Historically sulfur (S) application has not been recommended on Iowa soils for com and soybean production. Prior research has not determined a consistent need for S fertilization in Iowa, with field research indicating no com or soybean yield response to applied S at virtually every site studied (Thorup and Leitch 1975; Webb, 1978; Alesii 1982; Killorn, 1984; Sexton et al., 1998; Mallarino et al., 2000). The soil supply, in combination with sources such as manure and atmospheric deposition has apparently met com and soybean S needs. Sulfur deficiencies have been reported over the years in various areas of the Midwestern USA, examples including Alway (1940), Hoeft and Walsh (1970), Thorup and Leitch (1975), Rehm (1976), Hoeft (1980), Hoeft et al. (1985), Stecker et al. (1995), and Lamond et al. (1997). However, positive yield responses are often infrequent, for example Hoeft et al. ( 1985) and Stecker et al. (1995). Responsive sites are most often noted as eroded or low organic matter-coarse textured soils.

The ability of the top six- to seven-inches of soil to supply adequate S has been shown to be low in greenhouse studies where greater response to S application occurs than observed in the field (Dunphy and Hanway, 1972; Widdowson and Hanway, 1974; Hoeft et al., 1985; Sexton et al., 1998). Soil S levels or S supply may become depleted with prolonged crop removal, sulfate leaching, low atmospheric deposition, and declining soil organic matter. Sulfate-S in precipitation, for instance. is considerably lower in Iowa now than thirty years ago (Tabatabai and Laflen, 1976; NADP, 2002). The objective of this study was to determine if corn and soybean would respond to S fertilizer rate and material source at multiple sites across Iowa soils and climatic conditions.