Identification of important factors in the process of tillage erosion: the case of mouldboard tillage

Most of the experimental studies on tillage erosion by mouldboard tillage have hitherto focused on the characterisation of the erosivity of a given tillage pass for specific operating conditions. The effect of variations in tillage speed and depth on soil translocation and the associated tillage erosion has largely been overlooked. This paper reports on the results of a series of tillage experiments that were set up to investigate the effect of variations in tillage speed and depth on net soil displacement and the associated tillage erosivity for mouldboard tillage carried out in two contrasting directions: parallel to the contour lines (contour tillage) or perpendicular to the contour lines (up- and downslope tillage). The experimental data clearly show that the average soil displacement distance is not only a function of slope gradient, but is also strongly affected by tillage speed and tillage depth. A model to predict the average displacement distance and the tillage transport coefficient k from slope gradient, tillage depth, tillage speed and tillage direction information was constructed. The effect of tillage depth on mouldboard tillage erosivity depends on tillage direction: k-values increase exponentially with tillage depth for up- and downslope tillage, while the increase is linear for contour tillage. On the other hand, the effect of tillage speed on tillage erosivity is similar for both tillage directions considered, but less pronounced. Validation of this model using data on mouldboard tillage erosion experiments available in the literature shows that the effect of variations in tillage depth on tillage erosivity could be predicted very well. Although absolute values were slightly underestimated, which is probably due to the large variation in soil characteristics and implements used, this indicates that tillage depth, tillage speed and tillage direction are the dominant controlling factors on the magnitude of the tillage transport coefficient. Using this model structure, a series of nomograms were constructed, that allow one to evaluate the effect of changes in tillage depth, speed and/or tillage direction on the soil erosivity of a given mouldboard tillage operation. These nomograms, whereby the tillage transport coefficient k is predicted as a function of tillage speed, tillage depth and tillage direction, are a useful tool in evaluating strategies to minimise tillage erosion.