Gypsum should be applied before crop planting using a dry applicator that can spread a large volume of fertilizer, lime or litter. Fall application is common timing for gypsum. Both synthetic or natural gypsum products contain calcium and sulfur in the form of calcium sulfate dihydrate (CaSO4 2H2O). 

The common recommendation is for the applicator to have a stainless steel box with steep sides (45°angles) and a wide belt. In absence of a stainless steel box, graphite film coating or plastic liners are suggested to assure the gypsum slides easily onto the belt.

The Gypsoil company, which distributes a high-grade synthetic gypsum, provides some important tips. Because particle size is so small, gypsum products can easily bridge and not slide down to the belt and spinners in an applicator. Setting equipment properly is important. It is recommended that non-essential interior gates, partitions and braces be removed. Twin spinners are important to ensure a wide, uniform spread pattern, too.

The Gypsoil website notes the importance of the rear gate opening and calibration. “Raise the rear gate no more than three to five inches. Adjust spinners for best pattern. Calibrate spreader by loading spreader one-third full before first spreading. Check rate, flow and pattern and adjust belts and spinners as needed. Increase load size as you are comfortable with the pattern.”

The company also suggests when loading the applicator to feather or bed the gypsum rather than dumping it all in the center, and try to avoid piling it against the rear gate.

For long-term storage, gypsum should be stored under cover to avoid it from becoming saturated with moisture if rainfall. It can also be stored in the open on a turn-row or open lot for shorter storage or when delivered to a field.

“If storing in the open, place it on bare, scraped ground at least 200 feet from a stream or drainage ditch. It is a good idea to form a pile into a peak so rain runs off easily. As with all farming materials, keep away from livestock, pets and children,” warns Gypsoil.

Application timing as noted in general is before a new crop is harvested, but more precisely, Gypsoil provides the following rules of thumb about application if soil conditions are such that equipment can roll across the field:

  • After alfalfa cutting once hay is baled
  • After wheat harvest
  • After soybean or corn harvest
  • Before planting

Gypsum application rates depend on individual field situations and goals for the farmer. Typical rates are 300 to 500 pounds per acre for sulfur supplementation and one to two tons per acre every one to two years for soil amendment, according to Gypsoil.

If Cation Exchange Capacity (CEC) levels are being used to evaluate gypsum application, the suggested application rates are as follows:

CEC levels                                Rate

Less than 10                    0.5 tons/acre

10-15                              1.0 tons/acre

Greater than 15                2.0 tons/acre

“Cation Exchange Capacity is the amount of cations a soil can retain. Higher CEC soils have greater capacity to store plant nutrients. Soil CEC increases with more clay, more organic matter and is also influenced by pH,” the company explains.

Gypsoil gypsum is a co-product of the process that cleans the air from coal-fired plants and is sometimes called FGD gypsum or synthetic gypsum. This gypsum is also made as a co-product of certain other processing.

Clean Air Act amendments of 1990, reportedly required new scrubbing systems used by many coal-fired utilities to remove sulfur dioxide (SO2) from their emissions. These scrubbers produce high-quality and very pure FGD gypsum. Supply of this synthetic gypsum is still expanding.