Surface applied urea or urea-ammonium nitrate (UAN) solution is susceptible to significant loss of N by ammonia volatilization. As much as one-third of the N from urea can be lost if the fertilizer is not incorporated quickly by tillage or one-half inch of soaking rainfall. Many farmers, particularly no-tillers, count on rainfall to incorporate urea or UAN and thus minimize volatilization loss.
We have been seeing relative warm, dry, windy weather in many parts of the state over the last week or so, which has raised some questions about higher potential for ammonia-N volatilization loss from surface applied urea containing fertilizers. Applying any urea containing fertilizer to the soil surface during warm, dry, windy conditions will maximize the potential for N volatilization losses. This loss occurs quickly, starting within hours following application with most of the loss occurring within 2 days following application. If the N is going to be incorporated by tillage or rainfall, it is critical that tillage be done or rainfall occurs as soon as possible following application. To minimize losses this should happen within a day following application. One strategy is to delay application a few days if rain is in the forecast. However, an important consideration is, that under generally dry conditions with occasional small showers, a little bit of rain, a few tenths of an inch, can actually make the loss worse because it is just enough to dissolve the urea and activate the volatilization process, but not enough to soak the urea into the soil to minimize the loss. Management options under these conditions include immediate incorporation of the urea by tillage. In no-till or where immediate tillage is not possible, there are other options.
While most UAN applied at this time is sprayed with the herbicide program, if UAN is applied alone, dribbling the UAN rather than spraying can significantly reduce N volatilization. The other option is to use a urease inhibitor. Urease inhibitors slow the breakdown of urea allowing more time, usually 1-2 weeks depending on the product and the rate, for rain or tillage to incorporate the urea. Urease inhibitors can be used with both Urea and UAN to effectively reduce N volatilization losses. There are a number of N additives on the market that claim urease inhibitor activity. It is very important to choose a product that is a urease inhibitor, and is proven to reduce ammonia volatilization and apply it according to the product label. Be aware, that not all N "additives" are designed to reduce N volatilization. For example, some N additives are nitrification inhibitors, not urease inhibitors, these are good products, but will not do anything to reduce ammonia volatilization. Also, there are some products that make generic claims to "improve N efficiency and/or reduce N loss" but do not say specifically how they work or what losses they target. These may or may not be effective at reducing ammonia volatilization losses.
Efficient use of urea containing fertilizers to minimize N volatilization losses requires attention to details. Applications must be timed as much as possible to coordinate with tillage or rainfall. When ideal timing is not possible, alternative application methods such as dribbling UAN can help. Also, use of a proven urease inhibitor added to the fertilizer is another effective option under these conditions.