PNW CONSERVATION
TILLAGE HANDBOOK SERIES
Chapter
1 - Erosion Impacts, No. 2, October-November 1985
Crop Yields
Decline with Topsoil Loss
Crop Yields Decline with Topsoil Loss
Roger Veseth
Loss of crop yield
potential with topsoil erosion is particularly severe if the subsoil contains
a root-restricting layer, has a low level of fertility and has a low available
water holding capacity. An example of this type, of topsoil loss-productivity
relationship is described in a recent study by STEEP researcher David
Carter, USDA-ARS soil scientist and research leader at the Snake River
Conservation Research Center near Kimberly, ID.
The Soils
Soils of the study area were Portneuf silt loam and similar silt loam
soil series with a lime/silica-cemented hardpan or lime-enriched horizon
at 10 to 18 inches depth in the uneroded soil. The white hardpan or lime
horizon is 8 to 18 inches thick and contains 25 to 35 percent lime. These
soils represent more than 400,000 acres of irrigated cropland and much
more non-irrigated cropland in Idaho. Extensive areas of similar silt
loam soils occur in Oregon, Washington and other western states. Not all
of these similar soils have the highly cemented hardpan layer, but most
have low fertility subsoil layers.
Extent of Erosion
A recent survey of the Magic Valley area of southcentral Idaho indicates
that about 75 percent of the fields exhibit white eroded areas where the
white hardpan or lime horizon is exposed, or topsoil is shallow enough
that the material is brought to the surface by plowing. The average portion
of the white, hardpan-mixed soil areas on 14 farm fields studied by Carter
was 30 percent. If we assume that these are representative of the entire
area, approximately 23 percent of this furrow-irrigated region is seriously
affected by topsoil erosion. An additional percentage of the eroded fields
have topsoil losses that affect yield although the white material is not
yet incorporated in the topsoil with tillage. This level of erosion has
occurred in the 80 years since furrow irrigation began in the area.
Research Effort
The effects of topsoil depth on crop yield was determined using both
farm fields and experimental plots. During 1982 and 1983, field transects
of 14 farm fields were made to determine topsoil depth and subsequent
crop yield. Experimental plots were established at the Research Center
and the yields of six crops were evaluated over a topsoil depth gradient
of 4 to 26 inches in 1983 and 1984. The maximum yield harvested for each
crop on a farmer's field or plot area was considered 100 percent, and
all other yields in that field or plot area were converted to a percentage
of that maximum yield.
Yields were drastically reduced on all crops as topsoil depth dropped
below the original 15 inches. Fig. 1 shows an example of the research
results illustrating the yield reduction of wheat with declining topsoil
depth. As topsoil depth dropped from 15 inches to 5 inches, wheat yield
declined at about 3.5 percent per inch. Sweet corn yield decline was almost
identical to wheat yields. Barley, dry bean and alfalfa yields were similarly
affected by topsoil loss but not as severely as wheat or sweet corn yields.
Sugarbeets were least affected by topsoil loss, but still showed declining
yields. When topsoil depths were increased over the original 15-inch depth,
crop yields did not increase significantly.
Application to Dryland
Although this study was conducted under irrigation, topsoil loss under
dryland cropping conditions and similar soils would have a similar impact
on crop yield potential. Yield loss under dryland would possibly be even
more severe with similar soils since limited water availability was not
a yield factor under irrigation, but would be more critical under dryland
as topsoil was lost. More details of Carter's study can be found in University
of Idaho Cooperative Extension System CIS 761, Furrow Erosion Reduces
Crop Yields.
Fig. 1. Percent of maximum yield at various topsoil depths for wheat grown on farmers' fields and experimental plots (University of Idaho Cooperative Extension System CIS 761).
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