Evaluation of a fast method for determination of P and K fixation

McLean's “double equilibration soil test”; was evaluated as a method for determination of P and K fixation in selected Arkansas soils. Phosphorus and K fixation were determined simultaneously using Mehlich III (M3) or separately using Bray‐Kurtz 1 (Bl) for P and neutral normal Ammonium Acetate (AA) for K. Phosphorus and K added at McLean's prescribed rates (60 mg P/kg and 200 mg K/kg), double rates, and half prescribed rates were incubated for 16 and 1440 h. Addition of P and K together did not effect the fixation of either P or K. The amount of P or K fixed was proportional to the amount added for all extracting techniques and incubation times. Incubation time increased P and K fixation in most cases. Although, 16 h was sufficient time for P fixation when existing soil test levels were low enough to require fertilizer recommendations. Potassium fixation was observed within 16 h, when the extractant was Ammonium Acetate. McLean's method may become a valuable soil test procedure for identifying P and K fixation tendencies.