Vegetation Effects on Pedogenetic Forms of Iron and Aluminium and Mineralogical Properties of Basaltic Soils in the Southern Guinea Savanna of Nigeria

Abstract

This study investigates the effects of vegetation particularly fast growing exotic species like Tectona grandis on the Newer Basalt of the Jos Plateau. In particular it determines the effects exotic tree species have on the pedogenetic forms of Fe and Al and mineralogical properties of the basaltic soils age. Soil samples were determined at the depth of 0 – 30cm and 30 – 60cm under plantation of four different ages and subjected to standard laboratory analysis. The citrate bicarbonate dithionite (CBD) extractable Fe and Al slightly decreased with aging plantation, which was ascribed to the inhibitory effects of organic matter on the crystallization of Fe and Al. Similarly the active Fe value was less than unity, which implied increased crystalline forms of Fe and Al oxides are as a result of weathering intensity. Mineralogical results indicated that the soils under the fallow and 11 year old plantation had the dominance of magnetites, while those under 21 year old plantation had mixed mineralogy, consisting of magnetites, biotites and montmorillonites. The older plantations (31 and 36 year old) were dominated by kaolinites and traces of haematites and goethites. The trend suggested an increased level of pedogenesis with aging plantations. Increase in crystalline forms of Fe and Al, kaolinites, traces of haematite and goethite concludes that Tectona grandis deteriorates soil properties over time.