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.
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.
These losses can be greater if the soil has a high pH or if the soil is wet when the urea is applied. The reason high pH increases the losses is that the ammonium carbonate breakdown reaction is basic and a high pH drives the reaction faster. The reason wet soil makes the losses worse is twofold - one is that applying urea to wet soil insures the pellet dissolves almost instantly and makes the urea subject to attack by the urease enzyme that causes the loss (if the urea stays in pellet form, the enzyme cannot act upon it), and two is that the urease enzyme that is responsible for the loss is more prevalent in wet soil than in dry soil.
Temperature is important in estimating the amount of loss that may occur. The urease enzyme is not active in cool weather. That is why urea losses are very small in cool weather and much larger in hot weather. An exact temperature at which urea losses begin to occur is hard to pin down, but it is generally thought that the temperature must be higher than 70° F for a sustained period of time for losses to be severe.
The key to avoiding urea loss is to get the urea into the ground. This can be done by tilling the urea into the soil, irrigating the urea into the soil or getting a rainfall event within three to four days. It is generally thought that .25 inch of rain is sufficient. Heavy dew is not good enough to incorporate the urea into the soil.
In closing, urea is an excellent source of nitrogen if one has an understanding of how to use it (or better yet, an idea of how NOT to use it). Losses can occur if used incorrectly. These losses can be as high as 40 percent, but are generally about 20 percent.