Fig. 1. Flowchart of the sequential extraction procedure. The terms H₂O-P, NaHCO₃–P, NaOH-P, and HCl-P represent phosphorus that was extracted by deionized H₂O, 0.5 M NaHCO₃, 0.1 M NaOH, and 1 M HCl, respectively. Residual P was the phosphorus that could not be extracted by the above extractants.