Soils of the study obtained from surface (0-15 cm) and sub-surface (15-30 cm) of the plots of three treatments- control, NPK and NPK + CD of the long-term integrated nutrient management experimental field at BRRI farm, Gazipur. During the last eight years (1990-1997) control plot received no fertilizer, NPK plot received 192 kg P ha-1 as triple super phosphate (TSP) and NPK + CD plot received 392 kg P ha-1 (192 kg as TSP and 200 kg as CD), respectively. The soil samples were air-dried, crushed, and passed through 2-mm sieve and stored in polyethylene bags at room temperature.
Fraction of inorganic and organic P was performed on each soil by a modified P fractionation scheme of Saleque and Kirk (1995). The following soil P fractions were measured in sequence:
(1) Solution P, by shaking 1 g soil in 30 ml of 0.05MCaCl2 for 16 h, centrifuging, filtering, and measuring P in the filtrate.
(2) NaHCO3–P, by shaking the residue from (1) in 30 ml of 0.5 M NaHCO3 for 16 h, centrifuging, filtering, and measuring P in the filtrate.
(3) NaOH-Pi–P, by shaking the residue from (2) in 30 ml of 0.1 M NaOH, centrifuging, filtering, and measuring P in the filtrate.
(4) NaOH-P0–P, by digesting 5 ml of the filtrate from (2) in 6 ml of concentrated H2SO4 for 1 h, cooling, adding 5 ml of H2O2, and re-heating until the residue became white, determining P in the digest, and subtracting NaOH-Pi–P from it.
(5) Acid P, by shaking the residue from (3) in 30 ml of 1:1 mixture of 1 M HCl/1 M H2SO4, centrifuging, filtering, and measuring P in the filtrate.
(6) Residual P, by refluxing the soil residue from (5) in 6 ml of a 5:2 mixture of concentrated HNO3 and HClO4, and determining P from the digest.
All P was determined colorimetrically (Murphy and Riley, 1962) after neutralization when necessary with dilute HCl and NaOH and the neutral pH indicated by the light yellow color of the solution in the presence of P-nitrophenol indicator.