COMPARAISON DE DIFFERENTES METHODES D’EXTRACTION DU P ASSIMILABLE EN RELATION AVEC LES PROPRIETES CHIMIQUES ET PHYSIQUES DES SOLS DU QUEBEC

Abstract

The objective of this study was to compare different available-P extracting methods on 83 Québec soils. The alkaline Olsen’s method and five acid chemical extractants (Bray-1, Bray-2, new Mehlich, North Carolina double acid DA-4 and DA-10) were compared with each other and with two anion-exchange resin methods. The ratio of P-HCl/P-DAF of Mehlich was used to identify the group of soils with predominant P-Ca forms [Formula: see text]. Within this soil group, the Bray-2, DA-4 and DA-10 methods extracted more P and they were less correlated with all the other methods (r = 0.01 to 0.48*). The Bray-1 and Mehlich methods showed good relationships with Olsen, F−- and HCO3−-form resin methods (r = 0.80** to 0.89**) for this soil group, with the exception that Bray-1 solution was more affected by the presence of free carbonates. In the soil group containing more than 0.6% oxalate-Al, the Bray-1, Bray-2 and DA-10 methods extracted more P than the Mehlich, DA-4 or Olsen methods. The F−-form exchange resin extracted more P than that in the HCO3− form. The depressing effect of free iron oxides on the DA-4 method was also observed with the soil group containing more than 0.6% oxalate-Fe. The contents of DA-4-P were less correlated with those of other methods (r = 0.74** to 0.88**) in this soil group. On the other hand, the Olsen, F− and HCO3− resins extracted more of the P-Fe forms from these soils. The DA-4 method was further influenced by the soil organic matter (OM) contents. The relationships between P-Olsen, P-HCO3−-resin with the other chemical methods were also improved by taking into account the contribution of OM contents. Soil texture influenced mostly Bray-2, Bray-1, DA-4 and DA-10 extractable P. The Mehlich, Olsen and F−-resin extractable P was less affected by clay contents while the HCO3−-resin was the most consistant over a wide range of soil textures. Key words: Available-P, anion-exchange resins, new Mehlich extractant, P-forms, soil testing