Quantifying tillage translocation and deposition rates due to moldboard plowing in the Palouse region of the Pacific Northwest, USA

Abstract

Most of the erosion research in the Palouse region of eastern Washington State, USA has focused on quantifying the rates and patterns of water erosion for purposes of conservation planing. Tillage translocation, however, has largely been overlooked as a significant geomorphic process on Palouse hillslopes. Tillage translocation and tillage deposition together have resulted in severe soil degradation in many steep croplands of the Palouse region. Few controlled experiments have heretofore been conducted to model these important geomorphic processes on Palouse hillslopes. The overarching purpose of this investigation, therefore, was to model tillage translocation and deposition due to moldboard plowing in the Palouse region. Soil movement by moldboard plowing was measured using 480-steel flat washers. Washers were buried in silt loam soils on convex–convex shoulder, linear-convex backslope, and linear-concave footslope landform components, and then displaced from their original burial locations by a moldboard plow pulled by a wheel tractor traveling parallel to the contour at ca. 1.0 m s −1. Displaced washers were located using a metal detector, and the distance and azimuth of the resultant displacement of each washer from its original burial location was measured using compass and tape. Resultant displacement distances were then resolved into their component vectors of displacement parallel and perpendicular to the contour. A linear regression equation was developed expressing mean soil displacement distance as a function of slope gradient. Tillage translocation and deposition were modeled as diffusion-type geomorphic processes, and their rates were described in terms of the diffusion constant ( k). A multivariate statistical model was developed expressing mean soil displacement distance as a function of gravimetric moisture content, soil bulk density, slope gradient, and direction of furrow slice displacement. Analysis of variance (ANOVA) revealed a weak correlation between soil displacement and both bulk density and moisture content. Soil displacement was, however, significantly correlated with direction of furrow slice displacement. Tillage translocation rates were expressed in terms of the diffusion constant ( k) and ranged from 105 to 113 kg m −1 per tillage operation. Tillage deposition rates ranged from 54 to 148 kg m −1 per tillage operation. With respect to tillage deposition, the diffusion constant calculated from volumetric measurements of tillage deposits equals ca. 150 kg/m. The rates of tillage translocation and deposition are not completely in balance; however, these rates do suggest that soil tillage is a significant geomorphic process on Palouse hillslopes and could account for the some of the variations in soil physical properties and crop yield potential at the hillslope and farm-field scale in the Palouse region.