Magnesium (Mg) is one of the three secondary macronutrients that are essential for plant growth and is a part of the chlorophyll molecule, thus very important for photosynthesis. Without Mg, plants would not be able to use chlorophyll to capture light energy from the sun and turn it into energy in the form of sugars. Magnesium is also crucial for the uptake of other nutrients, such as phosphorus.
In the soil, Mg is held on organic matter and clay particles in the Mg2+ form, which is available to the plant. Instances of Mg deficiency are typically found in acidic and/or sandy soils when the soil pH is less than 5.5. Soil tests can be used to determine the amount of exchangeable Mg in the soil. Tissue samples may be taken during the growing season to determine Mg levels in the plant. To test tissue early in the season, sample the entire corn seedling for analysis. The first fully developed leaf from the top of the plant should be sampled and submitted after the seedling stage but before tasseling. To sample after tasseling, sample the leaves opposite and below the ear. For adequate Mg levels in the soil for corn production, Mg should be 15% of the soil’s total cation exchange capacity (CEC) value. Tissue test results should be used in conjunction with soil test results to best determine Mg levels. In a 2010 study from the University of Illinois, it was determined that a 230 bushel/acre corn crop needs 52 lbs/acre of Mg. Of the 52 lbs/acre that was taken up by the plant, 15 lbs were removed with the grain; therefore, an additional 15 lbs/acre of Mg should be applied to meet the yield goal of 230 bu/acre.1
Magnesium Deficiency and Fertilization
Corn that is Mg deficient may first start to lose its healthy green color because without Mg, the plant is unable to produce chlorophyll, which gives plants their green color. In addition, corn plants that are deficient in Mg will show symptoms of interveinal chlorosis in the lower, older leaves first because Mg is plant mobile and will be relocated to newer growth when the Mg supply is low. Plant tissue between the leaf veins will turn yellow (chlorotic) while the leaf veins remain green. Severe cases of Mg deficiency may cause the lower corn leaves to turn red or purple and/or necrotic (dead plant tissue) on the leaf tips and margins.
Dolomitic limestone is the most common Mg-containing fertilizer that is used to supply Mg before planting, typically in the fall. If the pH of the field is between 6.0 and 6.5, lime would not be needed and other sources of Mg should be used. Magnesium sulfate (Epsom salt) or potassium-magnesium sulfate may be applied in-season to help correct soils that are magnesium deficient.
1Bender, R.R., Haegele, J.W., Ruffo, M.L., and Below, F.E. Modern corn hybrids nutrient uptake patterns. PowerPoint® Presentation. Crop Physiology Laboratory, University of Illinois at Urbana-Champaign.
Kaiser, D. 2016. Magnesium for crop production. University of Minnesota Extension online article. https://extension.umn.edu/.
Oldham, L. 2019. Secondary plant nutrients: calcium, magnesium, and sulfur. Mississippi State University information sheet 1039. http://extension.msstate.edu
Schulte, E. 2004. Soil and Applied Magnesium. University of Wisconsin-Extension publication A2524 http://corn.agronomy.wisc.edu
Sneller, E. 2011. Magnesium deficiency issues in Michigan corn fields. Michigan State University Extension article. https://www.canr.msu.edu/
University of Illinois at Urbana-Champaign. Cooperative Extension Service. (2009). Illinois agronomy handbook: 24th edition. 24th. Urbana, Ill.: University of Illinois at Urbana-Champaign, College of Agriculture, Cooperative Extension Service.
Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
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