4th Strategic Farming: Let's talk crops! webinar focused on sulfur

Sulfur study near Albert Lea in 2019. No sulfur applied on left plot. Sulfur applied in a 2x2 band on the right.

By: Phyllis Bongard, Extension content development and communications specialist

Sulfur (S) might be considered a secondary nutrient, but it is essential for crop production. Dr. Dan Kaiser, Extension nutrient management specialist, and Jeff Vetsch, Soil scientist at the Southern Research and Outreach Center, joined Extension educator Ryan Miller for a wide-ranging discussion on new findings in sulfur fertility in the January 26 session of the webinar series, Strategic Farming: Let’s talk crops!

New findings in sulfur fertility

Recognizing a change in sulfur needs

Historically, sulfur fertilizer was primarily recommended on low soil organic matter soils and coarse-textured sandy soils. Then in a liming study that ran from 1999 to 2006, Vetsch started to recognize slight corn yield differences when S had been applied. One of the liming treatments in the Waseca study was gypsum, a fertilizer that contains sulfate. While the differences between the sulfur-containing gypsum and non-sulfur fertilizer treatments weren’t large, they were fairly consistent over the six-year study on a high organic matter, fine-textured soil.

In 2004, Gyles Randall, a retired soil scientist at Waseca, initiated a NPKS fluid starter study that included several sources. In the final year of the study, 2006, the treatments containing S showed a large, significant 15-18 bushel response.

Role of reduced sulfur emissions

What was behind the change in S needs on medium- to fine- textured soils? Researchers threw out several potential causes; perhaps it was higher yielding corn or perhaps fewer sulfur impurities in phosphorus fertilizers, but the cause that stands up to scrutiny is the role of reduced sulfur emissions from power plants.

As part of the Clean Air Act, the Acid Rain Program begun in 1995 required major reductions in sulfur dioxide emissions and set permanent caps for primarily coal-burning electric plants. Then in 2015, the Cross State Air Pollution Rule was enacted to reduce downwind fine particulate (soot) pollution, further reducing sulfur dioxide emissions.

How did the programs perform? Sulfur dioxide emissions have decreased from a level of 17 million tons per year in 1980 to less than 1 million tons in 2020. As a result, atmospheric deposition of sulfur today is relatively small and not a significant source for crops.

Additional sulfur research

With renewed interest in sulfur research, A 2010 study in Waseca opened researchers’ eyes to what a sulfur response on heavy, high organic matter soils could look like. In this source and timing study, ATS applied at planting at 5.6 lb S/A showed a significant 24 bushel response over the control. Preplant ammonium thiosulfate (AMS) or gypsum broadcast incorporated showed a 29 to 45 bushel increase over the control for 10 and 20 lb S/A, respectively. Gypsum applied in-season at V5 also performed very well (41 to 43 bushel response for 10 and 20 lb S/A, respectively). These results were a game changer for S research in Minnesota.

When Kaiser arrived in Minnesota 13 years ago, he didn’t expect to do a lot of research on sulfur, but it has comprised a third to a half of his research activity. Updated sulfur guidelines for corn is just one outcome of this research.

Considerations for sulfur management

Sources

Sulfur fertilizer source studies are at the core of Minnesota research today. While fertilizers that contain sulfate are preferred, Kaiser is interested in how elemental sulfur oxidizes and becomes available for the crop, particularly with a fall application.

In a current 3-year study, source responses have been more consistent at Rosemount and Waseca compared to Becker and Morris. At Rosemount and Waseca, potassium (K) sulfate and K-MST (Micronized Sulfur Technology) performed equally well and better than elemental S (Tiger 90). However, if finely ground elemental S can be dissolved, it can oxidize in our soils and become available for crop use.

Another source, irrigation water, provided roughly 20 to 25 pounds of sulfate per acre during the growing season at several UM study sites. At these levels, corn sulfate needs were met. If you irrigate, it could be interesting to see how much sulfate is provided through your irrigation water, especially if you already test for nitrate.

Crop response

Due to their responses on fine-textured soils, corn and alfalfa are the two crops that are targeted for sulfur fertilizer applications. For more information on alfalfa, see Should you apply sulfur fertilizer for alfalfa? and Does source of sulfur fertilizer matter for alfalfa?

Soybean response to sulfur has not been consistent, so S is recommended under very limited circumstances. See Soybean sulfur needs for more information.

Recent research suggests that sulfate-sulfur can carry over from one year to the next when it’s applied in excess of crop uptake. When this is the case, carry over sulfate-S may be sufficient to meet the following crop’s needs.

Rate

For corn, broadcast sulfate-S guidelines are based on crop rotation, soil drainage and soil organic matter. For more information, see the sulfur guidelines that are included in Fertilizing corn in Minnesota.

Timing

In Kaiser’s research, he has seen maximum corn yields when S was applied in-season at V5, but Vetsch’s work shows that these in-season applications can be accompanied by an uptick in grain moisture. This is an area that is being studied.

Final thoughts

Kaiser reinforced that there’s no one-size-fits-all approach for sulfur fertilization. Since this is a dynamic system, there are many options to best fit your crop rotation and tillage operations. If you see anything that might be interesting for follow-up research, contact him at dekaiser@umn.edu.

  

Join the webinar series

University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series features discussions with specialists to provide up-to-date, research-based information to help farmers and ag professionals optimize crop management strategies. For more information, visit z.umn.edu/strategic-farming.

Thanks to the Minnesota Corn Growers Research and Promotion Council and the Minnesota Soybean Research and Promotion Council for their generous sponsors of this program!