That is why earth worms are good. Roots can break rocks, but they are happier when they can spread. make the hole at least as deep at the roots and as wide as you can. spread the roots out in the hole if you can. My shrubs died when I didn't break up the pot soil a bit. so I do that and mix the soil with soil moist as long periods without rain are now common.
If you have a choice between a $20 hole and a $5 shrub or a $5 hole and a $20 shrub, go with the $20 hole and $5 shrub.
That was the way it was explained to me.
Also, when digging a hole, you might want to have it a bit higher where the shrub is resting, then dig a moat around it. This will give the roots more room to grow while preventing the shrub from settling too deep into the ground.
Dig a hole 3 times the diameter of the root ball and 2/3rds as deep as the root ball is in height. Place the root ball in the center of the hole and pack the removed soil around it so that the tree or shrub remains upright. The top of the root ball should be exposed by 1/3rd. Cover the hole with about 3" of mulch.
Water frequently until the roots are well established. This may take weeks or months depending on how big the root ball is. Resist the urge to stake unless you have problems with it leaning, and even then just stake it long enough so the tree or shrub remains upright.
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.
too wet or you end up sealing the sides of the hole.
One year my husband and I needed to put in a new fenceline. We'd had a drought for a couple of years and I was trying to use one of the lower pastures that was not as burned off from the heat.
I usually ran the tractor and he would guide the auger. We lined it up and tried to drill a post hole - only the very tip went into the ground. So my husband climbed on top of the auger, thinking it just needed a little push. The auger began to polish a little pock mark into the clay.
With the brakes locked on the tractor I got off and climbed onto the other side of the auger. It just polished the pock mark a little more.
We put the tractor up and set up a soaker hose along where we needed to put the fence posts. Left that running for two weeks and tried again. The pock mark increased to two inches deep and the wet clay stuck to everything - but the water had only made it to that two inch depth.
We never did get that fenceline in that year. The next summer after a couple of tropical storms came up into the NE Gulf we finally got enough rain to soak the ground so we could get into that clay.
Sometimes the only time you CAN dig is when the clay is wet!
mix the top layer of the native soil w/compost and or peat along with
some hardwood bark mulch at a 1/3 to 1/3 to 1/3 ratio a couple of
pounds of espoma organic fert should be added to the planting mix.
Vesicular Arbuscular Mycorrhizal is your plant's friend.
If it is a container plant cut two sides of the root ball with a knife or
spade ...... this stops circular growth of roots.
If you are in heavy clay plant the plant about 1" above grade.
Stay away from dyed mulches ..... especially the black types.
Some plants such as queen of the prairie and arborvitae like
to stay damp and you want to plant them lower.
Avoid non native invasive plants such as norway maple, burning bush,
privet, english ivy, callery pears, and so on.
A good spade really helps ..... http://www.amleo.com/spades/c/P01H/