The genesis, classification and erosion susceptibility of the soils of the semi - arid Sirima and Mukogodo catchments, Laikipia District, Kenya.

Abstract

The two study catchments are Mukogodo and Sirima in Laikipia District. The objectives of this study were: to study the influence of soil forming factors on the development of the soils of the study areas; to study the genetic relationship of the soils in time and space; to characterise the morphological, physical, chemical and mineralogical properties of the soils inorder to differentiate them; to classify the soils using the latest methods of classification; to assess the erosion susceptibility of the soils with an aim to identifying areas that need urgent measures to curb the erosion. Sirima and Mukogodo catchments cover 365 and 252 hectares respectively. The catchments show diversified morphological, physical and chemical soil characteristics. The soils of the Sirima catchment range from excessively drained to imperfectly drained, shallow to deep. red (2.5 YR) to black (10 YR), friable to firm clay soils with vertic characteristics. These soils have high exchangeable bases, high cation exchange capacities, humic topsoils with organic carbon content ranging from 1.2 to 4.6%. The soils are deficient in phosphorus. The major soils are Luvisols, Cambisols and Vertisols. The soils have a very low to very high erosion susceptibility as evaluated by the FAO qualitative method. These soils are developed on mixed igneous rocks of Miocene to Pleistocene age and occur in agroclimatic zone IV. The soils of the Mukogodo catchment are excessively drained to moderately well drained, very shallow to extremely deep. The surface of these soils is generally gravelly to rocky. The soils have red colours (5 YR to 2.5 YR) and are generally friable with sandy textures. These soils have low cation exchange capacities, low exchangeable bases and low organic carbon on topsoils ranging from 0.28 to 1.9%. These soils classify as Cambisols, Lixisols and Fluvisols. The soils have a moderate to very high erosion susceptibility as evaluated by the FAO qualitative method. These soils are developed on metamorphic rocks of Pre - Cambrian age and are dominantly gneisses of various compositions. The catchment is in agroclimatic zone V. The differences ln morphology chemical and physical characteristics of the soils of both catchments are related to the Parent materials (or geology), Climate (past or present), Topography, influence of organisms (human and vegetation) and Time (age of the landscape).