Stricter EPA air-quality standards that reduce sulfur emissions are good news for public health. But they could put a hit on forage yields.
Forage crops, big users of sulfur, have been getting a considerable portion of the nutrient through emissions by heavy industry and motor vehicles. However, between 1988 and 1997, air concentrations of sulfur dioxide nosedived nearly 40% nationwide. Emissions dropped 12% in that same period. And more reductions are expected as even tighter standards kick in.
Sulfur levels from precipitation in Wisconsin decreased 42% between 1969 and 1987, and have dropped another 40% since the mid-1980s, say University of Wisconsin soil scientists. They recently saw a yield response by alfalfa to sulfur fertilizer at a research station where responses haven't been seen in the past.
Even without the reduction in air-borne sulfur, there would be good reason to monitor forages for this important nutrient, agronomists advise.
In the past, forages grown on sandy, low-organic-matter soils were most in need of sulfur fertilizer. But agronomists are finding that shortages are no longer restricted to coarse-textured soils.
"There is increasing evidence of deficiency in other types of soils," says Vivien Allen, Texas Tech University agronomist. "The best responses have occurred in warm-season grasses. They have come primarily in areas of long-term cropping to corn or sorghum silage, and even on heavier soils."
Kansas State University agronomist Ray Lamond reports that bromegrass grown on soils with 3-5% organic matter has shown fairly steady yield responses to sulfur fertilizer.
"Assuming brome hay garners $75 a ton, we're consistently seeing additional income of $20-30 an acre after topdressing with 15 lbs of sulfur, which costs less than $5 an acre," says Lamond.
Growers in the Northern Plains get big responses to sulfur, even on 3% organic matter soil, if soil temperatures are cool, notes University of Minnesota agronomist George Rehm.
Jeff Polenske, an independent crop consultant at Appleton, WI, routinely tests for sulfur on up to 400 alfalfa fields annually. He uses plant tissue analysis, the recommended method for diagnosing sulfur deficiency.
"We have about one-third red clay, one-third loamier ground and one-third sandy soils," Polenske reports. "We find almost no deficiencies on the clay and a moderate number of deficiencies on the loamier ground. Most deficiencies are on sandy fields, especially those that haven't had manure for three or four years."
When Polenske finds a sulfur deficiency in alfalfa, he recommends that sulfate sulfur be applied before the next cutting. Agronomists point out that sulfate sulfur is more readily available than elemental sulfur. But elemental sulfur can be a good choice if applied annually, they say.