Under the outer walls that support the roof and/or additional stories there will be a sizeable footing that should go below the frost line. But inside the footing where the foundation is just supporting the first floor you only need a few inches of concrete.

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I’ve found bits of plastic trash in almost all of the potting soil I’ve bought. I’m at the point where I think a heavily filtered hydroponic setup is one of the only ways to really minimize microplastics.