What I've found when planting on our farm which is on a red clay ridge, if I dig a big hole in the clay and fill with improved soil, it's almost like putting the plant in a pot. Sure it might grow good for a couple of years but then it will stall. Even when it seems as though it's well established, the roots never make it out of the original hole to seek deeper moisture and can end up root bound exactly the same way that plants left in pots do. The other problem is that in wet years those cavities in the clay hold water for much longer than the surrounding clay and you end up with root rot.
I almost never buy large plants anymore, keeping to one gallon or less. When I dig a hole, I reserve the original soil even if it is solid clay and replace it in the hole when setting the plant in place. Even better I get bare root plants and trees, just cut a slit deep enough for the roots, slide them in and stomp the slit closed.
Almost forty years ago I planted a few thousand bare root trees with this last method and most of them are still growing nicely - aside from the lightning struck ones or the ones we had to cut down next to the house.
Apparently the bare root plants develop roots more capable of getting into the crevices in the clay if they are never coddled with a cushy starter hole.
The areas on the farm with more loamy or sandy soil got more traditional treatment for planting - big holes with lots of extra space. But we built the new house on the area where the pig pens had been before we purchased the farm and it was hard red clay that even forty years of grass, horse manure and fertilizers never improved. So all the plants we are putting in these days are going into that clay.
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