Phosphorus Sorption and Uptake from Sri Lankan Alfisols

Soil organic P as well as inorganic P quantity and capacity factors have been identified as P availability determinants in tropical soils. The effects of these factors on P uptake from 12 Sri Lankan Alfisols were determined. For the analysis, a P quantity variable ( Q , µg g −1 soil) was P extracted by 0.5 M NaHCO 3 (16‐h shaking). The tangential buffer capacity inverse (× 10 3 ), computed at points on P sorption isotherms corresponding to Q , was the capacity variable [ B , µg solution P × 10 3 g −1 solution/(µg sorbed P g −1 soil)]. Phosphorus extracted by 0.1 M NaOH (16‐h shaking) after removal of labile P was a moderately labile organic P variable ( O p , µg g −1 soil). Phosphorus uptake by foxtail millet [ Setaria italica (L.)] was determined from Neubauer pots. A regression equation indicated significant influences by Q, B , and O p on P uptake. The effect of O p was through an interaction with Q . Moreover, applied inorganic P increased moderately labile organic P by only 11% but labile inorganic P by 255%. The significant interaction was evidence that applied inorganic P increased uptake from the labile inorganic P pool in soils with high NaOH‐extractable organic P relative to uptake from soils with low NaOH‐extractable organic P. The interaction between Q and O p was consistent with three alternative hypotheses regarding the contribution of organic P to soil P status. The first derivative of the regression equation with respect to Q was used to examine a soil × applied P interaction in the uptake data. Variables from the first derivative explained about two‐ thirds of the soil × applied P interaction of P uptake from the Neubauer pots, evidence that P sorption properties as well as O p differences among the 12 soils influenced P availability.