Just a caution here. If you are still thinking moderate light and no CO2, I can assure you that a plain sand or fine gravel substrate is more than adequate. There will be no real benefit to these other substrates. Allow me to explain.
Plants will grow fine in any substrate, provided it is not too dense (like mud) or the opposite, too "open" as in larger gravel. I have used fine gravel, pea gravel and play sand. The pea gravel was large enough that I did see some slight problem for the plants, but otherwise, didn't matter. Aquatic plants, like terrestrial for that matter, can only take up nutrients from the water via the roots and the leaves. Any minerals in the substrate material must leech into the water for the plant roots to assimilate them. This is why adding nutrients to the water column work just as well in terms of providing nutrients. The water flows down through the substrate (there is a natural flow in nature and the aquarium) carrying nutrients which are then taken up via the plant roots. Some nutrients are primarily taken up via the leaves. So adding nutrients to the water column does cover all bases.
Now, having said that, adding nutrients directly to the substrate can quicken this assimilation in some cases. Substrate tabs achieve this the same. Given the higher cost of enriched substrates, I do not myself see the benefit.
As for soil, the benefit here is in CO2. The organics in the soil are high, and bacteria establish to break them down, producing CO2 in the process. I mentioned this as the major source of CO2 previously. And in any substrate, this is going to occur, if we leave it alone; too frequent or deep cleaning is detrimental to this, obviously, as it is removing the organics. Soil contains high organics, so there is an initial release of lots of CO2. However, after about a year, any substrate will be of equal value in this. So the benefit of soil is initially. But with this there are other issues as the other member mentioned, like ammonia, unstable water conditions, etc. There are many soil-based planted tank aquarists who seriously suggest setting up a tank with soil and not adding fish for six months, solely because of this fluctuation. I have never had problems setting up new tanks with clean fine gravel or play sand; my plants take off after a couple weeks, and water conditions are relatively stable by comparison. As was said earlier in another's post, do your research and decide what you want before starting out. It can be as simple or as complex as you make it.
Byron.
.
