The availability of new technology for rapidly analyzing soil samples for nitrate in recent years has led to renewed interest in using the pre-sidedress nitrate test — PSNT — as a method for prescribing variable-rate nitrogen. A recent University of Minnesota study funded by the Agricultural Fertilizer Research and Education Council investigated its potential.
Here are six key takeaways from the study:
1. This study confirmed the results of an earlier study in Minnesota that was not able to correlate or calibrate the test. Variability of the numbers and inconsistency in response to applied sidedress nitrogen led our researchers to not endorse this technology for Minnesota. Iowa State did release recommendations that are appropriate for Minnesota, but the biggest concern is trusting PSNT numbers. In our study, we found that numbers can vary dramatically over a short period of time.
2. Some of the difficulties with the PSNT include previous fertilizer applications, immobilization of nitrogen, and predicting availability from organic forms of nitrogen. In most cases, the PSNT is being used to prescribe a variable rate of sidedress nitrogen after a base rate of fertilizer was applied pre-plant. This can cause “hot spots” from application bands, and there could be large amounts of applied fertilizer still in the ammonium form which is not measured. This depends on soil temperature, application timing, and whether a nitrification inhibitor was used.
Additionally, the test won’t account for nitrogen from manure, cover crops, or decomposing residue that hasn’t mineralized yet. Another problem is that the process of decomposition of residue can temporarily immobilize nitrogen, meaning it’s still there, but no longer measurable.
3. Protocol for most PSNT programs call for a soil sample up to one foot deep, but there can be significant amounts of nitrate below one foot. Nitrate will leach, with water infiltration through the profile. Most PSNT programs only call for a one foot sample, but in a wet year, the nitrate could be below that, causing an under-reporting of nitrate in the PSNT leading to over-fertilization. In our study, we sampled to two feet and found that this happened on several occasions.
4. Proper sample handling is critical if you are going to use the PSNT. Most commercial PSNT programs analyze the soil sample “wet.” A moist soil sample has to stay cold to prevent loss from denitrification. If you are doing your own testing with a portable unit, be aware that the numbers will be affected by differences in moisture content. A wet sample is likely to have a lower number than a dry sample, meaning that you should attempt to correct numbers based on soil moisture.
5. How the numbers are interpreted and recommendations made is just as important as getting an accurate test. It doesn’t matter if the test works if the numbers are not used properly. Beware of interpretation apps based on long season corn hybrids. These will not be accurate for Minnesota.
6. Remember to consider economics in your evaluation. This whole process can be time consuming and expensive. Previous work on variable rate nitrogen shows limited potential for increased yields, meaning in most cases reduced fertilizer rates will need to cover the cost of the PSNT