I'm glad you're testing this out with un-occupied containers.
There's no end of pathogens & other potentially harmful stuff in typical "topsoil" sold in garden & hardware stores. I'd advise NOT to use that, no matter how innocuous it may look. Just don't. Same goes for dirt from your yard...there will likely be parasites (worms) & germs aplenty from wildlife (birds, squirrels, etc.)
What kind of heat are you using? If UTH, then you need to avoid thick substrate (1/2" or deeper) over or near that, as it will "insulate" & not allow heat to rise where it's needed.
I can understand you wanting your snake to enjoy digging around- the best way to do that is to put deep substrate in a large container that either goes within the living quarters IF it's big enough (& if there's enough room on the UN-heated side) or in a separate "snake playground" that some of us provide.
For lower humidity, consider shredded paper (I shred all my clean business papers to use as substrate- it allows heat to rise, & adds NO humidity.) There are many substrates you can buy that use some sort of "paper" also. Carefresh is actually hygroscopic, but I wouldn't use that for a BP- best for snakes that like it dryer. It also depends on your local humidity too. No way to guess a "ratio" for you- you'll have to test it for yourself. But you could use paper shreds, & also offer a humid hide (with damp moss) for example, & let the snake choose what it needs/wants.
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