David Hare-Scott wrote:

I find that any organic matter will improve the texture of clay soil
and there it is hardly possible to have too much.

I happen to live where we have hard red clay several feet deep. I also
happen to live within 4 miles of the county composting station. Since
I live in the county, I can get all the compost I want free. After
screening the compost to 1/2 inch size, I add 4 to 6 inches every year
& till it in. Very little fertilizer or watering needed. This soil is
now loam to about 14 inches.

Check and see if there is such a composting station in your area.

Tom J