Pedogenesis and Classification of Soils Under Teak (Tectona grandis Linn. f ) Plantation of Various Ages in the Southern Guinea Savanna of Nigeria

Plantation trees play a major role in soil formation and development, thus this stimulated the need to determine the effects of Tectona grandis in pedogenic processes and to classify the basaltic soils of the Jos Plateau underneath trees of various ages. The study showed that the main pedogenic processes were braunification, mineralization and lessivation. Floral pedoturbation encouraged illuviation, thus resulting into the formation of clay films on ped surfaces and Bt horizons. The soils were reddish brown to dark reddish brown with hues between 2.5 to 5YR and chroma less than 4. The texture varied from gravelly clay loam to gravelly clay, the structure ranged from weak fine to moderate subangular blocky and non sticky to sticky consistence with aging plantations. The soil pH slightly varied between 5.6 and 6.3 with plantation age and soil depth. Organic C content did not reveal a definite trend with plantation age, though values were lower in subsurface horizons and decreased with depth. Available P and Total Exchangeable Bases (TEB) linearly decreased with aging plantations. The lowest TEB value (8.52 cmol(+)/kg) was obtained under the oldest tree stands. The exchange acidity (Al and H) value was highest (2.40 cmol(+)/kg) in the underlying horizons under the oldest plantation, thus indicative of the deteriorative effects of trees on the soil properties. The soils were classified using the USDA soil taxonomy as Humic Dystrustept (11year old), Oxic Dystrustept (21year old), Kanhaplic Haplustult (31 year old) and Typic Haplustult (36 year old). Using the FAO/UNESCO the soils were classified as Humic Cambisols (11 and 21 years old), Humic Acrisols (31year old) and Chromic Luvisols (36 year old). Applying the WRB, the soils were classified as Chromic Umbrisols (11 and 21 years old), Umbrihumic Acrisols (31 year old) and Chromic Luvisols (36 year old).