Soil Potassium Relationships as Indicated by Solution Equilibrations and Plant Uptake

Four Ohio soils of wide range in K release capability were initially K‐treated (0, 100, 200, 800 ppm) and successively cropped three times with corn in a growth chamber. Aliquots of the soils with and without K added and before and after cropping were assayed by extraction, equilibration, and plant uptake techniques for the effects of cropping and K‐treatment. Uptake of K by the corn was closely related to the K release capabilities of the soils. Exchangeable K increased as rate of K increased. Initial cropping decreased the exchangeable K in all soils eliminating the effect of K treatments on exchangeable K. However, after initial cropping where no K was added, exchangeable K varied sixfold from the lowest to the highest. Nonexchangeable K removed with H 2 SO 4 initially reflected K additions, but after the first crop, the values decreased progressively as K release capabilities of the soils increased. Parameters from Q/I isotherms of the soils gave correlations with percentage and total K contents of assay crops less satisfactory than for exchangeable and H 2 SO 4 extractable K. There was a tendency for prior cropping to increase the fixation and reduce the plant recovery of subsequently applied K in the two soils high in clay and total K.