Influence of Topography and Land Use on Physical Soil Properties at the Wundanyi Catchment Area in Taita Hills, Kenya

Tropical Africa has been under intense land cover change during the last decades. Human activities have had an impact on natural ecosystems and this has accelerated changes in them. Soil can be considered as an ecosystem and changes in natural soil formation can lead to soil loss and erosion. Taita Hills located in south-east Kenya are no exception. Almost all recoverable land is used for agriculture in these subtropical hills despite the great altitude differences and steepness of the hill slopes. Wundanyi catchment at the altitude 1400 - 2100m above sea level was studied in order to identify certain physical parameters of the soil in the hills and to define if these could be explained by changes in the physical environment like alteration in altitude, slope angle and land uses. A total of 68 study points were selected within the catchment in order to represent different physical environments. At each study point, the slope angle was measured, an approximately one meter deep soil cut was dug into the hill slope of which the soil profile was drawn and the subsamples were taken for the grain size analysis and bulk density determination. At 21 study points, the soil was not sampled due to a thick humus layer. The soil hydraulic conductivity was determined experimentally at three study points with an infiltration ring and theoretically for the rest of the study points by estimation, using other physical soil properties obtained from grain size analysis. The soil at the Wundanyi catchment was found to be massive, chemically weathered fine residual soil originating from the weathered gneissic bedrock. It was discovered that generally the soil is quite homogeneous within the catchment area and the physical parameters of the soil are similar despite the different land cover classes. The most common soil type at the catchment is fine sand. The deepness of the organic layer varied greatly, the mean being 0.7 meters. The thickest organic layers were found in indigenous forests and in places that had not been used for agricultural purposes. Soil hydraulic conductivity at the studied area is low. The degree of correlation between soil grain size distribution and varying physical environments was found low. However, there seems to be moderate correlation between the elevation and the proportion of clay and the aspect and the proportion of clay. Topographic environment mainly defines the land use and land cover within the studied area. Only the steepest slopes and areas that are not accessible have avoided the excessive exploitation of the soil. The vegetation protects the mineral soil and, therefore, the removal of natural vegetation exposes the soil to impacts of climatic conditions. The exposition of the soil together with fine soil texture can reduce the infiltration making the soil compacted. This increases the surface runoff, which can increase the sediment transportation and can lead to environmental problems such as erosion, gullying, silting of rivers and transportation of nutrients to rivers. Signs of these environmental problems can already be seen at the Taita Hills, proving that soil exploitation is not sustainable.