Certified GKH Gardening Expert
Your local extension office will do this for you. Extension services are normally run out of your local university with a horticultural program or through a local Master Gardener program. They will typically do soil tests for free or low cost. There are also soil testing kits available at most garden supply stores.
If you haven't already read it, here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/trees/crepe-myrtle/crepe-myrtle-not-blooming.htm