Last week, I told you that it was time to investigate. Today, let’s talk about how exactly to do that:
The best investigation is a thorough one, which in our situation makes for a two-fold approach: we need to know how much nitrogen is actually left in our soil and we need to know how much we need to support a maximum realistic yield. That means soil testing and stand evaluation and a little math.
For in-season nitrogen testing, there’s an old school option—pre-sidedress nitrate testing (PSNT)—and a new school option—a 360 soil scan. Either option yields reliable results, though the soil scan cuts out a day of wait time and does the math for you.
If you’re feeling old school, the PSNT is a straight-forward process that lets you know how much nitrogen is left in your soil and what you need to apply to support your crop for the rest of the season. The Iowa Method, which is a certain way of performing this test, starts with soil samples pulled during V5 or V6 from two soil depths: 0-12 inches and 12-24. The samples are sent to the lab, you wait a day or so, and soil nitrogen levels are reported back to you in terms of parts per million (ppm).
Then comes some fairly simple number crunching, with three possible outcomes:
1) If your results show 25 or more ppm in the 0-12 inch depth range, you’ve probably got enough nitrogen to last the season (though read on for the instance in which that isn’t true).
2) If your results show less than 25 ppm in the 0-12 inch depth range, add the reported ppm for ammonium so long as that number is greater than 5. If the sum of the two numbers is 25 or greater, again, you’ve probably got enough nitrogen to last the season.
3) If you can’t hit that 25 ppm threshold in the 0-12 inch depth range, add the reported ppm for nitrate for both the 0-12 inch depth range and 12-24 inch range. If that sum is greater than 40, then once again, you’ve probably got enough N to last until harvest.
If you don’t reach the threshold in any of these scenarios, that’s when another application of nitrogen is called for. Look at your results for the equations in #2 and #3 above and focus on the one that yields the sum furthest below the target number. If #2 yields the lowest answer, then you can figure the number of pounds of additional N to apply with the following equation: (25-reported ppm for nitrate in the 0-12 inch depth range) x 8. If #3 yields the lowest answer, then the equation changes accordingly to: 40-combined reported ppm for nitrate in the 0-12 inch depth range and 12-24 inch range) x 8.
The math isn’t difficult, but if you’re finding the whole process of sampling/waiting/adding/subtracting/ multiplying to sound like a bore, then the super cool and new school 360 soil sample might be your bag. This method of soil nitrogen testing brings the lab into your field. We’ll essentially show up on site with a microwave sized lab, make a slurry out of a sample of your soil, and let the thing do its work right then and there. Its LP certified, just like labs are, so gives similarly accurate results with no wait time and no subsequent number crunching. It’s pretty exciting new technology that I’d love to bring to your field, so give me a call if you’re interested.
This is the part that really makes this investigation cutting-edge. While we have to start the investigation with soil nitrogen levels, the investigation shouldn’t end there. Here’s why: the thresholds and recommendations outlined above assume a yield of 220 bu/acre. Our tightening margin for error makes such assumptions a bit uncomfortable for us, but fortunately, friends, our advancing technology and know-how can save us from relying on potentially damaging estimations.
Enter the NESP.
As I’ve explained in a previous article, the NESP gives us good insight what our actual yield potential is. If you make a nitrogen application (or not) based solely on results of the PSNF or 360 soil sample, you yet have the potential to over- or under-apply depending on your yield potential. And though I’m sure you realize, it’s worth saying again: over- or under-applying costs you money, one way or another.
The adjustments warranted by high yield potential again present some easy math. Instead of looking for a target of 25 ppm in situations 1 or 2 above, you adjust to a target of 30. Likewise, instead of looking for a target of 40 ppm in situation 3, you adjust to a target of 47.
That’s the bulk of it, but none of this would be complete without an appeal to you to hash things out with your Central Valley Ag Field Sales Agronomist. When numbers come close, your FSA is someone who can help you sort through those gut feelings pulling you one way or another. He or she is a person who can provide insight as to how your particular hybrid might be affected by your particular situation. And when an application is called for, your FSA is someone who can help you determine the best method of application to bring home the hard work you put into making the decision.
Determining the details of your soil nitrogen situation takes a little work, but it pays off either in terms of peace of mind or effective action. Taking the time and care to sample your soil and determine your yield potential is the best way to ensure that your nitrogen management is efficient, accurate, and poses the smallest risk to your bottom line.