No...There is no need for a soil mix. Adding any biological materials could negate the benefit of the island should it leach or spill into the water.

I am constructing 2 islands, one planted with hydroponic seedlings and another of just seeds. I am hoping in the future, I can embed seeds into the matting and just toss them out. Like a Ronco oven, toss it and forget it!

Both islands cost me less than 80 dollars in plants and materials. I was lucky to purchase a roll of Poly Bio Flo material off CL for 20 bucks. It is usually about 4 dollars/sq ft.