Field Measurements of Ammonia Loss from Surface Applications of Urea Solution to Bare Soil1

Factors influencing ammonia (NH3) volatilization from surface‐applied urea have been studied extensively; however, almost all research has been done in a manner that modifies the environment compared to field conditions. Results from such studies, therefore, may incorrectly assess actual losses in the field. Our objective was to evaluate the magnitude of NH3volatilization and how soil temperature, soil water, and other environmental factors affected rates of loss. Three studies were conducted to assess the volatilization of NH3from surface‐applied urea, two on a silt loam and one on a very fine sandy loam soil. A mass‐balance micrometeorological method, which produces very minimal disturbance of the natural environment was used in all studies. Diurnal patterns of NH3loss, which coincided with the fluctuations in surface temperature and water content, were found in each study. In all studies, loss followed most closely the pattern of urea hydrolysis. Urea hydrolysis in the surface soil was slowed by dry conditions and approached zero when the soil was near “air dryness.” Rainfall of 0.86 cm did not prevent further NH3loss from Muir silt loam soil (fine‐silty, mixed, mesic Pachic Haplustolls). Three simulated rainfalls with 0.5 cm of irrigation separated by 3 and 4 days did not curtail loss from Haynie very fine sandy loam soil (coarse‐silty, mixed, mesic Typic Udifluvents), but rather enhanced the rate of volatilization, probably due in part to an increased rate of urea hydrolysis.