What's New?
Measuring the amount of nitrate-Nitrogen in the surface 30 cm (12.in.), usually in the first two weeks of June has proven to be a reasonably good indicator of potential soil N supply for a corn crop. This test is commonly referred to as the Pre-Sidedress Nitrogen Test or PSNT and was first introduced into Ontario in 1993.
Over the last twenty years we have examined the PSNT nitrogen fertilizer recommendations for corn and now, in 2015, we have arrived at a superior recommendation approach using this test. This new approach or calibration will be significantly more accurate than the older recommendation especially in higher yielding corn fields.
What's Different?
The main difference in the new PSNT recommendation system is that sidedress fertilizer N rates will hinge not solely on the soil nitrate value (i.e. 15 PPM) but will also factor in the expected yield for the field or part of the field.
In the former PSNT approach this was always a concern and some of you have called over the years to express this concern. How could a 15 PPM soil nitrate test mean the same amount of recommended nitrogen regardless of whether it came from a field where the yield expectation was 135 bu/ acre or from a field that was going to yield 235 bu/acre? Thanks for those calls; they inspired us to keep working at improving the PSNT.
So now Table 1 illustrates the new recommendations for sidedress N application rates based on both the soil N test and yield expectations.
What's Not Different?
The actual testing procedure has not changed just the recommendation system. So all of the old rules still apply:
- Samples must be from a depth 30 cm (12 in.).
- It is important that all cores in a field be taken to the same depth.
- It is very important to ensure that sampling avoids any previously broadcast nitrogen fertilizer or starter fertilizer bands.
- Sampling should occur when corn is 6-12 inches (15 - 30 cm) tall, and usually in the May 25 - June 15 window.
- Soil nitrate samples tend to be less accurate when sampled after periods of significant rainfall. If possible allow a day or two for the soil to dry before taking PSNT samples.
- Place soil cores in a clean plastic pail, crushed by hand and well mixed.
- Microbial action in the sample can change the nitrate content quickly if it is not handled properly. Chill or freeze samples as soon as possible. For shipping, pack samples with insulating material to keep them cool and send them by courier to ensure quick delivery to the lab.
Source: OMAFRA