I have two 15+ year old magnolias that are in a shady spot in my front yard. They have always bloomed well, but are now becoming root-bare and the leaves are less full leaving open spaces in the tree. What type of soil or mulch should I be using to cover the roots and help the tree regain its former health? Should I be trimming them so the leaves don't touch the ground?
I'd recommend applying an organic mulch such as wood chips to a depth of 3 or 4 inches over the root zone of your magnolias. The thinning leaves are likely due to increasing shade. Consider thinning the trees that cause the shade in order to improve the health of the magnolias. Magnolias prefer full sun.
It's not necessary to remove the lower branches on magnolias, even if they touch the ground. However, if you'd like to create some clearance you can cut them off back at the main trunk or juncture with a side branch.