Magnolia

I have a Magnolia Soulangeana and a Magnolia Stellata which i have grown in large pots of ericaceous compost for about 5 years. Both plants are currently in flower although Stellata always flowers more profusely. I've noticed that a lot of gardens in my neighbourhood have large Magnolias growing in the soil which is slightly alkaline. I've read that Stellata prefers alkaline soil but Soulangeana prefers acid. I would like to plant both in my soil and I don't foresee any problem with Stellata, but can anyone tell me if they have grown Soulangeana successfully in slightly alkaline soil and will I be ok to do so if I add plenty of ericacious compost to the planting hole and apply regular doses of sequestrene.