This topic had been asked about many times in many threads, but I don't think anyone has ever really dedicated a thread to it...at least I have not seen one.
Not only in the world of NPT, but also in Dutch and in other tank-keeping styles/methods hobbyists often choose to use regular soil from outdoors. I have a couple of times, but one time it ended up being a real pain. This was in my 125. I think I had mentioned it, but never really discussed it, so...I'm providing these thoughts to help keep others from making my mistake:
...Here is what I did wrong. I found a local clay-loam and decided to use it due to its nutrient richness and high cation-exchange-capacity (CEC). I "mineralized" it per that thread on mineralizing topsoil, even though I didn't need to (outdoor soils are already "mineralized"...having been wet/dried thousands of times in as many years).
Anyway, I forgot about flocculation. This particular soil in my area doesn't flocculate well...meaning, in water, the particles tend to disperse. I could have chosen a heavier clay soil...but I preferred "loam" because it was less dense...easier for plant roots to get through.
What I ended up with was tiny particles floating about my tank...with no way to filter them out. Even with 100% water changes or temporary canister filter use...the water would be crystal clear for a few hours and then get the particles floating about again.
From my limited experience and from reading other people's threads...I have noticed that the soils high in organic matter (those purchased at garden stores) tend to have less problems than mineral soils (from the yard). I am also starting to think that it doesn't matter how nutrient rich a soil is. I believe (and please feel free to correct me) the majority of available nutrients in any soil are going to be released in a matter of weeks (at least as far as N ), and after that, all nutrients that plants are getting are from fish food, waste, and decomposing OM...and added ferts.
What I SHOULD have done was: 1) Either add a flocculating element to the soil, such as dolomitic limestone (I think AaronT talks about this in his "mineralizing topsoil" thread)... OR 2) just gone with "MiracleGro Organic Choice" like I did with other NPT's... OR 3) used ADA Aquasoil (Amazonia).
If I do it again; for low-tech I'll use the "MiracleGro Organic Choice" and mineralize it per that thread, and cap it with something pretty. For high-tech I'll stick with the ADA substrate. It releases lots of nutrient load at first, but after it settles down, it is the quickest route to a stable substrate.
On my daughter's "Rita's Princess Tank" thread, I just re-did hers and we used "topsoil" from a garden center. It's anything BUT topsoil...looks basically like composted peat and other vegetation...very fiber-heavy and zero mineral soil. We put that in sloping from 0.5" in front to 2" in back and capped with regular gravel. Total depth is 1.5" at front and 4" at back. No problems at all with her tank now except when I try to fertilize it (gets algae on the glass when I add ferts...so maybe I shouldn't do that
Anyway, these are just some thoughts I felt would be useful for others who are considering a soil layer in their planted tanks.