Solid granular or prilled urea (45-46% N) has become widely available in Kentucky, as has 28% nitrogen solution (UAN solution) which is half urea and half ammonium nitrate. When applied to a field, urea is transformed to ammonium N soon after application. The rate of transformation is greatly increased by urease, the enzyme found to varying degrees in soil and on plants and plant residues. Due to the chemical reactions involved as urea nitrogen undergoes transformation, there is some degree of likelihood for volatilization loss of some N. Whether such a loss will occur, and how much N will be lost if it does occur, relates to temperature, moisture, degree of contact with soil, and pH of the soil.
Wells, Kenneth L., "Using Urea Nitrogen as a Fertilizer" (1984). Soil Science News and Views. 153.