Well, what I did was tear away some of the roots that were at the bottom of the pot and then I put a little sprinkling of bonemeal at the bottom of the hole. I would say we put another foot around the plant, so if the root ball was 1.5 feet in diameter, the hole was 3 to 3.5 feet in diameter. The native soil is a sandy loam and the medium it was in was more humusy. I tried to mix some of the pot soil into the fill soil.
Here's another question: any plants that make good companions for the area around the trunk? Right now I've covered the soil with bark mulch.
Cheers.