Well, at least you have clay UNDER your rocks! Central Kentucky is known for the "Bluegrass" area. The reason why the grass has that famous blue hue is the amount of lime in the soil. That's from all the limestone deposits under us. Go on up in to southern Indiana, and you get into an area known as the "Limestone Belt". (Ever here of "Bedford Stone"? Bedford Indiana is the capital of cut limestone quarries.) (Watch the movie "Breaking Away" sometime.) Look around the area and you'll see a BUNCH of stacked limestone fences around older farmsteads.
As far as "some tractors just plow better than others", truer words were never spoken. My 2440 Deere is 60 + HP, 7900 lbs, and has more than enough iron to handle a plow, yet my 150 Massey will plow circles around it. (Granted, with one less bottom on the plow) Along about the mid 70's, plowing with smaller tractors just wasn't fashionable anymore. They weren't designed to do heavy tillage. Those older Masseys and Fords were. Better draft control is about the only reason I can come up with.