Agronomy: Soybeans Require Sulfur

Do soybeans respond to sulfur? That is a question that has been on my mind recently. Do we really know if they do, and if so, what is the best way to provide it? Understanding whether soybeans respond to sulfur is a bit like understanding how soybeans respond to nitrogen. Response depends on growth and yield potential, nitrogen fixation, available and mineralizable sulfate and the weather.

Soybeans, like all crops, require sulfur (S). Sulfur is a secondary nutrient alongside calcium (Ca) and magnesium (Mg), and is behind the primary nutrients of nitrogen (N), phosphorus (P) and potassium (K). Sulfur plays a key role in protein synthesis and is essential for many plant functions since it is a component of amino acids, protein and peptides. When it comes to sensitivity to sulfur deficiencies, soybeans seem to rank third behind alfalfa (1st) and corn (2nd).

So, besides being an important component in proteins that are made of long chains of amino acids (some which contain sulfur), Mg also is an important storage protein in soybean seed. Cysteine and methionine contain S, and are essential amino acids in plants and limiting amino acids in soybean meal, meaning that nutritionists have to supplement soybean meal with synthetic cysteine and methionine.

Sulfur has become more important as a limiting nutrient in crop production in recent years for several reasons. These include higher crop yields that require more sulfur, fewer sulfur impurities in modern fertilizers, less use of sulfur-containing pesticides, reduced industrial sulfur emissions to the atmosphere, soil organic matter levels that are too low to provide enough sulfur and a greater awareness of sulfur needs. We can’t rely on significant atmospheric deposits but instead must rely on what is mineralized from soil organic matter or what can be added through supplemental fertilizer.

But sulfur also is important to nodulation and nitrogen fixation in soybeans. Could a lack of sulfur or sulfate be a limiting factor to nitrogen fixation? Larry Purcell, a crop scientist at the University of Arkansas wrote in an email, “You are correct in that nodules and N fixation do require a good bit of S. It basically comes down to nodules are very high in protein, and nitrogenase (also) has iron-sulfur cofactors. Because S is a component of two amino acids, S can limit N fixation and ultimately yield.”

He added “My soil colleagues tell me that S deficiency has increased in frequency since the U.S. is not burning high-S coal and since the U.S. has cleaned up its air. In most soils, however, sulfur is not a limitation; the exceptions would be sandy soils (with a high leaching potential) or muck soils. I would add in soils with 1.5% or less organic matter (e.g., sandy soils), not enough S is mineralized to provide a big enough supply to the crop.”

Soil tests for sulfur and sulfate are considered unreliable predictors of sulfur availability. However, tissue tests are more reliable indicators of what the plant is seeing. Building soil organic matter to 2% or greater and then having a biologically active soil that is continuously recycling carbon and nutrients is a better indicator of making sure the soil supplies enough sulfur to the crop.

Sulfur can be applied through manures, elemental sulfur, gypsum (calcium sulfate), ammonium thiosulfate or potassium thiosulfate, and it is often a component in many starter fertilizers.

Let us know in our forum if you are using sulfur on soybeans. If you do, when and how do you apply it?

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.