Most soils that would benefit from gypsum cannot sustain turf well.
Unless you are located in the southwestern United States, it is unlikely that gypsum should be applied to your lawn either before applying sod or after sod has become established. Southwestern soils tend to be high in salts, which the gypsum can help to mitigate. Apart from a few other specialty uses, which can only be determined by a soil test, gypsum should not be applied to sod as a matter of course.
What Is Gypsum?
The sulfur in gypsum seeks out the sodium molecules in sodic soils.
Gypsum is a combination of calcium and sulfur (calcium sulfate). It is a slow-release compound that is not quickly soluble in water. It can take up to six months to fully break down and incorporate itself into the soil. The calcium and sulfur molecules split off to do separate jobs. The calcium takes the place of the sodium in soils, and the sulfur bonds with the freed sodium, allowing it to be leached out.
What Gypsum Does
Sodium sulfate is formed from the dislodged salt and the sulfur split off from the gypsum.
Gypsum helps to dislodge many of the salts in sodic (high levels of salt) soils. Sodic soils are of poor structure and have poor drainage. These soil types need to be amended prior to installing sod. Any treatment of sod lawns on sodic soils should incorporate regular irrigation to flush the salts that have been dislodged by the gypsum. If sod is installed on sandy soils, in some instances the use of gypsum can help to replace calcium and sulfur that may have leached out.
Misconceptions About Gypsum
Gypsum was thought to act like a hoe on clay soils, helping to break them up.
Gypsum is not a cure-all. Many people believe or have been told that gypsum will help to loosen heavy clay soils. Packaging for gypsum used to make this claim, which has been unsubstantiated by numerous research experiments. Gypsum is sometimes thought to raise or lower a soil's pH. It does neither. Gypsum is sometimes recommended to counteract aluminum toxicity in lawns and soils. This is only true if the pH of your lawn is at or below 4.5.
Soil Testing
A soil test is a reliable and inexpensive method of determining if your lawn needs gypsum.
The only way to be truly sure if you should incorporate gypsum into your soil before applying sod or use it as an amendment on established sod lawns is to have a soil test professionally performed. A lab analysis of your soil will reveal the soil's exchangeable sodium percentage (ESP). Soils with an ESP greater than 15 percent are considered to be sodic and would benefit from applications of gypsum. Soil tests revealing a deficiency in either calcium or sulfur will also benefit from a prescribed application of gypsum.
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