I have replied to this onceut it wouldn't let me post
I have grown a magnolia stellata in a large pot biut would hesitate about puttin a soulangea in one as it is a large tree,
If you have to do so I would get something like a half barrel and plant in there, It would need amix of ericaeous compost with some JI2 0r 3 Baout 2/3 ericaeous to 1/3 JI
However I do not think it would be happy long term