If your GH and KH is high (hard water usually has high GH & KH), no soil will drop the pH because the KH will stop it going down.
Peat moss is acidic and will lower the pH of water. But again it comes down to the actual GH and KH of the water supply. If the GH & KH are high, then peat might not drop it much at all. Peat moss will also stop affecting the pH after 6-12months, depending on the GH & KH.
The GH (general hardness), KH (carbonate hardness) and pH of your water supply can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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Soil substrates are not all they are cracked up to be. Most release ammonia for 6-12 months and then offer little to no nutrients to the plants. In my opinion, they are not worth the money and you are better off just using gravel and aquarium plant fertilisers.
If you want to try an experiment, perhaps set up a tank and put a divider on the base, use soil on one side and plain gravel on the other. Grow the same plants in both halves and see what happens.
You can also grow plants in pots of trays and either have the trays buried in the gravel, or camouflaged with silicon and gravel or algae.
You can add peat, soil, clay, garden fertiliser, etc, to the trays and cover them with a thin layer of clay and soil to stop the nutrients leaching into the water. The plants get plants in the gravel and when their roots grow down and get to the other things, the plants usually take off.
Most aquatic plants take the majority of their nutrients through their leaves. Sword plants, Cryptocorynes and Aponogetons will take some nutrients through their leaves but also like nutrients in the soil.