There is soil underneath that is a black clay that is 6-8" deep over decayed limestine, caliche subsoil so the 5.5 inches is just additional soil, the roots will go down where the moisture is abundent. Being covered the clay soil will stay moist and be penatrable by roots. The more I grow over it the more penetration into it with roots and soil life and it will soften up.
This is one reason I am using cover crops and deep rooted Brassicas. I also live in a very hot area and did not wish to raise the bed too high and risk drying out but I wanted some elevation to prevent oversaturation in the clay soil.
You may be able to go with a higher bed because you are in a cooler area. That would be helpfull with the stooping.
My goal is to have a slightly raised bed that doesn't dry out and for the the soil I added and organic ammendments and cover crops to enrich the soil beneath it. My raised beds I instaled last year have great healthy soil, earthworms and the plants are doing great.
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