Of all nitrogen fertilizers, anhydrous ammonia is the least likely to leach initially after application, followed by urea. Third are UAN solutions, which are one-fourth nitrate. Half of ammonium nitrate is in the nitrate form, making it the fertilizer in which the greatest amount of nitrogen is ready to leach immediately after application. On sandy soils, the differences between these forms of nitrogen are especially significant because water and nitrate can leach quickly through such coarse-textured soils. On heavy soils, the differences may not be as significant-unless you use a nitrification inhibitor.
Inhibitors delay the transformation from ammonium to nitrate.
They are most effective if the soil is moderately or well-drained and is in an area of heavy rainfall or frequent flooding. Also, they offer the greatest potential benefits when you fall-apply nitrogen after the soil temperature has dropped below 60 degrees Fahrenheit and the soil is likely to be frozen for most of the winter. They work best when nitrogen is applied at or below optimum levels. Urease inhibitors slow the conversion of urea to ammonia. This reduces volatilization, the process in which ammonia on the soil surface is lost into the atmosphere as a gas.