Not sure how useful this is for others but I was at my wit's end for ideas for soil so did a thread on reddit where people came up with some really good suggestions:
Small basalt slabs from Clockwork work well in small planters and glass display boxes (set on their side).
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I just updated the reddit thread with a campfire for the base of trees and the wood elf bedding furnishings, which when flipped are one among the best for soil, I think.
That would be nice, but it should be possible as well to remove all existing grass from a plot. Sometimes it is just so annoying to have grass shining through your floor of a building or a terrace.