Advancing Sustainable Agriculture in the Pacific Northwest

Conservation Tillage Systems

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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 1 - Erosion Impacts, No. 8, Fall 1987


Technology No Substitute for Conservation

Roger Veseth

Scientists and farmers alike have long recognized that soil erosion reduces crop yield potential. The actual assessment of the erosion damage to soil productivity, however, has always been an elusive problem for several reasons. First, the erosion process and its effects on yields are usually gradual so it is difficult to see the damage in the short term. Second, annual yield variability from combinations of weather, damage from diseases, weeds and insects, and other factors obscure the continued reduction in yield potential from topsoil loss.

STEEP Research

Assessing the loss in soil productivity because of past and projected future erosion has been part of the STEEP conservation farming research program since it began in the three Northwest states in 1976. Two of the scientists who have been cooperatively involved in this STEEP research area are David Walker and Douglas Young, agricultural economists with the University of Idaho and Washington State University, respectively.

Technology vs. Erosion Damage

One of the aspects of their research focuses on another important factor which masks the damage of soil erosion, that of advances in crop production technology. In many areas, technical progress has increased Mop yields on a field basis faster than erosion has reduced them. This can easily give the misconception that erosion is of no consequence. The researchers point out that to correctly assess erosion damage, the effects of erosion and technology on yields must be separated and projections of their impacts made individually.

To help explain the separate impacts of erosion and technology, Walker and Young developed a 64-year wheat yield projection model based on field data from the Palouse area of eastern Washington. The yield projections assume a deep soil with only slight changes in physical or chemical properties between the original 18-inch topsoil layer and the underlying subsoil. *
*NOTE: With soils containing subsoils that have a much higher clay content, root restricting layers, high lime content or other less productive characteristics, sharper declines in yields with topsoil loss would occur than that shown by the examples in this article.

The following four figures show a comparison of yields with this model where soil depth has declined from the original 18 inches to 15.4 inches under a particular conservation system and to 5.2 inches with an erosive (conventional) system. Starting from a "no technology change" yield curve, the researchers have defined three types of changes in technology depending on their impacts on yields: (1) land-neutral, (2) land-complimentary and (3) land substituting.



No Technology Change

With no projected technology change, yields under an erosive system are projected to decline 19 bushels/acre (70 to 51 bushels/acre) as topsoil depth is reduced from the original 18 to 5.2 inches (Fig. 1). Under a selected conservation system, yield decline is projected at only 2 bushels/acre with the loss of 2.6 inches of topsoil. The following discussions of the impacts of advances in technology will focus in comparing erosion damage with a conservation system vs. with an erosive system. In this case of no technology change, the difference is 17 bushels/ acre.

Fig. 1. Yield damage from loss of topsoil under no technology change.



Land-Neutral Technology

Land-neutral technical progress provides an upward shift in the yield curve with an equal yield benefit regardless of topsoil depth (Fig. 2).


Fig. 2. Residual yield damage from loss of topsoil after land neutral technical change.


Walker and Young explain that this type of shift in the yield curve would occur only where the subsoil does not contain a root restricting layer, and physical and chemical properties are reasonably similar to the topsoil.

In this situation, the technical progress has not eliminated erosion damage but maintained the differences in yield potential between topsoil depth. The 17 bushels/acre yield difference between the conservation system and erosive system still exists after the technical progress. The researchers point out that land-neutral technology rarely occurs on highly erodible land in the Northwest since the subsoils commonly are less productive than topsoils because of root restrictive layers or other unfavorable physical or chemical properties.

Land-Complimentary Technology

The researchers define land-complimentary technology as technology which increases yields more on deep topsoils than shallow topsoil (Fig. 3). With this type of technology, yield increased 17 bushels/acre under an erosive system with 5.2 inches topsoil compared to a 32 bushels/acre increase under a conservation system with 15.4 inches topsoil. Consequently, this type of technology actually increases yield reduction from erosion.

Fig. 3. Residual yield damage from loss of topsoil after land complimentary technical change

An example of land-complimentary would be the development of improved crop varieties. New varieties usually attain their greatest genetic yield potential in a soil environment with unrestricted water and nutrient supplies; conditions more often found in areas with deep, uneroded topsoil.

Land-Substituting Technology

Technology that boosts the yield curve more on shallow topsoil than deep topsoil is called land-substituting technology (Fig. 4). Compared to no projected technology change, yield damage because of topsoil loss decreases with land-substituting technology. Yield under the erosive system is only 11 bushels/acre less than under the conservation system with the land-substituting technical progress compared to 17 bushels/acre with the static current technology.


Fig. 4. Residual yield damage from lose of topsoil after land substituting technical change.


An example of this type of technology might be a residue management practice that increases plant-available water by reducing water loss from evaporation and runoff. Because topsoil serves as an important part of the crop's water storage reservoir, water deficiency is more likely to occur in shallow, eroded topsoil areas than where topsoil is deep. Thus, technical advances that conserve water are likely to proportionally boost yields more on soils with shallow topsoil.

Nature of Past Technical Progress

Although there is no foolproof method for predicting the type of technology to expect in the future, an evaluation of past trends helps to shed some light on the possibilities. By analyzing historical yield and topsoil data in the Palouse of eastern Washington, Walker and Young, and other cooperating researchers, have evaluated the impact that technical change has had on yield. At the heart of the Palouse region in Whitman County, advances in technology have increased average county wheat yields from 25 bushels/acre to more than 60 bushels/acre between 1930 and 1980. A closer evaluation, however, reveals that while the average field yields have increased, the increase has come primarily from the uneroded portions of the fields. The effects of erosion on yield potential are being masked by the yield increase on a field basis.

Fig. 5 represents a statistical evaluation of about 900 farm field measurements of winter wheat yield and topsoil depth between 1952-53 and 1970-74 in eastern Whitman County. Point estimates of yields from the two yield curve functions reveal that technical advances over the two decades boosted wheat yields by 22.9 bushels/acre (51.0 to 76.9 bushels/acre) on the 25-inch topsoil depth, but only 14.4 bushels/acre (25,6 to 40 bushels/acre) where there is no remaining topsoil and the subsoil is exposed. Technical progress boosted yields about 60 percent more on deeper topsoil than where no topsoil remained.


Fig. 5. Comparison of winter wheat - topsoil depth relationships from the 1950's and 1970's in eastern Whitman County, WA.

Technology Examples

A statistical comparison of the two yield curve functions at the 90 percent probability level revealed that the technical advance during the two decades had been primarily the land-complimentary type where yields are increased more on soils with deep topsoil than with shallow topsoil. Probably one of the best examples of land-complimentary technology in this region has been the development of higher yielding semi-dwarf wheats, beginning in the 1950's. Their highest yield potential is realized on deep topsoil areas where water and nutrients are not major yield limiting factors. Eroded soils are likely to have more water loss to runoff, a lower available water-holding capacity and lower nutrient availability, and contain restrictive layers which inhibit root penetration. Furthermore, poor seedbed conditions, which are common on subsoils or mixes of subsoil and shallow topsoil, also restrict genetic yield potential by reducing stand establishment and early plant vigor.

Another example of land-complimentary technology which has occurred throughout the country is the dramatic increase in the use of inorganic nitrogen fertilizer beginning in the 1950's. Although nitrogen and other fertilizer nutrients have increased yields on eroded soils, problems with seedbed conditions and water availability can often be more limiting than the fertility level. Consequently, the use of fertilizer technology may increase yields more on uneroded soils where it compliments the greater amount of available water and superior seedbed conditions.

Improved weed control with the advent of new herbicides and tillage equipment has also increased wheat yields, However, like higher yielding semi-dwarf wheats and fertilizer use, the greatest benefit to improved weed control is where the highest yield potential exists - in areas where topsoil is deep enough to provide adequate water storage and nutrient supplies and a conducive growth environment.

Conclusion

Walker and Young conclude that most of the yield enhancing technical advances in agriculture have been land complimentary and have not helped offset erosion damage to yield potential. Yields have increased on a field average basis, but the increase has come more from the areas with deeper topsoils than areas with shallow topsoil or exposed subsoils. Consequently technology in the Palouse, and perhaps nationally, has largely intensified the problem of erosion, rather than reduce it. The researchers stress that the greatest yield gains in the future can only be achieved by developing sound conservation practices which maintain productive topsoil in order to take full advantage of future technology, Technology must not be viewed as a substitute for soil conservation, but rather a compliment to conservation practices.

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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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