Abstract
The study characterized the benchmark soils within the rainforest region of southwestern Nigeria and established their taxonomic (USDA & FAO/UNESCO) and fertility capability classes. Eight profile pits were established and described. Rock and soil samples were collected for laboratory analyses. Soil samples were analyzed for some physical, chemical and mineralogical properties. Fine sand fraction was separated into heavy and light minerals with bromoform. Thin section of the bedrock sample and slides of fine sand fraction were prepared and examined under a petrographic microscope. Data were subjected to descriptive and inferential statistics. The soils were fairly deep, intensely weathered and well-drained with lithologic discontinuity at the lower slope. Bulk density values increased with profile depth varying from 1.17 - 1.67 g/cm-3. Acidity was low, ranging between 5.1 - 6.5, organic matter content ranged from low to medium, with higher values at the surface horizons. Total nitrogen content ranged from low to medium, while available phosphorus ranged from medium to high on the surface soils and from low to medium in the subsurface horizons. The three methods used to determine cation exchange capacity (CEC) showed significant positive correlation (P <= 0.01) with organic matter. However, values of CEC by BaCl2-TEA (pH 8.2) method were higher and significantly different from those by NH4OAc (pH 7) and effective cation exchange capacity (ECEC) methods. Quartz dominated the primary minerals in the bedrock. Rutile and opaque minerals dominated the heavy mineral portion of the fine sand fraction. The upper and mid slope soils were classified as Typic Isohyperthermic Paleustults and Plinthic Isohyperthermic Paleustults (USDA) while they are Chromic Lixisol and Plinthic Lixisol, respectively, in the FAO/UNESCO system. Fertility capability units were SCehik and SCrehik, respectively. Agronomic constraints of the soils were high leaching potential (e), high soil acidity (h), high P fixation potential (i), low content of weatherable minerals (k) and high gravel content (r).
