There are many thoughts regarding how much nitrogen is lost from applying urea on pastures and hay fields. If applied incorrectly, up to 40 percent of the nitrogen applied as urea can be volatilized (vaporized) and lost as a gas. If applied correctly, little, if any, nitrogen will be lost. Obviously, the key is to apply the urea correctly, writes Eddie Funderburg for The Samuel Roberts Noble Foundation.
To go in reverse order, let's look at the worst way to apply urea to pastures. The worst way is to apply urea on the soil surface without incorporating it with tillage or irrigation, have hot temperatures and not have rain within three to four days after application. If you have all these things happen, you can have urea losses of up to 40 percent, but 20 percent losses are more common. A key to remember is that ALL three things have to happen for the urea losses to occur. If you till in the urea within three to four days, or irrigate it into the soil within that time frame, or have rainfall within that time frame, or it is cool (less than 70° F sustained), urea losses will be insignificant.
Urea is lost for the following reason. It reacts with water to form ammonium carbonate. Ammonium carbonate is unstable and breaks down into carbon dioxide and ammonia gas. If the urea is in the ground, the ammonia gas will quickly combine with soil water to form ammonium hydroxide which is stable and not subject to volatilization loss. However, if the urea is on the soil surface when the breakdown occurs, the ammonia gas is lost to the atmosphere.
To read the entire article, link to The Samuel Roberts Noble Foundation.