Farm Progress

Corn Source: Here’s how to fine-tune your nitrogen management for corn this spring.

February 23, 2017

5 Min Read
NITROGEN FERTILIZER: It’s one of the largest variable costs for corn. Applying “more than enough N” is no longer “cheap insurance” and it’s not environmentally friendly. Cost and environmental impact encourage growers to critically evaluate their N management, including application rate and timing.

By Terry Basol

It’s hard to believe that in the next 30 to 60 days, we’ll be back at it, planting corn, and ensuring all the winter planning and decision-making gets us one step closer to efficiently optimizing the yield for each respective field.

One of those critical decisions each spring is determining a nitrogen rate, as well as timing of application or applications (single application before or near planting vs. split application).

To obtain a high yield and economic profitability, it’s critical to supply adequate N for the corn crop. Without it, substantial yield loss potential will occur. Recent long-term research conducted at Iowa State University (2000-13) shows corn yield will average only about 60 bushels per acre for continuous corn and 115 bushels per acre for corn following soybeans when corn isn’t fertilized with nitrogen.

Optimally fertilized corn will easily yield more than 200 bushels per acre, indicating the large yield increase possible with nitrogen application to cornfields.

On the other hand, we don’t want to apply too much N, as that will decrease optimum profitability and increase the potential of N loss and a negative environmental impact.

Calculate application rate
As we look at the corn yield response when we increase nitrogen rates, it can be explained by the law of diminishing returns.

As the nitrogen rate increases, the incremental yield increase gets smaller and eventually there is no further yield increase at higher and higher N rates. Because of this, the most profitable fertilizer rate will be just under the N rate in which the highest yield will be obtained. The ultimate goal is to apply nitrogen at a rate that has the greatest economic return.

A web tool that can help is the Corn Nitrogen Rate Calculator, found at It was updated last spring and has a fresh new look. Here, current nitrogen application rate guidelines from Iowa State University Extension are based on extensive data from nitrogen response trials to help determine the economic return to nitrogen application.

The suggested application rates from the calculator are determined from many research trials, where multiple N rates provide grain yield response to applied nitrogen, and then economic return calculated from the response determining the most profitable application rate.

This approach is called the maximum return to nitrogen (MRTN). In fields given on the website, information such as fertilizer product, state or sub-state area, rotation, fertilizer N price, and corn price can be entered to determine the suggested nitrogen rate called the MRTN rate.

A most profitable N rate range is also provided, which gives nitrogen rates within $1 per acre of the maximum return. This profitable nitrogen range provides flexibility in choice and rate to apply, based on production risk tolerance, water quality concerns and maintenance of soil nitrogen. The MRTN rates are derived from rate trials with good nitrogen management practices; for instance, trials with spring or sidedress nitrogen application.

Consider application timing
Another discussion point in winter meetings or one-on-one conversations with farmers I’ve had this past winter is the consideration of sidedress and split application of nitrogen.

Sidedress and split application is when a portion of the total nitrogen needs for the season is applied before or at planting, and the remainder of the N is applied later as a sidedress application before corn gets too tall — as opposed to applying all the nitrogen needs before or at planting. Growers I’ve talked with primarily want to improve use efficiency in corn and reduce N losses.

According to John Sawyer, professor of agronomy and ISU Extension specialist in soil fertility and nutrient management, research conducted in the last 12 years at ISU has shown it’s been difficult to achieve a frequent and consistent positive response in corn production regarding a split application of nitrogen for Iowa’s soils and conditions.

In general, data has shown whether all the nitrogen needed for the growing season is applied at or near planting, or whether it is sidedressed and split-applied, the results have been similar. That is, similar with each other in terms of the economic optimum nitrogen rate and yield at the economic optimum nitrogen rate. Exceptions to this or when sidedress has proven to be beneficial are course-textured soils, with irrigation, or in areas of high rainfall and poorly drained soils.

When does sidedress work?
In consideration of springs with high rainfall and fields with poorly drained soils, sidedress or in-season applications of nitrogen can prove to be of benefit. Sidedress applications can certainly avoid some early-spring nitrogen loss.

On the other extreme, if you have corn in dry soil conditions and lack rainfall, especially after sidedress application, reduced yield can occur due to poor nitrogen uptake.

The combination of weather and soil properties will influence corn response to springtime nitrogen application timing. You should not expect one or the other’s timing to always be the best. Also, unless there is a nitrogen deficiency in the soil system and the crop can respond, an in-season application of nitrogen will not improve yield. 

As nitrogen management decisions are finalized for the 2017 growing season, the Corn Nitrogen Calculator can be used as a research-based tool to help calculate an optimum economic return to nitrogen rate. The calculator uses current research information from nitrogen response trials, as well as taking into consideration fertilizer price and expected corn price.

We have lots of opportunities in spring for nitrogen application — from preplant through midseason timing with tools to help adjust rates. If sidedressing and split application is a consideration for 2017, soil and weather conditions (too wet or dry) can be a factor for successful timing of application for optimum corn yield. Some aspects to consider are labor, time and equipment to effectively and timely cover the required acres.

Basol is an ISU Extension field agronomist at Nashua in northeast Iowa. Contact him at [email protected].


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