Michigan State University Extension
Soils & Soil Management - Fertilizer - 06029708
07/10/97
Fertilizer Recommendations for Field Crops in Michigan Extension Bulletin E-550A, March 1992
D. R. Christenson, D. D. Warncke, M. L. Vitosh, L. W. Jacobs and J. G. Dahl, Department of Crop and Soil Sciences
Magnesium (Mg)-Magnesium deficiency is most likely to occur in acid soils with sandy surface texture and a subsoil as coarse or coarser than the surface. It also occurs in similar soils limed with calcic limestone or marl. Responsive field crops are potatoes, oats and corn.
Applying magnesium is recommended if a soil test indicates that the exchangeable magnesium level is below 75 pounds per acre; or when potassium exceeds magnesium as a percent of the total exchangeable bases (calcium plus magnesium plus potassium, expressed as milliequivalents per 100 grams of soil); or when the soil magnesium as a percent of total bases is less than 3 percent.
On acid soils where magnesium is needed, apply at least 1000 pounds of dolomitic limestone per acre. On non-acidic soils, magnesium deficiency may be corrected by broadcasting 50 to 100 pounds Mg per acre, or banding 10 to 20 pounds Mg per acre. Magnesium sulfate (Epsom salts), potassium-magnesium sulfate and finely ground magnesium oxide are all satisfactory sources of magnesium.
Magnesium can also be applied as a foliar spray. The suggested rate is 1-2 pounds of Mg per acre. This can be supplied by using 10 to 20 pounds of magnesium sulfate (Epsom salts) applied in at least 30 gallons of water.
Magnesium deficiency may be induced by high rates of potassium fertilizer. In some states, agronomists strive for at least 10 percent magnesium saturation of the total exchangeable bases (equivalent basis). These high rates are aimed at preventing "grass tetany" disorders in livestock that feed on lush grass. If you are concerned with grass tetany, avoid excessive rates of potassium fertilizer and feed legume hay, which is generally higher in magnesium. Contact your animal feed specialist for specific recommendations concerning supplemental Mg in the ration.
Calcium (Ca)-Well-limed soils generally contain sufficient calcium for the growth of field crops. Even soils needing lime to correct acidity will contain sufficient calcium for plants. The poor growth of plants on acid soils is usually due to excess soluble manganese and/or aluminum, rather than calcium deficiency.
Studies in Ohio, Indiana, Michigan and Wisconsin show alfalfa and corn to yield equally well at a wide range of calcium to magnesium ratios. Therefore, adding calcium to improve the calcium to magnesium ratio is not necessary.
Sulfur (S)-Sulfur is an essential nutrient found in plants in about the same concentration as phosphorus. Current intensive cropping systems, use of higher analysis fertilizers with lower sulfur concentration and the cleanup of industrial smokestacks point in the direction of more widespread need for sulfur. However, field studies with several sulfur responsive crops on numerous sites do not show any benefit from sulfur applications. Even though the surface soils on some of these sites tested low, the subsurface soils supply more than adequate quantities of sulfur to meet plant needs. Sulfur deficiency would most likely occur on alfalfa grown on sandy soils.