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. 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 denitrification (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," says Jim Fawcett, ISU Extension field agronomist in eastern Iowa. "This provides farmers with the opportunity to cut their nitrogen application rates this spring unless we have an unusually wet spring."
Sample soils this spring and have them tested for nitrate content
Rather than just guessing how much nitrate-N has carried over from last year, Fawcett recommends that growers pull some soil samples this spring and get them tested to estimate the amount of carryover. Before any spring nitrogen fertilizer is applied, he advises that you take the following steps to estimate nitrate-N carryover:
1) Pull 1-foot soil samples to at least a 2-foot depth (0 to1 foot and 1 to 2 foot) before the spring nitrogen fertilizer or manure is applied. A 3-foot depth is preferable. Pull 15 to 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.~~~PAGE_BREAK_HERE~~~
2) Take the soil test result (ppm nitrate-N) and multiply it times 4 to calculate pounds per acre of N.
3) Add up the N in each foot and subtract the "normal" carryover of 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 application 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 and testing this spring, 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."
Best economic nitrogen application rates for corn production: The corn nitrogen rate calculator is an exceptional research-based tool to help decide on N application rates for corn production. The calculator allows you to input your N fertilizer cost and your predicted corn value to calculate a maximum return to N rate. The calculator is very easy to use and is located here.
For more information, contact your ISU Extension field agronomist located in your area of the state. Field agronomists are listed at this link.
ISU Extension resources for crop-related issues during a drought can be found on the Dealing with Drought 2013 website. Current ISU Extension crop news is available from ISU's Integrated Crop Management News, an online newsletter.