Applying Gypsum to Wisconsin Soils

Applying Gypsum to Wisconsin Soils

Dick Wolkowski, Extension Soil Scientist

The interest in the use of gypsum in crop production has increased in Wisconsin for a variety of reasons. The installation of “scrubbers” at coal-fired power plants has reduced S emissions substantially, thereby significantly reducing the amount of S deposited in precipitation. Consequently many producers and consultants are now seeing evidence of low S for the first time and the use of S-containing fertilizer is on the rise. While there are numerous S fertilizer sources (e.g. ammonium sulfate, potassium sulfate, sulpomag), gypsum is typically the least expensive. Furthermore, a byproduct of some scrubbers is high quality gypsum, which is coming from SE Wisconsin powerplants operated by We Energies. This material is known as FGD (flue gas desulfurized) gypsum and is being marketed as GypsoilTM .

Gypsum or calcium sulfate dihydrate (CaSO4·2H2O) in its purest form has 23 % Ca and 18 % S, although most agricultural sources have slightly lower analyses. Calcium fertilization is unnecessary for field crops grown on Wisconsin soils. The majority of Wisconsin soils are routinely limed to maintain pH values greater than 6.0 and in doing so approximately 300 to 400 lb Ca is applied per ton of lime. Considering that crop Ca removal is 25 to 100 lb Ca/a/year and that Ca leaching is limited, a well-managed soil would rarely become Ca responsive.  Soils with a Ca soil test less than 400 ppm on sands or less than 600 ppm on medium-textured soils are likely to respond to Ca applications. One situation where plant response to Ca application has been demonstrated in Wisconsin is on potato, which is commonly grown on moderately acidic, low cation exchange capacity (CEC) soils. Furthermore, calcium-treated tubers have been shown to resist bacterial soft rot in storage because the role of Ca for forming strong cell walls. Sulfur behaves similar to N in soils, as it associates with organic matter and the sulfate anion is subject to leaching. Traditionally soils in western and northern Wisconsin have been more responsive to S fertilization as they receive less S from precipitation. However, the entire state is receiving less S in precipitation and thus, the potential for S response is increasing state-wide.

Because gypsum contains Ca some believe that it can be used as lime to increase soil pH. This belief is erroneous because it is the carbonate anion (CO32-) in liming materials that actually neutralizes the H+ in the soil. Note that in the literature, situations where gypsum increases pH have been shown, but these responses were observed on highly weathered soils containing high level of exchangeable Al. Such conditions would not be expected in Wisconsin soils.

GypsoilTM has suggested application rates of 1 – 3 tons per acre and claims improvement in soil physical conditions (reduced compaction, improved water holding capacity), deeper rooting, enhanced biological activity, and maximized growth and yield. These claims have not been substantiated by current Wisconsin research, although studies in other north central states have shown some benefits. Research in Wisconsin has not consistently shown benefits from the application of gypsum or other Ca containing materials in crop production beyond the expected nutritional benefit of Ca or S. “Balancing” Ca:Mg levels with gypsum or other Ca containing materials is neither economically or scientifically sound.   One promising potential benefit for gypsum application at these higher rates is the reduction in dissolved reactive P, which could result in reduced P in runoff. Research to confirm this response is on-going.

Gypsum is an excellent source of Ca and S for crops. Where S deficiency is suspected use plant analysis to confirm the need for S fertilization. Consult UWEX Publication A2809 for the appropriate rate of S application for the crop to be grown.  Producers that choose to apply high rates of FGD gypsum should consider leaving several untreated “check” strips to evaluate the soil and crop response to the material.

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