Is a nitrogen credit really a rotation benefit?

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The “maximum return to nitrogen” (MRTN) approach for recommending nitrogen for field corn is based on numerous nitrogen response studies across the corn producing areas of Michigan. The MRTN recommendation suggests a nitrogen rate that will most often provide the best economic return for the investment in nitrogen fertilizer. The MRTN rate is lower when corn is being grown after soybeans and small grains than for corn after corn. This apparent N credit from the previous soybean or small crop is primarily due to increased net mineralization (release) of nitrogen which is part of the rotation effect rather than a direct nitrogen contribution. Soybeans harvested for grain actually remove more nitrogen than is fixed from the atmosphere. This is different than direct nitrogen credits that result from other previous legume crops (such as alfalfa or clover) or applied manure. “Nitrogen credits for corn” is discussed in an article by Larry Bundy (University of Wisconsin) in an article in the March-April issue of Crop & Soils published by the American Society of Agronomy.

When corn is planted no-till into heavy residue, the soil warms more slowly than in tilled fields. The lower soil temperature results in slower mineralization of N. In this type of cropping system, corn may benefit from an additional 25 to 30 lbs N/a, especially applied in the band placed fertilizer.

For those seeking to gain a better understanding of the MRTN approach to N recommendations for field corn, check out the paper by Sawyer et al. “Concepts and rationale for regional nitrogen rate guidelines for corn” PM 2015. It can be found at: www.extension.iastate.edu/Publications/PM2015.pdf

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