Magnolia roots

Hi all, I have a small magnolia soulangeana planted in my garden for the last two years, it was probably in a pot before sale for a year or two and has really struggled. I decided this evening to lift it and to my surprise its roots are very poor. My soil is a mix of bog standard heavy clay soil, good draing topsoil and ericacous soil for acidity.

Any thoughts on how I can improve the roots? Thanks