Comparison of soil test methods for predicting cotton response to soil and fertilizer potassium on potassium fixing soils

Cotton (Gossvpium hirsutum L.) yields are limited by K deficiency in extensive areas of California where soils contain minerals that fix K in nonreplaceable forms. A survey study was conducted to evaluate the relationship between seed cotton yield and four soil test methods for K by regression analysis. A total of 38 on‐farm sites over two years were included in the study. Intensity parameters obtained from two soil tests that measure solution‐phase K+ concentration in soil/solution suspensions were the best predictors of cotton yield across soils, sites, and years. Quantity parameters based on 1 M NH4‐extractable K or K extracted by 5 N H2SO4 were significantly less precise in predicting yield. Three K fixation isotherm methods were also compared for their ability to predict the proportion of added K that is fixed. Only one method that included a soil wetting and drying cycle provided estimates of K fixation and solution‐phase K+ concentration that were in reasonable agreement with actual values measured in a 3‐yr field study. These results indicate that K fertilization recommendations would be improved using a solution‐phase K+ soil test to identify soils where a response to added K is likely, and an appropriate fixation isotherm method to estimate the fertilizer K requirement.