What the NRCS may be calling "dispersive clay" sounds like colloidal clay. The proper amount of moisture is key for compaction of clay. Hydrated Lime is ionically positive and colloidal clay is negative.
Gypsum only works about half the time in clearing colloidal clay, and it takes a LOT of it. and like lime, it tends to soften the clay.
The easiest way to check your clay is to roll it into a golfball size and toss it a couple feet in the air, letting it land on something solid and hard. If it breaks up, or won't hold it's share to begin with, it won't compact/seal well. The clay ball should be slightly moist, but not wet, and stay in one piece with a flat spot from the landing.
You can also roll some clay out, hotdog style bout 6" long and 1/2" in diameter, hold it by one end, vertically, and it should not break apart.
