QUANTIFYING TILLAGE EROSION RATES DUE TO MOLDBOARD PLOWING

Tillage erosion, the movement of soil downslope by mechanical implements, has been recognized as a problem, but primarily in a qualitative manner. This study was initiated to quantify soil movement by tillage on a hillslope landscape by computer simulation of long-term moldboard plowing. Regression equations describing soil movement as affected by slope gradient were developed from a hillslope at the University of Minnesota Southwest Experiment Station where individual plots could be located on the frontslope, apex, and backslope. Based on these equations, yearly soil movement calculations were made for a moldboard plowing operation over two hillslope landscapes. The first landscape was generated to give concave, linear, and convex slope gradients. The second landscape was a measured line segment from the experimental site. From a starting elevation and slope, one-dimensional calculations were based on forward moment of soil blocks, 0.24 m (plow depth) x 0.46 m (plow share width) x 1.5 m (increment length), over the hillslope to simulate 1 year's moldboard plowing. Elevation and slope of each soil block position was recalculated, and forward movement was then calculated in the opposite direction to simulate the next year's plowing. This process was continued to simulate 100 years of moldboard plowing at opposing direction during alternate years. Results from this analysis showed that a net loss in soil will occur on convex slope positions (tillage erosion), soil accumulation will occur in concave slope positions, and little change occurs in linear slope positions regardless of slope gradient. Calculated average annual net soil movement rates away from convex slope position were up to 30 t ha-1 year-1 and could easily account for the presence of observed lighter colored soil in convex slope positions on many landscapes. The magnitude of net soil movement calculated strongly suggests that soil movement by tillage is a serious problem.