I obtained a magnolia alba in 2011 which has been kept in a pot and has done fairly well and blooms much of the year. The plant is almost 7 feet tall now and suddenly had a lot of leaf drop and the leaves are turning almost yellow although there is new growth on the ends of the branches.

I would like to prune the roots which I've read is a delicate process since there are fleshy roots. However, I haven't found any specific information on this root pruning process in searching YouTube and Google. Please let me know if you have specific tips on such root pruning.