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Effect of Pyrophosphate on Phosphate Sorption and Ammonia Volatilization by Calcareous Soils Treated with Ammonium Phosphates¹

ABSTRACT

Techniques 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.

¹ Contribution from the Fertilizer Technology and Agro-Economic Divisions, International Fertilizer Development Center, Muscle Shoals, AL 35660.

² Visiting Soil Scientist, Research Chemist (Soils), and Soil Scientists, respectively. The address of the Senior Author is Soil and Water Science Dep., Univ. of Alexandria, Alexandria, Egypt.

Received for publication November 25, 1981. Accepted for publication March 1, 1982.