Purdue News

March 2, 2001 Use it and lose it? Nitrogen fertilizers vary in soil activity WEST LAFAYETTE, Ind. – Corn plants aren't picky when it comes to nitrogen – they'll take it from whatever source is available. Farmers need to be a bit more discriminating, however, before they choose and apply fertilizer to their corn crop, say Purdue University agronomists. In a market beset by high prices and limited supplies, farmers may be unable to afford or find adequate amounts of anhydrous ammonia, the most popular nitrogen fertilizer. While other nitrogen fertilizers work well with corn, each behaves differently in soil, says Bob Nielsen, Cooperative Extension Service corn specialist. "The agronomic differences among N (nitrogen) fertilizer sources lie mainly in their relative risk for nitrogen losses due to leaching, denitrification and volatilization," Nielsen says. Among the alternative fertilizers is urea, a 46 percent nitrogen solution; urea-ammonium nitrate, or UAN liquids, 28 percent to 32 percent nitrogen; ammonium nitrate, 34 percent nitrogen; and ammonium sulfate, 21 percent nitrogen. None are as nitrogen-rich as anhydrous ammonia, which is 82 percent nitrogen. Nielsen says UAN liquids and ammonium nitrate are susceptible to leaching and denitrification losses the day they are applied to the field. Urea-based fertilizers convert quickly to nitrate forms of nitrogen and are vulnerable to similar nitrogen losses. "Consequently, these forms of fertilizer N are not well-suited to early preplant applications, but rather to later preplant or sidedress applications," Nielsen says. Another characteristic of urea-based fertilizer is its tendency to vaporize when applied to the surface and not be absorbed into the soil, especially in high-residue tillage systems, Nielsen says. "Part of the conversion of urea to nitrate involves the formation of ammonia, which is very volatile," he says. "When this conversion occurs on the surface, quite a bit of the nitrogen may 'disappear into thin air' and not be available to the developing crop." Surface-applied nitrogen also is used by soil microbes to decompose plant residues, delaying its nutritional benefit to the corn plant until later in the crop season, Nielsen says. "For these two reasons, urea-based fertilizers should be injected below the surface trash, or at least applied in concentrated bands over the surface, as opposed to broadcast surface applications in high-residue tillage systems," he says. Applying urea to soils in the winter months, when temperatures are cold and volatilization is minimal, may do little good, says Sylvie Brouder, Extension soil fertility specialist. "If you apply urea now it may very well rain before it gets warm, but incorporation into the soil will not occur if that rainfall is onto frozen or snow-covered ground," Brouder says. "Surface runoff will carry away your product. "Also, isolated warm weeks in March are not uncommon and, in this situation, volatilization loss of unincorporated fertilizer will be pronounced." Urease inhibitors – products that slow the breakdown of urea in soil – are helpful if used properly, Brouder says. "Between the time of application and incorporation – by rainfall or with tillage – they can minimize N volatilization loss of surface urea applications," she says. "But you only get a benefit from this product if volatilization loss is the problem. If there is no incorporation delay, then there is no benefit." For additional information about nitrogen fertilizers, read Purdue publication AGRY-01-01, "Nitrogen Decision$ 2001: The Soil Fertility Perspective." The publication can be downloaded from the web. Sources: Bob Nielsen, (765) 494-4802; rnielsen@purdue.edu Sylvie Brouder, (765) 496-1489; sbrouder@purdue.edu Writer: Steve Leer, (765) 494-8415; sleer@aes.purdue.edu Related Web site: The Corn Growers' Guidebook To the Purdue News and Photos Page