That bottom photo is a classic photo of a tree too deep in the ground, coming straight out of the ground like a post, no flare in sight. It's like it can't breathe when the flare isn't exposed. Use a trowel or something that will allow you to carefully pull the dirt away from the tree. If it has been way too deep for a while there may be some small tangled false roots called "adventitious" roots. Work past them to the flare, the large woody support roots that spread at the base of the tree. Expose the flare as you can see in some of the link photos.
As bad as that bark looks, you might have a girdling root under there, a root that wrapped around in the pot and is still in that position in the ground, grown larger, and now strangling the tree.
The sick tree treatment
is in order once you have the root flare uncovered.