THE SUSCEPTIBILITY TO GULLY EROSION OF BENIN CITY SOIL IN THE NIGER DELTA BASIN OF NIGERIA

Mechanical and chemical analyses of soil samples were applied in the study of soils in the gully erosion sites to determine their susceptibility to gully erosion. Mechanical analysis includes the determination of texture and sand + silt/clay ratio, while the chemical analysis includes the determination of pH, sodium adsorption ratio (SAR) and organic matter (OM) content values of topsoil and subsoil. The topsoil has percentages of sand > 76.85% and clay < 15.64% and is classified as sandy soil, while subsoil was classified as heavy clay soil with percentages of sand < 29.71% and clay > 59%. The pH of soils ranged from 4.34 to 6.42, indicating acidic soils. The sodium adsorption ratio (SAR) of topsoil and subsoil ranged from 2.08 to 2.25 and 2.33 to 2.53, respectively. Therefore, the soils can be described as saline and dispersive, with dispersiveness increasing from topsoil to subsoil. The total organic matter (TOM) and organic carbon (OC) ranged from 0.25 to 0.86 meq/100g and 0.16 to 0.56 meq/100g, respectively. The generally low values of OM content indicate unstable soil aggregates and poor structure that cannot resist erosion. The average monthly rainfall amount/intensity is enough to induce water saturation of soils. Slope level ranged from 4° to 6.4°. Therefore, based on high SAR, low OM, gentle slope and high rainfall amount/intensity, the topsoil and subsoil are susceptible to gully erosion when the former is exposed to the raindrops with vegetation removal and the latter is exposed to runoff with the erosion of the topsoil.