Acid or alkaline?
Does Ruellia thrive in acidic or alkaline soil?
Certified GKH Gardening Expert
They thrive in acidic to neutral soil, but will tolerate most soil conditions. It is best to aim for a pH of 5.5 to 7 with 6 being just about optimal.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-care.htm