First let me start by saying that in my view, it is entirely natural. In nature, soil becomes more nutrient dense the deeper you get, just like how in a compost pile the compost is formed from the bottom up. By layering (not solid layers, just a general trend of more nutrients the deeper you go) you allow the plant to access different levels of nutrients at different stages in its life, the same way it would in nature where it grows through nutrient bare topsoil, comprised of depleted soil and decomposing material, into nutrient dense soil comprised of fully decomposed material/humus.
I use my own version of super soil similar to Sub's but mine is from an old Tomato planting technique for planting in nutrient bare soil, where you dig a deep hole and add stuff to the bottom of the hole for the plant to access when it's fully grown (In the recipe I learned it was 3 fish heads, an aspirin, 2tbls blackstrap molasses and 1 cup of kelp meal per plant. Orange/banana peels optional.).