Characterization and assessment of erosion susceptibility of the soils of Mount Marsabit forest ecosystem
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The study was conducted in the Mount Marsabit forest ecosystem to determine the soil types, erosion susceptibility and to recommend appropriate soil conservation measures. The observation sites were selected based on aerial photograph interpretation and the observed field conditions. Soil classification was done based on the F AO (2001). The soil erosion susceptibility evaluation was carried out based on Kenya Soil Survey (1987) guidelines. The relative soil erodibility (K) factor was determined by the equation developed by Gachene (1995). The soils studied falls in two major soil classification groups. The soils units identified were Nitisols and Cambisols. All the soils in the site are clay textured. The soil pH increased down all the profiles while the CEC of the soils decreased down in all the profiles. The Nitisols had the highest carbon content of 5 percent while the Cambisols had the lowest carbon content of 1.9 percent in all the topsoil. Dispersion ratio was found to have the highest influence on soil erodibility (r2 = 0.997). The dispersible clay is the most erodible because it is the clay that can easily be brought into suspension by raindrops. The forest Nitisols had the least erodibility index of 4.2. This is because of its high (8.64 percent) organic matter content, which acted as a binding agent and low dispersion ratio of61 percent. The Badassa Cambisols with the highest dispersion ratio of98.6 percent were the most erodible. Soil erodibility index differs between terrain and land units. The erodibility is greatest on slope gradients greater than 10 percent. The mountain Nitisols had the highest erosion susceptibility. Relatively, all the soils recorded low soil erosion susceptibility except for the Nitisols and Rhodic-hyperdystric Cambisols with humic phase. The observed present soil erosion conditions correlated well with the predicted erosion susceptibility for Karare and Songa areas. High rates of soil erosion were found to occur in areas with little or no protective vegetation cover and those on steep slopes. The results justified the hypotheses in this study that; soil types, biophysical parameters and land use determine the extent and severity of soil erosion susceptibility. The recommended soil conservation measures varied with topography. In this study, gabions are recommended to control the gulleys of Sagante; terraces and planting of grass strips along the dry riverbeds of Ajamarko and Karare is proposed. In Songa, contour cultivation is proposed and finally, a general re-forestation is recommended. The Comparative Particle Size Distribution (CPSD) index was determined to estimate the extent of lithological discontinuity within and between the profiles. The relatively high CPSD indices (>60%) within and between the profiles, and the fact that quartz dominated the rock mineralogical composition shows that there is little variation in the parent material of the site.