Soil fertility management issues following drought
As a conservative approach, and due to uncertainty with either estimation method, a minimum rate recommendation of 50 lb N/acre should be considered. If fall/spring precipitation is well above normal, then the carryover nitrate would not be likely, especially in soils with high leaching potential. Sandy soils are not likely to retain carryover nitrate.
Spring soil profile sampling for nitrate-N is an option, especially with concerns about residual nitrate remaining after the fall/winter. In addition, such sampling could allow for a spring preplant or sidedress N application based on spring profile nitrate-N results, and instead of a fall application. Use of the late spring soil nitrate test (LSNT) to determine carryover nitrate may miss considerable nitrate deeper than in just the top foot. Therefore, it would be better to sample the deeper profile before planting.
There could be considerable variation in nitrate levels across fields, due to yield level, banded N application, and soil/topography. Therefore, many cores (at least 12) should be collected per sample, and multiple samples per field from representative areas. Since the cores are by one-foot depths, mixing in the field will be needed to obtain a representative sample for each depth. Keep the soil from each depth as a separate sample to send to the lab.
Stalk nitrate testing
The end-of-season lower corn stalk nitrate test can be useful for determination of excess plant available N from the soil (i.e. concentrations above 2,000 ppm nitrate-N). However, that interpretation is for normal weather and production conditions. In 2012, test results could be abnormally high due to the dry conditions and severe impact on plant growth and grain production. Therefore, it is suggested to not use the stalk nitrate test this year, or to use it as a measure of potential nitrate carryover.
Timing of fall N application
With the potential for early fall harvest this year, carefully consider the risks of early N fertilizer or manure application. With typical warm soils in the late summer and early fall, conversion of fertilizer and manure ammonium to nitrate will be rapid. This places the applied N at risk for loss if wet conditions develop. For many years now the ag industry in Iowa has followed the “wait until 50 ○F and cooling” before anhydrous ammonia application. That would also be a good practice for manure with high ammonium-N content.
Corn yield response to N rate and needed fertilization rate decreases in years with below normal rainfall. This effect can persist across periods (years) of dry conditions, and even for year(s) after rainfall returns to normal (but not excessively wet). If below normal rainfall conditions continue, then consider using the low end of the Corn N Rate Calculator profitable range for corn N rate recommendations.
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