I recently had my garden soil tested. If I have 42 ppm of nitrate-nitrogen in the soil where I plan to plant corn, do I still need to use a starter basic fertilizer?

The pre-sidedress soil nitrate test (PSNT) is a soil nitrogen test developed by Dr. Fred Magdoff from the University of Vermont to help make an accurate nitrogen (N) fertilizer recommendation for a corn crop.

The test assumes the soil has had an application of manure, a cover crop that contributes to the nitrogen in the soil, or has had a previous application of nitrogen. The PSNT test displays the needed concentration of nitrogen in the nitrate-nitrogen (NO3-N) form.

Research done in many states has shown that when 25 ppm of NO3-N is present in the top foot of soil, additional N applications are not necessary. However, if the NO3-N level in the top foot of soil is less than 10 ppm, the full recommended rate of N would most likely be necessary.

In other words, for a corn crop to have all the N it needs to grow properly, the soil needs to have a concentration of 25 ppm of NO3-N or greater.

Soils with a concentration of NO3-N below 10 ppm would need to be fertilized at the full recommended rate, whereas those with a concentration of NO3-N between 10-25 ppm may need supplementary fertilization (that is, less than full strength).

From your soil sample, we know the concentration of NO3 (nitrate) in the soil is 42 ppm. However, to use the PSNT with your soil test results, we need to convert your NO3 value into a NO3-N value.

To convert NO3 to NO3-N, we multiply the NO3 value by 0.226. So, in your case, the NO3-N value for your soil is 9.49 ppm (42 ppm x 0.226 = 9.492 ppm).

As you can see, your NO3-N value is less than 10 ppm. This means it would be a good idea to use a fertilizer at its full recommended rate to help raise the concentration of NO3-N in your soil.

Keep in mind, it is possible for the concentration of NO3-N to build up over time with the addition of organic matter (be it manure or cover crop) or fertilizers. Therefore, it is a good idea to test the soil for NO3-N concentrations each year prior to planting as your soil’s NO3-N concentration could raise above 10 ppm over the course of the year.