Physicochemical properties as related to mineralogical composition of floodplain soils in humid tropical environment and the pedological significance.

Abstract

Floodplains represent a huge but poorly understood and hence underutilised agricultural resource in the tropics. Insight into the pedogenesis of the soils could guide their exploitation. This study assessed the physicochemical and mineralogical properties of floodplain soils and explored the interrelationships among these properties for informed inferences on contemporary pedogenic processes. Surface (0-20cm) and subsurface (69-200cm) horizons of four pedons of River Benue floodplains (central Nigeria) on shale/alluvium were sampled and analysed. The physicochemical and mineralogical properties were examined for relationships whose pedological significance was discussed. Silt and clay contents were in the ranges of 117-614 and 50-500gkg-1, respectively, being generally higher in the surface and sub-surface horizons, respectively. The soils are young with one surface horizon being silt loam. The surface horizons had higher soil pH (5.9) but lower soil organic carbon (12.63gkg-1), total nitrogen (1.05gkg-1), effective cation exchange capacity (18.58 cmol kg-1), and available phosphorus (5.50mgkg-1) than the sub-surface horizons. The minerals followed the order quartz < illite < kaolinite. Quartz related inversely to the clay minerals (kaolinite and illite), but none of these minerals influenced the physicochemical properties. Instead, soil textural/acidity indices influenced those defining colloidal activity and fertility status, implying greater dependence on their mixed parent material than overall pedogenesis. It is inferred from the mineralogical relations that illitization prevails in these fast-weathering soils. The lesser influence of pedogenesis on their inherent fertility calls for effective management using the multifunctional sawah ecotechnology. The illitization may not cause environmental problems due to clay activity. Alluvial deposit-mediated increases in silt could promote carbon sequestration; however, off-site detrimental effects of nutrients associated with this erosion-susceptible silt would be expected.