- Following the surface application of urea to soil, there is the potential for nitrogen (N) loss from ammonia (NH3) volatilization until sufficient rain or irrigation has moved it into the soil. Many studies measuring NH3 loss have used laboratory and microplot field studies. The limitation of these techniques is that they do not represent NH3 loss under natural conditions because enclosures alter air movement, humidity, and soil and air temperature inside the chambers. However, they do allow researchers to study the individual factors controlling ammonia loss. To overcome the limitations imposed by enclosures, field-scale passive flux methods have been developed that do not change field conditions. Field and lab studies were initiated to quantify NH3 volatilization from surface applied urea, determine if the addition of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) could reduce NH3 loss, and explore the relationship between soil properties and NH3 volatilization. A total of three field trials were conducted 2014 and 2015, in which urea or urea + the urease inhibitor Agrotain was applied at a rate of 150 lb N/acre, and NH3 loss measured using a passive flux method. In the lab, NH3 loss from surface applied urea applied to six soils with varying soil properties was measured using static chambers. In the field, ≤1.6% of applied fertilizer N was lost as NH3 within 6 to 8 days after application despite ideal conditions for loss (i.e., rapid dissolution of urea that was applied to moist, warm soil). Agrotain did not reduce ammonia loss. In the lab study, ammonia loss for soils with a CEC ≥19 cmol(+)/kg (n=5) was ≤8.4% of added urea-N, and ammonia loss from the soil with the lowest CEC (6 cmol(+)/kg) was 25.6%. The soil property that most strongly controlled NH3 loss was CEC (which was related to soil pH buffering capacity), not pH or urease enzyme activity. This study indicates that NH3 loss from surface applied urea is expected to be minimal for most Willamette Valley soils, which have moderate to high pH buffering capacity.