PNW CONSERVATION
TILLAGE HANDBOOK SERIES
Chapter 2 - Systems and Equipment, No. 10, Spring 1988
Maintaining a portion
of the residue from the previous crop on the soil surface through conservation
tillage has been shown to be very effective in reducing soil erosion.
Consequently, the development of new technology for conservation tillage
systems has been the main focus of STEEP and related conservation farming
research in the Northwest. One should not interpret that other farming
practices
cannot help conserve and improve soil as well. Contour strip cropping
and divided slopes, uphill plowing, terraces, the use of grass or legumes
in crop rotations and other practices can also help to conserve soil and
maintain or improve productivity.
Two adjacent fields with long histories of different crop rotation and
tillage systems are located on a south-facing slope near Dayton, WA, in
a 20-inch average annual precipitation zone. Both fields have been in
production for more than 80 years. A STEEP research project was conducted
in 1986 and 1987 to compare the impacts of the two different farming systems.
It was conducted by Ann Rodman, graduate research assistant in soils at
Washington State University in Pullman, in cooperation with WSU soil scientists
Bruce Frazier and Alan Busacca.
Farming Practices Compared
Two adjacent fields, designated as D 1 and D2, had sustained substantial
soil erosion losses before the first conservation practice began about
40 years ago. Field D 1 has been farmed with contour strips approximately
400 feet wide for the past 40 years, had bluegrass as part of the rotation
with winter wheat and spring peas for the past 15 years, and has been
uphill-plowed, so that the plow furrow is turned upslope on the contour,
for the past 10 years. Field D2 has not been farmed with contour strips,
has been cropped in a winter wheat/fallow or winter wheat/spring pea rotations,
typically with the pea residue removed, and has been conventionally-plowed
so the plow I furrow was turned downslope.
The fields were compared at specific slope positions on a south-facing
slope (Fig. 1). Soils were sampled and described on a series of transects
across the fields. The steepest part of the slope was about 15 percent.
Bedrock is within about 16 inches of the surface on the shoulder of the
slope, so loss of soil depth by water and tillage erosion is of particular
concern.
Organic Carbon Content
Soil organic carbon content of the surface 6 inches of soil was determined
in the study instead of organic matter content because it is a more accurate
measurement for research purposes. (Percent organic carbon can be roughly
converted to percent organic matter by multiplying the organic carbon
percentage by 1.7.)
Organic carbon content of the soil averaged 0.5 percent higher on the
south-facing summit, shoulder and upper backslope positions of field D
1 than on similar slope positions of field D2 (Fig. 2). The two fields
had similar , organic carbon content in the surface soil on lower slope
positions.
Fig. 1. Schematic Illustration of the south-facing slope and slope
positions of the study area on fields D1 and D2 near Dayton, WA (Rodman,
Frazier and Busacca, WSU).
Fig. 2.Comparison of percent organic carbon (0. C.) content of
the surface 6 inches of soil on the southfacing slope of adjacent fields
under conservation (Dl) and conventional (D2) practices near Dayton, WA
(Rodman, Frazier and Busacca, WSU).
Depth to 1% Organic Carbon
The depth to 1 percent organic carbon content was used to determine
the approximate thickness of the A horizon (and transition AB horizon
if present), or organic matter rich surface layer of soil. This method
helps to reduce some of the subjective variation in visually determining
the boundary between the A and B horizons. Fig. 3 shows that the A horizon
of the soil on field D1 was an average of about 5 inches thicker on the
summit and 3 inches thicker on the lower backslope positions than on the
same slope positions in field D2.
In describing soil profiles in field D 1, the researchers noted that the
boundary between the A and underlying B horizon was diffuse, commonly
spanning about 12 inches,
Fig. 3. Comparison of thickness of the A horizon, measured as
the depth to 1 percent organic carbon content, on adjacent fields under
conservation (field Dl) and conventional (field D2) practices near Dayton,
WA (Rodman, Frazier and Busacca, WSU).
In contrast, the horizon boundary was often abrupt in field D2. They
explain that the abrupt boundary indicates that water and tillage erosion
had removed most of the A horizon, and the subsoil B-horizon material
is being mixed into the tillage zone. In other words, the boundary between
the A and B horizons is now the depth of soil mixing by tillage.
Soil structure and aggregation were more evident in surface soils on field
D 1 than on field D2. The researchers attributed this to the higher organic
carbon content in field D 1 and reduced mixing of the A horizon with the
B horizon, as the B horizon has a lower organic carbon content. Good soil
structure and aggregation are important for reducing soil erodibility,
maintaining an adequate water infiltration rate, reducing surface crusting
potential and other productivity-related factors. Also, the higher clay
content of the B horizon mixed with the A horizon tends to reduce the
rate of water infiltration and increase surface crusting.
Conclusion
From this study, it appears that the combination of contour strips,
grass in the crop rotation and uphill plowing have helped to increase
organic carbon content of the surface soil and reduce soil loss by water
and tillage erosion compared to conventional farming without those practices.
Without soils data from the fields before the conservation practices were
initiated, or a zero-erosion comparison, it is difficult to accurately
evaluate the effects of these practices. However, the researchers feel
that the conservation practices are maintaining the organic carbon content
and topsoil depth which were present before the conservation practices
were initiated. The inclusion of grass in the rotation may have even increased
the organic carbon content.
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