A GEOPEDOLOGICAL APPROACH TO SOIL CLASSIFICATION TO CHARACTERIZE SOILS OF UPPER KABETE CAMPUS FIELD, UNIVERSITY OF NAIROBI, KENYA

Abstract

Background. The study area exhibits a first class catena having homogenous parent material and forming a spatial continuum. Functionally and taxonomically distinct soils result from differences in drainage and lateral movement of materials in the soil. Objective. To classify the soils using a geopedological approach which involves a strong relationship between pedology and geomorphology. Methodology. The area was delineated into Soil Mapping Units (SMUs) through augering into soils defined by different macro-relief. Mapping units were demarcated according to slope categories namely 0 to 5%, 5 to 8%, 8 to 16%, 16 to 30% and >30% connoted as flat to gently undulating (AB), undulating (C), rolling (D), moderately steep (E) and steep (F) respectively. Profile pits were dug in the five identified mapping units using Stratified Random Sampling technique. Identified SMUs include UmIr/F, UmIr/E, UxIr/D, UxIr/C and UxIr/AB in the order of decreasing slope gradient. The first entry represents the physiographic unit (Uplands, U), followed by physiographic position (lower middle uplands, m or uplands, undifferentiated levels, x), geology (I), color (r) and slope class respectively. A soil map with a legend describing the mapping units was produced using a scale of 1:10000. Topographic influence on soil properties was presented by Pearson’s correlation coefficient (r) with p-value included where the influence was significant. Statistical analysis was done using IBM SPSS 25th edition and MS Excel. Results. All the mapping units are well drained and deep to very deep (>80 cm). The color of the upper B horizon is predominantly dark reddish brown. The texture of top horizon is clay in UmIr/F and UmIr/E and is clay loam to clay, sandy clay loam to clay and loam to clay loam in UxIr/D, UxIr/C and UxIr/AB respectively, lucidly exposing the influence of topography on the depth of clay illuviation (clay: r = 0.724; p ≤ 0.01). The structure is predominantly subangular blocky throughout the profiles with the top horizon of cultivated areas having predominantly granular structure. Saturated hydraulic conductivity (Ksat) generally decreases with increasing clay content down the profiles and the bulk density ranges from 0.9 to 1.2gcm-3. Means of soil reaction of top horizons generally slightly decrease with decreasing gradient (r = 0.231) having lower values in cultivated areas. Percent organic carbon regularly decreases down the profiles with higher values in uncultivated, steeper areas (r = 0.521; p ≤ 0.05). In the top horizon: Total nitrogen is predominantly medium across the study area ranging from 0.2 to 0.56% (r = 0.185) and follows the organic carbon trend; Available phosphorus is deficient (<20 ppm) in the study area. Bases are sufficiently to richly supplied while micronutrients are richly supplied. All soils are non-saline and non-sodic; the cation exchange capacity (CEC) soil is predominantly medium across the profiles ranging from 15 to 27.6 cmol(+)/kg with values increasing slightly with increasing slope (r = 0.320). Based on data collected from description of the profiles and physicochemical data of the soils and according to IUSS Working Group WRB (2014) soil classification legend, the soils were classified as Mollic Nitisols. Implications. The soils are generally fertile for crop production but organic manure is recommended to buffer the acidic soil reaction, improve nitrogen and phosphorus sources. Conclusion. Soil characterization, land evaluation and precise input application are encouraged.