If the veins in the leaves are a deep green but the rest of the leaf is pale, it indicates a chlorosis due to lack of iron. If the veins are light green, your plants need nitrogen. Petunias will generally be happy in the acidic soil inherent to the Pacific Northwest so I don't think the pH is a problem. Try feeding with a liquid fertilizer (such as Miracle-Gro) and you should see almost immediate results.
Good luck with your petunias!
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