I've been a fan of gypsum for over a decade. Gyspum (calcium sulfate) has become a more common soil and water amendment and a good source of sulfur.
There are mined (natural) sources and synthetic sources, which are mostly a byproduct of industry. I've always heard the quality of these products was virtually equal, but is that true? My own testing of multiple products tells me they are substantially equivalent with slight differences in composition.
Mined gypsum, known by its mineral name selenite, comes from deposits left when ocean water high in calcium and sulfate slowly evaporates. It is most commonly used in wall board, plastics and other industrial uses, but also gets used in agriculture. It ranges from 21% to 24% calcium and 18% to 20% sulfur and contains other minerals such as some lime, dolomite, clays and other minerals that affect its purity and its color (from white to gray to yellow [S] or pink [Mn]).
Mined gypsum can vary in purity (calcium sulfate content) ranging from below 50% to above 95%. In other words, if you buy a 50% gypsum, you only get about 1,000 pounds of calcium sulfate per ton compared to 1,900 pounds in a 95% material. So it definitely pays to know what you are buying so that the price reflects the amount of gypsum actually being purchased.
Mined gypsum starts as soft rock and then is crushed into ag gyp, also known as a spreadable powder. This powder is difficult to work with and mostly applied in California. Growers in California spread 2 to 4 tons per acre to keep sodium flushed below the root zone and find this their most economical source.
Ag gyp can be further milled to solution grade which is injected into irrigation water where it immediately dissolves and can be dispensed through microsprinklers and emitters. It can also be granulated or pelletized and marketed through the fertilizer distribution channel. Its size and density allows it to be accurately broadcast alone or it can be blended and spread with other dry fertilizers like urea, MAP, DAP and potash.
In the Corn Belt, the most well-known gypsum is a byproduct of flue gas desulfurization (FGD) produced at coal-fired power plants. The Clean Air Act Amendments of the 1970s led to reductions in sulfur emissions and deposition. Further refinements in the 1990s gave rise to new scrubbing systems used by many coal-fired utilities to remove sulfur dioxide (SO2) from their emissions.
The scrubbers create gypsum via a wet process that first forms calcium sulfite (CaSO3 * 0.5H2O), which is then oxidized into gypsum (CaSO4 * 2H2O) and partially dried. The best-known FGD gypsum is Gypsoil brand. FGD gypsum is very similar to mined gypsum, but carries slightly more calcium carbonate into the final byproduct and is a bit lower on sulfur content as a result.
There are other industrial processes that also generate byproduct gypsum such as citric acid and lactic acid production, neutralization of waste sulfuric acid and wastewater treatment (high in sulfate). ProCal 40 sold by Soil Solutions in Iowa markets ProCal 40, a byproduct of lactic acid producton. Calcium sulfate from these processes is very pure, usually over 98%. These byproducts, similar to FGD gypsum, are usually moist, tend to cake and therefore have special handling requirements.
Phosphogypsum is created as a byproduct when rock phosphate is treated with sulfuric acid to make phosphoric acid. Phosphoric acid is a basic ingredient in phosphorus fertilizers and other industrial products. Unfortunately, phosphogypsum is usually radioactive due to the presence of naturally occurring uranium or radium in the rock phosphate ore and can't be land applied for agricultural uses.
The presence of metals, and in particular heavy metals like arsenic, cadium, lead or mercury, is an important concern and a reason to ask exactly what you are buying. Contrary to what you might think, the FDG synthetic products come with some safeguards in this regard. Fly ash is removed before the flue gas is desulfurized. EPA also must issue a permit to land apply byproduct gypsum from power plants. Some plants with the right scrubbers can apply gypsum and some cannot.
Two years ago I paid to have Wallace Laboratory in California analyze eight sources of gypsum -- six were mined, one was FGD and one was from lactic acid production. All the heavy metals tested below EPA thresholds; calcium and sulfate were similar among all products and calcium carbonate was higher in the FGD gypsum. Other mineral content was extremely variable, but insignificant in quantity compared to calcium and sulfur.
I have learned that for all practical purposes, gypsum products are similar in terms of calcium sulfate and all are safe to apply. If synthetics are being sold, they probably have been approved for land application, but don't hesitate to ask its status and see a complete mineral test.
Dan Davidson can be reached at AskDrDan@dtn.com
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