Question: I’m doing PSNT sampling for corn, what happens when I mix response zones?

Answer: The first step is ensuring you are pulling a representative sample in the field. Why?

This 45ish acre field had 5 nitrate samples pulled. They were pulled in very particular positions given how those areas of the field respond and perform to applied inputs. If you had taken the sample by accident in the depressions, you would have put no nitrogen on the field. If you have taken the sample on the upper slopes, you would have significantly over applied for a large portion of the field.

When consider the 4Rs of nutrient management (right rate, right time, right place, right source), getting the right rate on the field is site specific when it comes to nitrogen (both for the supply and sink side), especially with variable rate (VR). Add a soil moisture probe or historical yield data to the map below, and you have a fairly accurate way of fine-tuning your N-recommendations (see the over/under difference comparing flat rate vs VR nitrogen).

The map below is what actually got sent to the rate controller for this field, as the sprayer was unable to do an extremely wide rate range. In the future using a dry spreader to top-dress might be more appropriate for wide rate ranges. The client was not comfortable putting a zero rate on in the depressions, so we put the lowest possible rate the equipment was capable of.

Summary

It is fundamental to collect samples from areas that are representative of the field and SWAT MAPS are the best way to delineate zones that should be managed similarly. VR applications based on SWAT zones help to not mix response zones and therefore follow the 4R nutrient management principles. If you are interested in learning more about nitrate sampling in corn or SWAT MAPS management zone creation, you can reach me at the email address below.

Jonathan Zettler
Owner, Fieldwalker Agronomy
jz@fieldwalker.ca