The collected soil samples were homogenized, air-dried, and passed through a 2-mm sieve before analyses. Soil pH was measured with distilled water in a 1:2 soil and water suspension by a glass electrode.Oxalic acid (0.1 M) and ammonium oxalate (0.175 M) solution, equilibrated at a pH of 3.0 (McKeague and Day, 1966) were used as the extracting agent to determine oxalate-extractable P (Ox-P), Fe (Ox-Fe),and Al (Ox-Al). The suspension was shaken in a mechanical shaker andequilibrated for 4 h in the dark. The equilibrated suspension was centrifuged and filtered through a 0.45-μm filter, and analyzed for P,Fe and Al by inductively coupled plasma spectrometry (Thermo Jarrel Ash ICP 61E, Thermo Elemental, Franklin, MA) (Loeppert and Inskeep,1996). Soil samples were extracted with Mehlich 3-extracting solution(0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.13 M HNO3 + 0.001 M EDTA) by shaking for 5 min at a 1:10 soil to solution ratio to determine P (M3-P), Fe (M3-Fe), and Al (M3-Al) (Mehlich,1984). Water-soluble P (WSP) was determined by extracting each soil sample with deionized water at 1:10 soil-to-water ratio, and analyzing P on the filtrate collected after passing through a 0.45 μm filter. Watersoluble P concentrations were determined on an autoanalyzer followingthe Murphy and Riley (1962) procedure.