Michigan State University Extension
Soils & Soil Management - Fertilizer - 06039718
07/10/97
Tri-state Fertilizer Recommendations for Corn, Soybeans, Wheat and Alfalfa Extension Bulletin E-2567 (New), July 1995
M.L. Vitosh, Michigan State University; J.W. Johnson, The Ohio State University; D.B. Mengel, Purdue University; Co-editors
Calcium (Ca), magnesium (Mg) and sulfur (S) are the three secondary nutrients required by plants. They are less likely to be added as fertilizer than the macronutrients (N-P-K). Most soils in Indiana, Michigan and Ohio will adequately supply these nutrients for plant growth. The standard soil test measures the relative availability of Ca and Mg in soils. There is no accurate soil test for S at this time. A plant analysis is the best diagnostic tool for confirming S availability.
If the exchangeable Ca level is in excess of 200 ppm, no response to Ca is expected. If the soil pH is maintained in the proper range, then the added Ca from lime will maintain an adequate level for crop production.
The required soil exchangeable Mg level is 50 ppm or greater. Low levels of Mg are commonly found in eastern Ohio and southern Indiana and on acid sandy soils in Michigan. High levels of exchangeable K tend to reduce the uptake of Mg. Therefore, if the ratio of Mg to K, as a percent of the exchangeable bases, is less than 2 to 1, then Mg is recommended for forage crops. Most Mg deficiencies can be corrected by maintaining proper soil pH using lime high in Mg. The ratio of Ca to Mg should be considered when lime is added to a soil. If the ratio, as a percent of the exchangeable bases, is 1 to 1 or less (less Ca than Mg), a high calcium/low magnesium limestone should be used. Most plants grow well over a wide range of Ca to Mg soil ratios.
Excessive use of K fertilizers can greatly reduce the uptake of Ca and Mg. High K/low Mg forages can cause grass tetany, milk fever, hypocalcemia and other health problems for ruminant animals. For these reasons, the tri-state K recommendations for alfalfa and corn silage do not follow the maintenance plateau concept above the critical K soil test level. Potassium recommendations above the critical level are less than crop removal so as to discourage luxury consumption of K and improve Mg uptake.
Sulfur is taken up as sulfate by plants. Sulfate sulfur is supplied primarily by microbial decomposition of soil organic matter. Sulfate is a negative ion and easily leaches in soils. Most soils in Indiana, Michigan and Ohio will adequately supply needed sulfur for plant growth. Sandy soils low in organic matter that are subject to excessive leaching may not supply adequate sulfur. Crops such as wheat and alfalfa that grow rapidly at cool temperatures when mineralization of S is slow are most likely to be S deficient. If elemental sulfur is used, it should be applied at least 2 months before the crop is planted. This would allow time for the S to be converted to the plant-available sulfate form by the soil bacteria. Sulfur should be added in the sulfate form if added less than 2 months before plant uptake.