For substrate I will be using my existing AS plus some RootMedic Goodness and a bag of miracle grow potting mix to give the tank a super charged substrate.
Craig, I know a lot of people use it but I would URGE you very wholeheartedly to NOT put the potting mix in there.
I used the potting mix and it was doing fine. However, I went to shift my wood at one point, and I broke the sand top layer. A volcano of dirt and gases erupted into the water. I almost lost my fish, I lost pretty much all of my shrimp a few cories and an otto.
When I started taking the dirt out it smelled awful. Apparently it broke down something awful and wasn't venting properly, even though I made sure to keep the sand to no more than 1" deep.
Right now I'm running it with straight sand (and the plants are suffering) until I can move later this summer and put together an order for AS.
I know some people have luck with dirt, but considering the size of this tank any issues like I had would be a train wreck.