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 gneis s), Cretaceous sediments (mainly shale and sandstone), and Quaternary alluv ium, and which are typical of the major agricultural soils in the Lower Ben ue Valley (Nigeria) were studied with the objective to determine their over all potassium (K) reserves and any relationship between these and other soi l 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 positiv ely with the clay, and negatively with the sand contents of the soils and i s also influenced by their parent materials. The order of abundance accordi ng to parent material is: alluvium- > Basement Complex (gneiss)- approximat e to shale- > sandstone-derived soils. The concentrations of readily availa ble K (RAK) in the soils are quite low, accounting for only between 0.30 an d 7.8% of the total K in the soils and less than 4.0% of their exchange cap acities. Based on critical limits established for many Nigerian soils, the soils derived ti om sandstone are clearly deficient in RAK, while soils dev eloped from gneiss, shale and alluvium parent materials have moderate to su fficient levels for a wide range of crops. Non-exchangeable or moderately a vailable 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 s oils have the most levels. However, the potassium supplying power (KSP) of the soils may be considered to be generally high. Although this bears no pa rticular 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 cont ents of DAK, while the highly weathered sandstone soils have the lowest. Si mple correlation analysis shows that, irrespective of parent material and K form, clay content and CEC are the most important soil properties influenc ing 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 most ly in the lattice structures of K-bearing minerals, with accumulations in t he subsurface horizons. Its plant-available or supplying status is low on s andstone-derived soils and moderate to sufficient on soils derived from Bas ement Complex rocks, shales and alluvium.