By Robert Mullen
As agronomists, we are constantly looking for better tools to improve our decision-making. One potential tool for improved nitrogen management is the pre-sidedress soil nitrate test (PSNT) or the late-spring nitrate test (LSNT).
When should the PSNT be used? The usefulness of the PSNT is almost exclusively isolated to fields that have received recent manure applications (say, the previous fall) or fields that were previously planted to long-term legume forages. The PSNT/LSNTs conducted on soils that do not have a manure history or previous forage legume crop rarely return levels high enough to impact nitrogen rate decisions. Pre-sidedress soil nitrate tests should not be taken when preplant commercial fertilizer nitrogen has been applied. Sampling fields that have received commercial nitrogen can result in an elevated PSNT level, indicating a low probability of nitrogen response that may be incorrect.
How do you collect PSNT samples? Soil samples submitted for PSNT analysis should be collected using similar protocols for general soil sampling – collect an adequate number of samples (a minimum of 15) to constitute a composite, collect the samples in a random fashion, and be consistent about sampling depth. The biggest differences between PSNT sampling and routine sampling are the depth of sample collection and the timing. Soil cores should be sampled to a depth of 12 inches for PSNT sampling; PSNT samples are obviously collected during the growing season. One word of caution: Do not collect PSNT samples too early. This can lead to an underestimation of the PSNT and may not be reflective of what will be occurring in the soil as temperatures warm. The general rule of thumb is, collect soil samples when the crop is at least 12 inches tall. If starter was applied with the planter, avoid sampling areas where the starter band was applied.
What does the PSNT mean as far as nitrogen rate decisions? Depending upon the PSNT/LSNT level, we get an estimate of the likelihood of seeing a response to additional nitrogen fertilizer, but we do not necessarily get an actual nitrogen recommendation. PSNT/LSNT values near 25 to 30 ppm are unlikely to benefit from additional nitrogen fertilizer; and the higher the value, the less likely the need for supplemental nitrogen. The problem arises when PSNT/LSNT values are less than 25 ppm. PSNT/LSNT values below this level may or may not respond to additional nitrogen fertilizer, but the stock recommendation would be that they do require more nitrogen. There can be sites that have a low PSNT/LSNT value but show no response to nitrogen fertilization. This is a major limitation of the PSNT/LSNT. The bottom line: If the PSNT values are above 25 to 30 ppm, adequate nitrogen should be available for this year’s corn crop, meaning no need for additional nitrogen input. If it is less than 15 ppm, the normal nitrogen rate should be applied. Between 15 and 25 ppm, other factors should be considered before reduction of the normal nitrogen rate.