Aquarium substrate for healty plants?

If they are true underwater plants, you don't need anything but gravel. The problem with putting in fine substrate under the gravel is that it can get stirred up and cloud the water, especially if you have bottom feeders, and they can root just fine in gravel. The most important thing for the plants is the lighting. I only keep sword plants and floating plants but they have all thrived and spread with the use of plain aquarium gravel and good lighting.

too complex a mix...stick with a pound of gravel per gallon of water

I don't know a thing about that stuff, I have a black thumb. But here is a forum all about planted tanks, http://www.plantedtank.net/forums/