Pics for Al of newly potted mango, take 2. Image link:

FYI Ken, I actually have seen them basically grow back together on these vigorous Red-Silver Hybrids from a very thin saw cut. Cutting out a section helps to make manipulate the remaining root easier. The root needs to be cut back to a bend, where the cut end is pointing away from the trunk, and not across it. If you leave the root pointing across the trunk of the tree, new roots from the cut end in the same direction i.e. back across the trunk of the tree, and you can end up in the same boat again. Arktrees

I for one am not an advocate of bare rooting Japanese maples at any time other than when fully dormant. Their roots are extremely sensitive to disturbance and the trauma of bare rooting, shipping and replanting at this time of the season is sufficient to send any JM into shock. My preference would have been to pot it up and baby it for awhile before planting out in the ground in full-on sun, specially in mid June. My nursery advocates extreme care when planting even well-established containerized or B&B maples at this time of year and our summers are notoriously late to start and NOT hot when they do arrive, if rather dry. I think David's advice is sound. Without seeing your soil and planting conditions firsthand, it is only speculation but the amending you have done in "heavy clay" soil may be creating a poor drainage situation - too much moisture remaining in close contact with the roots and not percolating down through the soil level. Generally, if working with a heavy soil it is recommended to dig a very wide but shallow planting hole adding no amendments. Plant the tree high, backfill with the soil you removed and add the amendments as a mulch to smooth out to the top of the roots. This wide, dished planting hole will allow for adequate drainage without creating a bathtub effect of a deeper, amended hole. Personally, after such a short period of time in the ground, I'd be inclined to lift the tree and pot up into a container where you are able to more closely monitor the amount of sun and water it will receive and give some much needed TLC. Forget the fertilizer - stressed plants are unable to metabolize much anyway. And you can add me to the list of those who think that SuperThrive (or SuperJive, as a friend refers to it) is pretty much just very well-marketed snake oil. The active ingredient (primarily B1) is present in such an infinitesimal amount and then is further diluted by water that it is providing virtually no measurable benefit to the plant.

glad to hear of a dog who respects maples as much as i do ... i cant really tell much from the pic .. but i doubt what you see.. is the main root ... what that means.. i dont know ... ken