Tillage Erosion – Changing Landscapes and Productivity

Roger Veseth

Chapter l -Erosion Impacts, No.4, Fall 1986

The high rate of topsoil erosion on much of the sloping Northwest cropland has been a serious problem for decades. Most people attribute the soil loss to water erosion through surface runoff from rapid snowmelt or heavy rainfall, particularly on frozen soils. Water erosion rates of over 150 tons of soil per acre, about 1 inch of topsoil, in one winter season are not uncommon on steep slopes. Although water erosion is one of the most important and most visible mechanisms of soil loss, there is another erosion mechanism which is also significantly reducing topsoil depth on certain landscape positions – ” tillage erosion”.

Erosion from Tillage Implements

Tillage erosion, the movement of soil downhill during tillage operations, has not received as much attention as water erosion. Over time, however, tillage erosion is believed to be responsible for a significant reduction in topsoil depth and increased subsoil exposure on hilltops and ridges in agricultural land.

Although other tillage implements can move soil downhill, the moldboard plow causes the most tillage erosionifthe furrow slice is turned downhill, called “downhill plowing,” In contrast, “uphill plowing, ” where the furrow slice is turned uphill, may help hold the soil in place on the slope or may possibly reverse the tillage erosion process. Traditionally, downhill plowing has been more popular since it has lower power and fuel requirements, and buries more of the crop residue than uphill plowing. But, growers are eventually paying the price in reduced yields at ridge tops as topsoil depth decreases.

Repeated downhill plowing acts as a slow motion conveyor belt as each furrow slice is moved downslope. This downcutting effect is most apparent on hilltops and ridges where soil removed is not replaced by soil moved down from a higher landscape position.

Besides the increasing exposure of less productive subsoils, there is other evidence of the tillage erosion process underway in the Palouse and other Northwest farming areas with sloping cropland. Old fence lines often form steep walls up to 6 feet or more in height where the uphill farmer has plowed toward the fence and the downhill farmer plowed away from the fence (Fig. 1).

Estimates of Tillage Erosion

Experimental studies on topsoil loss attributable to tillage erosion are scarce, One of the early reports stressing the importance of tillage erosion in the Palouse was by Verle Kaiser, a USDA-SCS agronomist in Whitman County, WA, in 1961. Kaiser determined that there was a 2-foot loss of soil depth on a Palouse hilltop during 31 years of farming between 1911 and 1942. An additional 2 feet of soil depth was lost during the next 17 years between 1942 and 1959. He attributed a substantial portion of that loss in soil depth to tillage erosion, in addition to water erosion.

STEEP researcher Don McCOO1, USDA-ARS agricultural engineer at Pullman, WA, recently made some measurements of soil loss that resulted from downhill plowing with the moldboard plow. MCCOO1 found there was a 21-inch downhill movement of the furrow using 16-inch plow shears set at 5 inches depth, a tractor speed of slightly
over 3.5 miles per hour and a 25 to 30 percent slope. Over a 100-foot slope length, the average soil loss was determined to be about 13-tons-per-acre.

Uphill plowing, under the same condition, resulted in an uphill soil movement of 9 inches. MCCOO1 states that this uphill soil movement may compensate for some or most of the downhill movement inherent in secondary tillage operations with field cultivators and rod weeders and other implements.

MCCOO1 stresses that the amount of downhill soil movement with downhill plowing would increase as the percent slope and tractor speed increased. Cropland slopes of 20 to 35 percent are common in the Palouse region with some slopes reaching 45 to 55 percent or more. This 13ton-per-acre soil loss measured from a single downhill plowing is comparable to an average overwinter soil loss to water erosion under winter wheat with a low-residue, conventionally-tilled seedbed. Besides increasing exposure of subsoils, MCCOO1 points out that tillage erosion increases slope steepness because soil is removed more rapidly than it is replaced from the flatter, upper portion of the slope.

Increasing Subsoil Exposure

Most hilltops and ridges on the Palouse cropland and other areas of the Northwest now have only shallow topsoil depth, if any topsoil is present. Exposure of less productive, clayey or highly calcareous subsoils has greatly increased due both to tillage erosion and water erosion.

In a 1978 Palouse Cooperative River Basin Study, it is estimated that all of the original topsoil has been removed from 10 percent of the Palouse cropland, exposing clayey or calcareous (high lime) subsoils. Another 60 percent of the cropland has lost 25 to 75 percent of the topsoil.

Washington State University soil scientists and STEEP researchers Alan Busacca and Bruce Frazier currently have a research project underway to estimate the proportion of Palouse landscape which has potential for exposure of clayey or calcareous subsoils as topsoil depth is reduced. Through historical and modern aerial photography, special imagery and on-site monitoring, they plan to quantify the current rate of increasing subsoil exposure.

Eventually, they hope to develop predictive equations for several Palouse sites that will estimate the actual rates of increased subsoil exposure under different rates of tillage and water erosion.

The two WSU soil scientists and research assistant Ann Rodman are also evaluating the impacts of long-term uphill plowing on a farm near Dayton, WA. Two adjacent fields, located on one landscape position, are being evaluated for organic matter and organic carbon content, depth to subsoil, depth to bedrock and other properties. One field has been uphill plowed over the past 35 years while the other field has been downhill plowed during that time.

Control Options

Tillage erosion can be reduced to a minimum with changes in management practices. Some growers, who feel they need to continue using the moldboard plow, are now uphill plowing on the contour. Uphill plowing helps counter the downslope tillage erosion problem and also leaves slightly more of the crop residue on the surface. The residue helps reduce the potential for water erosion during the following crop year. Removing trash boards from the plow will further increase the surface residue level.

Substituting chisel plows, sweeps or other non-inversion tillage implements for the moldboard plow effectively reduces tillage erosion. The higher surface residue levels of minimum or reduced tillage and no-till systems effectively control water erosion and increase soil water storage for a higher yield potential. Many growers are demonstrating that minimum tillage and no-till are also very efficient, productive farming systems. Recent advances in crop management technology including seeding equipment, fertilizer placement, weed and disease management strategies, variety development and alternate crops are helping to ensure the success of these conservation farming systems.

Tillage erosion and water erosion together are sharply reducing topsoil depth and corresponding production potential on the Palouse and other sloping cropland areas of the Northwest. Control options are available and are constantly being improved. It is time to explore some alternatives.

Roger Veseth is an Extension Conservation Tillage Specialist with Washington State University and University of Idaho.