Soil Use and Management Effects on Aggregate Stability and Hydraulic Conductivity Within River Njoro Watershed in Kenya.
Isaiah I.C. Wakindiki, Zachary G. Mainuri, and Maina Gichaba. Egerton Univ, Njoro, Kenya
There has been tremendous changes in land use and management in the River Njoro Watershed during the last three decades. Formally large scale farms have been converted into smallholder farms and plantation forests have gradually been lost. These changes in land use have brought in different soil use and management approaches that have triggered soil erosion and other forms of land degradation. Up to 8.6 kg of soil loss per ha from the cultivated soils has been reported in one storm. This massive soil loss was probably due to reduced aggregate stability and hydraulic conductivity. The objective of this study was to trace the changes in land use and determine their effects on the aggregate stability and hydraulic conductivity. A semi detained soil survey of the watershed was undertaken following a three-tier approach comprising image interpretation, field surveys and laboratory analyses. The measured variables in the soil were analysed using a two-way ANOVA and correlation analysis. The major land uses were found to be forest, agriculture, grassland and wetland. A strong soil-landscape relationship was observed within the watershed. Soils of slopes were moderately to severely eroded, shallow and less developed whereas those on summits, pena plains, uplands, plateaus and valleys were deep and well developed. Aggregate stability was in the order of forests > grassland > agriculture > wetland. The mean weight diameter in the various land uses was 0.68, 0.64, 0.58 and 0.41 respectively. Hydraulic conductivity was in the order of forests > agriculture > grassland > wetland. Hydraulic conductivity significantly correlated negatively with bulk density and clay content. We concluded that land use changes that reduced the amount of organic matter significantly reduced aggregate stability. Also soil use and management activities that increased the soil's bulk density significantly lowered hydraulic conductivity and therefore may contribute to soil erosion and other forms of land degradation.