The forms and content of sesquioxides (Fe and Mn) were studied in soils developed on basement complex rocks within Northern Guinea Savanna of Nigeria. The study showed that variation in parent materials and pedogenetic development significantly influenced content of dithionite, oxalate extractable forms of iron and active iron ratio in these soils. The mean values of active sesquioxides (Fe and Mn) ratios between the soils varied in the order of soils on mica schists > quartzites > older granites > migmatite gneiss suggesting that the soils pedogenic development was lowest in soils on mica schist and highest in soils on migmatite gneiss. Increase in pedogenetic development increased sesquioxides crystallization and was indicated by the highly significant correlation between active Fe ratio and iron crystallinity index (r = - 0.507**). Pedogenetic development and sesquioxides crystallization were significantly higher in the subsoil compared to surface horizons. Clay and organic matter had strong influence on movement and distribution of the forms of sesquioxides in the soils.
Key words: Sesquioxides, basement complex, iron oxide, manganese oxide, crystallinity index.
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