Corn prices are higher. Is this the year to add more nitrogen through a late-season application? The typical approach is to ask questions about how much N was applied, weather patterns, yield potential of the field and stand counts.
Some growers are trying a more quantitative approach, which involves pulling soil samples. This test works in fields where manure wasn’t applied. For manured fields, use the Pre-sidedress Soil Nitrate Test. However, the PSNT won’t be accurate on fields where manure wasn’t applied.
Collect 12-inch-deep soil samples seven to 10 days before the planned application. Target the collection by soil type and/or land position. If the nitrogen was banded, collect multiple cores perpendicular to the band to get an average of the band width. Collect 15 to 20 cores to represent 20 to 25 acres. Keep samples cool and ship to the lab using next-day shipping.
Request both ammonium and nitrate analysis to get total plant-available nitrogen. When the lab directly reports NO3-N and NH4-N in parts per million or milligrams per kilogram, these values can be added together and multiplied by four to estimate nitrogen available to the crop.
Estimate future crop needs
Once you know what you have, how much nitrogen does corn need to achieve your yield goal? Realistic total N requirements range from 0.8 pound of N per bushel to 1.4 pounds.
Narrow the range based on management practices. Taking multiple actions to reduce nitrogen loss on soils that efficiently retain nitrogen favors the low end. Less efficient practices like applying all nitrogen preplant favor the high end.
Previous experience can refine this estimate. Multiplying this nitrogen-use factor by a realistic estimated yield goal based on the current growing season will result in a season total nitrogen requirement.
Now you have a solid estimate of how much corn needs and how much the soil currently has, but it’s well into the growing season. Check this nitrogen uptake chart. Determine what percent of nitrogen was taken up at various growth stages. This identifies what percent of nitrogen is yet to be taken up.
For example, suppose you applied 180 pounds of N per acre. Today, a realistic yield target is 225 bushels per acre. Based on nitrogen application timing and source and soil characteristics, you estimate you need 1 pound of nitrogen per bushel. By simple math, we need a season total of 225 pounds of nitrogen.
As you’re deciding whether to add more N, corn is at V10. Based on research at the University of Illinois, only a little over 25% of the total nitrogen was taken up so far. The crop still needs 75% of its total nitrogen, or about 170 pounds.
Since only 180 pounds of nitrogen was applied, more may be needed. In this example, the soil test result showed a combined nitrate and ammonium result of 34 ppm. So, 34 times 4 equals 136 pounds of N per acre.
There’s an estimated 136 pounds of nitrogen available, and an estimated need of 170 pounds of N to finish the season.
Suppose your ag retailer suggested applying another 40 pounds. In this example, the extra 40 pounds of N looks to be on target.
Bultemeier is an agronomist and Indiana certified crop adviser with A&L Great Lakes Lab, Fort Wayne, Ind.