Potassium Fixation by Oxidized and Reduced Forms of Phyllosilicates

Abstract

Core Ideas Understanding how soils fix and release K is crucial for soil fertility. K fixation and release are controlled by layer silicate collapse and expansion. K fixation is affected by layer charge, charge distribution, and Fe redox. The greater the negative layer charge, the greater the amounts of K fixation. Tetrahedrally derived charge favors more K fixation than octahedrally derived charge. 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.