[I Buried My Guns]

"...If you are bound and determined to grow grass under the oak trees you might try applying 10 lbs. of pelletized gypsum (calcium sufate)..."

Thank you for the info.

Anyway: isn't gypsum itself acidic and will contribute to acidity in the soil?

I put gypsum in my clay soil and it is not conducive to low-acid tolerant plants, like Lantana.

[panaxanax]

You are correct to some degree. The addition of the organic matter in the form of well-composted manure will act in conjunction with the CA Sulfate to improve the soil and raise the pH.
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Gypsum Increases the pH of Acidic Soils:

“One mechanism in which gypsum can increase soil pH enough in some acid soils to sufficiently decrease the level of soluble aluminum to grow crops satisfactorily is replacement of hydroxyl ions from some clay lattices by sulfate ions.”

http://www.cmtmi.com/gypsum.asp