The federal Clean Air Act of 1990 has dramatically reduced the amount of sulfur coming from industrial smokestacks and automobile tailpipes. Some agronomists suggest that this reduction requires the addition of sulfur to sulfur fertilizers. Other researchers say studies are mixed and response of corn to the addition of sulfur is difficult to anticipate.
Nitrogen (N), potassium (P) and phosphorous (K) are the three main nutrients found in starter fertilizers. These elements are called macronutrients. Micronutrients --- minerals needed in lesser amounts --- include sulfur (S) and zinc (Z). The Clean Air Act has led to an estimated 75 percent decrease of sulfate deposits on soil in some areas. However, agronomists at Michigan State University and the universities of Nebraska, Iowa and Minnesota all say it is not profitable for farmers to add sulfur starter fertilizers to corn except in sandy soils and those with low organic content.
Soil that is high in organic matter usually provides the sulfur that corn needs. Agronomists at Michigan State say 1 percent of organic matter in 1 acre will usually provide 2 to 3 lbs. of sulfur. Coarse-textured, sandy soils also leach sulfur more quickly than finely textured soils. Michigan State agronomists conducted sulfur studies over a two-year period and found that applying sulfur increased yields about 20 percent of the time. Similar studies in Iowa and Minnesota showed increased yields only on sandy soils low in organic matter.
Microbes in the soil decompose organic matter and in the process convert sulfur into the sulfate form needed for corn to grow. To flourish, these microbes require warmth and moisture. Agronomists at Michigan State University say many growers have begun to plant corn earlier in the spring to increase potential yields; the microbes haven't had enough warmth and moisture to provide the sulfate needed by corn seedlings.
Over the years, the purity of starter fertilizers has improved, thus removing many sulfur impurities they once contained. Fewer growers often use animal manures, a rich source of sulfur, to fertilize their corn.
Agronomists at the University of Nebraska recommend adding sulfur only to soils that are sandy and low in organic matter. A study by Ohio State University agronomists on one site for one year concluded that adding sulfur to starter fertilizer on corn did not increase the yield. The Ohio State agronomists say adding sulfur to starter fertilizer on corn when not necessary increases the cost of production and reduces profit. A three-year University of Illinois study of starter fertilizers on no-till corn concluded that adding sulfur did not increase yields.