Fig. 5. Relationship between CaCl₂–extractable total phosphorus (CaCl₂–TP) or CaCl₂–extractable reactive phosphorus (CaCl₂–RP) and the ratio of NaHCO₃–extractable phosphorus (Olsen P) to phosphorus sorption index (PSI) or ammonium lactate–extractable phosphorus (P-AL) to PSI. The term *** indicates significance at the 0.001 probability level.