Presidedress Soil Nitrogen Test

The PSNT is a soil nitrate test that has been developed to help make an accurate nitrogen fertilizer recommendation for a corn crop that has had an application of manure, a cover crop that contributes nitrogen to the soil, or has had a previous application of nitrogen such as a row starter, weed and feed, or broadcast treatment. This test was originally developed by Dr. Fred Magdoff at the University of Vermont for field corn production. In recent years there has been research done for a number of other crops such as cotton, tobacco and sugar beets. Since its successful introduction, the PSNT has been calibrated for a small number of vegetable crops, and the work continues on other corps. Research has been done in several states that when 25 ppm nitrate-N is present in the top foot of soil, response to additional side dress N is not likely. However, if the nitrate-N level of the top foot of soil is less than 10 ppm, response to full recommended rates of N would be expected.

Nitrogen Soil Test Procedure

The following is the procedure for the PSNT as recommended by several universities.

1. Apply a minimum of N in the spring as a starter fertilizer or herbicide carrier.
2. Soil samples are to be taken when the corn is in the 4 to 6 leaf stage (6 to 12 inches tall) or at least a week before planned side dressing.
3. Fields are sampled by taking a composite soil sample of 20-25 soil cores at random throughout the field. The cores should be taken to a 12 inch depth. Avoid areas where row starter or banded fertilizers have been applied, as well as where manure may have been injected. A large number of cores are recommended because of the variability in nitrogen content of many types of manure, and non-uniformity in spreading. If manure or fertilizer has been banded, 10-15 sets of cores, each set consisting of three cores, one three inched to the right of the corn row, one in the middle of the rows and one three inches to the left of the next corn row, should be collected for the composite sample.
4. The cores should be mixed thoroughly and crumbled. Approximately 1 cup is needed as a sub sample for analysis. Heat the sub sample in an oven or microwave (in an oven at 212º F or in a microwave on a paper plate a full power for 3-8 minutes or until completely dry. NOTE: This is required to stop the bacteria from converting other N form to the NO₃-N. The heating will also destroy the sample as far as using it for other normal soil analysis.
5. Mark history and soil sample bag clearly that the sample is submitted for "NO₃-N analysis only." And send to Spectrum Analytic Inc for analysis.

Interpretation of Soil Nitrate And Ammonium Nitrogen Results

The majority of the University interpretive data considers only the NO₃-N. Their work has found little NH₄-N in the soil, and it has not been significantly correlated with N response. The "Normal" or "Background" level of NO₃-N is often considered to be about 5 to 10 ppm, and full N rates are recommended. Universities vary, but most do not make side dress N recommendations when the soil NO₃ -N is above 21 to 25 ppm in the top 12″ of soil. A limited amount of recent information suggests that there may be a reason to increase this "target for soil NO₃-N to about 30 ppm. University recommendations for N when the soil NO₃-N is intermediate tend to be proportional (when soil NO₃-N is at mid-point in the range, the N recommendation is about 50% of their full recommendation for the corn crop). Factors that inhibit nitrification could cause an individual sample to have low NO₃-N and high NH₄-N. In this situation, the grower may want to adjust the interpretation due to the higher NH₄-N level. If high levels of NH₄-N are found, there is no research calibration data available, on which to base the interpretation.

When the Pre-sidedress Nitrogen Test was introduced, Spectrum began evaluating the test with our normal long term fertility plots. We found good correlation with yields at the normal interpretations. However, at that time we were also evaluating nitrification inhibitors, and we found that where nitrification inhibitors were used, we sometimes had significant amounts of NH₄-N in the soil. In these situations, ignoring the NH₄-N resulted in unreasonably high recommendations for sidedress N. Our current program includes a factor that gives some credit for the soil NH₄-N when it is present in large amounts, but not when amounts are low.

Our program also includes increase N recommendations for N applications when the Pre-sidedress Nitrogen Test results are between 25 and 30 ppm and the request is for a high yield goal. This is to account for the limited data mentioned earlier, which suggested the need for a higher critical level for corn.

Nitrogen fertilizer recommendations based on the Pre-sidedress Nitrogen Test are valid only for corn at 10″ to 12″ tall and a 1 ft. deep soil sample. Extended periods of conditions causing N loss after testing, or significant amounts of undiscovered soil N below the 12″ sampling depth could change the yield response.

*Sample N Content calculated as follows:

NO₃-N: 1 ppm = 1 ppm

NH₄-N: If NH₄-N <16 ppm, then NH₄-N = 0

If NH₄-N >16 ppm then 1 ppm NH₄-N = 0.5 ppm NO₃-N

Add the measured soil NO₃-N to the calculation conversion of NH₄-N to NO₃-N, and then use the corresponding crop tables.