Answer: Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. (Lack of nitrogen is a more common problem than lack of iron.) Soil should be kept moderately moist (but not wet). Finally, transplant shock can contribute to yellowing. Petunias are typically very vigorous plants so I would simply pinch or cut off the yellowing leaves to encourage the development of healthy new growth. I think you'll see a significant change in another week or two.
Best wishes with your petunias!
Q&A Library Searching Tips