EVALUATION OF MEHLICH-III EXTRACTANT TO ESTIMATE THE AVAILABLE P IN QUEBEC SOILS

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 Ń 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 Ń 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 Ń than Olsen and approximately 1.2 times of that desorbed by HCO-3-resin. The amount of Ń 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 Ń 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 Ń testing in the laboratory for different kinds of soil because of i t s 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 soilfertility levels established by Bray-II, Bray-I and Mehlich-II to that of Mehlich-III were proposed for the purpose of Ń fertilizer recommendations. © 1990, Taylor & Francis Group, LLC. All rights reserved.