Will soybeans respond to sulfur?

Sulfur (S) is becoming a more deficient nutrient, following phosphorus and potassium. Alfalfa and corn producers are already adding sulfur into their fertility program. Should we be including sulfur in our fertility program for high yield soybeans?

Sulfur deficiency symptoms include stunted plants; pale green color indicating nitrogen deficiency except chlorosis is apparent on upper leaves while nitrogen deficiency appears first in lower, older leaves.

Sulfur has become more important as a limiting nutrient in crop production in recent years. These include higher crop yields that require more S, less S impurities in modern fertilizers, less use of pesticides containing S, reduced industrial and automotive S emissions to the atmosphere, soil organic matter levels that are low to provide enough S, and a greater awareness of S requirements.

Atmospheric sulfur deposition in 1990. Atmospheric sulfur deposition in 2012

After the introduction of the Clean Air Act decades ago, atmospheric deposits continue to decline. Growers can no longer rely on significant atmospheric deposits and or what may be mineralized from soil organic matter to meet their yield requirements.

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 rank third behind alfalfa (1st) and corn (2nd). And in crops like soybeans where one of the end products is soybean meal and has two amino acids containing sulfur – cysteine and methionine. These two amino acids are considered essential and limiting in soybean meal.

Sulfur is also important to nodulation and nitrogen fixation. Larry Purcell, Ph.D., 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 the fact that 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.”

There are plant tissue and soil tests for sulfur. Andy Wycislo, Ph.D., Midwest Division Agronomist for Waypoint Analytical, will be posting a blog this spring on soil and tissue tests for sulfur and relevance to soybeans.

Sulfur can be applied through manures, elemental sulfur, gypsum (calcium sulfate), ammonium sulfate, ammonium thiosulfate or potassium thiosulfate. If you apply elemental S, microbes need to convert it into sulfate to be taken up by the plant. Apply sulfate with elemental sulfur to ensure some sulfate is readily available for crops. Liquid thiosulfate products supply sulfur but half of the sulfur in thiosulfate is in the elemental form and half is sulfate.

Sulfate also mineralizes from soil organic matter. Mineralization is contingent on microbial breakdown of organic matter. Cool and wet soils slow mineralization and increase the need for sulfur in sensitive crops in the spring. Poorly drained soils also can show signs of sulfur deficiency early in the growing season. Application of as little as 10 pounds of sulfur per acre can go a long way to alleviating sulfur deficiency early in the growing season.

When considering S, remember the 4Rs – the right source, at the right rate, at the right time and in the right place. Sulfate acts very similar to nitrate in the soil environment and can leach or volatilize.

Agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agrwrite.com.