
soil surface, the shorter the distance at which maximum flow occurs and
the narrower the fields must be to keep the soil loss at a tolerable
level. Therefore, an effective means of wind erosion control is to reduce
field width. It is also important to work fields as close to perpendicular
to the prevailing wind erosion direction as possible.
Residue or growing vegetation on the soil surface reduces wind velocity at
the ground surface. As the quantity of residue on the surface increases,
the wind velocity decreases. Pound for pound, residues with a stem of a
smaller diameter result in more surface area of residue and therefore
produce more friction. The greater the friction, the greater the reduction
in wind velocity. Standing residues reduce wind velocity more than those
lying flat.
Therefore, residues are more effective in reducing wind erosion if:
-
Large quantities of
residues are left on the soil surface.
-
Stalks are small in
diameter or have a large surface area per unit of weight.
-
The residue is left
standing or nearly so.
In
summary, when designing a wind erosion control system, one should consider
cropping and management that will:
-
Result in large soil
aggregates;
-
Include ridging;
-
Establish narrow
fields, wind strip cropping, and, where feasible, windbreaks that are as
nearly perpendicular as practical to the prevailing winds during the
critical wind erosion periods;
-
Leave large amounts
of residue on the surface--standing if possible--at least during the
most critical periods.
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