in nutshell..no. you do not need to add a different substrate.
part of what happens depends on the plant.
some will feed from the water column. anacharis, wisteria, java moss, java fern, anubias. to name a few.
some are root feeders (swords to name one) and will feed from the substrate.
early on in the tank there is not much in the gravel for root feeders to feed on. but that changes as the tank matures.
in general it is a good idea to add fert tabs to the substrate for root feeders.
what plants do you have? how much light do you have? these are probably a bit more imprtant questions.