Well my experience with soil in a planted tank is not that well lol, I actually was able to grow mold in the soil, I wouldn't reco it but thats just my experience. When I looked at a good substrate I did alot of bashing of the dollar sign and looked and researched what would be the best for my needs. My needs being I wanted it to last forever basically or at least 10 years!
Take a look in my signature about my 55 and see how I did the substrate I ended up using vermaculite mixed with peat, Clay and Silica sand!
Enjoy Not sure If i posted all reasons but the point was for it to last and when the clay exhausts itself it actually reasorbs nutrients back into to exhaust again a repeat process.