DISPONIBILITÉ DU PHOSPHORE DANS LES SOLS NEUTRES ET CALCAIRES DU QUÉBEC EN RELATION AVEC LEURS CARACTÉRISTIQUES CHIMIQUES ET PHYSIQUES

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Abstract
Seven chemical methods (Bray-II, Bray-I, North Carolina DA-4, DA10, Mehlich II, Mehlich III and Olsen) and two anion exchange resins (F− and [Formula: see text] forms) were evaluated on 43 soils with pH varying from 6.4 to 7.88. A greenhouse experiment was carried out with ryegrass receiving two treatments (without P fertilization and 150 mg kg−1 soil) on these soils. The Olsen's method and the two anion resins used as reference ones were effectively the better correlated with P uptakes (r = 0.83**–0.87**) and relative yield (R2 = 69.3–70.2%). The Mehlich II, Mehlich III and to a lesser extent Bray-I methods showed good correlations with these reference methods (r = 0.77**–0.83**) and with plant P uptakes (r = 0.73**–0.79**) or relative yields (R2 = 48.9–55.1%). The Bray-II, DA-4 and DA-10 methods may dissolve some calcium phosphates such as apatites or tricalcium phosphate in some soils and thus overestimated their available P content. The P amounts extracted by these methods were less correlated with P uptakes (r = 0.57*–0.67**) or relative yields (R2 = 14.4–31.4%). On the other hand, the DA-4 extractability was diminished in soils containing more than 3% of carbonates. Although the Ca-P was the predominant form in these soils, the P fixation capacity (M) and maximum P buffer (Mb) capacity were mostly related with Al and Fe-oxalate contents. However, the low Mb values obtained from these soils indicated that the P concentration in soil solution is more favored with respect to adsorbed P. Organic matter and clay contents, pH (H2O), exchangeable Ca and carbonate contents had some effects on the extraction capability of some methods without affecting plant P uptakes. Key words: Available P, soil testing, anion exchange resins, soil properties, P fixation

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