MODIFIED KELOWNA TEST FOR AVAILABLE PHOSPHORUS AND POTASSIUM IN SOIL

A recently described soil testing procedure for simultaneous estimation of plant-available phosphorus (P) and potassium (K) in soil uses an aqueous solution containing ammonium fluoride, ammonium acetate, and acetic acid (the ''modified Kelowna'' or KM extract). Before adopting a KM method at our laboratory (in 1988) for routine soil testing, we compared its ability to predict crop response to phosphate fertilizer using data from field sites on a wide range of soils in Alberta and Saskatchewan with that of the Olsen and the Miller-Axley (Bray-type) soil testing methods. All three methods were unable to predict with great accuracy actual optimum rates of phosphate fertilizer in these trials, but they predicted the likely crop yield response to phosphate reasonably well. The main advantages of the KM test over the others are its analytical convenience and its applicability to calcareous soils.