Fig. 5. (a) Ratio of P extracted by resins at 1 h (Q_1h) to the maximum extractable P (Q_max in the Johnson–Mehl equation) as related to the ratio of P extracted by CA-EDTA + dithionite (Ca-EDTA-P) to the combined of P fractions (Díaz-Espejo et al., 1999) and to (b) the ratio of P extracted by citrate (Cit-P) to the combined P fractions (Ruíz et al., 1997).