This week’s column discusses the macronutrients known as secondary nutrients: Sulfur (S), Calcium (Ca), and Magnesium (Mg). Calcium and magnesium are found as cations, positively charged ions in the soil, while sulfur is typically found as sulfate, an anion, negatively charged.
While are three are important, sulfur is especially critical to producers. Proteins are made up of amino acids and all plant proteins need sulfur containing amino acids. Sulfur deficiencies result in lower protein levels in grains such as wheat. Certain vitamins such as thiamine, necessary for human health, contain sulfur. It is important in chlorophyll formation and aids in seed production. Plants such as garlic, mustard, and onions produce sulfur containing volatile compounds giving them their characteristic odor and taste. Plants deficient in sulfur are often a pale green and unlike nitrogen (N) and potassium, the symptoms show up on younger leaves first. Poor, spindly growth may also occur. Sulfate-S is not held by soils and is subject to leaching like N and a soil test is to a depth of two feet like N-tests is needed. Sulfur is primarily found in organic matter. Deficiencies are becoming more common for several reasons. Fertilizers are purer now and no longer have sulfur as an impurity. Scrubbers have been placed on coal burning power plants and S has been removed from fuels so S no longer comes from acid rain here. And finally, crop yields are much greater and we are removing more sulfur with crops. Sulfur deficiencies are more pronounced in sandier soils low in organic matter. More and more producers are using sulfur fertilizers, especially to boost wheat protein levels, in alfalfa production, and oil seeds. The most common sources are ammonium sulfate, calcium sulfate (gypsum), thiosulfates, and K-Mag. Elemental sulfur is also used but more cumbersome to work with and a slow release source. Since these are readily available, it is possible to use S fertilizers as in-season rescue treatments if early enough.
Grass crops need little calcium compared to broadleaf plants. It is an important component of plant cell walls; is involved in cell elongation/division, membrane permeability, and enzyme activation. Even under fairly acid conditions, there is normally adequate Ca available. Limestone, calcium carbonate, is calcium source, however it is used primarily to correct acid soil conditions not fertilize for Ca.
Magnesium is critical in photosynthesis as it is the atom that captures the energy of the sun in the light reaction so it can be used to make sugar in the dark reaction. Mg is also involved in oil and protein synthesis, and enzyme activation. Deficiencies show up as interveinal yellowing on older leaves and stunted growth. Lack of magnesium in grasses grazed by cattle can result in grass tetany, staggers, which is prevented by Mg supplementation. Ca and Mg come from the materials forming the soil as they weather and some from the decomposition of plant residues. Severe Mg deficiencies aren’t common and can be corrected with magnesium sulfate (Epsom salts) or K-Mag. Next week finishes this series with the micronutrients.
Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.