Corn Nitrogen Rate Calculator Update

July 22, 2014

By John Sawyer, Department of Agronomy

The Iowa N response database in the Corn Nitrogen Rate Calculator was recently updated, with response trials added from 2013 research. There are now 246 trials for corn following soybean and 133 trials for corn following corn. Being able to easily update the database with recent data is one of the many advantages to this dynamic database approach for corn N rate guidelines. Having new response trial data allows rapid updating with changing hybrid genetics, rotations and climatic conditions.

With the updated database, recommended N rates have increased slightly from previous years, the result of a higher N fertilization rate requirement due to the wet 2013 springtime. The table below gives the N rate at the maximum return to N (MRTN) and the most profitable N rate range from the updated calculator for several N: corn grain price ratios (price of N fertilizer in $ per lb N divided by the price of corn in $ per bu), using anhydrous ammonia prices as an example. You can work with any price of N and corn you wish when running the calculator. Output information includes the N rate at the MRTN, the profitable N rate range, the net return to N application, the percent of maximum yield and the selected N fertilizer product rate and cost.
Nitrogen rates determined from the calculator are directly the total fertilization amounts for each rotation, with no need to further adjust rate for previous crop. That is, for the soybean-corn rotation, there is no need to subtract a "soybean credit" as the rotation effect is already accounted for by the N rate trials that the database is derived from.

If you are not familiar with the Corn Nitrogen Rate Calculator

The Corn Nitrogen Rate Calculator is an online tool that allows determination of nitrogen (N) application rates for corn production and is helpful in determining the effect of fertilizer and corn price on needed rates. The method for calculating suggested N rates is based on a regional (Corn Belt) approach to N rate guidelines. Details on the approach are provided in the regional publication Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn. This approach and the Corn Nitrogen Rate Calculator is the current university N rate recommendation system used in seven states across the Corn Belt: Iowa, Illinois, Indiana, Michigan, Minnesota, Ohio and Wisconsin.

Resources for N Rate Decisions

Corn Nitrogen Rate Calculator

ISU Agronomy Extension Soil Fertility

The regional publication Regional Nitrogen Rate Guidelines for Corn (PM 2015) can be downloaded or ordered through any ISU county extension and outreach office, from the ISU Extension Online Store or by calling (515)  294-5247.

John Sawyer is professor with research and extension responsibilities in soil fertility and nutrient management. He can be reached at or (515) 294-7078.

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