Researchers explore sulfur, micronutrient fertilizer applications

The chemical elements nitrogen, phosphorus and potassium may be well understood among producers as important parts of their fertilizer regimens. The LSU AgCenter is studying some lesser-known elements that may play a vital role in producing viable, profitable soybean and grain crops.

Brenda Tubaña, professor at LSU’s School of Plant, Environmental and Soil Sciences, has been working to determine how the application of sulfur and micronutrients affect plant health, and ultimately crop yields.

“We need to make sure our fertilization guidelines for sulfur are up to date. With the cases of sulfur deficiency in crops increasing, it (sulfur) is now commonly included in crop fertilization programs,” said Tubaña.

Tubaña said sulfur is a vital nutrient for plants because it facilitates the production of amino acids — the building blocks of proteins that are found abundantly in soybeans. Sulfur also aids plants in developing a compound that helps the plant use nitrogen. Sulfur deficiency has not always been an issue for crop production. Tubaña explained how a trade-off of cleaner air has contributed to deficiencies of atmospheric-derived sulfates.

“In the past, sulfur wasn’t really a major issue in crop production in terms of supply,” Tubaña said. “There is less sulfur dioxide in the atmosphere due to the Clean Air Act enacted way back in the 1980s, thus we don’t see a high concentration of sulfate in precipitation.”

The LSU AgCenter’s efforts in studying sulfur application rates are focused on making sure the sulfur fertilizer rates producers are applying to their crops are adequate considering the decrease in sulfates provided by precipitation.

In field trials, Tubaña and her colleagues tested soil treated with sulfur, in various forms, at rates of 20, 40 and 60 pounds per acre. What they found is that due to the nutrient’s mobility, no accumulation of sulfur in the soil was observed, regardless of the rate applied. Like nitrogen, sulfur is a mobile nutrient in the soil; sulfur losses often occur following precipitation events. Proper application techniques are being studied to determine how to decrease nutrient losses caused by leaching or runoff.

Tubaña said one of the sources being studied is pelletized elemental sulfur. In this form, sulfur must be first converted into sulfate, which is the form that plants uptake of this vital nutrient. The benefit to using pelletized elemental sulfur is that it provides a delayed-release characteristic that can slow down sulfur losses from runoff and leaching. The downside to applying this compound in pelletized form is that it takes longer for the crop to be able to uptake sulfur in sulfate form.

Tubaña and her colleagues are testing blends of compounds that can assist producers with application timing and nutrient uptake.

“The pelletized elemental sulfur is considered a slow-release fertilizer, so we are mixing it with the sulfate form, so a fraction of the blend has an immediate source of plant-available sulfur,” Tubaña said. “With the sulfate being mobile, the soil might start losing it, but the presence of elemental sulfur in the blend will later supply the plants to avoid deficiency.”

Other water-soluble compounds being tested for sulfur applications include calcium sulfate and ammonium thiosulfate.

Tubaña said research has shown that grain crops experiencing deficiencies of micronutrients can lead to yield losses similar to crops with deficiencies of nitrogen, phosphorus and potassium. The AgCenter is exploring applying solution and suspension forms of different micronutrient blends of zinc, manganese, copper and molybdate. The micronutrient causing the deficiency is sometimes hard to determine during a crop’s growth stage because of the similarity of micronutrients’ symptomology.

“While you may not see abnormalities, or discoloration, of the crops, the deficiency can be in a state called ‘hidden hunger,’” Tubaña said. “In such a state, a plant is already experiencing deficiencies and starts reducing its yield potential before the visible symptoms occur.”

The functions of the micronutrients being tested are to aid crops in activating enzyme reactions and to prevent lipid peroxidation that may occur naturally during the photosynthesis process. Tubaña said oxidation can lead to damage in plant cells and tissues resulting in impaired growth and yield losses. Micronutrients also play a role in establishing adequate lignin production which gives plant cell walls their rigidity.

“The reinforced mechanical structure of the plant is quite important to reduce lodging and to help control diseases,” Tubaña said. “With stronger structure, pathogens and insects will find it harder to have entry into a plant cell.”

Because micronutrients are needed in such small amounts, as their moniker suggests, a single application of a solution or suspension containing no more than 3%, at most, of a certain element by volume should be sufficient in preventing yield losses caused by micronutrient deficiency. Derek Albert

8/17/2022 7:51:45 PM
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