Soil drought, that can be enhanced by global warmin g increases ammonia (NH 3) volatilization. This laboratory study was conducted with two soils: Krome Gravelly Loam (KGL) from Florida and Warden Silt Loam (WSL) from Washington State and two fertilizers: Ammonium sulfate [(NH 4)2SO 4] or ammonium nitrate (NH 4NO 3). Two water regimes including 20 and 80% Field Capacity (FC) were used at 20°C which is the average temperature in the potato growing se ason in Washington State. The data demonstrated that variation in NH 3 volatilization subject to different soil water reg imes can be explained by changes in Gibbs free energy of Nitrogen (N) fertilization in soils with varying water contents. The absolute values of Gibbs free energy of (NH 4)2SO 4 or NH 4NO 3 applied to soil at 20% FC soil water regime were 5 -fold greater than at 80% FC. Accordingly, the equilibrium constant (K) of deprotonation of ammonium ions in soil solutio ns at 20% FC was 3,000- or 50-fold greater than that a t 80% FC for (NH 4)2SO 4 or NH 4NO 3, respectively. Nitrogen loss via NH 3 volatilization was 4-to 7-fold greater at 20% FC t han that at 80% FC. This study suggests potential acceleration of NH 3 volatilization from soils under drought. Therefore , optimal water management is critical to mitigate NH 3 volatilization from agricultural soils.