Determination of Available Phosphorus in Acid and Calcareous Soils With the Kelowna Multiple-Element Extractant

The principal objective of this investigation was to evaluate 0.25, 0.5, and 1.0 N HOAc with and without 0.015 N NH4F at 1:10 and 1:5 vol/vol soil-extractant ratios and 5-, 15-, or 30-min extraction times for determining available P in acid and calcareous soils. To that end, P concentrations (ranging from about 5 to 150 μg ml-1, NaHCO3) were extracted from 78 soils (40 had pH values between 4.2 and 6.9; the remainder from 7 to 9.2) and contrasted with reference values by means of graphing, correlation, and regression techniques. Reference P values were obtained with the Bray-1 P solution for soils with pH values < 7.0 (H2O), and NaHCO3 (Olsen et al.) solution for all soils. Results established that NH4F, 5-min extraction times, and 1:10 soil-solution ratios were necessary for optimizing the relationships between P concentrations for acid and, to a lesser extent, calcareous soils. Substituting from 0.025 to 1.0 N HOAc for the 0.025 N HCl in the Bray solution resulted in a P extractant that compared well with the Bray solution on acid soils, and with the NaHCO3 solution on both acid and calcareous soils. As higher acid concentrations were unnecessary, the 0.25 N HOAc + 0.015 N NH4F solution with a 1:10 soil-solution ratio and 5-min extraction time was selected as the Kelowna extractant. It removed similar amounts of P as the Bray −1 P but 2.5 times more than the 0.5 N NaHCO3 solution (r values ≥ 0.98**). Extractant pH was maintained as effectively with 0.25 N HOAc at 1:10 ratio and 5-min extraction time, as with more concentrated acids, even on a soil containing 38% CaCO3 equivalent. The Kelowna extractant has been used since 1984 to simultaneously extract NO3-N and P, K, Ca, Mg, Na, and S from acid and calcareous soils.