Not really - magnolias will grow well in any good, humus rich garden soil, except chalky soil. You can use it if you like, or a mix of it and John Innes No. 2. Or multi purpose and John Innes No. 2.

I've got a stellata and I use liquid ericaceous feed once a year in the summer. It flowers magnificently, but to be honest, it probably would flower the same without the liquid!! (I bought the feed in the £ shop - guess how much???)

Welcome to GoY by the way

Mine flowers quite happily without so far having been fed at all. My soil is only very slightly acid. I have seen a large one in a totally neglected front yard that was an absolute picture year after year.