Earth Digging area

Hello! On behalf of our two gerbils, I’m asking about dig areas. Some set-ups I’ve seen include earth filled boxes.

Our set up is a mix of hey and woodchips, with cardboard rolls daily thrown in which of course get quickly demolished and added to the mix.

So what Earth do people use for a more earthy area? What is safe to buy and is it more for the play area or in a section in the main cage?

I’m not in US so I may not be able to get all suggested brands, but it will help to see anyway what they consist of