You can. I have done many tanks with a one inch layer of organic potting soil under 2-3 inches of sand. There are some advantages to this: The soil slowly leaches ammonia for a few weeks as it converts to an underwater biome, which will cycle your tank for you and help your plants get off to a really good start. It also releases a lot of CO2, which gives new plants a better start. And the whole thing feels much more natural than adding fertilizers...even though, practically, that's all you're doing anyway.
There are some disadvantages, too. If you have any species that burrow, you are in for a gigantic mess. Same if you have to uproot a large plant, or if a powerhead comes loose and blows the substrate around, or anything like that. And if you ever decide to tear down your tank, removing all that old, rotten topsoil is awful.
In the long run, you might be better off with just plain sand. That's what I've done for my last couple of builds, and I actually like it better. You can add root tabs for plants that are heavy root feeders (my swords and larger crypts especially seem to appreciate them).
