How Do We Estimate N Carryover for 2013?

How Do We Estimate N Carryover for 2013?

The drought of 2012 has likely increased the carryover of nitrate-nitrogen (nitrate-N) into the 2013 season. In addition to less N being used by last year’s crop, the reduced rainfall in 2012 resulted in less nitrate leaving the soil through leaching and de-nitrification. Although it is common for about 50 pounds per acre of nitrate-N to carryover from one season to the next, soil samples pulled in the fall of 2012 indicate we will likely have fields with more than 100 pounds per acre of carryover nitrate-N into the spring of 2013. We can take advantage of this carryover N by pulling soil samples this spring to estimate carryover N and reduce spring N rates accordingly for the 2013 corn crop. Failure to do so may result in increased nitrate losses into water systems in 2013 and future years.

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2012 Drought May Mean Less Nitrogen Needed for Corn in 2013

            One thing that we learned from the 1988 drought was that it resulted in less nitrogen needed for many corn fields in1989, according to Jim Fawcett, ISU Extension Field Agronomist. It is common for about 40 to 50 pounds of nitrogen per acre to carry over from one season to the next, but soil samples taken in the fall of 2012 indicates that we have fields this year where there may be over 100 pounds per acre of nitrogen that carryover from last year, Fawcett reports. Not only was less nitrogen used by the 2012 crop because of lower yields, but the low rainfall resulted in less nitrate-nitrogen (nitrate-N) being lost from fields by leaching or de-nitrification (loss by gas into the atmosphere). This provides farmers with the opportunity to cut their nitrogen rates this spring unless we have an unusually wet spring.

            Rather than just guessing how much nitrate-N has carried over from last year, it is best to pull some soil samples this spring to estimate the carryover. It is recommended that 1-foot soil samples be taken to at least a 2-foot depth. This should be done before any spring nitrogen is applied. Because nitrate-N can be quite variable across a field, a minimum of 15 cores should be taken for each sample and multiple samples taken in a field. The soil samples should then be sent to the lab for nitrate-N analysis. The lab will report the results in parts per million (ppm) of nitrate. Multiply the ppm nitrate by 4 to convert to pounds per acre of nitrogen. If greater than 40-50 pounds per acre of nitrogen is present, the nitrogen rate can likely be reduced.

            If farmers are not able to do the soil sampling, I would recommend that they at least cut back their nitrogen rates to be on the low end of the range of recommended rates, says Fawcett. If we fail to account for this carryover nitrogen and put on a full nitrogen rate this spring, it may result in increased nitrate losses in 2013 and future years. One tool available to calculate nitrogen rates for corn is the corn nitrogen rate calculator found at For more information contact your county extension office or see the article by John Sawyer and Antonio Mallarino at


Jim Fawcett, Ph.D.

Iowa State University Extension

Field Agronomist


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