Question
Stores in our area (Northeast Ohio) sell all 3 products and I have not been able to get any explanation from the people that work in the stores that satisfy my curiosity of when should I use one or the other - all I have gotten is that gypsum is better but noone can tell me why or why not limestone or lime.
My soil is all clay with about 1" of topsoil that I put on top. I am also surrounded by trees and lots of leaves which makes it acidic.
Thanks,
Bill

Answer
Hi Bill,
Limestone usually refers to crushed (powdered) lime (calcium carbonate/magnesium carbonate) also called agricultural grade lime.  This is what farmers use to raise the pH of soil. How quickly it works depends on how finely it is ground.  It's cheap, but difficult to spread for homeowners.
More commonly found is the same product but pelletized for use with spreaders.  This is what most homeowners will use on their lawns and gardens.
Hydrated lime is a white lime. It's used for pickling, odor control and occasionly in the garden when we need to raise the pH immediately in soils that need to be planted soon. It's not wise to use it around existing plantings.
Gypsum is calcium sulfate.  I've read many times that it helps "separate" clay soils.  I've never found that to be true.  I've found that it seals soils and slows drainage turning clay into muck.  In fact, gypsum is used to pour in lakes to help seal leaks.  In the southwest U.S., it is used to help take soluble salts out of the soil, but it probably does more harm than good in most clay soils.
That said, I know people who use it but I think they are adding so little that they see no harm.
I would incorporate your top soil into the top 4-6" of clay.  Clay is good.  It's the best for water and nutrient retention, it just needs to be "separated" with organic matter and I'd use your leaves.
For lawns, veggies, most flowers, perennials and roses, your pH should be around 6.5.  For hollies, azaleas and dogwoods, aim for 5.5.
Jim