Measurement of Potassium Fixation Potential On Air-Dried Vs. Field-Moist Soil Materials.

Potassium fixation has been identified as a possible source of concern for managing fertility in granitic soils in the San Joaquin Valley of California. Previous work in our lab has demonstrated that vermiculite in the silt and fine sand fraction is predominantly responsible for observed K fixation in these soils, and that air-drying of soil materials after the application of K in solution results in an increase in K fixation potential relative to samples maintained moist. In order to better understand the effects of drying on K fixation potential, we collected soil material from 14 pedons at two depths each, representing a range of K-fixing and non-K-fixing soils. Collected samples were immediately sealed and refrigerated, providing materials that never experienced drying beyond field conditions. Subsamples were removed and air-dried.  K fixation potential (Kfix) and ammonium acetate-extractable K (NH₄OAc-K) were measured for both field-moist and air-dried samples, demonstrating the effect of a single drying event on soil K measurements.  Most soil materials showed an increase in Kfix with drying, though to varying extents. Changes in NH₄OAc-K values were generally small and did not consistently increase or decrease with drying.