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Nitrogen Loss Update

June 22, 2001
Most Illinois farmers demonstrated their concern about the environment and about the economics of crop production by delaying the application of anhydrous ammonia in the fall of 2000 until the soil temperatures were low enough to minimize the potential for nitrogen loss. Unfortunately, rains arrived at about the same time as the cool temperatures, prohibiting many farmers from getting their N on last fall. This created serious concerns about whether or not the fertilizer supply needed for the 2001 crop could be delivered on time to meet the needs. As it turned out, getting an adequate supply in a timely manner was not a problem, but the price was markedly higher. The high price was the result of the weather being colder than normal in December, which raised the price of natural gas; since ammonia production requires considerable amounts of natural gas, the price of ammonia was driven up as well.

Those farmers who had applied N earlier before the temperatures were cool enough were pretty smug about making a good decisionsmug until the excessive late spring rains arrived and caused significant N loss through the processes of denitrification (N lost back to the atmosphere) and leaching (some N lost to the water supplies of the state). The exact amount of N lost is hard to predict, but it is estimated that those who applied N in early to mid-October probably had at least 90% of the N converted to nitrate by mid-May, if they didn't use a nitrification inhibitor, and 50% converted if they used an inhibitor. On the other hand, those who waited until November to apply the ammonia would have had less than 60 and 40% converted to nitrate if they did not and did use an inhibitor, respectively. This doesn't mean that they lost all the N that was converted, but if they had fields saturated with water for more than 5 days in late May, they would probably have lost 25 to 40% of the nitrogen that had been converted.

Waiting until spring to apply N doesn't mean that it was safe from loss in all cases. Those who were able to apply in late March or early April and did not use a nitrification inhibitor may have experienced significant loss as well. If it was applied before April 10 without an inhibitor, as much as 50% of the N may have been converted to nitrate by mid-May. If they used an inhibitor with the early spring application, the conversion to nitrate by mid-May would have been in the range of 25 to 30%. Most (80 to 90%) of the ammonia that was applied after mid-April, with or without an inhibitor, would have still been in the ammonium form by the time the excess rains occurred in late May.

Farmers that have observed N deficiency in their fields in 2001should consider applying additional N. Prior research has shown that yield response to additional N will occur when N is applied up to tasseling or a little after, providing that rain is received to move the N down into the soil. On most fields, an additional 60 pounds N per acre will be adequate to optimize yield. However, one should calculate the potential amount of N lost by taking the amount of N applied times the estimated percentage of N nitrified (given in the preceding paragraphs) and then multiply that number times 0.05 for each day the field has been saturated. That will give the estimated pounds of N lost. If the number is less than 40, no additional N is suggested. If the number is between 40 and 80, apply an additional 60 pounds of N, and if the number is over 80, apply an additional 90 pounds of N. As an example, assume that 190 pounds of N (amount recommended for that field) was applied on October 10, 2000, without a nitrification inhibitor and that the field has been saturated with water for 8 days. The calculation to determine if additional N is needed would be: 190 x 0.9 = 171 pounds converted to nitrate; soil was saturated for 8 days171 x 0.05 x 8 = 70 (68) pounds lost. Consider applying an additional 60 pounds N. Be sure to dribble the N between the rows, keeping it off the vegetation. If the same field had received the same treatment with a nitrification inhibitor 1 month later, the calculation would be: 190 x 0.40 = 76; 76 x 0.05 x 8 = 30. In this case, no additional N would be needed.--Robert G. Hoeft

Author: Robert Hoeft

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