By Sjoerd Willem Duiker
A recently published study using remote sensing showed that 35% of Corn Belt cropland has lost the entire A horizon, far exceeding past estimates of the damages of soil erosion. This loss is not uniform across a field, and actually involves translocation of soil from the tops in the landscape to depressions. In our landscape we readily recognize this as ‘clay knobs’ that don’t have any topsoil in higher parts of the field, while in the lower parts we may have 2 or even 3 feet of topsoil. This is a major cause of yield variability in undulating landscapes. A 2005 study compared corn yields of Iowa soils that had lost varying amounts of topsoil. The study found that corn yield was reduced approximately 16 bu/A if topsoil thickness was reduced from 15 to 0 inches on loess soils, and 26 bu/A on till-derived soils. The latter would probably better reflect typical yield reductions on Pennsylvania soils that have limited depth to bedrock. In previous Field Crop News articles , we have discussed different types of soil erosion by water in cropfields: sheet and rill erosion and ephemeral and classic gully erosion. There is another type of erosion, however, that works in tandem with water erosion, called Tillage erosion. It is the major contributor to the loss of A-horizon in undulating landscapes. Tillage erosion is defined by the Soil Science Society of America as the downslope displacement of soil through the action of tillage. It ‘collaborates’ with water erosion, where tillage erosion as well as water erosion cause soil loss in the upper-slope and upper mid-slope, cancel each other out in the lower mid-slope, and cause soil accumulation towards the end of the lower-slope. However, when runoff does occur, the loose, poorly structured soil in lower-slope positions is now easily dislodged, causing ephemeral gullies to form in the landscape. The end result is that soil loss is greater when tillage and water erosion work in tandem than if we consider each one on their own. There are different factors that impact the magnitude of tillage erosion. Slope is one, but the three-dimensional shape of the landscape also impacts it. Moldboard plowing typically causes more tillage erosion than chisel plowing or disking. Reducing the depth of tillage helps to reduce tillage erosion. Up-and-downslope plowing causes more tillage erosion than plowing on the contour. Plowing the furrow downslope causes more tillage erosion than plowing soil uphill. Higher speed of tillage causes more tillage erosion than tilling at slow speeds. Contour strip cropping can accentuate the effects of tillage erosion because topsoil is slowly plowed from the top to the bottom of each strip. The best way to control tillage erosion is to stop tilling.
Source : psu.edu