Serving as a building block for protein synthesis, and especially for the formation of amino acids, sulfur (S) impacts the quality and quantity of protein available in forage for livestock. Adequate sulfur levels are also required for plants to metabolize nitrogen properly, creating healthy, high-yielding forage. University of Wisconsin-Madison Division of Extension educator Scott Reuss identified the important role that sulfur plays in a forage’s diet in a Midwest Forage Association (MFA) Clippings article.
In the 1990s, the Midwest was maintaining its cropping system sulfur needs due to pollution-induced atmospheric deposition. According to the National Atmospheric Deposition Program, sulfur deposition across Wisconsin neared 10 pounds per acre per year in the 1990s. Fast forward to today, and that number has dropped significantly to now less than 4 pounds of atmospheric sulfur deposition per acre. Supplying 2.8 pounds of sulfur per acre per percent organic matter, S can be available naturally. However, natural S levels do not cut it, simply because of the more concentrated manure applications typical of Midwest operations.
A recent MFA-funded study in northern Wisconsin tested whether yield and protein content would decline in alfalfa if there was not enough S. The study confirmed this theory, showing that adding additional sulfur boosted yield and protein content in alfalfa. “Across the study’s tested sulfur application rates (0,15,30, and 45 pounds S per acre), the 30-pound rate achieved the highest return on investment. The study utilized elemental S and maintained a strongly positive yield effect for at least 18 months after application,” Reuss explained.
In terms of how much and when we should be applying S, the answer depends. As sulfur compounds are converted by soil bacteria into sulfate, and this makes it almost as mobile as nitrate in the soil. “Thus, we need to plan on applying sulfur annually, with exceptions to higher organic matter content soils, or those with clay subsoils, which can bind sulfate and allow somewhat deep-rooted plants good S availability. The ‘how much’ S fertilizer portion of the questions depends primarily on the crop being grown and if manure has been applied recently,” noted the educator.
Manure should supply 1 pound of S per ton solid and 1 to 2 pounds per 1,000 gallons of liquid. If manure is not a part of the fertilization plan, S applications can range from 10 to 25 pounds per acre applied as sulfate. It is recommended to incorporate 25 to 50 pounds of S per acre before seeding an alfalfa stand, although these rates will vary across the Midwest.
