Investigating the effect of grain size and extractant type on the different forms of potassium in icaceous minerals and potassium feldspars

Potassium (K) as a macroelement is frequently made available by weathering of aluminosilicates such as K-feldspar and micaceous minerals in the soils. The objectives of this study were to determine the effect of mineral grain size and type of extractant on the different forms of K of micaceous and K-feldspar minerals and to find out a relationship between aqua-regia extractable K and type of K-bearing minerals. Water soluble, exchangeable, non-exchangeable and aqua-regia exteractable K of 50-100 and <50 μm sizes of biotite, phlogopite, muscovite, zanjan and yazd K-feldspars were investigated. The amount of released K was measured by flame photometer. The results indicated that the amounts of soluble and exchangeable K of trioctahedral micas (biotite and phlogopite) and feldspars are not greatly different, but in this regards, diotahedral muscovite is less important even than feldspars. Soil reservoir of non-exchangeable potassium is significantly higher in trioctahedral micas (up to 17 g kg-1) than muscovite and feldspars (up to 3 g kg-1). The small particles (<50 μm) had a significant effect on the different forms of K in all minerals. The amount of aqua-regia extractable K from trioctahedral micas exceed to 81 g kg-1. This value was 4 g kg-1 for other minerals. The mineral size had no effect on the aqua-regia extractable K in trioctahedral micas while in muscovite and feldspars this form of K increase up to 3.5 times with particle size reduction. More investigations are necessary to propose aqua-regia extractable K as an indicator for assessing type and amount of K-bearing minerals and soil potassium supplying power.