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Ammonia Volatilization from Surface Applications of Ammonium Compounds on Calcareous Soils: I. General Theory

Fenn, L. B., Kissel, D. E.
Soil Science Society of America journal 1973 v.37 no.6 pp. 855-859
ammonia, ammonium carbonate, ammonium chloride, ammonium hydroxide, ammonium nitrate, ammonium nitrogen, ammonium salts, ammonium sulfate, application rate, barium, calcareous soils, calcium, calcium sulfate, carbon dioxide, fluorides, magnesium, soil pH, solubility, volatilization
Volatilization of NH₃ from NH₄⁻-N salts applied to the surface of a calcareous soil was investigated. Ammonia losses from surface applied NH₄F, (NH₄)₂SO₄, and (NH₄)₂HPO₄ were 68, 54, and 51%, respectively, of the added NH₄⁻-N in 100 hours. Ammonium nitrate, NH₄Cl, and NH₄I lost 18, 18, and 16%, respectively, of the added NH₄-N in 100 hours. An increase in pH at the soil surface resulted in higher loss of NH₃. Applications of 550 kg N/ha as NH₄F and (NH₄)₂SO₄ increased the pH from 7.6 to 8.9 and 8.1, although pH decreased rapidly with time. Soil pH decreased to 6.8 by the addition of 550 kg NH₄-N/ha as NH₄NO₃ and changed very little in 50 hours. Ammonium sulfate applied to soils saturated with Mg²⁺ and Ba²⁺ gave similar results. Solubility of the apparent reaction product was the major factor regulating NH₃ volatilization. Formation of a Ca²⁺ reaction product, such as CaSO₄, tends to drive the reaction to the right and favors formation of (NH₄)₂CO₃. A proposed mechanism of the observed loss of NH₃ is as follows: when an ammonium salt dissolves in a calcareous soil, (NH₄)₂CO₃ and a Ca²⁺ salt of varying solubility forms. Ammonium carbonate subsequently decomposes, losing CO₂ at a faster rate than NH₃. This causes formation of NH₄OH and an increase in pH and greater NH₃ losses.