digging holes for trees
by jim deal
(bedford,ohio)
Doug says you don't add a thing to holes for tree and shrub planting (other than a hand-full of compost if you have it - or the compost can go around the top of the plant after backfilling) But modern research has shown that modifying planting holes is bad for long-term tree and shrub growth.
And yeah, no matter what the ads tell you, the packaging or those who don't know any better. Dig a much bigger hole than the roots - but don't add anything to that hole when you plant. It's enough to aerate the soil.
In this case, it sounds as if you're planting on a blue-clay swamp. And your success will be determined by the plant you use - not how you plant it.