Effect of Pyrophosphate on Phosphate Sorption and Ammonia Volatilization by Calcareous Soils Treated with Ammonium Phosphates

Abstract

AbstractTechniques are needed to simultaneously reduce phosphorus (P) precipitation and ammonia (NH3) volatilization from MAP and DAP (monoammonium‐ and diammonium phosphate). The purpose of this study was to investigate the inhibitory effect of sodium pyrophosphate (NaPP) on P precipitation and NH3 volatilization in a pure calcium carbonate (CaCO3) system and in two calcareous soils from Egypt.In the CaCO3 system, additions of NaPP‐P equivalent to 4 and 8% of MAP‐P and DAP‐P, respectively, reduced the amounts of P sorption and NH3 volatilization. Solution ammoniacal‐N was higher with PP than without it for both MAP and DAP. Pyrophosphate acts as a nucleation and/or crystal growth inhibitor for dicalcium phosphate dihydrate (DCPD), thereby maintaining higher solution P concentrations. Because NaPP inhibits the formation of DCPD, the pH of the solution was lowered by its addition, which, in turn, reduced NH3 volatilization. Pyrophosphate did not affect NH3 volatilization from NH4Cl in the CaCO3 system since no insoluble reaction products were formed. The solution pH values were about the same for NH4Cl with or without NaPP in the CaCO3 system. The reaction of DAP with CaCO3 did not directly affect NH3 volatilization but indirectly increased solution pH which, in turn, controlled NH3 volatilization.In the soil system the inhibitory effect of NaPP was relatively more effective in reducing P sorption and NH3 volatilization from MAP than from DAP, and its effect lasted longer in the soil with less CaCO3 content than in the other soil. The effect of NaPP increased when the rate of PP‐P was increased from 2 to 4% of MAP‐P, but no further reduction in NH3 volatilization was noted between 4 and 8% on either soil. The inhibitory effect of PP on P sorption from MAP, however, did increase with an increase in PP‐P from 4 to 8%. Sodium pyrophosphate reduced the P sorption and NH3 volatilization from DAP more efficiently if applied in solution form or applied to the dried soil surface followed by the addition of water than if NaPP was applied to the premoistened soil surface. This is probably related to the dissolved PP concentration in the soil solution. In summary the results obtained with the soil system are in agreement with the results obtained with the pure CaCO3 system.