Comparisons of phosphorus availability between anion exchange resin and Mehlich-1 extractions among oxisols with different capacity factors

Acid and dilute-salt chemical extractants are commonly used to assess and a djust phosphorus (P) availability in soils. Soybean was grown for 45 days u nder greenhouse conditions on samples of 10 Oxisols treated with variable l evels of soluble P to compare critical soil P levels between an anion excha nge resin (AER) and the Mehlich-1 extractant, and to evaluate the influence of the soil P capacity factor (PCF) on the critical soil P levels. Variati on among soils in the quantities of extracted P and critical P levels with the AER was less than that with Mehlich-1. Low correlation values between s oil characteristics related to PCF and measurements of AER soil P suggested that the AER method was not as sensitive to variations in soil PCF as Mehl ich-1. Interpretations of AER critical P levels, therefore, do not need aux iliary measures of PCF as with the Mehlich-1 extractant. The reverse was tr ue, however, for P uptake. Therefore, the criteria used to assess plant res ponse to P (P uptake or dry matter production) in soils with variable PCF w ill influence the results obtained in correlations between methods used to extract soil P.