The Skinny on Sulfur

Evidence mounts that you should consider sulfur fertilizer for corn, particularly in fields low in organic matter. It can take as little as 10 pounds per acre of sulfur, typically at a cost of 50¢ per pound or less to give a corn yield response, says Dan Kaiser, University of Minnesota Extension agronomist. Sulfur is particularly beneficial on soils under 2% organic matter.

"Sometimes, 20 pounds per acre gives the best response," he says. "It doesn't take much, and the crop doesn't care when it gets it. It tends to carry over, too. If you apply sulfur at 20 to 25 pounds per acre, you can get two years' response from that one application in medium- and fine-texture soils."

In tests done on southern Minnesota farms, a sulfur investment of 20 pounds per acre bumped yields by 7 bushels to 20 bushels per acre. Sulfur fertility principles should work anywhere else in the Corn Belt, Kaiser says.

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Response to sulfur fertility has increased in recent years for two reasons.

1. Farmers apply less livestock manure. Since livestock manure contains significant amounts of sulfur, fewer applications may be the reason crops respond to sulfur applications in some years.
2. Anti-pollution laws work. Smokestacks used to churn out plenty of sulfur that fell from the sky in the form of rainfall. As power plants have adopted cleaner-burning fuels, they discharge less sulfur.

"The effect of industrial emissions can be far-reaching," says Kaiser. "However, laws have restricted the emissions. The total amount of sulfur applied through rainfall has greatly decreased."

In fact, maps from the National Atmospheric Deposition Program show that sulfur in rainfall in the Corn Belt is about 10 pounds per acre less than 30 years ago. Sometimes, says Kaiser, enough sulfur gets applied incidentally from impurities in other chemical applications or in irrigation water. Beyond that, it comes from mineralization of the soil, manure applications, or fertilizer.

Kaiser is now interested in learning how sulfur interacts with other crop nutrients. There doesn't appear to be an interaction with potassium and phosphorus levels, he says. However, a lack of sulfur in the soil could affect nitrogen uptake of some crops.

"We have seen two instances of this interaction," says Kaiser. "First, when sulfur is applied and nitrogen (N) is limiting, the sulfur will not be effectively utilized. This occurred in one field site in southeastern Minnesota where corn grain yield was increased by 25 bushels per acre with sulfur – but only when N supply was adequate.

"Second, in studies where sulfur is deficient, it is not uncommon to observe visual sulfur deficiencies on plants in the middle- to late-growing season. In this case, growers may sidedress nitrogen when they should be applying sulfur."

Limited soybean response
Kaiser has seen more limited response of soybeans to sulfur. "In the 2014 crop year, we did see a response in soybeans in two of our research locations. It was not from sulfur applied in 2014; it was from the year before when it was applied to the corn crop. At one location, we got a 22-bushel corn yield advantage from 20 pounds of sulfur in 2013. In 2014, we got 5.6 bushels on the soybeans.

"Cool, wet years like 2014 may be more conducive to sulfur deficiencies in crops like soybeans, which normally don't respond to the nutrient," says Kaiser.

Sulfur is a component of proteins, and soybeans are high in protein, he points out. They normally don't respond with higher yields. Soybeans may respond only in certain years and in fields with very low organic matter (2% or less in the top 6 inches), Kaiser says.

"My main concern with soybeans is, if you overapply nutrients, you can increase vegetative biomass, which is not necessarily a good thing considering disease pressure or drought years," he says. "I suggest that farmers apply sulfur before corn, which has a higher response rate. Then, any sulfur not used will likely carry over to the soybean crop, particularly if the soil is medium to fine texture."

He prefers to use organic matter levels to determine applications of sulfur rather than using soil tests.

"Organic matter relates well to the pool of sulfur that is available to your crop. It will give a better assessment of whether you'll have enough sulfur in a given year," he says.

READ MORE: Sulfur gets mixed reviews

The skinny
Corn uses 20 to 30 pounds of sulfur per year. One half of the sulfur gets harvested; one half goes back into soil in the stover. It's found naturally in soil organic matter. It's taken up in sulfate form and is typically applied as ammonium sulfate, calcium sulfate (gypsum), or elemental sulfur (must be oxidized to sulfate before it can be utilized). It can be surface-broadcast ahead of the growing season.

If you apply manure to meet N recommendations, you probably will meet sulfur needs.
Dan Kaiser recommends:

• Applying 10 to 25 pounds of sulfur in soils with less than 2% organic matter.
• Applying 10 to 20 pounds of sulfur in soils with 2% to 4% organic matter.
• Applying 0 to 20 pounds of sulfur in soils with more than 4% organic matter.

Faster drydown
Another benefit University of Minnesota Extension agronomist Dan Kaiser has seen from sulfur applications comes at corn harvest.

"With added sulfur, we sometimes see an earlier drydown of the grain. The savings in drying costs may pay for the sulfur."

Kaiser isn't sure if sulfur actually speeds up corn maturity or if the lack of sulfur delays it.

"We can see stress in Minnesota some years early in the season due to the cool temperatures," he says. "A temporary deficiency in sulfur may slow down growth, which could delay silking. In some instances, we see lowered grain moisture at harvest with sulfur applications.

"It may be due to the fact that a slight deficiency in sulfur delays maturity," he adds. "I see that in roughly 15% to 25% of my trials where we don't get a yield increase due to sulfur."