In a field study, percentage of nitrogen lost as ammonia from a jack pine (Pinusbanksiana Lamb.) soil increased with increasing rates of urea application between 112 and 448 kg N/ha. After 7 days, losses amounted to 18–28% of a 224 kg urea-N/ha application, representing 60–87% of the total losses measured over a 6-week period. Maximum volatilization rates occurred between the third and the fifth day after fertilization, at which time urea hydrolysis was virtually complete. Negligible ammonia losses were measured in plots treated with ammonium sulfate, ammonium nitrate, and sulfur-coated urea. Applying superphosphate with urea markedly depressed ammonia volatilization, an effect which was enhanced by a joint application of K2SO4•MgSO4. Reduction of volatilization by artificial precipitation was significant and increased with increasing precipitation when the latter was applied soon after fertilization; decreases in volatilization were then related to the amount of residual urea subject to diffusion into the humus layer or to leaching towards the underlying soil horizons.