Urea based N fertilizers are in an organic commercial form that requires a biological enzyme to promote degradation to ammonia. Ammonia exists as a gas at normal temperature and pressure, thus it may be lost by volatilization if not exposed to water. Ammonia loss potential by volatilization for incorporated urea products is negligible because soil holds enough water to capture ammonia as ammonium that can be held on the soil's cation exchange complex. Surface applications of urea are at risk of loss because there is no opportunity to capture the ammonia as it is produced.
Volatilization risks increase with warmer temperatures, particularly with rapidly drying soil surfaces (ammonia from the urea attaches to the water as it vaporizes from the field). High soil pH also increases the risk of volatilization. We generally recommend not to apply urea-based fertilizers to the surface of fields recently treated with lime.
Urease inhibitors can have different modes of action, and the first question we should ask is do they work? The active ingredient in the inhibitor can act as a substrate for the urease enzyme, thereby protecting free urea by allowing it to stay in solution longer, or the inhibitor can inactivate the enzyme. Agrotain® is the most common commercially available urease inhibitor. The active ingredient in Agrotain® is N-(n-butyl) thiophosphoric triamide. The mode of action is not clearly defined, but it is thought to act as a substrate for the urease enzyme. Regardless of the mode of action, laboratory evidence has shown that it does allow urea to be retained in the soil longer.
Other urease inhibitors are marketed, some may have some activity, but it is your job as a producer/consultant to determine whether or not the proposed mode of action makes sense. We would also encourage you to inquire about lab data indicating that the material being marketed does what it is supposed to do. Some legitimate products have limited data and may not be recommended by universities until more information and testing becomes available. North Dakota State University has a good discussion on recent urease inhibitors that may be found at the following URL: http://agprofessional.com/sites/default/files/sf1581.pdf
Even if a urease inhibitor has been demonstrated in a laboratory to have some inhibition properties on the enzyme urease, the agronomic question still remains as to its usefulness in a field setting. It really depends upon how N is to be applied (and the form) and the rate of nitrogen being applied. Higher rates of urea nitrogen (under most conditions) likely do not require urease inhibitors. Surface application of dry urea in high residue situations is a good place for the use of urease inhibitors. Dribble applications of liquid UAN may benefit from a urease inhibitor in high residue situations, but clean till fields are less likely to benefit. Injected liquid UAN does not require stabilizers based upon current research. Also keep in mind that the urease inhibitor will only benefit the urea component of the UAN solution.