What is Gypsum?
Gypsum is a mineral that has been used in agriculture for a long time. Its chemical name is calcium sulfate dihydrate (CaSO4 • 2H2O). It provides a sulfur source in the plant available form, sulfate, and provides calcium – both essential nutrients in crop production.
SO4 is naturally mined gypsum
SO4 is pelletized from gypsum that is naturally mined in northwest Iowa. Gypsum deposits were left behind when inland seas that used to cover Iowa dried up and receded.
Synthetic gypsum is a byproduct of burning coal
In contrast, synthetic gypsum (photo above) is a byproduct of burning coal. This source is commonly referred to as synthetic or flue gas desulfurized (FGD) gypsum. Power plants have ‘scrubbers’ that control emissions from their flue stacks. The process in its entirety is called flue gas desulfurization.
In short, these scrubbers filter by forcing sulfur dioxide and calcium carbonate (limestone) to react with one another, which creates calcium sulfite (CaSO3). Most power plants also use an additional step called ‘forced oxidation,’ whereby the calcium sulfite is oxidized to calcium sulfate, or synthetic gypsum. The resulting moist material is either landfilled or used in various industries around the U.S. – wallboard for instance.
Challenges with synthetic gypsum
There are a few challenges with synthetic gypsum worth considering:
1. Coal contains heavy metals, which are generally isolated in the scrubbing process but occasionally can end up in the synthetic gypsum, raising obvious concerns about agricultural applications.
2. In bulk form, the material contains high moisture levels, making it difficult to spread and manage. As a result, recommended application rates are in the 1,000+ lbs/A range, which can create imbalance in the soil. These rates lack scientific evidence supporting their use in Midwest agriculture.
3. The purity of synthetic gypsum is only as good as the starting feedstock (limestone) and the system that produces it, creating highly variable chemical characteristics. Because of its synthetic/by-product nature, it will never be registered for organic use.
Synthetic gypsum is difficult and expensive to pelletize due to its fine particle size and requires the use of specialized binders and additives. This results in slow breakdown and activity in the field.
In summary, natural gypsum is mined from the earth while synthetic gypsum is a byproduct of burning coal. SO4 is pelletized, natural gypsum. It’s consistent pellet size allows it to blended and applied with other dry fertilizers.