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