Dynamics of Ammonia Volatilization during Furrow Irrigation of Maize

Abstract In irrigated agriculture, supplemental nitrogen (N) is commonly applied by dissolving ammonia (NH 3 ) gas in the irrigation water. The operation is convenient and labor‐saving, but because NH 3 is volatile, the potential for loss is high. A micrometeorological method was used to measure losses of NH 3 gas during applications of water‐run NH 3 to short (0.9 m) and tall (2.1 m) maize crops. The equilibrium NH 3 vapor pressure of the irrigation water and wind speed both affected the loss, but the influence of the latter was very marked, volatilization increasing with the approximate square of the wind speed. Average volatilization losses were 7% of the N present in the irrigation water per hour from the short crop, and 1% from the tall crop, the difference arising from the greater attenuation of wind in the tall crop. Results for both crops were reconciled by considering transfer through an air layer close to the ground. Volatilization rate could then be expressed as the product of a wind‐dependent exchange coefficient and the NH 3 vapor pressure difference between the water surface and 0.3 m. Volatilization losses resulted in very uneven applications of N. The nitrogen content of the irrigation water decreased by 84% over a distance of 400 m along the furrow in the short crop, and by 59% in the tall crop. Some practical remedies are suggested, including irrigating at night. In these experiments, night‐time volatilization rates were only one‐half those observed by day for similar aqueous NH 3 concentrations.