Applying Nitrogen Prior to Rainfall

Photo courtesy of John Deere. 

By Edwin Ritchey, Extension Associate Professor

Some people are concerned about making a sidedress nitrogen (N) application prior to the big rain event predicted for later this week. Are we going to lose the N we apply just before the rain? Well, as many extension answers start – it depends. It depends on what form of N is applied, how the N is applied, how much rain we receive, how intense the initial rainfall is, and how long the soil stays saturated. Let’s dive into the N cycle to get a better appreciation of the risks involved.

The major N loss mechanisms in Kentucky are volatilization of ammonia from a urea-based fertilizer, denitrification and/or leaching of nitrate N, and loss of surface N in moving water (runoff). Ammonium containing fertilizers, gas, solid and liquid, are the primary forms of N used for corn production (and sidedressing in particular) in KY. Urea will convert to ammonia (NH3) just prior to forming ammonium (NH4+). The NH3 is subject to volatilization loss. Rain will reduce the potential for volatilization loss by dissolving/moving applied fertilizer N into the soil.

Solid urea will need to dissolve and then infiltrate into the soil. Once in the soil, the N will largely be in the NH4+ form, which is not prone to leaching or being lost in runoff water. However, if the initial rain event is very intense the solid urea can be washed from the point of application. Urea-ammonium nitrate (UAN) is a liquid that contains urea, ammonium, and nitrate N. A surface application of UAN is much less likely to be lost in runoff than is solid urea, but is at a greater risk than injected N. Many producers inject UAN about 2 inches below the soil surface. This greatly reduces runoff potential and essentially ensures against volatilization loss.

Denitrification is the final loss mechanism that needs to be considered. Denitrification occurs when the soil is depleted of oxygen and organisms normally using oxygen as they grow begin utilizing the N contained in nitrate (NO3-). Typically, 3 to 4 days of soil saturation are needed before any appreciable amount of denitrification occurs, and then only the NO3-N is subject to this loss mechanism. Denitrification is enhanced with warmer temperatures, a pH > 7, readily available carbon (food for hungry microbes), and can account for a substantial amount of N loss with prolonged soil saturation.

To wait or not to wait to sidedress N comes down to a risk management decision. There is little potential for N loss if the rain starts slow and doesn’t wash recently surface applied fertilizer off the application site. For less than well-drained soils that can stay saturated over 4 days, denitrification potential increases, but only recently applied NO3-N (the nitrate in UAN) is subject to this loss. All these factors should be considered, along with how big the corn will be if you delay your sidedress N application until after the rain passes.