Forms and profile distribution of potassium in typical soils of the Lower Benue Valley of Nigeria

Six profiles, derived from Precambrian Basement Complex rocks (mainly gneiss), Cretaceous sediments (mainly shale and sandstone), and Quaternary alluvium, and which are typical of the major agricultural soils in the Lower Benue Valley (Nigeria) were studied with the objective to determine their overall potassium (K) reserves and any relationship between these and other soil properties including their parent materials. Total K in the soils varies from 0.13-27.1 g kg-1 with average 6.64 g kg-1. This correlates positively with the clay, and negatively with the sand contents of the soils and is also influenced by their parent materials. The order of abundance according to parent material is: alluvium- > Basement Complex (gneiss)- approximately equal to shale- > sandstone-derived sons. The concentrations of readily available K(RAK) in the soils are quite low, accounting for only between 0.30 and 7.8% of the total K in the soils and less than 4.0% of their exchange capacities. Based on critical limits established for many Nigerian soils, the soils derived from sandstones are clearly deficient in RAK, while soils developed from gneiss, shale and alluvium parent materials have moderate to sufficient levels for a wide range of crops. Non-exchangeable or moderately available K(MAK) in the soils is also relatively low (0.020-8.59 mmol(c) kg-1); while the sandstone derived soils have the least MAK, the alluvial soils have the most levels. However, the potassium supplying power (KSP) of the soils may be considered to be generally high. Although this bears no particular relationship to soil parent materials, the sandstone-derived soils have the lowest KSP. The bulk of the total K reserves in the soils (55-88%) exists as difficultly available or structural K (DAK). The alluvial soils first, then the gneiss- and shale- derived soils next have the highest contents of DAK, while the highly weathered sandstone soils have the lowest. Simple correlation analysis shows that, irrespective of parent material and K form, clay content and CEC are the most important soil properties influencing the overall K supplying status of these soils. It is concluded that in major agricultural soils of the Lower Benue Valley of Nigeria K exists mostly in the lattice structures of K-bearing minerals, with accumulations in the subsurface horizons. Its plant-available or supplying status is low on sandstone-derived soils and moderate to sufficient on soils derived from Basement Complex rocks, shales and alluvium.