Characterization and Classification of Soils Derived from Coarse-Grained Granite and Gneiss in Humid Rainforest Area of Southwestern Nigeria

Abstract

This study investigated soils derived from coarse-grained granite and gneiss in southwestern Nigeria, focusing on their characterization and classification. Eight soil profile pits were established across varying topographic positions, and samples were collected from generic horizons. Standard procedures were used to analyze physical and chemical properties. The soils were generally deep and well-drained, except at valley bottoms where they were shallow and poorly drained. Surface horizons displayed colours ranging from dark reddish brown (5YR3/4) to dark yellowish brown (10YR3/4), while subsurface horizons varied from yellowish red (7.5YR5/6) to yellowish brown (10YR5/8). Textures ranged from loamy sand to gravelly sandy clay at the surface, and sandy loam to clay in the subsoil, with moderate, medium, sub-angular blocky structures. Gravel content varied widely (8.5–67.1%), bulk density ranged from 1.17 to 1.81 g/cm³, and porosity values spanned 38.07–54.63% at the surface and 31.88–55.63% in the subsurface. The soils were slightly to moderately acidic, with variable levels of exchangeable bases, organic matter, nitrogen, and phosphorus. Organic matter was particularly important for nutrient availability. Classification revealed upper and mid-slope soils as Typic isohyperthermic Paleustults, lower slope soils as Plinthic isohyperthermic Paleustults, and valley bottom soils as Aquic Psamments under USDA taxonomy. FAO/UNESCO categories included Lixisols, Plinthic Lixisols, and Fluvisols. Overall, the soils exhibited low to medium fertility, highlighting the need for sustainable management. Recommended strategies include applying fertilizers, incorporating crop residues, and using organic manures to enhance productivity and ensure long-term soil resource sustainability.