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The PSNT is a soil test for nitrate-nitrogen (NO3-N) developed for use at the 4 to 6 leaf stage of corn to help in making more accurate N fertilizer recommendations at sidedressing time. The test, designed by Dr. Fred Magdoff at the University of Vermont, was initially developed to help in estimating the amount of N available where manure or other organic wastes have been applied, or corn is grown in rotation with a forage legume. The PSNT was initially developed to identify fields that would not be expected to respond to additional N. The test uses the NO3-N content of the top foot of soil as an estimate of the amount of N available to the crop. Research in a number of states has confirmed that this test can be useful in managing N on corn. Work in Massachusetts, California and Florida has also shown the PSNT to be useful for some vegetable crops. By far, the major use of PSNT is in the production of field corn, which is the focus of this paper.
As the PSNT has been adopted by agriculture, there has been a great demand to make calibrated N recommendations based on the test results. In other words, agronomists have been asked to make N recommendations that are adjusted for, or inversely proportional to the amount of residual NO3-N found in the soil. Given that the original purpose of the PSNT was simply to identify soils that were non-responsive to N, we may be asking a lot of this test by using it to accurately calibrate soil N status levels or N recommendations. However, some progress in this area has occurred. There is agreement on the general range of soil NO3-N that is considered critical for adequate corn growth without additional N applications. This range is from 21 to 30 ppm NO3-N in the top 12 inches of the soil when the corn is in the 4 to 6 leaf stage. Many Universities consider the critical NO3-N value to be 25 ppm.
There is less agreement on how much supplemental N to recommend when the PSNT result is between zero and the critical level. In other words, how do we calibrate N recommendations to PSNT results? Most of us understand that agronomists often disagree on fertilizer recommendations, and the PSNT is no exception. There is some research that supports higher N recommendations when the PSNT result is low and lower recommendations when the result is closer to the critical level. However, many factors of weather, soil conditions, corn hybrid, crop management, and others may change crop response to a particular PSNT level. It is not within the scope of this paper to thoroughly discuss all of the factors that can influence the reliability of N recommendations based on PSNT results. However, like all soil testing, the PSNT should be used as a guide, not a guarantee. Neither the PSNT result nor the N recommendation should be over-simplified to a simple recipe. The PSNT is simply one part of the complicated process of crop management.
Another occasional point of confusion occurs when we try to reconcile the ppm of NO3-N with the amount of N required by a corn crop, or the amount of N applied prior to taking the PSNT sample. The PSNT result is an “index” of available N, not a calculation factor. You should not try to perform calculations to reconcile the amount of N applied or crop uptake with PSNT results, because they will not likely “add up”. We must keep in mind that a crop typically utilizes N from below the 12 inch sampling depth; it will receive N from mineralizing soil organic matter; it may lose access to N due to denitrification or leaching later in the season; or other factors may change the N available to the crop, either before or after the PSNT is taken.
It is common to hear people discuss fertilizer recommendations in terms of crop removal, plus or minus soil test buildup as if they were somehow disconnected from each other. In fact, they are two aspects of the same subject. Most of us fall into the habit of thinking that we are fertilizing plants. Except for foliar fertilizer or tree trunk injection, we do not fertilize plants… we fertilize soil. Because of this, soil chemistry will determine how much of the applied nutrients the plants will be able to take up. If a soil is low in phosphorus (P) or potassium (K), it will tie-up or “fix” much of the applied fertilizer P and K (P2O5 and K2O) into forms that are not available to the plants. This nutrient fixation is simply another way of saying that the soil is trying to build itself up in these nutrients… whether that is your intention or not. The soil is a reservoir for the nutrients that have been applied or generated by other means over the years. The nutrients that a plant gets in any one season are likely to be ones that have been in the soil for many years. Therefore you can think of fertilization as putting nutrients into one side of a reservoir while the plants are taking them out of another end. What happens inside of this nutrient reservoir is soil chemistry and microbiology. These processes, along with weather, determine how much access the plants have to the nutrients within the reservoir.