I don't think there is a "regular potting soil". When you buy potting soil you get whatever mix of ingredients that particular manufacturer wants to sell as potting soil. There is nothing wrong with them doing that, since there are so many different possible mixes that will work fine for growing plants outdoors. I suggest going to the el Natural forum and searching to see what has been said on this subject over the past year.
Agree with Vaughn. Most, if not all potting soil contains a lot of organic matter and fertilizers. This will cause havoc during the first two months of setting up. You might be better off by getting cheaper top soil from HD or Lowes to begin with.
As the organic matter start to decompose, all the nutrients will leech into the tank water. As you start seeing the leeching nutrients, change the water. This will help control the algae explosion. After 1 1/2 months you can start slowing down the water changes and after 2 months or so, you can go a couple of months without water changes.
Top soil seems to be the choice for most of those who practice the EL Natural method of setting up an aquarium. Top soil usually does not contain the extra ferts, perlite, etc., found in regular potting soil.
I cannot speak for other peoples' experiences, especially considering that many followers of the EL Natural Method have been very successful. Regrettably though, my experience was very negative(massive fish and shrimp deaths, little or no plant growth, major plant die off, even when I got the point where I was adding ferts to try and stimulate plant growth, inspite of the fact that water parameters tested normal). I believe that Top Soils vary from brand to brand and this could make or break you. If you are going to go with this approach, you may want to use the exact same brand of Top Soil that someone else has been successful with or better yet, try Aaron's Mineralized Soil Approach as many claim success with this method. http://www.aquaticplantcentral.com/...-mineralized-soil-substrate-aaron-talbot.html
I've tried potting soil as a base before, covering it with gravel.
One of the problems is gas (methane?) when it starts to decompose.
This went on pretty much continuos over the course of a year, fish seemed to be ok with it, but hindsight, I'd say you be better off with gravel and added fertilizer sticks or other dosing.
Wouldn't try it myself now for sure.