By Roy Ulrich, technical agronomist for Dekalb and Asgrow, Southern Ohio
Adequate rates of nitrogen available to a corn plant during the entire growing season is a foundation to a successful harvest. This fact is foundational that Fred Below from the University of Illinois in his “Seven Wonders of the Corn Yield World” ranked nitrogen as the second most important factor in corn yield, only to be outdone by weather.
If nitrogen is that critical to a successful crop, then what is the correct rate of nitrogen for an acre of corn? The old school approach would be to take a yield goal and multiply it by 1.25 pounds per bushel so a 250-bushel per acre yield goal would require an application rate of 312 pounds per acre of nitrogen. As most know, nitrogen isn’t quite this simple and isn’t this cut and dry when it comes to final yield.
When it comes to actual nitrogen rates, like most good agronomy answers, when it comes to nitrogen needed “it depends” is the correct answer. The correct rate varies vastly on many factors including but not limited to soil type, drainage, application timing, nitrogen source, nitrogen rate, weather, application method, manure history, potassium availability, and the use of nitrogen stabilizers. All these variables impact the total amount of nitrogen needed and its length of availability to the plant.
What about when a corn plant needs nitrogen during the growing season to add another layer to this already complex input? From planting to V6, a corn plant only uptakes roughly 10% of its total needs for the entire growing season. V6 is the start of the rapid growth phase in which uptake is the greatest and the ability to add additional nitrogen is fairly limited with traditional application methods due to the fast growth rate of the corn plant.
During this rapid growth, large amounts of nitrogen are required to keep up with plant development with the peak occurring at VT. During this time corn plants have gone from 10% total nitrogen uptake to 66% uptake in a short span of time. However, this still leaves a third of the total nitrogen needed for the plant to be pulled in during the final 60 days of the growing season which normally occurs from mid-July to mid-September which can coincide with dry weather that can limit nitrogen uptake.
It is easy to understand why nitrogen is a complicated variable when it comes to corn production and why it changes from growing season to growing season and from grower to grower and one acre to the next. So, with complexity of nitrogen, the cost of nitrogen, and its weighted importance on final yield, one can see why it is important to monitor the growing season and nitrogen applications.
So, what about nitrogen and the 2022 growing season? The delayed start to the 2022 growing season may have changed a few plans for timing of application and the cost of nitrogen products may have caused growers to think about the overall rate of nitrogen. But, for the most part, we have avoided many of the large rain events that keep the soil saturated and drive denitrification leading to nitrogen loss. The good news is there are some in-season tools that can help fine tune applications like pre-side dress nitrate tests (PSNT) which can give the amount of nitrogen still available before additional nitrogen is applied. This can be useful where manure was applied, or a large percentage of the total nitrogen was applied early in the growing season.
A PSNT can give you the amount of ammonium and nitrate in the soil which can help determine the amount of additional nitrogen needed. Utilizing a tissue test at this time will tell you how much nitrogen is in the plant, but since such a high percentage of the total nitrogen needed must still be pulled into the plant, this does not give any information to adjust additional applications. There are some tools available to help growers make decisions supported by data. One of these tools is from Iowa State and can be found at http://cnrc.agron.iastate.edu/ . This will allow you to input grain prices and nitrogen prices to help calculate the maximum rate of return.
Careful management, using the right tools and sound agronomics can take some of the guesswork out of this vital input for corn production that is never cut and dry when it comes to final yield.