I don't know if this helps, but I am currently reading Peter Hiscock's book "Encyclopedia of Aquarium Plants" (and also his book "Aquarium Designs Inspired By Nature"). I am a beginner regarding planted tanks, and it seems to explain a lot of things that confused me. I have read parts of Walstad's book and a lot of material on fins.actwin.com, and I find the technical information overwhelming. It is sort of a lot to take in at once. Perhaps I will go over this material again once I've fully digested the above books.
Pushak, the author of the post, has a lot of info on substates on his site:
http://home.infinet.net/teban/ (see
http://home.infinet.net/teban/substrat.htm)
Hiscock seems to prefer substrate heating cables and a layered substrate. He recommends a layer of silver sand to just cover the heating cables to aid heat distribution. On top of this he recommends a thin nutrient rich layer. Above the thin nutrient layer, he recommends a layer of plain gravel. The reason for the plain gravel on top is that it provides a good rooting medium, and the top area of the roots is primarily used for storage - nutrient exchange occurs lower down (hence the nutrient rich layer).
I've seen aquarists in various groups and forums highly recommend eco-complete. It doesn't need rinising, provides a good rooting medium (though some say it is a little light), and contains a lot of nutrients. It seems people get great results and it's very simple.
Others have recommended using a small amount of peat under a gravel like flourite, though I have also seen people discuss plain gravel (1-3mm grain size, I believe) or sand.
HTH, and if anyone finds any of the info above is inaccurate, please correct me!