Several phone calls to my office have concerned the use of urea fertilizer as a nitrogen source for summer perennial pastures such as bermudagrass and introduced bluestem. The callers recalled a problem with using urea fertilizers on pasturelands but couldn't remember exactly what it was. Well, what they were trying to remember is that under certain conditions the nitrogen in urea easily changes to a gas; therefore, it can be lost to the air like anhydrous ammonia if not incorporated.
Granular or prilled urea (46-0-0) is used extensively as a nitrogen source in the U.S. It is white and is typically applied to the soil surface of pasture- and cropland. When urea is put on a moist soil, it dissolves and in the presence of urease, a naturally occurring enzyme found in the soil and on plant litter, is transformed to ammonium carbonate. This compound is unstable and rapidly decomposes into ammonia (NH3), carbon dioxide (CO2) and water (H2O; figure 1). The NH3, which is a gas, can be lost to the atmosphere in a process called volatilization. Whether such a loss will occur and how much nitrogen will be lost relates to the soil's pH, temperature, and moisture, and the degree of contact with the soil.
Generally, NH3 volatilizes more readily on alkaline soils or those with pHs higher than 7.0, and losses increase as temperature increases, which is more likely on sandy soils and pastures that have plant residues. To avoid the possibility of NH3 loss, avoid summer applications of urea on pasturelands. If the cost per pound of nitrogen in urea is considerably cheaper than that of the other nitrogen sources and you are willing to take a risk, then try to apply the urea fertilizer when the soil surface is dry and 0.25 inch or more of rain is expected in the next 48 hours.