Figure 14.1. A waterway scoured into a gulley on a midwestern cornfield
Figure 14.2. Erosion on steep lands in Central America. Removal of the
after erosive spring rains. Photo by Andrew Phillips.
fine topsoil left mostly boulders behind. Sorghum plants show drought
stress due to lack of rain and low water storage capacity in soil.
with good aggregation. This is reflected in the soil erod-
the rate of topsoil creation can balance this loss. The
ibility ratings, which soil conservationists use to plan
soil loss tolerance amount is reduced for soils with less
control practices.
rooting depth. When it is less than 10 inches, the toler-
A small amount of erosion is acceptable, as long as
able rate of soil loss is the same as losing 0.006 inch per
new topsoil can be created as rapidly as soil is lost. The
year and is equivalent to 1 inch of loss in 167 years. Of
maximum amount of soil that can be lost to erosion
course, on agricultural fields the soil loss is not evenly
each year, while maintaining reasonable productivity, is
distributed over the field, and areas of water conflu-
called the soil loss tolerance, or T value. For a deep soil
ence experience greater losses (figure 14.1). Also, many
with a rooting depth of greater than 5 feet, the T value
conservationists would argue that any amount of erosion
is 5 tons per acre each year. Although this sounds like a
is unacceptable, as the off-site damage to water and air
large amount of soil loss, keep in mind that the weight
quality may still be considerable.
of an acre of soil to 6 inches of depth is about 2 million
When soil loss is greater than the tolerance value,
pounds, or 1,000 tons. So 5 tons is equivalent to about
productivity suffers in the long run. Yearly losses of
.03 inch (less than 1 mm). If soil loss continued at that
10 or 15 tons or more per acre occur in many fields.
rate, at the end of 33 years about 1 inch would be lost.
In extreme cases, as with croplands on steep slopes in
On deep soils with good management of organic matter,
tropical climates, losses of five or ten times that much
erosIon: A short-terM MeMorY ProbLeM?
It’s difficult to fully appreciate erosion’s damage potential, because the most severe erosion occurs during rare weather events and climate anomalies. Wind erosion during the Dust Bowl days of the 1930s, which resulted from a decade of extremely dry years, was especial y damaging. And about one-third of the water erosion damage that occurs in a particular field during a thirty-year period commonly results from a single extreme rainfall event. Like stock market crashes and earth-quakes, catastrophic erosion events are rare, but the impacts are great. We must do our best to understand the risks, prevent complacency, and adequately protect our soils from extreme weather events.
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Building SoilS for Better CropS: SuStainaBle Soil ManageMent