Gypsum is a fertilizer product and supplies the crop-available form of calcium (Ca2+) and sulfur (SO42-). If these forms are deficient in soil, then crop productivity will benefit if gypsum is applied. This is a big “if” for Iowa soils. Research has not shown deficiency of Ca and normally any potential problem with low Ca levels is taken care of with application of limestone (CaCO3). Acidity problems will occur before a deficiency of Ca, so liming effectively takes care of Ca also. Table 1 lists typical exchangeable Ca levels of several Iowa soils, and they are very high. For calcareous soils (containing free lime) the soil system is saturated with Ca, and Ca supply and soil pH is controlled by the free lime.
For S, it’s basically the same. Research conducted for more than 35 years in numerous field trials across Iowa has shown only isolated and very small corn or soybean yield response to S fertilization (two positive and one negative). Table 2 gives results for recent S trials on corn and soybean conducted in 2000 and 2001 at six sites across Iowa. These results are typical of research conducted for many years in that there was no yield increase to applied S, gypsum, or Ca. So, if there is no need for fertilizer application of Ca or S, then gypsum application is simply not needed for fertilization reasons.