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 adjust phosphorus (P) availability in soils. Soybean was grown for 45 days under greenhouse conditions on samples of 10 Oxisols treated with variable levels of soluble P to compare critical soil P levels between an anion exchange 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. Variation 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 soil 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 Mehlich‐1. Interpretations of AER critical P levels, therefore, do not need auxiliary measures of PCF as with the Mehlich‐1 extractant. The reverse was true, however, for P uptake. Therefore, the criteria used to assess plant response to P (P uptake or dry matter production) in soils with variable PCF will influence the results obtained in correlations between methods used to extract soil P.