Influence of elevation, soil depth and climatic variables on soil physicochemical properties: a case study of farmlands on the eastern slopes of Mount Kenya

This study investigated the variation of soil physicochemical properties across elevation gradients and soil depths in farmlands on the eastern slopes of Mount Kenya. Using a stratified systematic sampling approach, 30 composite soil samples were collected along an 18 km transect from three elevation zones: lower farmland (1000–1200 m), mid farmland (1200–1450 m), and upper farmland (1450–1700 m). Samples were taken from two depths: 0–20 cm (topsoil) and 20–40 cm (subsoil). The samples were analyzed for bulk density (BD), pH, texture, soil organic carbon (SOC), and total nitrogen (TN) using standard laboratory procedures. Results revealed significant variation in the selected soil properties across both elevation ranges and soil depths. The BD values increased with soil depth and decreased with elevation, ranging from 0.73 ± 0.10 g cm⁻³ in the upper farmland’s topsoil to 1.03 ± 0.09 g cm⁻³ in the lower farmland’s subsoil. Soil pH was acidic across all elevations, with slightly higher values being recorded at the 1000–1200 m elevation (5.80 ± 0.10) compared to the 1450–1700 m elevation range (5.24 ± 0.47). SOC and TN content increased significantly with elevation, with the highest concentrations occurring in the upper farmland’s topsoil (3.10 ± 0.48% SOC and 0.27 ± 0.02% TN) and the lowest in the lower farmland’s subsoil (1.30 ± 0.19% SOC and 0.13 ± 0.01% TN). Soil texture varied along the elevation gradient, with sand content decreasing as elevation increased, while silt and clay contents were higher at upper elevations compared to lower ones. The carbon-to-nitrogen (C: N) ratio ranged from 9.61 to 11.60 and showed no significant variation across elevations or depths. SOC and TN were positively correlated, while both were negatively correlated with BD. Precipitation significantly influenced both TN and SOC (p = 0.01), while temperature showed a marginally significant positive effect on both TN and SOC (p = 0.06). These findings highlight the strong influence of elevation-driven climatic factors, along with soil depth, on soil fertility indicators, underscoring the need for elevation-specific soil management strategies in mountainous agricultural landscapes.