Chemical Compositions of Soils in Parts of Edo State, Southwest Nigeria and their Relationship to Soil Productivity

Thirty eight (38) soil samples taken from locations accessible to the Benin - Oluku - Ifon - Uzebba - Afuze - Auchi - Ukpilla - Okene highway, covering a distance of approximately 185 kilometres and an area of about 12,000 square kilometers were subjected to chemical and mineralogical analyses with a view to determining the variability of their fertility status within the rainforest and savannah vegetational zones. Results show that the soils of the area consist predominantly of quartz, kaolinite, feldspar and sesquioxides of aluminium and iron, including goethite. They are generally acidic with very low cation retention and buffering capacities. Higher concentrations of the major oxides MgO, CaO and K₂O were recorded in soils of the savannah zone while the soils of the rainforest zone are relatively deficient in these oxides. A chemical productivity index (CPI) of MgO + CaO + K₂O/Al₂O₃ is proposed for the soils of the area of study.