Urea (46% N) and urea-ammonium nitrate (28% UAN) have typically been about 10¢/lb. of nitrogen (N) more expensive than anhydrous ammonia. Recently, urea prices have fallen substantially to be competitive with ammonia and less expensive than UAN. Lower prices have resulted in an interest among farmers in using urea instead of ammonia or UAN.
Is this a good change to make, or not?
Urea's main advantage over ammonia is it can be applied faster and with less disruption to the soil. More N must be applied to Indiana farmland this spring because fewer acres than usual received ammonia in the fall. With preference being given to timely planting, urea can accelerate application.
Unfortunately, in most situations urea is not as good a fertilizer N source as ammonia. Urea is often inferior to UAN when surface-applied, but equivalent or slightly better than UAN when incorporated into the soil.
Surface-applied urea can be converted to ammonia, which can be volatilized to the air. Under the worst of conditions, up to 60% of N in urea can be volatilized; more commonly, losses are 30% or less. Since half of 28% UAN is comprised of urea, ammonia volatilization losses from 28% UAN are typically about half that of urea.
Results of studies on several no-till fields in Indiana illustrate the impact of ammonia volatilization on grain yield (table below). Nitrogen loss due to surface application of urea or 28% UAN reduced corn yield an average 16-21 bu./acre in comparison to injected UAN and anhydrous ammonia, respectively. Ammonia volatilization losses could not be calculated from these studies, but they likely ranged from 30 to 80 lbs. N/acre.
Typically, ammonia volatilization losses are expected to be greater with urea than with 28% N, but this did not hold true in these studies. More recent research in Illinois showed much greater yield reductions with broadcast urea than 28% UAN.