Soil Mineralogy and Potassium Supply in Lodi Winegrape District Soils.

Soils in the Lodi Winegrape District of California’s Central Valley have a complex array of properties due to source rock mineralogy, depositional and erosional history, and landscape age.  Potassium (K) supply and management needs in vineyards vary widely across the district as a result of the complexity of the soilscape.  Soil mineralogy, especially vermiculite content, dominates the supply, exchange, and fixation of K.  Our soil/landscape model of the district includes five regions that are characterized by variations in soil texture, parent material mineral composition, and landscape age as predictors of K supply. Fine-textured soils of Region 1 are smectite-dominated in upper horizons and fix little K; subsurface horizons appear to be parts of older, truncated and buried soils that contain vermiculite and fix K.  Region 2 soils, weakly weathered, coarse-textured soils in our original concept, generally are intermediate K-fixers if formed from granitic alluvium but low K-fixers if formed from metavolcanic and metasedimentary sources, reflecting the presence or absence of abundant vermiculite in clay and silt fractions. Region 3 soils are more strongly weathered, mostly fine-loamy and fine soils that are the strongest K-fixers and contain abundant vermiculite.  Region 4 soils are very diverse and encompass the range of soil texture, mineralogy, and landscape age found in the District.  Generally, strongly weathered soils on remnant depositional landscapes are kaolinite-dominated and do not fix K, whereas less weathered soils on erosional/depositional hillslope components have widely varying K-fixation properties that are difficult to predict.  Region 5 soils are formed from andesitic lahar material, and have substantial K supplies from feldspars, but lack vermiculite and don’t fix K.