2021 nitrogen loss in Missouri cornfields

Published: August 17, 2021

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.

Accumulated Precipitation (in)
June 16, 2021 to June 30, 2021

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.

Danger zone for nitrogen loss is shown in crosshatch. Numbers show estimated yield loss due to N deficiency for individual fields at the locations shown. These fields are among the worst 25% of fields in their area.

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.

Vernon County July 30 satellite image--predicted field-average yield loss is 18 bushels/acre.

Bates County July 30 satellite image--predicted field-average yield loss is 29 bushels/acre.

Johnson County July 29 satellite image--predicted field-average yield loss is 27 bushels/acre.

Ray County July 29 satellite image--predicted field-average yield loss is 22 bushels/acre.

Carroll County July 29 satellite image--predicted field-average yield loss is 23 bushels/acre.

Saline County August 2 satellite image--predicted field-average yield loss is 17 bushels/acre. The best 20% of the field is assumed to have sufficient N, but that may not be true and actual yield loss may be higher.

Boone County August 3 satellite image--predicted field-average yield loss is 20 bushels/acre.

Audrain County August 3 satellite image--predicted field-average yield loss is 20 bushels/acre.

Montgomery County August 2 satellite image--predicted field-average yield loss is 20 bushels/acre.

Take-home messages

1. N loss happened late this year (at least one commercial N model disputes this conclusion, but I stand behind it).
2. 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.
3. 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 (see map near top of article).
4. N deficiency is patchy in the fields where it exists, because N loss is always patchy (most in the wettest parts of the field).
5. It is probably too late to do anything to help fields with N deficiency.
6. My opinion is that late-planted fields suffered the most N loss because
1. Plants pumping out the excess water were smaller
2. These fields tended to be wetter to start with
7. Fields that got top-dress or sidedress N applications should be in good shape.