I have a Magnolia stellata, which I purchased from mail order. Have had it for about 18 months (planted in the garden) but it did not hardly develop. Now I have moved it in a pot with some sand to improve drainage, but I think it is still struggling as the roots are in a clay ball. Could I uproot it, wash the clay from around the roots, and then replant it in a alkaline based soil? Can I add lime to the soil to make it alkaline? What should be the pH?
This article night answer some of your questions: https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-tree-care.htm
You can locate other articles by entering "magnolia" into the search box at the beginning and end of the article.
If your plant is only 18" tall, you might consider planting in a nursery pot with a 50-50 mix of good topsoil and potting soil until the root system is more developed. Even after planting a tree with a good root system it can take 2-3 years before you will see optimum growth. (Rule of thumb: first year, it sleeps; second year, it creeps; third year, it leaps.)