recent field experiments have demonstrated that certain soils in the San Joaquin Valley need to be fertilized with heavy applications of potassium (K) o obtain maximum production. The question of what effect these high K additions have on the nutritional status of 3ther plant-essential elements for cotton has never been answered, but K-induced magnesium (Mg) deficiencies had been demonstrated for a number of crops by previous investigators. A greenhouse experiment to evaluate the effect of K on the Mg status of the cotton plant, under California conditions, was conducted at Riverside. Distinct K- and Mg-deficiency symptoms, along with the associated plant tissue analyses, were developed.