Norder Blog

Nitrogen Soil Sampling


Proper management of nitrogen is one way Norder Supply assists growers on managing input costs, and maximizing net return per acre. Nitrate is just one of the chemical compounds of nitrogen found in the soil, and conversion from one form to another occurs rather easily throughout the year.  Soil testing is an excellent tool at providing an estimate of nitrogen availability, but soil samples taken at different times of the year can give different nitrate results, especially as the soil warms up in the spring and more nitrogen converts from organic forms and ammonium to nitrate.

To calculate your nitrogen fertilizer requirement, you can use the nitrate result from your soil test in an equation. 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 1.2nfactor) – Nitrate – Legume Credit – Manure Credit

Universities, soil labs, and agronomists may have equations that are more complex.  An equation is only a starting point in the process of determining how much N you will need to apply.  There are some potential pitfalls to only relying on an equation like this.  Soil samples collected in the fall after harvest will typically have nitrate levels less than samples collected in the same field during the spring. Fall samples may report 20 pounds of nitrate per acre while the spring soil sample could have 70 pounds per acre. If you only use this equation, you would need to apply 50 additional pounds of fertilizer according to the calculation when using the fall soil sample result.  Do you over apply nitrogen using the fall result or under applying using the spring result?  An adjustment factor is often used to alter the nitrogen recommendation depending on the time of the year the sample is collected.  Additional adjustments may be appropriate when switching between fall applied anhydrous and spring applied UAN or split application in when nitrogen utilization is more efficient.

Spring soil samples are best when using an equation to calculate nitrogen needs, but fall soil sampling provides better insights about the previous season.  With a fall sample, re-evaluate your yield goal and back calculate your actual yield to nitrogen applied ratio to fine tune next season’s fertilizer needs.  If yields exceeded expectations and soil test results comeback with 30 or even 40 pounds per acre of left over nitrogen, you may be applying close to the optimum rate.  If yields were better than expected and soil test results exceed 50 lbs. per acre, consider backing off the rate of nitrogen applied the next season.  If soil test nitrates comes back low, re-evaluate your nitrogen needs and consider increasing the nitrogen application the next season. Equations cannot accurately calculate the impacts of weather or plant availability of other nutrients may have on yield.  Weather has the greatest impact on nitrogen availability, which means having a back-up plan in place to apply additional nitrogen in season is recommended.

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.  Collect a pre sidedress nitrogen test (PSNT) soil sample 7 to 10 days before a sidedress application to determine if an additional nitrogen application is necessary, or to fine-tune a preplanned application. Corn plants will have already extracted nitrogen from the soil and accumulated it in the leaves by the time you take a PSNT sample.  This is why an accompanying tissue test may also be of value when taking a PSNT soil sample.

A soil test of 24” or 36” sample is always recommended. Norder Supply agronomists use the PSNT test and collect tissue samples before any additional fertilizer is applied.  Our goal is to maximize yields with the least amount of input dollars and avoiding excess fertilization.

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