The Ohio State University (Dick, Chen & Kost) has conducted a study in which they measured the response to adding gypsum to the fertilizer program to increase nitrogen efficiency in corn. As the results below indicate the 120# rate of nitrogen with 200# of gypsum produced a significantly higher yield than the 180# nitrogen rate by itself. This could be due to a nitrogen/sulfur interaction, a calcium/nitrogen interaction or simply a gypsum/nitrogen interaction. These soils were silt loam soils and had a pH near neutral.
This would support the idea of a more balanced fertility program to produce higher yields rather than just increasing nitrogen rates as some have suggested.