Detailed Soil Survey and Spatial Variability of Selected Soil Properties in Upper Kabete Campus Field, University of Nairobi, Kenya

Abstract

The study area exhibits a first-class catena having homogenous parent material and forming a spatial continuum. This study aimed to classify the soils using a geopedological approach which involves a strong relationship between pedology and geomorphology. The area was delineated into Soil Mapping Units (SMUs) through augering into soils defined by different macro-relief. Mapping units were based on 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. This technique was used because delineation was based on slope categories that acted as stratum within which profile pits were dug. 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), colour (r) and slope class, respectively. 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 SPSS software for correlation and descriptive statistics. All the map units were well drained and deep to very deep (>80 cm). The colour of the upper B horizon was predominantly dark reddish brown. The texture of top horizon was clay in UmIr/F and UmIr/E and 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). Clay in the top horizons ranged from 24 to 66%. The structure was predominantly subangular blocky throughout the profiles with the top horizon of cultivated areas having predominantly granular structure. Saturated hydraulic conductivity (Ksat) generally decreased with increasing clay content down the profiles and the bulk density ranged from 0.9 to 1.2 gcm-3. Means of soil reaction of top horizons generally slightly decreased with decreasing gradient (r = 0.231) having lower values in cultivated areas. Percent organic carbon regularly decreased down the profiles with higher values in uncultivated, steeper areas (r = 0.521; p ≤ 0.05). It ranged from 1.66 to 4.03% in the top horizons. In the top horizon: Total nitrogen was predominantly medium across the study area ranging from 0.2 to 0.56% (r = 0.185) and followed the organic carbon trend; Available phosphorus was deficient (<20 ppm) in the study area. Bases were sufficiently to richly supplied while micronutrients were richly supplied. The Cation Exchange Capacity (CEC) was 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. The findings of this study show that the geopedological approach to soil characterization is valuable in soil management. Spatial variability of soil properties was investigated in a selected farm (Field 3). Selected soil properties varied spatially in the field which indicates the need to blend fertilizers with targeted nutrients. Variable input application is also recommended. Soils of the study area are generally fertile for crop production but application of organic manure is recommended to buffer the acidic soil reaction and to improve nitrogen and phosphorus sources. Organic sources will help in efficient use of these nutrients and also improve soil resilience.

Keywords: First class catena, Soil Mapping Units, Stratified random sampling, Mollic Nitisols, Spatial variability.