Phosphorus Dissolution Kinetics and Bioavailability of Water-Insoluble Fractions from Monoammonium Phosphate Fertilizers

Abstract

Impurity compounds, with low P bioavailability, are present in monoammonium phosphate (MAP) fertilizer due to the presence of Fe, Al, Mg, and F in phosphate rock (PR). The kinetics of P dissolution from water-insoluble MAP fertilizer fractions, at initial pH 5, were studied to determine if a kinetic parameter from pseudo-first-order or Elovich equations could be used to predict P bioavailability. A pseudo-first-order kinetic equation did not describe P dissolution well, which may have been due to solution-pH and ionic-composition changes with time and a heterogeneous solid phase. The Elovich equation described P dissolution well with R² > 0.99. A low β constant in the Elovich equation, d[P]/dt = α exp(−β[P]), is associated with a higher P-dissolution rate (d[P]/dt) and a greater buffering of P-dissolution rate with increased P concentration, compared with a high β. A significant inverse relationship was found between P availability of water-insoluble fractions of MAP to sorghum-sudangrass (Sorghum bicolor [L.] Moench) and the β constant for P dissolution. The significant relationship between P bioavailability and β did not establish P dissolution as a rate-limiting step to P uptake because an inverse relationship also exists between β and equilibrium P. The Elovich β constant may be a useful parameter to predict P bioavailability in soils and fertilizer materials if the P-dissolution rate is proven to be rate limiting to P uptake by roots.