EROSION DUE TO CULTIVATION OF CALCAREOUS CLAY SOILS ON HILLSIDES IN SOUTH-WEST FRANCE .2. EFFECT OF PLOWING DOWN THE STEEPEST SLOPE

The erosion of calcareous clay soils from Terrefort in south west Fran ce is not exclusively due to run-off water. An experiment was carried out in situ to quantify the effect of ploughing on the displacement of soil down the slope. In a field experiment, a trench 0.4 m X 0.4 m X 20 m was dug parallel to the contours and filled with crushed gravel. Gravel distribution on the surface and within the soil mass was examin ed after ploughing downhill and then uphill. Mathematical analysis of the data showed that a much greater total soil mass was displaced down wards by ploughing downhill than by ploughing uphill. This resulted in soil removal from the top of the plot (0.32-1.62 kg m for a single pl oughing) and much greater accumulation at the base of the slope than e stimated for surface water erosion (i.e. 0.04-0.25 kg m(-2) year(-1)), according to measurements made by other authors in the same region.