Sulfur deficiency may already be showing up in some southwestern Wisconsin alfalfa fields, reports Carrie Laboski, soil fertility and nutrient management specialist with University of Wisconsin Extension.
Sulfur-deficient alfalfa plants are lighter green, shorter and more spindly with smaller leaves than healthy alfalfa. “Some fields have patches of lighter green and may be associated with eroded knolls, while other fields have an overall lighter green color.”
Sulfur deficiency can be confirmed with a tissue test. If sulfur concentration is less than 0.25%, an application of 25 lbs sulfur/acre is needed, and should be done before there is substantial regrowth. In general, yield responses to applied sulfur will be greater as the sulfur concentration decreases, although the recommended sulfur rate does not change.
“Also keep in mind that, if tissue concentrations are 0.25% or greater, there will be no significant increase in yield or profitability from sulfur applications,” says Laboski.
Potassium sulfate, calcium sulfate or ammonium sulfate can be used. When choosing a sulfur source, Laboski advises considering overall nutrient need and price of the fertilizer material.
Potassium sulfate can be a good choice for Wisconsin producers, she says, because most alfalfa stands probably would benefit from the potassium (K) it supplies. “The price of sulfur in potassium sulfate is 77¢ a pound if the value of K in this fertilizer is taken into consideration.”
On the other hand, the nitrogen in ammonium sulfate has no benefit to alfalfa, making it a relatively expensive source of sulfur at 93¢/lb. For alfalfa, potassium sulfate is a better buy than ammonium sulfate. Calcium sulfate – mined gypsum – is an even better buy, according to Laboski, because the sulfur is 59¢/lb, assuming no value to the calcium. See a price comparison for sulfur fertilizers.