I'm often asked about "my technique" for planting trees. It's a technique used for years with great results in this area's gumbo/clay soil, and it's one that can be applied to large shrubs as well.
While most landscape plants should be planted in raised beds, trees will do well in poor soil if they're just given some help in acclimating to the environment.
As many people have found, just digging a hole and popping in a tree's root ball doesn't always work. That's because, in most cases, the hole is dug just big enough for the root ball. Consequently, the roots are immediately faced with the need to penetrate the hard clay soil, so the tree stagnates and doesn't seem to grow much at all.
Some folks dig more accommodating large holes, but they back fill with peet, humus, mulch or even fluffy potting soil mixes. That normally leads to an area that's continually too wet, so the root system can't breath and the tree yellows and dies.
So, the idea is to help the tree properly adjust to the existing soil conditions. Since clay isn't really a very good environment, we have to add a permanent soil amendment*.
First, dig a hole two to three times wider than the root ball, and 1½ times as deep as the ball's height. For example, if the container is 10 inches across and 10 inches deep, you will need a hole 20 inches across and 15 inches deep. Throw the dirt on a drop cloth or tarp as you dig.
Next, add the amendment to the dirt you've dug out. Every amendment has different dirt-to-amendment ratios, but you're almost always safe at 6-to-1 or 5-to-1.
Add enough mixture to the bottom of the hole so that when the tree is inserted, the top of the root ball is at ground level. Center the tree in the hole, and fill in around the ball, tamping down continuously as you go. Don't worry about "compacting" the area ... the permanent soil amendment you've added will essentially keep the dirt "aerated."
Finally, build a mulch ring on top, and water in. In fact, water once a week for the first year of the tree's life.
As the tree's roots acclimate to their new environment, they'll strengthen enough to penetrate the harder clay soil beyond the zone you established.
*PERMANENT SOIL AMENDMENTS
- Tru-Gro (kiln-fired rocks, a concrete byproduct)
- Schultz Soil Conditioner
- Fertilome Natural Guard (also known as Revive)
- Non-Commercial (homemade) Alternatives
- Small pea gravel (the smallest you can find)
- Pure clay kitty litter
- Granular gypsum (pelletized gypsum)