My soil has some pretty discrete layers:

The top 4-6 inches are pretty decent topsoil, but the next 4-6 inches are heavily compacted sand, and below that is clay that goes on forever.
If I want my "soil" tested, do I want to test the top layer? The sand layer? A mix of the two? Or of the three?
When I use my broadfork:

I loosen pretty much the entire sand layer and bring up a few inches of clay, and then the rototiller mixes it together.
It's been suggested that I apply some gypsum to prevent it all from reforming into hardpan again.
I have no idea how much I should add, nor how to actually apply it.