Should I use garden soil in Square foot garden?

I found great composite wood look heavy duty snap together raised beds at sam's club very reasonable, but now have to fill them with soil. Square foot gardening book recommends 1/3 coarse vermiculite, 1/3 peat moss and 1/3 compost - I'm worried this will dry out very quickly in our Florida sun and am tempted to use at least a good percentage of Miracle grow garden soil Does anyone have experience with "Mel's mix" soil and can you tell me if it dries out rapidly? Our prime gardening season (fall) is coming up and I plan to be ready with my new raised beds! The beds will be in mostly sun and placed directly on the ground.