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Nitrogen Loss for 2002

May 3, 2002
The warmer-than-normal winter, coupled with a wet spring, has many wondering whether nitrogen loss is or will be greater than normal for the 2002 crop year. Fortunately, research conducted at the University of Illinois over a number of years has provided a data base on which to make an informed decision about the amount of N loss that has occurred or that might occur in the next few weeks.

Nitrogen loss associated with excessively wet soils will occur only from that portion of the fertilizer that is in the nitrate form when soils become saturated. Because most fertilizers are applied as ammonium or a form that quickly converts to ammonium, you must first determine how much of the applied nitrogen has been converted to nitrate. This rate of conversion of ammonium to nitrate--a process called nitrification--is primarily dependent on soil temperature. Nitrification does not occur when soils are frozen, but it does occur at temperatures above freezing and is faster the warmer soils are. Equations that define the relationship between soil temperature and nitrification have been developed for two Illinois soils, a Drummer silty clay loam and a Cisne silt loam. These equations, using daily soil temperature data provided by the Illinois State Water Survey for the Drummer at DeKalb and Bondville and the Cisne at Brownstown were used to estimate the amount of fall-applied nitrogen that had been converted to nitrate during the winter and spring of 2001-2002 (Table 6).

As expected, there were significant differences in the rate of nitrification, dependent on location, with higher values the farther south in the state. Addition of a nitrification inhibitor, N-Serve, substantially reduced the rate of conversion of ammonium to nitrate.

The conversion of ammonium to nitrate does not mean that it has been lost from the soil system but rather that it is susceptible to loss in fields that have been or may become saturated with water for several days. When soils are excessively wet, nitrogen will be lost through the process of denitrification or leaching. As of April 1, the amount of nitrate-nitrogen lost from tile lines was less than 6% of the equivalent of the total fertilizer nitrogen applied without a nitrification inhibitor in a central Illinois experiment.

Denitrification is the major nitrogen loss mechanism in most Illinois soils, particularly in medium to heavy textured soils. Illinois research has shown that 4 to 5% of the amount of nitrate-nitrogen present (note that this is not 4 to 5% of the total nitrogen applied) will be lost via denitrification for each day that soils are saturated when soil temperature is above 65 to 70 deg F. At temperatures less than 55 deg F, it is estimated that denitrification will be closer to 1 to 2% of the nitrogen that is in the nitrate form. Prior to April 25, soil temperatures were less than 55 deg F all but 8 days in central and northern Illinois and 15 days in southern Illinois.

Assuming 7 days of saturated soils in late April and 160 pounds of nitrogen applied without a nitrification inhibitor on November 1, 2001, the loss potential would be 160 lb N/acre x 59% [.59] (nitrification rate) x 19% [.19] (7 days saturated at 2% denitrification per day + 5% leaching) = 18 lb N/acre loss for a central Illinois location. If a nitrification inhibitor had been used, the loss potential would be reduced to 160 x .23 x .19 = 7 lb N/acre. For a southern Illinois location, the comparable situation would be 160 x 100% x .19 = 30 lb N/acre without an inhibitor, or 20 lb N/acre with an inhibitor.

The bottom line of this analysis is that most producers need not worry about N loss up to this time. They can save that worry for later in the season if soils become saturated.

This information will be updated as the season progresses. Additional nitrogen is not being recommended at this time to compensate for loss. Producers should wait until late May when additional information will be available to better predict the need for additional nitrogen.--Robert Hoeft

Author: Robert Hoeft

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