Agricultural gypsum is 'slag'. It is RUMOURED to improve heavy clay soils, but it simply doesn't. IF your soil have lots of sodium in it and poor drainage because of the sodium, gypsum does act to remove it. However, most clayey soils are not sodic.

The 'fix' for clayey soils is lots of organic matter.

My soil is reasonably fertile but sticky when wet due to a heavy clay content, which is not beneficial to camellias which need well drained soil. (We get about 70 inches of rain per year.) At the suggestion of an agronomist, I have tried and found both gypsum and slag, when mixed into the soil, very helpful in making it fluffier -- more granular and better draining in the root zone. It has some effect on the molecular properties of the soil particles, making them stick less to each other, which someone else would have to explain -- I just use it. I also have top dressed with gypsum in existing beds and my lawn, which produces results, albeit not as good as mixing into the soil. Some lawn experts recommend an annual top dressing of gypsum, and it does help. Slag is slightly alkaline, something to keep in mind as camellias need slightly acid soil. This is not a problem in my heavily acidic soil, but gypsum has a more neutral pH and thus may be safer for you if you are unsure of your pH. You can often find both gypsum and slag in quantity at local garden centers and farm suppliers, very cheap. I also second dorie's comment about adding organic matter -- rotted leaves in as high a quantity as you can mix into the soil in a new planting area, and some builders sand if available. Think of the exercise!

Forrest, your coastal clay is a perfect candidate for a gypsum treatment. Aerosol salt, salt water intrusion, and high saline water tables can all work to turn almost any soil (clay or otherwise) into something rather gunky.

It's the saline that reduces the ability of the soil to flocculate and form essential aggregates. Sodium will bind to the gypsum, allowing it to be leached away to where it can't do any harm.

Golf courses in coastal areas will typically bring in granulated gypsum by the truck load! I even used it in a container nursery I had for awhile....because my wells began to become a bit sodic. All I had to do was top-dress the containers. Of course, in that situation, the sodium was causing injury to the plant roots rather than it being a soil problem. Gosh, the difference was like night and day!