Short and medium term assessment of tillage erosion in the Uluguru Mountains, Tanzania

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Soil translocation due to shallow tillage by manual hoeing appears to be one of the most important erosion processes in the Uluguru Mountains. In order to quantify erosion rates caused by manual hoeing in the area a tillage experiment was set up and an on-farm survey was conducted during the dry season of the years 2000 and 2001, respectively. Soil flux rates on eight slope gradients (31–67%) were monitored by measuring the tillage step characteristics using Trapezoid-step method and by collecting soil material lost in Gerlarch troughs. Soil flux rates due to medium-term (30 years) manual hoeing along contour bands with grass barrier were also monitored by measuring volumes of tillage step below and colluvium accumulation above the surface of the original slope on six slope gradients (51, 52, 55, 56, 58 and 60%). Average tillage depth for superficial tillage was 5.2 cm. The results obtained by the Trapezoid-step method ranged from 43 to 70 kg m À1 per tillage pass with a mean tillage transport coefficient (k) of 107.5 kg m À1 on the tested slopes. Mean soil flux rates obtained by Gerlarch trough method were slightly lower than those obtained by Trapezoid-step method with values ranging from 14 to 77 kg m À1 per tillage pass and a tillage transport coefficient (k) of 83.9 kg m À1 per tillage pass. The rates measured by both methods showed an increasing soil flux with slope gradient. Results on soil flux rates due to the medium-term tillage operation (step measurements) showed a negative trend with increasing slope gradient. Soil flux ranged from 148 to 42 kg m À1 per year for slopes between 51 and 60%. Soil flux due to colluviation behind grass barriers showed a similar trend with values higher than those obtained by step measurements. The soil flux rates behind grass barriers ranged from 153 kg m À1 per year on slope of 51% to 67 kg m À1 per year on a 60% slope in approximately 30 years of cultivation. A reasonable correspondence between calculated displaced soil (area under original slope) and the accumulated colluvium (area above the original slope) was obtained indicating significant contribution of tillage erosion. Contribution due to water erosion processes ranged from 7 kg m À1 per year on slopes of 51% to 25 kg m À1 per year on a slope of 60%. The study demonstrated that tillage translocation rates due to manual superficial tillage are very high and could partly be held responsible for the development of shallow soils observed on steep slopes and the accumulation of colluvium behind grass barriers along contour bands in the Uluguru Mountains.



Short and medium term tillage erosion, Uluguru Mountains, Tanzania, soil translocatiion