The determination of potentially available phosphorus by chemical extractors is difficult in variable charge soils because there are several degrees of bound energy between phosphorus and colloids. The objective of this work was to estimate the minimum number of successive extractions with the Mehlich-1, Mehlich-3 and anion exchange resin (AER) methods to obtain values of potentially available phosphorus. Thirteen successive extractions were accomplished with Mehlich-1, Mehlich-3 and AER methods in samples of a Rhodic Hapludox under a no-tillage system and increasing phosphorus doses. Results showed that one single extraction with Mehlich-1, Mehlich-3 and AER extracts only 29, 35 and 37 % of the potentially available phosphorus, respectively. To obtain the quantity factor, seven successive extractions with AER and four with Mehlich-1 and Mehlich-3 were necessary. The AER method was more efficient to extract phosphorus in treatments with low contents while the Mehlich-1 and Mehlich-3 methods were more efficient in treatments with high phosphorus availability.
Mehlich-1; Mehlich-3; anion exchange resin; desorption; no-tillage