ABSTRACT

We developed a map showing the location of potentially K-fixing soils in cotton-producing areas of Fresno, Tulare, Kings, and Kern Counties in the southern San Joaquin Valley, California. The map was produced from seven digitized soil surveys (NRCS SSURGO database) and a California Department of Food & Agriculture cotton database. The criteria for soil map unit selection included (1) age of soil as indicated by degree of horizonation, with younger soils presumed to have more potential for K fixation (2) formation of the soil on Sierra Nevadan parent material, which is more likely to contain vermiculite (the K-fixing mineral) formed from biotite mica, and (3) coarse to medium texture, as indicated by soil taxonomy family level texture. Laboratory measurement of K fixation by a modified Cassman incubation method and conventional x-ray diffraction supports this model, but further studies, including measurement of K fixation in a larger number of fine-textured soils with Sierra Nevada genesis and measurements across a chronosequence must be completed in order to confirm it.