When I was in Tucson, we had a houseplant that hung from the eave in the front of the house by the front door. It was one of my duties to water that thing at LEAST twice a day! You who don't live in the desert don't understand what the heat is like there. It can be over 110 every day for weeks at a time. Soil is only one factor. You can give a plant beautiful organic soil and water it, but it won't stand a chance in those temperatures, it will simply cook unless you can provide some kind of shade for it and protect it from those temperatures. If you want to grow a sweet potato there, you need to plant it in a pot in your air-conditioned house.
Edit: Gardening in those areas consists of finding native plants that can survive there.