Assessment of tillage erosion rates on steep slopes in northern Thailand

The switch from shifting cultivation to more permanent highland cropping systems in northern Thailand led to an increase in soil tillage intensity. In order to quantify soil losses by tillage erosion, a tillage experiment was set up and an on-farm survey was conducted. Soil fluxes due to manual tillage on five slopes (32-82%) were measured by monitoring tracers, by measuring tillage step characteristics and by collecting soil material in a trench. The trench method yielded soil flux values that were significantly smaller than those obtained by the tracer or the step method. Soil fluxes resulting from one manual tillage pass ranged between 39 and 87 kg/m on the tested slopes. On slopes up to 60%, there were no significant differences in soil fluxes. However, on slopes steeper than 70%, soil fluxes increased significantly because the angle of repose for soil clods was exceeded. The soil fluxes are used to construct a nomogram for estimating soil loss rates resulting from manual tillage erosion as a function of slope and plot length. Rates on a typical upland field (slope 30%-50%, slope length 30-50 m) range from 8 to 18 t/ha . tillage pass, so tilage erosion is a significant contributor to the total soil loss. It dominates on short fields and fields with buffer-strips, whereas water erosion is the more important form of soil loss on middle size and long fields. Increasing land pressure will result in increasing tillage erosion rates, and these need to be considered when assessing soil degradation rates or when studying hillslope evolution. (C) 1997 Elsevier Science B.V.