Alfalfa fields may need more sulfur as the natural occurrence of the nutrient continues to decline, says Mark Sulc, an Ohio State University Extension forage specialist.

In a four-year study, alfalfa fields with sulfur added to the standard fertilizer recommendation yielded about a half-ton per acre more dry matter each year than those with only recommended fertilizer.

Sulc and colleagues compared fields with no fertilizer added to those with annual applications of phosphorus (P) and potassium (K), P and K with 75 lbs/acre of elemental sulfur applied only at planting, and P and K with 1,470 lbs/acre of gypsum added on the surface each year.

“Alfalfa responded positively to the addition of sulfur,” Sulc says. “When elemental sulfur was applied with P and K, the response was not seen in the first year, but averaged 0.7 ton/acre more than P and K alone thereafter.

“Elemental sulfur must first be converted to the sulfate form before it is available to plants and that takes a little while,” he says. “The annual broadcast applications of FGD (flue gas desulfurization) gypsum plus P and K were slightly higher yielding than P and K alone in the first year, and then averaged 0.5 ton/acre more.”

The sulfate within the gypsum likely produced that yield increase, he explains.

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The amount of sulfur reaching fields has been declining for decades because of a reduction in acid rain caused by sulfur emissions, Sulc says. Sulfur deposited from those emissions has declined 31% since 1980 as a result of the Clean Air Act.

The increased use of fertilizers containing little or no sulfur, higher crop yields that remove more sulfur from the soil, and fewer sulfur-containing pesticides have all contributed to the decrease in sulfur in ag soils.

An increase in specialized grain-crop farms that have no animals with manure to apply to the fields has also contributed to the decrease.

“Growers in Ohio (where the study was conducted) may need to be aware of the increasing potential for sulfur deficiencies in alfalfa and perhaps in other forages,” Sulc says. The study was conducted at only one site, but researchers have found positive responses to sulfur application in previous studies. “If the crop doesn’t seem to be coming up to expectations, a tissue test should be able to show if there is a problem.”

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