Phosphorus Fractions and Availability in Sri Lankan Alfisols

Abstract

Abstract Phosphorus is the second most limiting element in the Alfisols of tropical Asia and Africa. The relationships between P fractions in 12 Sri Lankan Alfisols and P taken up by plants in a greenhouse experiment and P removed by five chemical extractants were examined. Soils were selected to represent different land uses and catena positions. Sufficient P to elevate soil solution concentrations to 0.2 µg P g −1 was applied to one set of samples. A second set received 75% of the P applied to the first and a third received no P. Phosphorus uptake by foxtail millet [ Setaria italica (L.)] was determined after 25 d of growth in a Neubauer experiment. Soil P was fractionated sequentially into labile (removed by 0.5 M NaHCO 3 ) and moderately labile (removed by 0.1 M NaOH) inorganic and organic P as well as more stable soil P components. On average, 45% of total soil P was in the NaOH fraction and, of this, 72% was in an organic form. Plant P was significantly related to labile inorganic (NaHCO 3 ‐P i ) and moderately labile organic (NaOH‐P o ) fractions. Double‐acid P (extracted with 0.5 M HCl plus 0.0125 M H 2 SO 4 ) was significantly related to the same fractions. Moreover, double‐acid P was better correlated with plant P than was P removed by those extractants highly correlated only with labile inorganic P. The study suggested that the moderately labile organic P fraction as well as the labile inorganic P fraction contributed significantly to P uptake from tropical Alfisols and that the double‐acid extractant was superior to soil P extractants that are uncorrelated to the moderately labile organic P fraction.