Crusis, I believe as bacteria breaks peat down it releases organics into the substrate which in turn keeps nutrients available to plant roots. When placed under the substrate it does not move pH much. In the end I tried mulm and peat under my newest aquarium simply because more experienced aquarists do it, but I believe it is a redox reaction.
Alternatively, ADA Aquasoil is an acidic substrate that will lower KH and pH in the water column without clouding or tinting the water. Pricey though (I am using it specifically to grow Tonina and Eriocaulon sp, two species known to love acidic substrate, after failing with Tonina sp. Belem in my other high light tank with aged Schultz).
Of course, a great way to lower pH while helping plants is CO2 injection.
HTH
Alternatively, ADA Aquasoil is an acidic substrate that will lower KH and pH in the water column without clouding or tinting the water. Pricey though (I am using it specifically to grow Tonina and Eriocaulon sp, two species known to love acidic substrate, after failing with Tonina sp. Belem in my other high light tank with aged Schultz).
Of course, a great way to lower pH while helping plants is CO2 injection.
HTH