As a generalization, clay soils are typically high in fertility although it depends on the clay. Clays tend to have high CECs (cation exchange capacity - the ability to hold onto nutrients). I looked up Yazoo clay and according to Mississippi State University:
"The Yazoo Clay is a green to gray calcareous clay of Eocene Age. It is high in montmorillonite clay and reflects near-shore marine deposition (Merrill et al., 1985). The soils developed in the Yazoo Clay are fine textured with a dark gray, clayey surface horizon and a lighter gray subsoil (Dixon and Nash, 1968; Lowe, 1911). Montmorillonite constitutes more than 50% of the noncarbonate clays, resulting in high shrink-swell potentials and cation exchange capacities (Dixon and Nash, 1968). Kaolinite, illite, and vermiculite comprise the remaining portions (Ramathirtha, 1971). ...........These soils have high natural fertility with Ca as the dominant cation."
http://msucares.com/pubs/bulle...
However, you're the one that has to work with the stuff, Ken! Last time I gardened on clay I used to drag wheelbarrow loads from the manure pile behind the barn and empty them onto the garden and the soil became lovely and friable after several years. Where I am now there is shallow sandy loam overlying heavy clay, and that can make drainage quite an issue in some spots.
