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Effect of Pyrophosphate on Orthophosphate Reactions in Calcareous Soils¹

ABSTRACT

Sodium pyrophosphate (NaPP) is known as a crystal growth inhibitor. Its effect on the kinetics of phosphorus (P) retention by calcium carbonate (CaCO₃) and by two calcareous soils was investigated using orthophosphate (OP) solutions ranging in P concentration between 0.1 and 1 x 10^–3M. Reaction of the most dilute solution with CaCO₃ involved initial rapid adsorption of P followed by a gradual change in solution composition toward equilibration with hydroxyapatite (HAP). For the more concentrated solutions, the P concentration vs. time curves exhibited an initial, rapid adsorption followed by an induction period that preceded a second drop which was proportional to the initial concentration, and finally a gradual decrease toward a solubility equilibrium value. Amounts of P removed during the initial drop were close to the calculated Langmuir adsorption maximum. The second drop in P concentration was presumed to reflect precipitation involving growth of calcium phosphate nuclei adsorbed on the CaCO₃ surface. Behavior of a desert soil (39% CaCO₃) was quite similar to that of CaCO₃, while the precipitation reaction was considerably less in an alluvial soil (4.1% CaCO₃).

Addition of NaPP at a concentration of at least 2 x 10^–5M P increased both the induction period and the final P concentration. Effectiveness of NaPP depended on its addition before the end of the induction period as well as on the pyrophosphate-to-orthophosphate ratio. Results obtained with triammonium pyrophosphate on the two soils were similar to those with NaPP.

¹ Contribution from the Fertilizer Efficiency Project, Soil and Water Science Dep., Univ. of Alexandria, Egypt. The project is supported in part by the International Development Research Center.

² Professor and former Graduate Student, presently Assistant Professor, respectively.

Received for publication September 29, 1980. Accepted for publication April 22, 1981.