Factors influencing the isotopically exchangeable phosphate in soils

Abstract

The labile phosphate in a non-calcareous and a slightly calcareous soil was determined by isotopic exchange in the presence and absence of 0.001 molar solutions of a chelating and a non-chelating organic anion. The rate of isotopic exchange curves were analyzed graphically to sub-divide the labile phosphate into 3 or 4 fractions. The half-lives of exchange for the ‘rapid’, ‘medium’ and ‘slow’ fractions were between 0.3 to 1.6 hours, 1.8 to 8.6 hours and 25.8 to 46.1 hours respectively. In addition, an ‘instantaneously-exchanging’ component was sometimes observed. In the presence of the citrate ion, the total labile phosphate was increased in the non-calcareous soil and decreased in the calcareous soil, whereas the diethyl barbiturate ion (DEB) anions decreased the total labile phosphate in both soils. In general, the citrate ion increased the ‘rapid’ and the ‘medium’ fractions whereas the DEB anion either did not affect them or decreased them. Again, whereas citrate always increased the phosphate in solution, the effect of DEB anions depended on the soil. The major effect of the organic anions was to greatly decrease the ‘slowly-exchanging’ fraction in both soils. The half-lives of exchange for the ‘rapid’, ‘medium’ and ‘slow’ fractions were in the order no organic anion > citrate > barbiturate and the rate constants for a first-order mechanism were in the order no organic anion < barbiturate < citrate. Small but significant differences were observed between the two soils.