Increasing demand for corn grain to meet ethanol production in Iowa has spurred interest in growing more corn following corn. What is the nitrogen (N) fertilization rate for continuous corn (CC), how does it compare to rates with corn following soybean (SC), and what rates are needed for second- (CCS) or third-year corn (CCCS) in rotation with soybean?

Nitrogen application rate

Soybean in the cropping rotation results in a soil system that supplies greater crop-available N. There are several reasons for this, but mainly it is due to the amount of crop residue, residue N content and time of return to soil, and soil microbial mineralization rate.

Results of research in Iowa indicate that the N fertilizer rate requirement is on average approximately 50 to 60 lb N/acre higher with CC than SC (Figure 1). The emphasis today is not on determining a "soybean credit," which is really a misnomer, and trying to equate an N rate for SC from CC, but instead the emphasis is on determining the N rate required for corn in a specific rotation.

In recent years, there has been a wide fluctuation in both N fertilizer and corn prices. The most economical application rate is influenced by the ratio of these prices (i.e., the $/lb N:$/bu corn grain price ratio). To aid in determination of suggested N rates, the Corn Nitrogen Rate Calculator Web-based tool was developed so that different N and corn prices, as well as crop rotation, could be used in choosing an N application rate. Research in Iowa also has shown that second- (CCS) or third-year (CCCS) corn after soybean has an N fertilizer rate requirement similar to that for CC (see related article, Increasing the frequency of corn in crop sequences: Grain yield and response to nitrogen--a research update in this issue). Therefore, N rates for CC can be used when fertilizing second- or third-year corn.

Yield

Nitrogen fertilization rate should be based on expected maximum economic return to N application rather than trying to achieve maximum production. It is just not possible to pay for the relatively large N-rate increase required to grow the last small yield increase from an economic optimum rate to a maximum yield-producing rate. Current N and corn prices result in recommended rates that produce yields quite close to maximum production (average 96 to 99% of maximum yield). Unless N prices increase and/or corn prices decline dramatically, recommended rates will not hinder productivity and will allow expression of yield potential for the growing season. Using economically derived rates also helps reduce nitrate loss to water systems.

Summary

1. Based on current N fertilizer and corn prices (price ratio of about 0.10; example $0.30/lb N:$3.00/bu), recommended N application for corn following soybean is approximately 125 lb N/acre (range 105 to 145 lb N/acre) and for continuous corn is 175 lb N/acre (range 155 to 195 lb N/acre). See Table 1 for N rates at other price ratios. Because of variation in N fertilization requirement between locations and years, and uncertainty in an exact maximum economic return to N (MRTN) rate, a range in suggested N rates is provided that gives similar economic return and is usually ±20 to 25 lb N/acre within the MRTN rate.

2. Second- or third-year corn in rotation with soybean has an N fertilizer rate need similar to continuous corn.

Table 1. Nitrogen rate guidelines in Iowa for different N and corn grain prices.

¹Price per lb N divided by the expected corn price. For example, N at $0.30/lb N and corn at $3.00/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).

Resources for N application decisions

The Corn Nitrogen Rate Calculator Web tool is at http://extension.agron.iastate.edu/soilfertility/nrate.aspx.

Concepts and Rationale for Regional Nitrogen Rate Guidelines for Corn (PM 2015) can be ordered through any Iowa State University Extension county office, on the Web through the Iowa State University Extension Distribution Center's online store or by calling (515) 294-5247.

The Soil Fertility Web site is located at http://extension.agron.iastate.edu/soilfertility.

John Sawyer is an associate professor with research and extension responsibilities in soil fertility and nutrient management.