Soil Mineralogy, Slope And Soil And Water Conservation Effects On Infiltration, Seal Formation, Erosion And Crop Yield Under Semi-arid Conditions

Abstract

Soil erosion and runoff are serious and widespread land degradation problems in many parts of the world especially in in arid and semi-arid regions, where water is the main limiting factor for crop production. This thesis presents the results of three studies conducted to investigate the effects of soil mineralogy, texture and slope gradient on seal formation and its properties and their effects on infiltration, runoff and erosion. Water and soil conservation effects on infiltration, erosion and crop yield under seal formation conditions in semi arid conditions was also investigated. In the 1st laboratory experiment, soils with different mineralogy and texture were subjected to 80 mm of simulated rainfall at 9% slope gradient. The mean weight diameter (MWD) values were 2.8 mm in the kaolinitic soil, 0.25 and 0.31 mm in the montmorillonitic soils, and 0.84 and 0.87 mm in the non-phyllosilicate soils. The final infiltration rate (IR) was 20.5 mm h-l in the kaolinitic soil and <9.3 mm h-l in the montmorillonitic soils. The kaolinitic soil had a thin crust (- 0.1 mm) containing large particles (- 0.1mm), while the montmorillonitic soils had thicker crusts (> 0.2 mm) comprising either small (- 0.02 mm) particles with a very developed "washed in" zone underneath, or large (- 0.2 mm) ones with fine material between them. Crust layer in the non-phyllosilicate soils was - 0.2 mm and composed of fine particles - 0.01 mm. The high aggregate stability and the low dispersivity of the kaolinitic soil, which decreased the soil detachment; and its low runoff, which decreased the transport capacity, limited the soil loss to 0.33 kg m-2• The low aggregate stability and high runoff of the montmorillonitic soils contributed to their soil losses of 1.24 and 1.14 kg m". The intermediate aggregate stability and the high runoff of the nonphyllosilicate soils accounted for their intermediate soil losses of 0.75 and 0.8 kg m-2. In the 2nd laboratory study kaolinitic and montmorillonitic soils were subjected to 80 mm of simulated rainfall at 9%, 15%, 20%, and 25% slope gradient. The slope factor (Sf) values of the kaolinitic soil at slope gradient > 9% were lower than the corresponding values of the montmorillonitic soils. A positive linear regression significantly fitted the relation between the relative MWD (the ratio of the MWD values at any slope gradient to that at 9% slope) values and the corresponding Sf values at 99% confidence level (r = 0.78). It was concluded that the higher Sf values in the montmorillonitic soils than in the kaolinitic soil was a result of the high runoff rate in the former soils than in the latter soil. In the field study big trash line (BTL), small trash line (STL), and stone line (SL) techniques, and control were evaluated. These indigenous soil and water conservation techniques (ISWC) were asigned to 12 runoff plots (2 by 6 m each) with 10% slope in the semi-arid Tunyai area in Kenya, during five consecutive rainy seasons. The ISWC techniques significantly decreased the runoff and the soil loss, and increased the maize and cowpea yields compared with the control treatment in most of the rainy seasons. The BTL was the most effective technique, but no consistent differences were found between the STL and SL techniques. In BTL, STL, SL and control, the seasonal average runoff for each treatment was 25, 31, 29 and 51 mm, respectively, and the seasonal average soil loss was 0.23, 0.33, 0.3, and 0.67 Mg ha", respectively. The seasonal average biomass was 4.8, 4.0, 4.0 and 2.5 Mg ha", respectively. The seasonal biomass increased linearly and significantly (a = 0.01) with increasing water infiltration. Therefore as more water infiltrated, more water was available for crop production, and this resulted in higher crop yields.