Editor’s note: The following was written by Peter Scharf, University of Missouri Extension nutrient management specialist, for the university’s Integrated Pest and Crop Management website.
The year started out with soils on the dry end of the spectrum over most of Missouri, suggesting easy planting and low potential for N loss. But, as often happens, it turned into a wet spring, starting in southern Missouri in April and spreading northward and westward through May and June.
By mid-June, there were some areas with N deficiency showing up on the ground and in satellite images, but relatively few given the amount of spring rains. This likely related to the dry soil conditions at the start of spring.
And then … much of northern Missouri got more than 7 inches of rain in the last half of June. Most of this rain came during the last seven days of June, and much of it came slowly, soaking into the soil (where your nitrogen was).
At the time this happened, most or all leaves were already made on corn planted in April. I didn’t see much sign of N deficiency showing up in satellite images in early July, but I heard some reports of N deficiency showing up on lower leaves.
This is pretty much what I’d expect if N was lost after the top leaves were mostly formed. Nitrogen deficiency on lower leaves shows up as a V-shaped yellow then brown arrow pointing toward the stalk — it is the process of the plant pulling nitrogen from the lower leaves to send it to higher leaves or ears.
My experience and knowledge of nitrogen is that it disappears fastest when soils are both warm and wet, and this was the case over the last two weeks of June. My opinion is that a lot of nitrogen was lost during this period.
I’ve heard that quite a bit of nitrogen was topdressed on corn this year, and in my opinion that was a great investment.
Overall, about half of Missouri ended up in my “danger zone” for nitrogen loss. Recently, I quickly scanned through satellite imagery across most of the span of the danger zone. In each area where I looked it was easy to quickly find a field with lighter areas that are consistent with nitrogen deficiency. The location of these areas within the field was consistent with parts of the field likely to be wetter — near drainageways and creeks.
- N loss happened late this year (at least one commercial N model disputes this conclusion, but I stand behind it).
- In fields where most or all leaves were made at the time of greatest N loss in the last week of June, it took a while for deficiency symptoms to be visible from above.
- A majority of fields look fine from above, but N deficiency is now visible in a large number of fields across the areas in the danger zone for N loss.
- N deficiency is patchy in the fields where it exists, because N loss is always patchy (most in the wettest parts of the field).
- It is probably too late to do anything to help fields with N deficiency.