Q. How does soil erosion affect crop yields in North
America?
A. Although erosion is widespread, it
is difficult to quantify its effect on agricultural productivity
and production. Past estimates of production losses to
erosion in North America (ranging from $300 million to
$27 billion) used simulation models or average values
for erosion and crop yields, disregarding differences
due to regional variations in soils and climate.
Den Biggelaar et al. estimated production losses and economic impacts
of erosion for corn, wheat, soybeans, and cotton using soil-based
extrapolations from existing erosion-productivity studies in the
soil science literature. Accounting for soil- and crop-specific
erosion rates and yield impacts, the estimated value of erosion-induced
production losses for these crops decreases from $83 million (using
national-average erosion rates) to $56 million. This estimate does
not include the extra costs for fertilizers, irrigation, and machinery,
and any off-farm effects of erosion.
While onsite productivity impacts are relatively small (less than
0.1 percent of the total value of annual U.S. agricultural production),
offsite impacts of erosion justify further research and continued
policy measures to encourage the adoption of conservation practices.
Erosion Rates
Concerns
about soil erosion in the United States date to the 1930's,
but erosion rates on U.S. cropland have declined markedly
in recent years, due in part to the adoption of conservation
tillage, the retirement of highly erodible cropland under
the Conservation Reserve Program (CRP), and other measures
encouraged by the 1985 farm bill. Annual erosion rates
vary widely on cropland and CRP land in the United States,
with extensive areas of relatively high erosion in the
western plains of Texas and Colorado and in the upper
Mississippi and Missouri River basins in Iowa and Minnesota.
Soil-specific
erosion rates are highest on Aridisols (which occur in
the dry areas of the Western States), but are much lower
on the soils that are most important to crop production
in the U.S., namely Alfisols (fertile soils that occur
primarily in the Corn Belt), Mollisols (fertile soils
that occur primarily in the Plains States), and Ultisols
(fertile but acidic soils that occur primarily in the
Southeast).
Yield Impacts by Crop and Soil Order
From
90 original field studies, we calculated mean erosion-induced
yield declines for each crop and soil order (see
data and methods). If continued over a 100-year period,
these annual yield declines would result in cumulative
yield losses ranging from 3 percent (for wheat on Alfisols
in the U.S.) to 31 percent (for wheat on Alfisols in Canada
and for soybeans on Ultisols in the U.S.).
Production Impacts
Aggregating
across crops and soils, the total gross economic value
of erosion-induced loss of production in maize, wheat,
soybeans, and cotton on Alfisols, Mollisols and Ultisols
in North America is estimated at $41 million per year,
of which $38 million occurs in the United States.
Assuming the losses on Alfisols, Mollisols and Ultisols are representative
of the losses on other soils on which these crops are grown, the
total annual economic value of erosion-induced losses for these
crops in the U.S. is estimated at $56 million. Accounting for crop-
and soil-specific erosion rates and yield impacts reduced this figure
from the estimate of $83 million that results from using the 1992
U.S.-average erosion rate of 13.9 metric tons per hectare per year.
This figure is smaller than earlier estimates by Pierce et al.,
Alt et al., and Crosson (1986), each of which was based on data
collected in the 1970's and 1980's, and considerably smaller than
that of Pimentel et al. Erosion rates were significantly higher
then than they are today. Agricultural prices have also been following
a downward trend in recent years, reducing the gross economic value
of yield losses. However, the costs in lost production are only
a part of the total costs incurred by farmers. They do not include
costs incurred to offset erosion-induced yield losses, some of which
were included in the estimates produced in earlier studies (Crosson,
1997).
Lessons
- With the steady increase in crop yields over time due to technological
advances, the relative reduction of yields per centimeter of soil
loss has declined.
- Yield impacts vary significantly by crop and soil order, and
losses estimated using soil-specific erosion rates are smaller
than those estimated using national-average erosion rates.
- Current estimated losses are small relative to the total value
of agricultural production of these crops, but the decrease in
productivity is cumulative and may cause a noticeable impact for
some crops, soils, and areas if erosion continues unabated over
a long period of time.
- Estimated losses in productivity are also small relative to
the offsite costs of erosion estimated by others, including Ribaudo,
in terms of water quality and other impacts.
- More precise estimation of actual losses due to erosion (as
opposed to the potential losses estimated here) will depend on
improved understanding of how farmers' optimal responses vary
in the face of changing physical, market, and policy environments.
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