Farm Progress

Corn Source: Thoughts on midseason nitrogen for Iowa corn in 2018.

Joel DeJong

July 18, 2018

5 Min Read
N LOSS: High amounts of rainfall in parts of Iowa this growing season has led to questions about N loss and the need to apply supplemental N to corn.

The Corn Nitrogen Rate Calculator is the system Iowa State University Extension uses to estimate the economically optimum nitrogen application rate that produces the maximum return to N a corn crop in Iowa will likely need during the growing season. Due to uncertainty in exactly where the MRTN falls, there is a most-profitable range of rates also provided.

This calculator is based on hundreds of replicated trials in recent years conducted throughout the state. This data set has shown that southeast Iowa has a higher MRTN rate than the rest of the state. When you look at the individual research locations yearly or over time, we do recognize that sites sometimes have an optimum N rate need higher or lower than the number reported in the N rate calculator.

Why? The weather isn’t the same at all sites, and we all know that weather variability alters the “right” nitrogen rate for a field. This is part of the reason for the most profitable N rate range.

Challenging year for N management
During the 2018 growing season, the weather has been unusual with the coldest April on record, followed by one of the warmest Mays, and for northern Iowa, one of the wettest June periods we have observed. Weather affects the right N rate, so this is a challenging N management year. By now, most corn has silked and late N applications don’t often help when applied after tasseling, but we can always learn from each year.

So, how can we fine-tune the amount of N needed for our crop when erratic weather arrives? I believe that understanding how N forms change in the soil, when they change, what forms are water-soluble and subject to leaching or denitrifying, and when the crop takes up nitrogen all help us when trying to make those management decisions.

In addition is variation in soil N mineralization, which is microbial-driven and will continue during the season with moist and warm soils. I can’t cover all of those topics in this article, so let’s first look at crop uptake. Here is the N uptake graph for corn. By the time corn tassels, the plant has taken up about 70% of the total N it needs.


Estimating how much N lost
As the ears fill with kernels, N is moved from other parts of the plant into the ear. If you see that classic V-shaped symptom of N deficiency on several lower leaves at the R3 (milk) stage or earlier, you know the plants are likely running short of N. If you observe just a little yellowing on lower leaves like that at maturity, you likely were OK and near or at an optimal N supply. While it likely will not be a “drought” year, yellowing of the lower leaves also occurs with limited late-season moisture supply.

Stalks store excess N not used, so high nitrate levels in the lower stalk at the end of the season is a good indicator we had more than needed during that year. However, low nitrates left don’t always indicate reduced yield, there still might have been enough N moved from the other parts of the plant to reach maximum yield.

What clues can we get about the correct N rate before the end of the year when adjustments are possible? One clue comes from research conducted at multiple research farm sites over time — that is site-years of data across variable weather conditions.

Rainfall totals April through June
When ISU soil fertility specialist John Sawyer evaluated spring rainfall amounts from seven long-term N rate trials at ISU research farms, he discovered that springtime rainfall was a major factor when the MRTN rate prediction was lower than the optimal amount. Those spring rainfall totals have about a 76% chance of determining the need for additional N application, or that N supply was adequate or more than adequate.

What are the triggers for additional N needs according to his work? For southeast Iowa, March 1 to June 30 rainfall totals over 17.8 inches indicated a need for more N than the calculator indicated, and for the remainder of Iowa, 15.5 inches from April 1 to June 30 was the trigger amount. According to 2018 precipitation maps, we exceeded those rainfall totals in the central to northern part of Iowa. Local rainfall measurements would provide more specific rainfall information. The more rainfall there is above those trigger totals, the more likely supplemental N would be needed as the rainfall triggers are just at the point where a higher N rate need would begin.


An exception to the rule
There is an exception to using rainfall totals if there are heavy, short-duration rainfall events. If water runs off the field, and does not get into the soil profile, then there should be a discounting off the total. In addition, if the rainfall reaches those totals in the early spring, there should also be some discounting off the total due to less nitrate buildup and less denitrification with cool soils.

For example, total rainfall amounts in just an individual month, like April or May, do not provide the same level of success as when June rainfall is included. Why? The entire springtime is important for potential N response in corn.

In June, most applied N would be in the nitrate form, and warm soils allow faster denitrification. After June when corn has taken up much of the soil nitrate and is approaching the silking stage (see corn N uptake graph), there is typically only a small pool of nitrate that could be leached or denitrified. With rapid corn transpiration, nitrate uptake from deeper in the soil would reduce chance of loss. The rainfall triggers are related to use of suggested MRTN rates from the Corn Nitrogen Rate Calculator. If higher or lower N rates were applied to fields, then the odds of needing additional N go up or down.

By the time you read this, it might be too late to alter what you did in 2018. Nevertheless, each year is a learning opportunity. Keep learning, for a similar year might be repeated in the future, although I know of several farmers who hope it isn’t!

DeJong is an ISU Extension field agronomist in northwest Iowa. Contact him at [email protected].

About the Author(s)

Joel DeJong

Joel DeJong is the Iowa State University Extension field agronomist for northwest Iowa. He works with farmers on integrated management of pest populations, crop management, and nutrient and manure management. DeJong also trains company agronomists on research-based crop production practices, and teaches certified crop advisers on production, nutrient and pest management issues.

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