A 20-ml volume of 0.11 mol L−1 acetic acid was added to the tube, which was then shaken at 30 rpm for 16 h at 22 °C. The extract was separated from the solid phase by centrifugation at 3000 rpm for 20 min. The supernatant was decanted into a 50-ml polypropylene centrifuge tube and stored at 4 °C before analysis. The contents of the extractable cations were determined with an inductive coupled plasma emission spectrophotometer (Thermo 6300, USA).