It all depends on what you want to do, if you're getting good growth out of inert gravel without dosing (I assume you don't add any ferts) then you'd more than likely get better growth if you replaced it with organic substrate, or began supplementing macro and micro nutrients. Either way your plants will run out of nutrients soon and you'll need to replace them.
Dirted tanks provide a natural look and low-cost nutrients that will last 6-12 months. Whereas sand or gravel tanks have no organics and cannot hold many organics, they are however, cleaner. You can supplement the lack of nutrients with several dosing methods, which can be rather cheap or rather costly depending on what method you use.
All of my planted tanks use Mineralized Top Soil with a gravel or sand cap on top to keep it from flying into the water column. I also supplement by dosing once a week. In my experience I have NEVER had a dirt tank that didn't get murky as hell. I've always needed a cap.
Coincidentally all of my tanks look like riverbeds. Soil, sand, pebbles, driftwood, plants.
I really want to discourage you from using nothing but dirt as a substrate, but it's the most earthy look you can go for. Mainly because you will regret putting the dirt in there without something on top, but also because you'll kick up all the dirt after it settles while planting, hell, even your fish could kick up a nice cloud.
Last edited by AVN; 10-28-2012 at 08:34 PM.
Reason: moar stuff