Underestimation of Available Phosphorus by Resin-Bicarbonate and Olsen Tests in Calcareous Soils treated with Gypsum

In the present study, Olsen [0.5M sodium bicarbonate (NaHCO(3)), pH 8.5] and resin-bicarbonate (HCO(3)) tests underestimated available phosphorus (P) in calcareous soils treated with gypsum (CaSO(4)). The reaction of CaSO(4) and HCO(3) - ion or resin-HCO(3) to form calcium carbonate (CaCO(3)) precipitate reduced the strength of the Olsen NaHCO(3) extractant and resin-HCO(3) strip for P extraction. The iron (Fe) oxide-impregnated filter paper (Pi strip) was independent of CaSO(4) influence and thus correctly estimated soil-available P with respect to plant response to soil-available P. Two greenhouse experiments were conducted with maize and wheat grown on calcareous soils treated with different rates of CaSO(4). The results confirmed that Olsen and resin-HCO(3) tests should not be used to measure available P or labile P in the P fractionation scheme in the calcareous soils containing significant amounts of gypsum.