Get a Handle on Drought's Impact on Nutrient Removal

Whether growers use an estimate or take numerous measurements, crop fertility should be adjusted for drought-affected fields.

Published on: Aug 27, 2012

Producers are asking Fabian Fernández, a University of Illinois assistant professor of crop sciences, about nitrogen rate adjustments for the 2013 corn crop.

In areas where corn yields were somewhat reduced but overall production practices were normal, he advises no changes to regular practices should be needed for next year's crop. The Corn N Rate Calculator (extension.agron.iastate.edu/soilfertility/nrate.aspx) can be used to calculate the maximum return to nitrogen or profitable nitrogen rate range.

Whether growers use an estimate or take numerous measurements, crop fertility should be adjusted for drought-affected fields.
Whether growers use an estimate or take numerous measurements, crop fertility should be adjusted for drought-affected fields.

For corn following soybean, even for fields with little or no yield, no changes to nitrogen-management decisions should be needed because soybeans do not leave excess nitrogen in the soil.

As a general rule, corn following soybean needs less nitrogen because the quantity and quality of the soybean residue reduces the amount of immobilization and increases the amount of nitrogen mineralization from crop residue and the soil. The Corn N Rate Calculator adjusts for this, showing that, compared to corn following corn, nitrogen rate for corn following soybean for the north, central, and south regions of Illinois can be reduced by approximately 50, 30, and 20 pounds of nitrogen per acre, respectively.

Decision making is more complicated for fields in which the drought severely affected the corn crop and the amount of nitrogen taken up was reduced. Large amounts of unused nitrate-N are probably left in these fields. The problem is to determine how much of that nitrate is likely to be available next year.

While there are no sure-fire methods, it is possible to estimate this amount. The indirect method is by determining how much nitrogen was removed in grain; the direct method is to measure soil nitrate-N (NO3-N) levels.

On average, a bushel of corn removes 0.66 pounds of nitrogen. To estimate the amount of remaining nitrogen, this removal rate is multiplied by the number of bushels produced per acre and subtracted from the amount of nitrogen applied per acre.

Not all the nitrogen will be plant-available in the next growing season. When nitrogen-loss potential is low (normal precipitation or drier conditions between harvest and next spring), 50% of the remaining nitrogen should be available for the 2013 crop.

For example, if a field received 180 pounds of nitrogen per acre and yielded only 70 bushels per acre, the remaining nitrogen is 180 minus 46 (multiply the 70 bushels times 0.66 to estimate the amount of nitrogen in the grain), which equals 134 pounds of nitrogen per acre. When the difference is multiplied by 0.5 (50% availability), the result is 67 pounds of nitrogen per acre to be subtracted from next year's nitrogen rate.

Going direct

"The direct measurement using soil nitrate-N levels is, in my opinion, more reliable than the indirect measurement," Fernández says. "However, there is still a lot of uncertainty in the estimation because nitrogen availability for the 2013 crop will be largely a function of how much nitrogen-loss potential exists between fall and spring."