For isotherm determination, 2.0-g of air-dried, homogenized soil was treated with 20 mL of 0.01 M KCl solution containing 0, 0.1, 1, 5, 10, 25, 50, and 100 mg P L−1 (Graetz and Nair, 1995, Nair et al., 1998) in 50 mL centrifuge tubes. The tubes were placed on a mechanical shaker for a 24 h equilibration period. At the end of the equilibration period, samples were centrifuged at 6000 rpm for 10 min and the supernatant was filtered through a 0.45 μm membrane filter. The filtrate was analyzed for soluble reactive P by colorimetric method (Murphy and Riley, 1962) using a Technicon™ Autoanalyzer (EPA 365.1). All extractions and determinations were done at room temperature (25 °C ± 1 °C).