Advancing Sustainable Agriculture in the Pacific Northwest

Conservation Tillage Systems

Information Resource

Chapter 2 - Systems and Equipment, No. 10, Spring 1988

Farming Practices Conserve and Improve Soil

Roger Veseth

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.


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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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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