Where it's available, expanded shale is great addition to soil, whether it's clay or sandy. Expanded shale is shale that has been heated to 1700 degrees. It looks like pea gravel, but when it's heated, it pops like popcorn and is full of tiny pockets that will hold air and water. About 60% of the volume of a pebble of expanded shale is made up of these pockets.
What we do in Texas is put down about 4 inches of finished compost and till it into the soil, then add about 4 inches of expanded shale and till that in. Mix it in real deep.
Finished compost is any carbonaceous material that has been allowed to break down and, yes, it takes nitrogen to break it down. Leaves, wood chips, manure or whatever. When it has fully composted, it smells very good.
I have beds that I built up using expanded shale and compost and things grow like crazy in them. For example, I have lemon grass that's seven feet tall right now (until we have a hard freeze). I need to dig it up and put it in pots to overwinter.
Expanded shale isn't available everywhere. I inquired about it for family members in Minnesota and Wisconsin several years ago and nobody there had ever heard of it, but if you can get it, it's great stuff.