Urea is also easier to store in bulk because it draws less moisture from the air and there is no risk of fire or explosion. Even in relatively cool soils, the conversion of urea to ammonium occurs within a few days after application. The conversion to nitrate begins almost immediately and will be complete within 3 or 4 weeks following application. As was mentioned above, when ammonia is dissolved in moisture, ammonium ions are formed. If this occurs at, or near, the soil surface, the ammonia will be lost to the air.
Many factors determine the extent of the nitrogen loss.
These include the nitrogen material used, the rate and method of application, the pH and buffer capacity of the soil, the weather, and the amount of crop residues. Under some conditions, up to half of the applied nitrogen can be lost in this way. As the amount of ammonium nitrogen in the soil increases, so does the potential for conversion to ammonia. The rate at which nitrogen changes from the ammonium form to ammonia depends on the soil pH. When the pH is below 7.0, almost no ammonia is released. As the pH increases, the production of ammonia increases exponentially.