Young soybean plants close up

When it comes to maximizing yields, farmers are always listening. However, soybean agronomist Dan Davidson said one nutrient is being overlooked.

“Sulfur in soybeans has, in my opinion, been unfortunately ignored,” Davidson said. “Farmers with corn, it’s like a no-brainer. But with soybeans, they still are not quite catching on.”

Davidson said that while grain crops like corn and wheat tend to be more responsive to sulfur application, getting the nutrient on soybeans is equally important.

“Soybeans need sulfur just as much as alfalfa and corn,” he said. “Two of the four most important amino acids in soybean meal tend to be deficient, and they contain sulfur. Methionine and cysteine are sulfur containing, and they tend to be short in soybean meal.”

Dan Kaiser, a nutrient management specialist with the University of Minnesota Extension, said there have been good and bad results from the direct application of sulfur to soybeans.

Soybeans are a “balancing act” when it comes to fertilization, he said, and sometimes sulfur can cause extra vegetative biomass.

“It tends to produce more green material and possible not produce as much grain,” Kaiser said. “In terms of nutrients, that’s one thing I’ve seen with sulfur, that with nitrogen it can stimulate more growth and really not see any advantage in terms of yield.”

He noted that with the increased biomass, he’s even seen cases where yields have decreased or disease pressure is raised because of a denser canopy.

He said that where he works in Minnesota, there tends to be a bigger benefit of direct sulfur application in coarse sand, but the irrigated ground in his state takes away some of the benefits.

He noted that interest has increased over the last few years as farmers see the benefits in corn and look to improve their soybean yield.

In the past, much of the sulfur could come from atmospheric deposits or soil organic matter, but Davidson pointed to the introduction of the Clean Air Act as a major factor for the change.

“It goes right back to the Clean Air Act,” Davidson said. “In the ’70s, we had to clean up our environment, and one of the pollutants was sulfurous oxides. We had to clean those up. They were getting in the atmosphere, creating acid rain and damaging vegetation and infrastructure.”

With cleaner air, there is less sulfur coming from the atmosphere — “miniscule” compared to what they used to be, Davidson said.

However, the answers of how much sulfur to apply, when to apply and whether it is worth it to apply directly on beans varies depending on who you ask.

Davidson said his recommendation is 15-20 pounds per acre per year on a lighter soil with lower yield potential, regardless of crop. On a heavier soil, he suggested 20-25 pounds per acre each year.

“It doesn’t really matter, corn or soybeans,” he said. “You really need to be thinking about applying that much every year. … By managing soybeans as well as corn, you will get the same kind of dividends in yield as you do from corn and can improve protein quality.”

However, Kaiser said applying that much ahead of a corn crop should leave enough for soybeans to see the benefits of the sulfur the next year, while limiting the negative biomass effects.

“Where I’ve seen some big yield gains in soybean fields is where we’ve put 20-25 pounds of ammonium sulfate ahead of corn,” Kaiser said. “We still get that yield advantage in the beans without getting the vegetative biomass. There are circumstances out there where it may benefit beans, but what I’ve been encouraging to growers is to think ahead and apply to corn, and not have as great a risk to the beans.”