Ammonium Nitrogen is absorbed directly by plants at lower rates compared to nitrate. The positively charged ammonium ions detect the minerals in the soil and are less mobile than nitrate. Most of the ammonium is converted to nitrate by soil microorganisms and used by the plant.
Urea nitrogen is slowly taken up by plants. Plant roots cannot directly absorb significant amounts of urea nitrogen. Urea is first hydrolyzed to ammonium by soil microorganisms between a day and a week, depending on the temperature. Moisture is required to hydrolyze. After the urea nitrogen is hydrolyzed to ammonium nitrogen, it is converted to nitrate nitrogen and taken up by plants. In order for the conversion to nitrate to be long-lasting and to fulfill the nitrogen needs of the plant, urea must continue to exist in the soil.