Phsophorus sorption and uptake from Sri Lankan alfisols
1992
Sod 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, microgram g-1 soil) was P extracted by 0.5 M NaHCO3 (16-b shaking). The tangential buffer capacity inverse (X 10(3)), computed at points on P sorption isotherms corresponding to 9, was the capacity variable [B, microgram solution P X 10(3) g-1 solution/(microgram 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 (OP, microgram 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 Op on P uptake. The effect of Op 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 Op 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 X applied P interaction in the uptake data. Variables from the first derivative explained about two- thirds of the soil X applied P interaction of P uptake from the Neubauer pots, evidence that P sorption properties as well as Op differences among the 12 soils influenced P availability.
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