Gypsum (CaSO4·2H2O) is a mineral mined from sedimentary deposits created by the evaporation of saline water, where sulfuric acid comes into contact with calcium carbonate. Agricultural gypsum is applied as a soil amendment and fertilizer containing roughly 22% calcium and 18% sulfur. 

Coarse-textured soils with low potential for sulfur mineralization from organic matter may be at risk for sulfur deficiency that can limit soybean yields. This scenario is becoming more common due to a decrease in atmospheric deposition of sulfur from air pollution since the Clean Air Act of 1963 (once as much as 8-15 pounds per acre annually) and the removal of sulfur-containing impurities from fertilizers and pesticides.

In 2014, a Soybean Management and Research Technology (SMaRT) on-farm research project was conducted in Rogers City, Mich., to evaluate gypsum as a source of calcium and sulfur for soybean production. Gypsum was surface broadcasted at 1/2 ton per acre just before planting at a cost of $22.50 per acre for the product and $9.29 per acre for application, to be compared with an untreated control. Plots were 40 feet wide and 200 feet long with Pioneer 91M01 soybeans drilled in rows 7.5 inches apart at 175,000 seeds per acre. 

The two treatments were randomized and replicated four times. The trial was planted May 23, 2014. The soil was Cheboygan loamy sand with a pH of 6.9 and Cation Exchange Capacity (CEC) of 5.1 meq/100g. The previous crop was wheat followed by an oat cover crop, and seed was treated with inoculant prior to planting. Uppermost fully developed trifoliate leaves were sampled Aug. 6 for nutrient analysis and soybeans were harvested Nov. 1.

Effects of gypsum on leaf nutrient concentrations and soybean yield in Rogers City, Michigan, 2014.

*Soybean yields were significantly different (p<0.05). Yield adjusted to 13 percent moisture.
**Nutrients analyzed in the first trifoliate leaf tissue.

For soybeans, the calcium sufficiency range is 0.35-2%. Both treatments provided sufficient calcium. Soybean’s sulfur sufficiency range is between 0.20-0.40%. The gypsum treatment brought soybeans from a state of mild sulfur deficiency to optimum sulfur sufficiency. It is important to note that application of 20-40 pounds of sulfur per acre will generally correct sulfur deficiency. 

Our gypsum treatment supplied 180 pounds per acre. This rate was selected to highlight gypsum’s reported advantages as a soil amendment capable of improving soil tilth and water infiltration. However, in coarse soils these physical properties are not of concern, and it is assumed that the yield increase measured was solely related to sulfur fertilization. If this is the case, a lower rate may have performed just as well.

Interestingly, in addition to addressing sulfur deficiency, the gypsum treatment also increased nitrogen uptake by soybean plants. This is likely due to the interrelated roles of sulfur and nitrogen in protein synthesis, where sulfur deficiency can limit efficient nitrogen uptake and utilization. 

Other less desirable nutrient interactions also occurred. The gypsum treatment limited magnesium and boron uptake by soybeans in our trial. Calcium is known to chemically compete with other cations like magnesium and inhibit the uptake of boron. 

Our gypsum treatment brought soybean plants from a state of sufficiency to one of mild deficiency in both of these secondary and micronutrients. Still, the plants treated with gypsum were anecdotally observed to be greener in color with longer internodes, a larger number of healthy root nodules and more pods per plant, though these variables were not measured. The yield difference between the gypsum treatment and untreated control was statistically significant, averaging 6.6 bushels per acre (p<0.05, see table). Total product and application costs were $31.79 per acre.

Gypsum contributed to attractive results in the context of this trial. However, other sources of sulfur such as ammonium sulfate (AMS), potassium sulfate, elemental sulfur and organic matter are available and may provide advantages over gypsum. For one thing, other sources do not contain large amounts of calcium that can compromise the uptake of secondary and micronutrients. In addition, AMS and elemental sulfur will acidify alkaline soils, while gypsum and potassium sulfate have little effect on soil pH. Many producers already use AMS as an adjuvant with glyphosate. 

Adding manure or cover crops to a cropping system as a source of organic sulfur can offer other benefits, such as improved water holding capacity and increased biological activity.