Contents and profile distribution of three forms of free iron oxides in three ultisols and an alfisol in Nigeria

The contents and pedogenetic distribution of pyrophosphate, oxalate, and di thionite extractable iron oxides were examined in three Ultisols and an Alf isol developed on older granites. The soils represented a bio-climatic sequ ence in Nigeria, the Sudan, Northern Guinea, and Southern Guinea savannas a nd the Rainforest zones. Pyrophosphate extractable iron (Fe-p) in the soils ranged from 0.14 to 0.69%, oxalate extractable iron (Fe-ox) ranged from 0. 03 to 0.36%, whereas the dithionite extractable form (total free iron oxide s) varied from 0.50 to 8.00%. The contents of pyrophosphate and oxalate ext ractable Fe, i.e., the amorphous forms were low and represented only 0.9 to 28% and 5 to 6%, respectively, of total free iron oxides. The soil in the more humid rainforest contained the highest amounts of all forms of free Fe oxides. In the savanna soils the contents of free iron oxides decreased in the order Northern Guinea > Sudan > Southern Guinea. Pyrophosphate and oxa late extractable Fe increased slightly with depth but dithionite extractabl e Fe gave large bulges in the argillic horizons.