2021 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 (map at right). Most of this rain came during the last 7 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 (crosshatch in the map to the right)–a very rough tool based just on rainfall. 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.
Some of the fields with deficiency showing seemed to be planted in April but the deficiency took a while to become visible at the top of the plant. Others were late-planted and the deficiency showed as the leaves developed.
For each field, I analyzed the satellite image to predict yield loss due to N deficiency. This prediction is based on an analysis of yield loss vs. corn color in satellite images from fields in past years. Each image and the corresponding prediction of yield loss is shown below starting from the southwest and proceeding toward the northeast.
REVISED: August 17, 2021