Reduction and Oxidation of Iron in Phyllosilicates As a Driver for Seasonal Fluctuations in Extractable Potassium in Kansas Soils.

Extractable potassium contents in soils of southeast Kansas have been observed to fluctuate inexplicably over time. These fluctuations have been related to seasonally wet conditions. This observation, in conjunction with evidence that potassium fixation and release can be affected by the oxidation state of structural iron in phyllosilicates, suggests that perhaps the observed soil extractable potassium fluctuations are driven by changes in the oxidation state of iron. To determine whether or not this is the case, the silt and clay fractions of two Kansas soils and four reference clays (kaolinite, illite, smectite, and vermiculite) were subjected to chemical reductions by sodium dithionite and oxidations by exposure to oxygen. The extent of oxidation and reduction was determined by using a colorimetric method using 1, 10-phenanthroline to quantitatively assess Fe(II) and total Fe. Potassium fixation was measured by shaking the treated samples with a solution of KCl, followed by an extraction with NH₄Cl.