I want to plant herbs in my old cast iron bathtub in the backyard. The
tub is too deep, so I need to fill the bottom with something to elevate
the plants. (This will also keep the plants from drowning when heavy
rain fills the tub.) Filling the whole thing with soil would require
too much soil. Anyone have ideas on what else I could use? I want to
avoid scratching or staining the inside of the white tub in case I want
to use it for something else in the future.

Thanks.