To obtain a reliable measurement of nitrate-N, collect at least a 12-core composite from representative portions of the field, and at different positions with respect to the crop-row, to a depth of 2 to 3 feet. Keep each foot of sampling depth as a separate sample. The 2-foot depth should be adequate where little nitrate movement is expected, while the 3-foot depth is most appropriate for quantifying nitrate-N in fields where rain might have moved the nutrient deeper in the profile.
In any given year, there is nitrate present in the soil at the end of the growing season. Research has shown that for a 2-foot sample, only the amount above 40 pounds of nitrate-N per acre should be subtracted from the application rate.
Due to the uncertainty on how much carryover nitrogen will be present for the 2013 crop, the best way to adjust nitrogen rates may be to collect soil samples in the spring to adjust pre-plant or sidedress rates. This approach implies that nitrogen applications are done in the spring.
As with fall sampling, levels can vary across a field. To obtain a reliable measurement, use a sampling strategy that represents the field. "In normal years, a 1-foot sample is recommended for spring samples, but with the present conditions, I would recommend that a 2-foot sample be obtained to quantify nitrate-N in the lower profile," Fernández advises.
Another approach that can be used to adjust nitrogen rates when the amount of carryover nitrogen is difficult to estimate is to choose the lower end of the profitable nitrogen-rate range in the Corn N Rate Calculator.
Finally, producers have asked Fernández about using the end-of-season lower corn stalk nitrate test to determine nitrogen rate adjustments for next year. "I do not recommend using this test. The values are likely to be artificially high because of the dry conditions that reduced plant growth and grain production," Fernández says.