Advancing Sustainable Agriculture in the Pacific Northwest

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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 1 - Erosion Impacts, No. 2, October-November 1985


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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Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA (208)885-5971
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