Phosphorus Fractionations in Acidic Piedmont Rice Soils

Piedmont lands in Bangladesh, India, Nepal, and many other Asian countries are important rice-growing soils, but most of the soils are potentially phosphorus (P) deficient because of low pH. Phosphorus fractions of rice-growing acidic piedmont soils were determined. Soil samples were amended with 100 and 200 mg P kg−1 soil, and a control soil without P amendment was maintained. The samples were analyzed for the following fractions: solution P, labile pool [sodium bicarbonate (NaHCO3) P], alkali-extracted inorganic pool [sodium hydroxide (NaOH) Pi], organic pool (NaOH Po), acidic pool [sulfuric acid (H2SO4)−hydrochloric acid (HCl) P], and residual P. About 98% of the applied P in soils was extracted by the sequential extraction employed in the present experiment. The mean total P concentration in 10 acidic Piedmont soils was 247 ppm, of which only 0.12% was in solution, 8% labile (NaHCO3), 16% NaOH-extracted inorganic, 32% resistant organic, 18% relatively recalcitrant acidic, and 25% residual. Application of P fertilizer increased mainly the labile P fraction, which would be easily available to wetland rice. Solution P was positively and significantly correlated with pHKCl (r = 0.64, P < 0.05) and negatively correlated with clay (r = −0.77, P < 0.01). A negative and significant correlation of NaHCO3-P was observed with pHH2O (r = −0.62, P < 0.05). Solution P showed a negative and significant relationship with NaOH-Pi (r = −0.63, P < 0.05). A significant and negative relationship of solution P was also observed with acid P (r = −0.78, P < 0.01) and residual P (r = −0.82, P < 0.01). The relationship of NaHCO3-P with NaOH-Pi was positive (r = 0.70, P < 0.05) and significant. Similarly, a positive and significant relationship (r = 0.89, P < 0.01) between NaOH-Pi and acid P was observed, and acid P was positively and significantly correlated with residual P (r = 0.84, P < 0.01).