By Jim Fawcett and Willy Klein, Iowa State University
The drought of 2012 has likely increased the carryover of nitrate-nitrogen (nitrate-N) into the 2013 season, according to field agronomists with Iowa State University Extension and Outreach. In addition to less nitrogen being used by last year’s crop, the reduced rainfall in 2012 resulted in less nitrate leaving the soil through leaching and de-nitrification (loss by gas into the atmosphere).
"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 indicate that we have fields this year that may have over 100 pounds per acre of nitrogen carried over from last year," said Jim Fawcett, ISU Extension field agronomist. "This provides farmers with the opportunity to cut their nitrogen rates this spring unless we have an unusually wet spring."
Sample Soils this Spring
Rather than just guessing how much nitrate-N has carried over from last year, Fawcett recommends that growers pull some soil samples this spring to estimate the carryover. Before any spring nitrogen is applied, take the following steps to estimate nitrate-N carryover:
1. Pull 1-foot soil samples to at least a 2-foot depth (0-1 foot and 1-2 foot) before the spring N is applied. A 3-foot depth is preferable. Pull 15-30 cores per sample on an area of no more than 10 to 20 acres. Mix thoroughly and send a subsample (standard soil sample size) to the lab to test for nitrate. Multiple samples per field should be collected.
2. Take the soil test result (ppm nitrate-N) times 4 to calculate pounds per acre of N.
3. Add up the N in each foot and subtract the "normal" carryover N (40 pounds per acre for 2 foot depth and 50 pounds per acre for 3 foot depth).
4. Subtract the carryover N from your usual N rate.
5. Regardless of lab results, apply no less than 50 pounds per acre if no N has been applied, to account for field variability.
"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," said 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. For more information contact your ISU Extension and Outreach field agronomist.