Soil contamination potential by trace metal elements in large-scale farms under different cropping systems in Western Kenya

Soil is one of the most important natural resources that play a central role in ensuring agricultural productivity. With the current increased application of agrochemicals, agricultural soils are under contamination threat in Kenya and other parts of the world. This study investigated the potential of trace metal elements contamination in agricultural soils under different cropping systems in large-scale farms in Trans Nzoia County of Western Kenya. From the selected farms, 204 soil samples collected were analyzed for pH, Electrical Conductivity (EC), exchangeable base cations (Na+, Mg2+, K+, Ca2+), Soil Organic Carbon (SOC), Total N, Cu, Pb, Zn, Cd, Fe and Cr. Base cations and trace metal elements were determined through Atomic Absorption Spectrophotometry while total N through calorimetry. Soil Organic Carbon was analyzed using Walkley-Black method. Statistical data analysis was done using Stata, version 15.0 while Trace Metal Element Pollution Potential (TMEPP) was computed by Metal Pollution Index (MPI). The study showed that the concentrations of Zn, Cu, Pb and Fe were lower than the globally acceptable levels in agricultural soils (MPI < 1) while Cd levels exceeded permitted levels (MPI > 1). Comparatively, maize and sugarcane cropping systems were the most susceptible to trace element pollution. In conclusion, the rank order of the pollution potential associated with the analyzed trace metal elements in the study area was Cd > Zn > Pb > Cu > Cr > Fe. The study recommends that there is need to regulate the application of Cadmium-containing agrochemicals and the types of crops cultivated in cadmium-rich soils to avoid possible transfer of this toxic metal to other trophic levels.