By Mark Johnson

If you are making a sidedress nitrogen application or a later in-season application of N for corn, the key question is: How much N should you apply per acre? The answer depends on how much N you may have already applied either last fall or this spring prior to planting, and how much of that N has been lost from the soil since it was applied.

Key Points

• Late-spring nitrate test can help you judge if more N is needed.

• Test at least five areas that are representative of 100 acres.

• Chlorophyll meter and canopy sensing can also test for nitrogen.

The article on Page 18 explains how to figure N application rates using the Corn N Rate Calculator. Here are two more ways you can go about determining the adequacy of the N supply for your corn crop.

• Late-spring nitrate test. Soil samples collected for the late-spring soil nitrate test (LSNT) must be representative of the surface foot of soil. To learn about this test, search for ISU publication PM 1714 at store.extension.iastate.edu.

“Nitrogen Fertilizer Recommendations for Corn in Iowa” explains how to take cores from the field to make up a sample. Follow the sampling protocol. That and proper handling of the samples are critical to success. Start with sampling about five areas that are representative of the first 100 acres.

Due to variability with soil organic matter, texture, moisture, temperature, etc., it’s hard to use the LSNT results for more than determining if the field would benefit from another N application, or if it has adequate N. That’s because the LSNT is best at indicating when adequate to excess N is available, which is when the reading is at or above the 25-ppm nitrate-N critical level when no additional N is needed.

The LSNT is less reliable when the soil test results are below that critical level of 25 ppm. When the soil nitrate reading is below 25 ppm, the N fertilizer application rate determination should be taken as a guideline, not an exact determination. In wet springs, the critical level can be reduced to 20 ppm. Also, see the publication for adjustments that need to be made when the LSNT is used with manured fields and with corn following alfalfa.

• Chlorophyll meter and canopy sensing. These tools can provide additional feedback on the nitrogen status of corn plants. For both types of sensors, a key is to know that any potential deficiency is actual N. That is, keep in mind the results need to be calibrated for each field and field area (such as corn hybrid, plant stand, drought, compaction, other nutrient deficiencies, etc.).

This can be done using check strips or field areas known to have adequate N at the time of sensing.

This means you need to have check strips or field reference areas for each hybrid that have enough N available to the corn plants at the time when the sensor is being used on the plants. So some planning is needed for best use of sensing results. With the reference areas, you can then compare the field readings to readings in the calibration strip. Another good reference is ISU publication PM 2026, “Sensing Nitrogen Stress in Corn,” at store.extension.iastate.edu/Product/Sensing-Nitrogen-Stress-in-Corn.

Johnson is the ISU Extension field agronomist in central Iowa.

This article published in the June, 2016 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2016.

Nutrient Management