A common method to determine nitrogen fertilizer needs is to use the nitrate result from the soil test in an equation to calculate your nitrogen requirement for corn. Most equations will require an expected yield goal, a nitrogen factor, and then subtraction of nitrogen credits, which would include your soil test nitrate nitrogen. An example of this would be as follows:
N Requirement = (Yield Goal x 1.2 n factor) – Nitrate – Legume Credit – Manure Credit
This equation is a useful starting point in the process of determining how much N you will need for a corn crop. There are some potential pitfalls to this equation. Soil samples collected in the fall have nitrate levels that are often substantially less than samples collected in the spring just before planting. The fall sample may report 20 pounds of nitrate per acre while the spring soil sample could have 70 pounds per acre. If you use the above equation, you would need to apply 50 additional pounds of fertilizer according to the calculation if using the fall sample result.
Spring soil samples are more suited for using an equation to calculate nitrogen needs, but fall soil sampling will provide good insights about the previous season. A fall collected nitrate result between 20 and 50 lbs. per acre is acceptable. If you met or exceeded yield expectations and soil test results are in this range, you may want to apply approximately the same amount of nitrogen as the year prior. If yields were better than expected and soil test results exceed 50 lbs. per acre, consider backing off the rate of nitrogen applied for the next season. If soil test nitrates come back low, re-evaluate your nitrogen needs and possibly increase the nitrogen application the next season.
Weather and availability of other plant nutrients will have an effect on final yields, which equations cannot accurately calculate. Have a back-up plan in place to apply additional nitrogen in season with a side dress application or through pivot fertigation. Corn accumulates most of its nitrogen between the V8 and VT growth stages. We want to limit nitrogen deficiency at this time to maximize yield. Norder Supply agronomists recommend a Pre Side dress Nitrogen Test (PSNT) soil sample before making an in-season nitrogen application. Conversion of organic and ammoniacal nitrogen compounds to nitrate is continuous throughout the growing season, and plants will have already accumulated a large sum of nitrogen by the time a PSNT test is collected. Do not expect a significantly high nitrate result with a PSNT test. Your Norder Supply agronomist will often recommend an accompanying tissue test when utilizing a PSNT soil sample to help dial in additional nitrogen needs.
Norder Supply has two exclusive nitrogen management programs to assist our customers in achieving maximum net return per acre. With Spot On Recs, your Norder Supply agronomist will develop a field by field nitrogen strategy and monitor crop needs throughout the season. With Spot On Scripts, Norder Supply agronomists use variable rate prescriptions for a more robust site specific management of nitrogen.