IMO you would want to avoid soil, or other organic matter, in your substrate. It is just too unpredictable, especially for a larger tank, especially if it is your one show tank.
For a smaller tank, experimenting with soil is great, and you get an awesome growth out of your plants, and when the soup starts to blubber, you just clamp your nose shut, throw the hole enchilada out and start over. It is a little harder with large tanks.
I have had tanks with soil, and for the low maintenance, low light, no fert and CO2 approach it works wonders. But if you like aquascaping, and want to pull out plants once in a while because they would look better in a different spot, basically mess around with your tank, again, I would not use soil.
I don't think the high tech substrates are worth that much money. You can grow awesome plants in enriched fine gravel, which costs about $4 for 50 lbs (versus $60 for a plant tank substrate). It is just as long term stable. To increase the nutrient binding capability, something like laterite, or fired clay substrate like Schultz Aquatic Soil (I love kitty litter though
will not break the bank.
Anyway... that's what I think and do, and there are many ways :mrgreen: