Calcium, Magnesium, and Sulfur

Calcium (Ca), magnesium (Mg), and sulfur (S) are the three secondary nutrients required by plants, but less likely to be added as fertilizer as N, P or K. Most soils in Indiana, Michigan, and Ohio adequately supply these nutrients for plant growth. A 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.

Table- Recommended Mehlich-3 Soil Test Critical Levels Ca, Mg,  and S for Field Crops in the Tri-State Region

Excessive use of K fertilizers can greatly reduce the uptake of Ca and Mg. Forage with high K or low Mg concentrations 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 concept above the critical K soil test level. Potassium recommendations above the critical level are less than crop removal so as to discourage excessive uptake (i.e., luxury consumption) of K and improve Mg uptake. This can sometimes result in fields rotating out of alfalfa to have depleted soil test K levels.

Sulfur removal rates are listed in Table 29. A typical corn and soybean rotation removes approximately 20 lbs of S/acre (9 lbs of S/acre with 180 bushel/acre corn; 11 lbs of S/acre with 60 bu/acre soybean). Recent field trials in the tri-state region have shown infrequent crop response to S fertilization. If S deficiency is expected, applying 10-20 lbs S/acre every year typically supplies sufficient nutrition for grain crops.

Sulfur Removal Rates in Field Crops

Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa

Executive Summary  |  Soil Sampling, Handling, and Testing  |  Soil pH and Lime Recommendations  |  Nitrogen  |  Phosphorus and Potassium  |  Calcium, Magnesium, and Sulfur  |  Micronutrients  | Additional Resources  |  Authors and Acknowledgements

This website provides a summarized version of the Tri-State Fertilizer Recommendations, reporting only the main points of the document, but lacking comprehensive detail. For complete information, please see the full version which will be available soon from The Ohio State University Extension Publications Store.