VALLEY BOTTOM SOILS OF THE SUBHUMID TROPICAL SOUTHWESTERN NIGERIA ON BASEMENT-COMPLEX - CHARACTERISTICS AND CLASSIFICATION

The valley bottom wetland soils of the narrow inland valleys of the basement complex soils of the sub-humid southwestern Nigeria were investigated for water table movement and the physical and chemical characteristics to discuss the genesis, classification and land use of the soils. The water table exhibits a cyclical movement of surfacing in the rainy season and going into a trough in the dry season. The soils are all poorly or very poorly drained but these wetness classes are required for dry season crop growth and are an asset, rather than a limitation to land use under this climatic condition. The soils are well supplied with nutrients, slightly acid to neutral, have a high base saturation but the cation exchange capacity, highly correlated with clay, is low to moderate. The high clay activity and wide C/N ratios are indicative of both the youthfulness of the soils and inhibited mineralization of the organic fraction caused by hydromorphism or simply low nitrogen content. The profile distribution of the dithionite and oxalate extractable Fe and Al oxides and easily reducible Mn2O3is a visible (morphological) and chemical indication of the process of pedogenesis which is useful for classifying the soils. The soils which are groundwater gley soils with weakly developed pedogenic features, can be classified as Entisols or Fluvisols. © 1990 Taylor & Francis Group, LLC.