Balancing sulfur for optimal crop growth

Sulfur (S) may not be needed in quantities required for nitrogen, phosphorus, and potassium, but it’s still important for crop production. It’s essential for forming beneficial amino acids, proteins, and oils, and it’s needed to produce chlorophyll.

“S is a nutrient that is present primarily in organic form, as very little is in an inorganic form,” says Dorivar Ruiz Diaz, a Kansas State University soil fertility specialist. “The organic form goes through a mineralization process similar to nitrogen and releases a sulfate that is taken up by crops. In most growing systems, soil is the main source of S. The second source is deposition from the air, and fertilizer is the third source.”

Crops used to be able to use atmospheric S. However, antipollution laws work and have reduced atmospheric S levels. Now, crops often need S supplementation.

“There are two main sources of sulfur,” says Daniel Kaiser, a University of Minnesota (U of M) soil scientist. “Sulfate forms include ammonium sulfate or calcium sulfate, which is gypsum. There’s also elemental S, which comes in multiple packages. I caution growers that it takes a while for elemental S to oxidize or become plant available.”

Kaiser adds the speed of oxidization can become an issue as growers will apply elemental S and still see S deficiency, as it does not oxidize quickly.

He and Ruiz Diaz recommend four ways to get top performance from S applications.

1. Know your crop.

Target S application to crops that are more sensitive to S deficiency. For example, U of M researchers found targeting fields in a corn year delivers a higher chance of return, as some S is carried over for the soybean year.

2. Know your soil.

Understand what’s in your soil and what type it is. Determine if atmospheric deposition of S can help meet S requirements. If you irrigate, learn how much sulfate is in the water. These affect how plants respond to available S in the soil, and they’ll help you determine if you need to add the nutrient.

“Check the level of S in your soil profile regularly,” Ruiz Diaz advises. “Go deeper than 6 to 8 inches, because there can be accumulation of S in heavier clay at 12 inches or 18 inches and the roots are going to get to it. Simply collecting the first 6 inches of soil won’t tell the whole story.

“Don’t test for S alone,” he says. “Understand the organic matter and texture of your soil because S is a mobile nutrient and test results can vary on different areas in a field.”

3. Know your S.

“Always understand the type of S you’re using,” Ruiz Diaz says. “Using S is relatively new in some areas, so farmers may not be completely clear on the different sources of fertilizers. Is it elemental S or a micronized sulfate? If the former, it’s important to apply it at the right time. Because if you apply elemental S just before planting, none of the S will be available for the early stages of crop development.”

Kaiser adds that most starter fertilizer options can cause issues with seed placement. “You can’t apply in-furrow,” he says. “Any of your sulfate sources, even a potassium thiosulfate, can damage seedlings. Source and placement must factor into your application method.”

4. Use the 4Rs to guide S applications.

Kaiser and Ruiz Diaz say the principles of the 4Rs — using the right nutrient source at the right rate, right time, and right place — are the proper guidance for S application.

S is one of the nutrient types better supported by 4R guidelines, says Ruiz Diaz. The source of S determines the best time for application. The placement goes along with the rate of application because of the nutrient availability. Plus, the interaction of S with other nutrients is important, as it is a key protein.

“If we don’t have enough S, the plant can’t build the right proteins, no matter how much N you apply,” he says.

Kaiser urges caution when formulating S application amounts.

He says most crops will remove about 25 pounds per acre of S, but two-thirds to three-fourths of that amount will come from the soil, so applying 25 pounds per acre is likely too much.