We have investigated the relationship between soil mineralogy and K-fixation in a wide array of soils in California's Central Valley.  Initial work on soils used for cotton production investigated the role played by soil mineralogy in affecting extractable K and K-fixation potential (Kfix).  As a first step, we developed a 1-hour lab method to assess Kfix, whose results were similar to those from a 7-day incubation method developed by other researchers.  Soils developed from dominantly granitic Sierra Nevada alluvium fixed significantly more K (740 mg K kg^-1 soil maximum) than soils developed from dominantly sedimentary Coast Range alluvium (260 mg K kg^-1 soil maximum), mostly attributable to vermiculite (from biotite weathering) in the silt and very fine sand fractions.  This work was extended to wine grapes and the development of a landscape-scale K supply model based on soil parent material composition and soil development as an indicator of weathering intensity.  Little K fixation occurred in weakly weathered soils (biotite not yet weathered to vermiculite) and intensively weathered soils (vermiculite weathered to kaolinite or HIV) formed from granitic alluvium, and in all soils formed from andesitic parent materials (no biotite).  Kfix increased to a maximum (580 mg K kg^-1 soil) in intermediately weathered soils, where vermiculite dominated the silt and very fine sand fractions.  We are calibrating the Kfix results with lab experiments testing the effects of wetting and drying and duration of Kfix incubation, and with greenhouse experiments to develop K fertilizer recommendations.