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Ammonia Volatilization from Surface Applications of Ammonium Compounds on Calcareous Soils: I. General Theory¹

ABSTRACT

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.

¹ Contribution from the Texas Agr. Exp. Sta., Texas A&M Univ., College Station, Texas 77843.

² Assistant Professor and Associate Professor, respectively, Texas Agr. Exp. Sta., Temple, Texas 76501.

Received for publication March 30, 1973. Accepted for publication July 24, 1973.