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Research Highlights
Increasing Sulfur Levels Can Improve Soybean Quality

By Carol Brown

For farmers, growing a successful crop requires the right balance of nutrients in the soil. Most focus on nitrogen, potassium and phosphorus.  Sulfur, which is needed in similar quantities to phosphorus, is a key nutrient for soybean quality but not often considered when growing the crop.

Ray Weil is working to improve soybean yield and quality by making sure the crop gets enough sulfur. The University of Maryland soil science professor has been working for many years to improve soybean quality by increasing its sulfur percentage. Through a current research project, supported by the Maryland Soybean Board, he has been exploring methods of applying sulfur to the crop as well as more efficient ways to measure sulfur amounts in the harvested soybeans.

Soybeans in field trials show sulfur response compared to no-applied sulfur. Photo: Ray Weil

“Humans and non-ruminant animals, including chickens and pigs, need essential amino acids to process protein. Soybeans contain all the essential amino acids, but are typically low in the two that contain sulfur: methionine and cysteine,” said Weil. “These often have to be added artificially to rations for livestock as soybeans don’t provide enough naturally.”

Since the 1990 amendment to the Clean Air Act, U.S. factories and power plants have greatly reduced their contribution to air pollution, especially sulfur dioxide gas. With coal-powered plants adding sulfur scrubbers to clean their emissions, there is less sulfur going into the atmosphere. This is good for humans as sulfur dioxide is toxic when we breathe it. But this decrease of sulfur in the atmosphere means plants aren’t getting as much as they used to.

Sulfur deficiencies are being found in Maryland’s sandier soils and increasingly across the U.S. as well, Weil said. Producers need to overcome sulfur deficiencies in their soils in order to grow a higher quality soybean crop.

Weil has been successful in his efforts to improve soybean quality by adding sulfur to deficient soils and by adding it as a foliar spray directly to the soybeans when they start to flower. And he has seen some yield bumps, too.

“We were able to improve soybean protein by nearly doubling the percentage of methionine and cysteine, two amino acids that affect the nutritional value of the soybean,” Weil said. “Also, farmers saw a 5 to 10 percent yield increase; getting a few extra bushels was nice — more than enough to pay for the sulfur treatment.”

Aerial image of field with sulfur application trials on soybeans in Maryland. Photo: Ray Weil

Weil and his students have been exploring different ways of getting sulfur to the soybean for the best yield and quality response. He compared plots treated with gypsum, a calcium sulfate; Epsom salts, a magnesium sulfate; and plots with no applied sulfur. He applied ground gypsum to the soil at planting time and sprayed Epsom salt solution on soybean fields at first flower. Both methods of applying sulfur worked to give a yield increase and to boost the methionine and cysteine levels in the seed.

“For a farmer, it may be easier to apply sulfur with a spray rig, rather than dealing with a dry powder,” Weil said.

Proving that sulfur amendments work

After the sulfur is applied, how does one know if it is working? Another prong of Weil’s research is to find a better way to detect nutrient levels in harvested soybeans. Current methods for analyzing amino acids are slow and costly. This is where the special X-ray gun comes in. Weil used a portable x-ray florescence analyzer to read sulfur levels in soybeans. Combining this technology with a simple coffee bean grinder, he and his research team at the University of Maryland could read sulfur levels in ground-up soybeans in as little as two minutes.

“For the day to arrive when farmers get paid a premium for growing high-quality soybeans with extra amino acids in them, somebody is going to have to test them to ensure the quality is there,” Weil said. “And it will have to be quick and easy. I envision someday we can test truckloads of soybeans with a simple x-ray scan.”

Weil also wants to improve the ability of soil tests to identify which fields need more sulfur and which don’t. When soil samples are sent to a laboratory, the lab usually reports levels of a number of nutrients extracted from the soil, including sulfur. Weil said that labs don’t have good crop response data for sulfur, unlike phosphorus and potassium, on which to base their recommendations. In other words, with currently available soil tests, lab results don’t show how much is enough and what is deficient. He and his team compared four different soil testing methods on 23 fields over three years to determine which test was most predictive of crop response to field-applied sulfur. Included in the four tests was the Mehlich-3 test, commonly used in soil test labs in the Mid-Atlantic region.

The good news is that all four tests were able to identify fields that had plenty of sulfur and would not benefit from adding more. Unfortunately, Weil said none of the tests were very reliable in identifying fields that would definitely benefit from adding sulfur. For now, Weil thinks chances are pretty good to get a profitable response to sulfur by soybeans grown on sandy soils and not treated with livestock manures.

The solutions Weil is finding for increasing sulfur amounts in soybeans will help farmers grow a higher-quality product. Someday farmers will be able to accurately check sulfur levels in their soils and have the most practical ways to apply it to their fields. Increasing soil sulfur content leads to higher levels of amino acids for improved soybean protein quality. All of this makes a higher quality food product for both livestock and humans.

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.