Hey Byron! i am thinking of aquasoil for a side of the tank, sand in the middle, and gravel at the other side, and using rocks to separate the substrates. Have not really looked into the brand of the soil etc, but thought it makes sense that soil is a better substrate for plant growth (correct me if im wrong), such as carpets. i am likely looking at plants that require low co2 as i wont be getting a co2 canister.
Another alternative is to having a sand cap over the soil to prevent stirring up of the soil by the bottom feeders.
First, I do not recommend different substrates. They tend to look artificial, and they draw attention to the size of the tank and make it look smaller, whereas a uniform substrate will make it look larger, especially when it is sand. In order to keep the substrates separate, you need to separate them completely, which means the "dividers" must be siliconed together so there are no gaps, and siliconed to the tank floor. Water moves through the substrate naturally, and until you do something like this you cannot imagine how much it will occur. But from a visual aspect, I would have one substrate throughout.
Second, soil generally has no benefit for plants, certainly not long-term. Now, there may be specialized soils that are different, and very expensive, but unless you are intending a high-tech method with an aquatic garden having high light, diffused CO2, and (hopefully) no fish, I would not waste the money. I tried one several years ago and after two years it got dumped in my back garden with my regret that it cost so much for no benefit and in fact detriment to my poor fish.
Sand is the overall best substrate both for fish and plants. It is easy enough to use substrate fertilizer tabs for large plants, and minimal liquid fertilizer for non-substrate-rooted plants (which obviously would derive no benefit from an enriched substrate anyway).
Substrate carpet plants are not easy except in a high-tech system, but there are a few that do the job to some extent. Depending upon the intended fish, you usually want some open areas anyway (cories, loaches, cichlids especially).
