No. 25 Article 5/December 5, 2008

Fall and Winter Applications of Urea for Corn

This fall a combination of factors, including late harvest and high fertilizer prices, resulted in reduced fall applications of anhydrous ammonia for the 2009 corn crop. Recently, in at least some locations, prices for urea (CO[NH2)]) have been lower than for anhydrous ammonia, and some producers have wondered if it is advantageous to apply urea this fall. The answer is no.

Urea is 46% N, all in the urea form. As such, it is very soluble and moves freely up and down with soil water. After application in the soil, NH2 changes to ammonia (NH3) either chemically or by the enzyme urease, and then to ammonium (NH4+). Because urea transforms quickly to NH3 and then to NH4+, some people might be tempted to assume it is similar to anhydrous ammonia, since both products undergo the same transformation in the soil. While it is true that both sources transform to NH4+, urea can be quickly nitrified--transformed to nitrate (NO-3)--even if applied late in the fall. Once N is in the NO-3 form, it is susceptible to denitrification or leaching the following spring. When anhydrous ammonia is applied in the soil, there is a large change in soil pH surrounding the fertilizer band. This change in pH inhibits nitrification by microorganisms and causes anhydrous ammonia to have a lower susceptibility to nitrification and potential loss compared to urea.

When the conversion of urea to NH4+ occurs on the soil surface or on the surface of crop residue or leaves, some of the resulting ammonia will be lost as a gas to the atmosphere. The risk of N loss through volatilization from winter application of urea for corn on frozen soils is too high to consider the practice unless urea is incorporated with a half inch of precipitation within 4 to 5 days after application. As much as 40 bu/acre yield loss has been observed when urea is surface-applied in frozen soils during the winter.

For the reasons described, urea application in fall or winter has not been as effective as fall-applied anhydrous ammonia for corn. While it is possible to see little difference between fall application of anhydrous ammonia and urea in some years, this seldom occurs. Over many years research and observations have shown that the risk of N loss from urea application in the fall or winter is simply too large to consider the practice, even if the price of urea is lower than that of anhydrous ammonia.--Fabián Fernández

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