the majority of them prefer a slightly acid soil. though there are a few that tolerate alkaline/chalky soils. If you don't have the right soil type then you can add peat /ericaceous compost and then give it a feed with a formula for rhododendrons/camellia [as per directions on the packet].

I have neutral soil and all my deciduous Magnolia's leaves go much darker green after a few acidic feeds and it really perks them up.

I have M. stellata, sanguinea and grandiflora on solid chalk, all thriving although i did chuck in lots of ericaceous when planting