Evaluation of Mehlich‐III extractant to estimate the available P in Quebec soils

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Abstract
The Mehlich‐III method used for the determination of available P in 82 Quebec soils was compared to five other chemical methods (Bray‐II, Bray‐I, Mehlich‐II, Mehlich‐I and Olsen) and to two anion exchange resin techniques (F‐ and HCO‐ 3) In general, very good correlations were found between Mehlich‐III, Bray‐II and Bray‐I methods on acid soils (r=0.95** and 0.98**). The Mehlich‐III‐P content is somewhat the same as that determined by Bray‐I and corresponds approximately to 80% of Bray‐II‐P. The Mehlich—III extractant was found to be more reliable than Bray‐II and Bray‐I on very acid spodosols having high P‐sorption capacity. The Bray methods, with high NH4F concentration, extracted strongly fixed Al‐P in these soils and may overestimate the available P. In some acid soils containing apatite and in calcareous soils, the Bray‐II and Mehlich‐I (double acid) easily dissolved these Ca‐P compounds and gave exaggerated high P values. The Mehlich‐III and the other methods were less vigourous for these compounds and were more accurate in estimating available P in these soils. On moderately calcarous soils, the Bray‐I and Mehlich‐I are partially neutralized by the free carbonates in the soil and become less effective in extracting soil P. The Mehlich‐III reactant is more buffered and less affected by carbonates in these soils. The Mehlich‐III and the Mehlich‐II were highly correlated for all soils having diverse properties (r=0.98**) and the amount of P extracted by Mehlich‐III was 117% of that extracted by Mehlich‐II. The Mehlich‐III method showed good correlation with Olsen and anion exchange resin techniques (r=0.89** to 0.93**). The Mehlich‐III extracted about three times more P than Olsen and approximately 1.2 times of that desorbed by HCO‐ 3‐resin. The amount of P desorbed by F‐‐resin was higher and corresponded to about 1.25 times that estimated by Mehlich‐III. The Olsen method and the F‐ and HCO‐ 3 ‐resins tended to extract Ca‐P and Fe‐P more easily than Mehlich‐III. However, on some acid spodosols probably containing phosphates covered with oxide coatings, the Olsen extractant and the F‐ and HCO‐ 3‐resins did not attack these compounds and gave relatively low P values as compared to the acid chemical methods. Among the acid chemical methods, the Mehlich‐III was considered the most appropriate and economic procedure to be used for soil P testing in the laboratory for different kinds of soil because of its capacity to extract simultaneously other soil nutrients. The Al content in the Mehlich‐III extract may give an estimate of P‐sorption capacity. Some possibilities concerning the conversion of soil fertility levels established by Bray‐II, Bray‐I and Mehlich‐II to that of Mehlich‐III were proposed for the purpose of P fertilizer recommendations.

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