There is also a general principle for all holes used for planting saying that the old soil should be mixed with the new soil in order to prevent the "well effect". One of the expert members had, in past threads, cautioned against simply digging a hole and placing the new plant in it. The roots will grow out to the edge of the hole and turn back into the richer soil which will, in time, cause the roots to strangle themselves or grow back across each other. I have a maple tree doing the exact same thing from having a raised bed around it.
