Although nitrogen loss potential has been very low this year, people are asking University of Illinois assistant professor of crop sciences Fabián Fernández how much nitrogen they should apply at sidedress and what tools are available to determine the amount needed.
The pre-sidedress nitrogen test (PSNT) is used to determine the need for additional nitrogen. This test measures the amount of nitrogen mineralized into plant-available forms from organic nitrogen pools plus the amount of applied nitrogen still in the soil (either from an earlier application or as carryover).
“This year, because of the low nitrogen loss potential, performing the PSNT is probably not going to provide information beyond what you already know regarding how much nitrogen is still needed to provide a full rate,” said Fernández.
Growers who want to use the PSNT should remember that the usefulness of the information it provides can be heavily influenced by how the samples are collected, handled, and processed.
“The PSNT is often more accurate in high-yielding environments and in fields that have received manure or other organic fertilizers in the recent past or that have had legume crops with high nitrogen content, such as alfalfa,” Fernández explained. When the value is 25 ppm or more, there is a high probability that no additional nitrogen is needed.
Soil samples should be collected when corn is in the 4th to 6th leaf stage to a 1-foot depth at eight positions perpendicular to the direction of the nitrogen applicator. “If you know the location of the knife application, start collecting there, and then at one-eighth, one-fourth, three-eighths, one-half, five-eighths, three-fourths, and seven-eighths the distance between the rows,” said Fernández.
If the location of the knife application is not known, start with the corn row and use the same sampling strategy. “I suggest collecting at a density of one sample per 10 acres,” Fernández said. All eight cores should be placed in the same sample bag. Because this is a large amount of soil, use large bags.
Once collection is completed, freeze or quickly air-dry the sample if it cannot be delivered to the testing lab on the same day. Freezing is easiest. To dry the sample, spread it on a paper, crush the cores, and place a fan to circulate air and speed up the process. When sending the sample to the lab, indicate on the package that it is for nitrate nitrogen analysis so the lab knows to dry and grind the entire soil sample before taking a subsample for analysis.
If the lab results show PSNT values of 25 ppm or higher, there is no need for additional nitrogen. If values are 10 ppm or less, a full rate of nitrogen is needed. For values between 25 and 10 ppm, adjust the nitrogen application proportionally.
A more practical way to determine whether additional nitrogen is needed is by using strip nitrogen applications (60 to 80 lbs. of nitrogen per acre) across the field to see if there is a response in growth or level of greenness. Corn in the strips that is greener or growing better is an indication that the rest of the field needs more nitrogen.
“The risk associated with this approach is that color differences may not develop until corn is in the grain-fill period, at which time it is too late to apply additional nitrogen,” warns Fernández. Another risk is that nitrogen needs significant amounts of rain to move it into the root zone, and large rain events are less frequent later in the season.
To reiterate, this year there should be little uncertainty on how much nitrogen still needs to be applied. The potential for nitrogen loss due to weather conditions was minimal if the applications were done correctly. Thus, if a full rate was already applied, additional nitrogen should not be needed. “If you were planning to apply a portion of your nitrogen at sidedress, then I recommend applying it now rather than waiting until later,” said Fernández.