I have had good luck using a mix of peat, vermiculite, earthworm castings, and playsand. I have about 2.5in of this under 1.5in of Tahitian Moon Sand (bottom layer is "Profile" aquatic clay mixed w/laterite). I run about 3.5 watts/gal of light, and the plants are doing fantastic.
The biggest issue I have encountered is some gas buildup from organic materials breaking down in the soil layer, every couple days I use chopsticks to disturb the soil to let gases escape. Over the past 2 months I have noticed the soil packing down more and producing less gas, I'm thinking it will be settled in another month or so.
The more of a "soil" mix you use, the more organic breakdown you will have. If steer more towards a "soiless" mixture- peat, humus, stuff where most of the organics have already broken down- you will have less gas-producing organic decay. This problem can also be averted by boiling the chosen "soil" mix for about 20 minutes, this will break down the organics as well so they do not foul your substrate/water and cause extra tank maintenance.
EDIT: I would just like to add that most commercial soil mixes, even those without added chemicals, are usually pretty heavily composed of organic material. The ones I found to be listed as "organic" and "free from chemicals" contained the MOST un-decayed organic matter, most notably manure, which is a VERY BIG NO-NO for your aquarium.
