Last fall the Corn Nitrogen (N) Rate Calculator Web tool went online. It is a resource that aids N-rate decisions for corn production and is helpful in determining the effect of fertilizer price on application rates. The method for calculating suggested N rates is based on a regional (Corn Belt) approach to nitrogen-rate guidelines. Details on the approach are provided in the regional publication, Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn , PM 2015. Background information and interpretation of suggested N-rate guidelines were previously provided in an ICM newsletter article .
Nitrogen response trial databases for each state are used in the calculator. The database for Iowa was updated this summer with 18 additional response trials from 2005 research. With the updated database, calculated N rates are similar but have changed slightly from last year. The table at the right gives the N rate at the maximum return to N (MRTN) and the profitable N-rate range for several N:corn grain price ratios. You can work with any price of N and corn you wish when running the calculator. Other updates to the calculator include ability to select a specific fertilizer product, and N prices can be entered by price per pound of actual N or per ton of fertilizer material. The 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 N-product rate and cost.
Fall Nitrogen Applications
The N-rate response databases used in the Corn N Rate Calculator are derived from spring and sidedress fertilizer applications. However, the same calculated N rates are suggested for fall applications. Remember, the timing of fall-applied N is after soils cool to 50 °F and the weather forecast is for continued cooling. The only fertilizer N suggested for fall applications is anhydrous ammonia, and an inhibitor should be considered to further slow conversion of ammonium to nitrate. Waiting for cold soils (the colder the better) helps reduce the risk of fall/early spring conversion to nitrate and will help increase success of fall applications. Those guidelines will not, however, guarantee success. Since it is impossible to predict weather conditions from late fall through early spring that might affect N conversion and loss, the only reasonable approach is to use the same rate as for spring preplant and sidedress applications.
If conditions become conducive for N losses, then adjustment can be made after evaluation of that loss potential. Applying more N at fall application to offset possible lower efficiency leads to lower economic return and increased chance for too much N in the soil if losses do not occur.
Resources for N Application Decisions
The Corn Nitrogen Rate Calculator Web tool is located at http://extension.agron.iastate.edu/soilfertility/nrate.aspx .
The regional publication, Regional Nitrogen Rate Guidelines for Corn, can be ordered through any ISU Extension county office, on the Web through the Iowa State University Extension Distribution Center , or by calling (515) 294-5247. An electronic copy of the publication is available at www.extension.iastate.edu/Publications/2015.pdf .
Web sites for soil temperatures are located at
- http://extension.agron.iastate.edu/NPKnowledge/ 
- http://mesonet.agron.iastate.edu/agclimate/display.php?src=/agclimate/daily_pics/4in-temp-out.png 
- http://mesonet.agron.iastate.edu/agclimate/display.php?src=/agclimate/daily_pics/soil-hilo-out.png 
The Don't Go fall N poster is located at http://www.extension.iastate.edu/Publications/IPM69A.pdf .
The Iowa State University Agronomy Extension Soil Fertility Web site is located at http://extension.agron.iastate.edu/soilfertility/ .
Nitrogen rate guidelines in Iowa for different N and corn grain prices.
|Corn Following Soybean||Corn Following Corn|
1Price per lb N divided by the expected corn price. For example, N at $0.25/lb N and corn at $2.50/bu is a 0.10 price ratio.
2Rate is the lb N/acre that provides the maximum return to N (MRTN). All rates are based on results from the Corn N Rate Calculator  as of Sept. 1, 2006.
3Range is the range of profitable N rates that provides a similar economic return to N (within $1.00/acre of the MRTN).
This article originally appeared on pages 229-230 of the IC-496(24) -- September 18, 2006 issue.