Growing emersed plants on tank sides?

I'm planting up a shrimp tank, and have bought some pots that clip to the rim of the tank, with the idea of growing either house, or other suitable plants to make it more of a feature.

What I'm not sure of, is what I put in the pots in terms of substrate. Do I dump the stem in there, use gravel, sand, or soil? I know they will get nutrients from the water column, but just stuck on what to use, if anything.