If an S deficiency is confirmed in alfalfa (through plant tissue analysis or field response trial), the amount of S fertilizer recommended is 20–30 lb S/acre. Where deficiencies occurred in the 2006 trials, the first 15 lb S/acre gave the largest incremental increase in yield, but the next 15 lb S/acre was still profitable at most sites. Also, S fertilizers do not need to be applied each year as alfalfa will respond to S applied in a prior year.

Corn grain yield increase to S fertilization has occurred with high frequency. Also, the magnitude of yield increase has been large. Across the two years of rate studies, 62% of the sites had a statistically significant yield increase to applied S fertilizer, with similar frequency for fine- and coarse-textured soils. The across-site yield increase averaged 19 bu/acre for the responsive sites. Analyzed across S rate, the economic optimum S rate was 16 lb S/acre for fine- textured soils and 23 lb S/acre for coarse-textured soils. This research indicates a change in need for S fertilization, especially in northeast Iowa and the associated soils, and that S application is an economically viable fertilization practice on many soils. However, the research also shows that corn does not respond to S application in all fields or field areas and that chance of S response decreases outside of the northeast Iowa geographic area.

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