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 CO²⁻₃ 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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