Hi Robert and welcome to GoY. If your magnolia has been in alkaline soil up to now I am not surprised it has brown blotches on the leaves. I take it that it is in a pot? In which case the change to an acidic compost should help. Make sure it is in moist, but free draining compost; semi shade and out of strong winds and not in an open east facing border where the buds can get frosted. Basically same as you would do with a camellia