Potassium Fixation by Oxidized and Reduced Forms of Phyllosilicates

Potassium fixation traps K+ ions in the interlayer region of phyllosilicates. This study determined if increased negative interlayer charge caused by structural Fe reduction leads to increased K+ fixation. The five reference clays used were illite (IMt-1), kaolinite (KGa-1b), montmorillonite (STx-1b), nontronite (NAu-2), and vermiculite (VTx-1). Soil clays were fractionated from the upper 15 cm of a Belvue loam and a Cherokee silt. Potassium fixation capacities were measured on clay samples of unreduced and reduced forms of each clay. Iron (II) and total Fe contents were determined, and K+ fixation was measured by K saturating the clays, followed by five washes of MgCl2 solution. Iron reduction significantly increased the amounts of K+ fixed by NAu-2 and VTx-1. An increase in Fe(II) content caused increases in layer charge and K+ fixation. Although NAu-2 exhibited a greater increase in Fe(II) content on reduction than VTx-1, the increase in K+ fixation on reduction was greater for VTx-1 because of the tetrahedral location of Fe in VTx-1. For IMt-1, KGa-1b, and STx-1b, Fe reduction did not significantly affect the K+ fixation capacities because of their low Fe contents. The Belvue loam released K+ in both unreduced and reduced forms. The Cherokee silt did not appreciably release or fix K+ in either form. Although much K is removed in the first wash, small amounts of K were removed in subsequent washes, especially for reduced samples of NAu-2 and VTx-1. The washing procedure caused reduced Fe to reoxidize, which resulted in K that was previously fixed to be released.