Answer: The moisture control soil will hold more moisture than regular potting soil, even though it might feel dry to the touch on the top of the pot. I think you are overwatering and that's why the leaves of your Supertunias are turning yellow. Try holding off on watering a day longer than you usually water. Watch the plants closely, though, and if they appear to be water stressed, water them right away instead of waiting. Once you figure out how thirsty the plants are and how quickly the soil is inclined to dry out you'll be able to provide just enough water to make your plants happy but no so much that they develop yellowing leaves. Good luck with your plants!
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