Abstract
The content of various forms of Fe and Al in six well-drained soil profiles sampled from different parts of Nigeria was determined by selective extraction methods. Dithionite-Fe (total free Fe oxides) content increases with the increase of depth. The oxalate-extractable Fe (amorphous Fe oxides) constitutes less than 10% of the total free Fe oxides throughout the profiles. The active Fe ratio decreases with the increase of profile depth, suggesting that larger proportions of Fe oxides are present as crystalline forms in the lower horizons of these well-drained profiles. Little or no relationships were found in the case of Al. The constant clay/dithionite-Fe ratio within the four profiles from the wetter southern part of Nigeria indicates the co-migration of clay and Fe oxides from the A horizon into the B horizon (lessivage). However, this relationship was not observed in the two soil profiles sampled from the drier northern part of the country. The need for expansion or alteration of the present U.S.D.A. system of soil classification is emphasized.
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