Effects of land use and management on aggregate stability and hydraulic conductivity of soils within River Njoro Watershed in Kenya

There has been tremendous changes in land use and management in the River Njoro Watershed during the last three decades.Formerly large scale farms have been converted into smallholder farms and plantation forests have gradually been lost.These changes in land use and management have brought in different approaches that have triggered soil erosion and other forms of land degradation.The objective of this study was to trace the changes in land use and determine their effects on aggregate stability and hydraulic conductivity.A semi detailed soil survey of the watershed was undertaken following a three-tier approach comprising image interpretation,field surveys and laboratory analysis.The measured variables in the soil were analysed using ANOVA and correlation analysis.The major land uses were found to be forestland,agricultural land,grassland,and wetland.A strong soil type_landscape relationship was observed within the watershed.Soils of slopes were moderately to severely eroded,shallow and less developed whereas those on summits,pen plains,uplands,plateaus and valleys were deep and well developed.Aggregate stability was the highest in forestland and decreased in the order of grassland,agricultural land and wetland respectively.The mean weight diameter under the various land use conditions was 0.68,0.64,0.58,and 041 respectively.Hydraulic conductivity was the highest in forest-land and decreased in the order of agricultural land,grassland and wetland respectively.There was significant negative correlation between hydraulic conductivity and the bulk density and clay content of the soils.Reduced aggregate stability and lowered hydraulic conductivity is likely to be responsible for some of the severe soil erosion and other forms of land degradation observed in the River Njoro Watershed.

Linking landforms and land use to land degradation in the Middle River Njoro Watershed

Land degradation is the decline in the productive capacity of an ecosystem.This mainly occurs due to processes induced by human activities,such as deforestation,poor farming practices,or enhanced industrial growth leading to various land degradation processes such as,flooding,drought and accelerated erosion.The objective of the study was to link landforms and land uses to land degradation.Soils in the catchments were distinguished on the basis of Physiographic,parent material/geology and soil characteristics.Eight soil mapping units were identified in the area.The validity of the identified soil mapping units were checked in the field using auger hole,mini pits,road and erosion cut observations.Representative profile pits were sighted in the major mapping units.The profile pits were described according to FAO(1977)and Kenya Soil Survey(1987).Soil classification was done according to FAO/UNESCO(1997).Soil mapping units were found to follow soil physiographic units/land forms.Soils on mountains and hills were found to be somewhat excessively drained,shallow to moderately deep.Those from uplands and plateaus were well drained,deep to very deep.Soils on plains fell on two extremes;those that were well drained,deep to very deep and those that were imperfectly drained to poorly drained,moderately deep to very deep.Physical,chemical and biological land degradation was found to take place in the different physiographic units/land forms at varying degrees.Soil erosion,nutrient depletion and vegetation depletion were found to be the most important degradation processes.Soil,physiographic units,soil susceptibility and hazard maps were drawn and their classes in the different landforms established.