Mineralogy and potassium release from some Western Australian soils and their size fractions

Abstract

Seven surface horizon soils and their sand, silt, and clay fractions were characterised for mineralogy, and K release by extracting samples with 1 mM CaCl ₂ solution daily for 10 days. The low silt content is characteristic of many Western Australian soils, which may provide a partial explanation of the paucity of available K in soils that contain little silt-size primary minerals. The sand and silt fractions were dominated by quartz and contained significant amounts of K-feldspars. The clay fraction was dominated by kaolinite, and some samples contained illite/mica, inhibited vermiculite, and gibbsite. On a per unit weight basis the clay-fraction released the largest amount of K followed in sequence by the silt and sand. The contribution of size fractions to total K release by the soil ranged from 50 to 87%, 2 to 7%, and 10 to 44% for the clay, silt, and sand, respectively. Linear plots of K release versus time ^1/2 for the soils, and the sand and silt fractions, indicated that a parabolic diffusion equation adequately describes the K release process. For some clay samples this diffusion controlled kinetic is not strictly obeyed during the initial period of K desorption due to rapid exchange of adsorbed K at sites on external surfaces. The Elovich equation plots show a discontinuity in slope and support the hypothesis of the multireactive nature of K exchange sites for these soils. The parabolic diffusion rate constant closely predicted K supply to plants as it has a close positive relationship (r = 0.99) with total K uptake by ryegrass for 260 days of growth.