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

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 (0-20 cm) and subsurface (69-200 cm) 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 50-500 g kg-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 (12.63 g kg-1), total nitrogen (1.05 g kg-1), effective cation exchange capacity (18.58 cmol kg-1), and available phosphorus (5.50 mg kg-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.

Long-Term Effects of Crude Oil Spillage on Selected Physicochemical Properties Including Heavy Metal Contents of Sandy Tropical Soil.

Crude oil spillage effects on the environment often wane with time, making late remediation of affected soils look irrelevant. Physicochemical quality of a sandy soil under 9-year-old spillage was compared with that of adjacent unaffected site in southern Nigeria. Soil bulk density and equilibrated water content were higher in affected than unaffected site, but permeability did not change. The spillage increased soil pH, organic carbon, total nitrogen and available phosphorus by about 7%, 1700%, 133% and - 16%, respectively. It lowered divalent exchangeable bases/acidity but raised base saturation. It increased total petroleum hydrocarbon (PHCt) and micronutrients/heavy metals (Fe, Mn, Zn, Cd and Pb), all of which were below their critical limits in soils by regulatory bodies. Soil pH, organic carbon and PHCt correlated positively with all five micronutrients/heavy metals; total nitrogen did so with Zn and Pb. Nine-year period may be insufficient for spillage effects in sandy soils to cease to be evident. Such effects for PHCt and heavy metals, however, are deemed tolerable for ecological safety.