SummaryTotal, dithionite-extractable (‘free’) and oxalate-extractable (amorphous) Fe as Fe2O3 forms were determined in eight Nigerian soil profiles selected to include four major parent materials and different drainage conditions.Total Fe ranged from 1·98 to 15·01 % Fe2O3, the average contents being 4·40, 8·62, 12·59 and 13·49 Fe2O3 for soils derived from the coastal plain sands, basement complex rocks, shale and basalt, respectively. ‘Free’ Fe ranged from 0·08 to 12·28% Fe2O3, the basaltic soils having the highest content, and those on shale the least. Generally the poorly drained soils had low contents, the free Fe oxide constituting about 61 % of total Fe in well-drained soils, and only about 9% in poorly drained soils.The oxalate-extractable or non-crystalline Fe oxide ranged from 0·04 to 1·21% Fe2O3. The proportion of amorphous iron oxide, indicated by the ‘active’ ratio ‘oxalate-Fe/DCB-Fe’, was higher in poorly drained than in well-drained soils.The clay/DBC-Fe ratio remained constant in the well-drained soils but increased with depth in the poorly drained profiles, indicating a co-migration of free Fe oxide and clay in the former but not in the latter soils.In general, the relative distribution of the different Fe forms appeared influenced by both the parent materials and the drainage of the soils.
COMPARISON OF DRAINMOD BASED WATERSHED SCALE MODELS