I don't think the soil mix is causing the problem, it's usually not that rich. But I definitely agree with Weedhound that you should find out the soil & water pH, and quick. If that's the problem, then the condition of the plants is just going to deteriorate with each watering. While you wait on getting your pH meter calibrated, you could pick up some of the liquid test kits at many stores. That will at least help determine where your pH is, then you can go from there.