Regulation mechanisms and field implications of ammonia bonding with various crystalline salts
Evangelou, V.P.
Ammonia volatilization following surface application of urea or inorganic NH4 salts can be reduced by the addition of CaCl2, MgCl2, or MgSO4 salts. The mechanisms for this phenomenon are reported to be: (i) precipitation of CO3(-2) by Ca2+ or Mg2+, preventing (NH4)2CO3 formation and (ii) Ca2+ depression of soil pH by suppression of the dissociation of the CaCO3-Ca(OH)2 buffer system. Using Fourier-transform infrared (FTIR) spectroscopy, this study demonstrates that, for systems containing limited quantities of water (adsorbed water only), the CaCl2 or MgCl2 salts act as proton donors to NH3, forming stable MH2OOH.H4N.Cl2 (M = Ca or Mg) compounds. However, the ability of a metal cation to act as a proton donor is diminished when the cation-anion bond increases in strength (ionic character of the bond is diminished while covalent character is increased). These findings are consistent with NH3-volatilization data reported in the literature. The present study shows, via a different mechanism, that Ca salts of low solubility (e.g., CaSO4.2H2O) should not be effective in reducing NH3 losses. On the other hand, Ca or Mg salts of high solubility ought to be effective in reducing NH3 losses from surface-applied urea or NH4 fertilizers.
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