The only "hard" southern yellow pine is old growth pine. You will be hard pressed to find that anywhere anymore. The difference is the number of annual growth rings per inch. Old trees grown in natural settings grow very slowly, thus many rings per inch. Compare this to what you can buy today and you can see how growing trees in rows and regular thinning to optimize growth rate greatly reduces the number of rings per inch, thus softer pine. The 2x4 studs (I mean by God 2x4s) in my 50 year old house must be pre-drilled to drive a nail, and the rings are so close together that its hard to count them.
