Herbs in Hard Clay Soil

I live in Scottsdale, Arizona. When I planted my herb garden (basil, rosemary, parsley, cilantro, mint, sage, chamomile, thyme and parsley) I was told not to add anything to the hard clay soil; the lady at the nursery told me herbs like bad soil. The mint is thriving but nothing else is. The basil is doing "just okay." Should I have conditioned the soil? Should I transplant them or re-plant them in better soil? I planted everything in full sun but can move them to filtered sun if recommended. I give them Miracle Gro every other week and they are watered daily. Any suggestions?

While there is some truth to the notion that herbs can grow in poor soil, most of the popular ones do not do well in heavy clay soils. They prefer soils that drain well, even if they are not particularly rich in nutrients. We recommend that you add sand to your soil, lots of it, and at the same time add organic matter such as well-rooted manure, peat moss or compost. The organic matter will help to break up the clay, and, in the case of the manure and compost (but not the peat moss), provide a better nutrient base than the liquid fertilizer you have relied on so far.
Do not move the plants to filtered sun; they can take the full sun – in fact, they prefer it.