To make a magnolia at home in Austin you need to take some special steps. Amend the soil by adding a few inches of peat moss and/or composted bark. The rose soil you mentioned is probably fine too. Choose a spot with lots of morning sun but some afternoon shade. Keep the soil moist but make sure the area drains well or construct raised planting beds high enough to insure the roots don't sit in saturated soil. This would usually be about a foot high as they will settle in time to about 8 inches high. Fertilize with a product for acid loving plants. Maintain even moisture and a surface mulch to deter weeds.
One more thing...
When you remove them from their pots check for circling roots. I suggest using a box cutter knife to make 3 or 4 vertical cuts through the root ball to encourage the roots to branch and move outward into the surrounding soil after planting. This sounds drastic I know but won't kill the plants and can be the difference between the plants establishing and growing or remaining stunted and performing poorly.
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