Any of the professional supply houses that sell to golf courses should have pelletized gypsum (Calcium Sulfate).
Yes, it is applied on top of the soil in orchards. Don't expect a quick response. Doing per the following instructions will probably show a visible improvement in plant health and fruit quality, within 1 1/2 years.
Apply it each each time you apply a mixed fertilizer. Since I tend to apply no Nitrogen to established mangos, I calibrate Calcium applications to the amount of Potassium being applied, so that, AT A MINIMUM, there will be slightly more Calcium than Potassium. (Many consultants recommend around ten times more.) Mr. Cookie Monster, on this forum, did a one-time application of a huge amount of gypsum, and got a rapid positive response.
When I put out one 50-lb. bag of mixed fertilizer which contains 16% Potassium (K), I put out at least 1/4 bag of pelletized gypsum.
When I put out one 50-lb. bag of mixed fertilizer containing 12% Potassium, I put out at least 1/5 bag of pelletized gypsum.
Remember to fertilize the entire area of the circle that is 1 1/2 times the diameter of the canopy (=twice the radius of the canopy).