Determining Corn Fertilizer Requirements Following an Alfalfa Crop

Utilizing alfalfa in a crop rotation provides several benefits to the following corn crop.  These rotational assets include reduced soil compaction, a reduction in soil-borne pathogens and insect pests, and greater corn yields. One other substantial benefit from alfalfa is the nitrogen (N) it provides for the following corn crop.

Alfalfa fixes N by taking it in from the atmosphere and/or soil and converts it to a usable form.  During its life cycle, alfalfa sheds and regenerates fine roots which, along with the decomposition of leaves and stems, adds N to the soil. Additional N is available during termination of the crop with further decomposition of its shoots, crown, and roots. This contribution of N from alfalfa is available to the subsequent corn crop and is known as the alfalfa N credit.

The amount of N that is supplied by the previous alfalfa crop is called a N credit.  According to various university studies, farmers planting corn after alfalfa can take N credits from 40 to 190 lb/acre, depending on such factors as the quality of the previous alfalfa stand, soil types, and other growing conditions.^1-7 As a recommendation, Midwestern universities suggest that applied N fertilizer for corn following a good alfalfa stand can be reduced by up to 100% for first-year corn and by up to 50% or more for second-year corn.²  

First-year corn yields rarely increase by adding supplemental N after alfalfa. By adhering to local alfalfa N credit guidelines and overall recommendations, the appropriate amount of N fertilizer can be applied which can help save money while maintaining expected yields. 

Alfalfa production can remove high amounts of available phosphorus (P), potassium (K) and sulfur (S). Therefore, a fertility soil test should be conducted after an alfalfa crop to determine the potential availability of P, K, and S for first-year corn. Application rates for these nutrients should be applied according to the soil test results and expected corn yield.  

• Potassium: 0 to 255 lb K₂0/acre may be needed according to university guidelines and soil test results.^1,4,7  

• Phosphorus: 0 to 160 lb P₂O₅/acre may be needed according to university guidelines and soil test results. Always make sure to credit N that may be applied with the P fertilizers when determining your N rates for corn.^1,4,7   

Sulfur soil concentration levels have steadily decreased because of decreased S emissions from coal burning power stations. If a S deficiency is confirmed by soil tests or has been noted in a previous crop, an application of SO₄-S at a rate of about 15 lb/acre is recommended for fine-textured soils and about 25 lb/acre is recommended for coarse-textured soils.^8,9,10

First-year corn rarely responds to additionally applied N. Assigning the proper N credit when calculating N fertilizer requirements for corn grown after alfalfa may result in a substantial decrease in the amount of fertilizer N rates needed without diminishing yield. Be sure to follow your local guidelines in establishing the proper alfalfa N Credit and calculating the appropriate N rates for corn following alfalfa.  

¹Clark, J., Gerwing, J., and Gelderman, R. 2019. Fertilizer recommendations guide. South Dakota State University Extension. https://extension.sdstate.edu/sites/default/files/2019-03/P-00039_0.pdf.

²Coulter, J., Yost, M., and Russelle, M. 2012. Nitrogen and tillage management for corn following alfalfa. Proceedings of the Integrated Crop Management Conference, Iowa State University. https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1107&context=icm.

³Fernandez, A.L., Sheaffer, C.C., Tautges, N.E., Putnam, D.H., and Hunter, M.C. 2019. Alfalfa, wildlife & the environment. 2nd Edition. National Alfalfa and Forage Alliance, St. Paul, MN.  http://www.alfalfa.org/pdf/alfalfaenvironment2.pdf.

⁴Franzen, D.W. 2018. North Dakota fertilizer recommendation tables and equations. North Dakota State University Extension. SF882. (Revised) https://www.ag.ndsu.edu/publications/crops/north-dakota-fertilizer-recommendation-tables-and-equations/sf882.pdf.

⁵Nitrogen credits for alfalfa and soybean in Wisconsin. A3591. 2012. Nutrient and Pest Management (NPM) Program. University of Wisconsin. https://ipcm.wisc.edu/download/pubsNM/Legume_Credits_1008.pdf.

⁶Wortmann, C., Milander, J., Jansen, J., Anderson, B., and Wilson, B. 2019. Adding alfalfa to corn-soybean rotation can increase profit, reduce nitrate leaching. CROPWATCH. University of Nebraska-Lincoln Extension  https://cropwatch.unl.edu/2019/adding-alfalfa-reduces-nitrate-leaching.

⁷Yost, M.A., Coulter, J.A., Russelle, M.P. 2021. Managing the rotation from alfalfa to corn.  University of Minnesota Extension.  http://corn.agronomy.wisc.edu/Management/pdfs/L001.pdf.

⁸Sawyer, J.E., Lang, B., and Barker, D.W. 2012. Sulfur fertilization response in Iowa corn and soybean production. Agronomy Conference Proceedings and Presentations. 43. Iowa State University.
https://lib.dr.iastate.edu/agron_conf/43.

⁹Camberato, J., Maloney, S., and Casteel, S. 2012. Sulfur deficiency in corn. Soil Fertility Update. Purdue University. https://ag.purdue.edu/agry/extension/Documents/Sulfur_Deficiency_Indiana.pdf.

¹⁰Peters, J. and Laboski, C. 2013. Is sulfur needed in alfalfa in Wisconsin? Focus on Forage. Wisconsin Team Forage. University of Wisconsin-Madison. https://fyi.extension.wisc.edu/forage/is-sulfur-needed-for-alfalfa-in-wisconsin/.

Web sites verified 5/20/21.