also is it possible to put organic substrate over my other substrate?
Possible but probably not worth it. The point of having a planted tank substrate is to provide nutrients to the roots. Roots in general grow down. So if you have an inert gravel on the bottom and only put a small layer of a nutrient-loaded substrate on top, it's not going to do much good because most of the plant roots are going to be growing downward and thus be mostly located amidst the inert substrate, not in the nutrient-loaded substrate.
If you go through the effort of buying some planted tank substrate, then you are much better served by yanking out the gravel you have in there right now and just putting in the new substrate by itself.
And just to clarify, what you are looking for is called a planted tank substrate...it is NOT called an "organic" substrate. There is no such thing as an organic substrate--you can't put something like potting soil or topsoil into your tank, you will create an absolute soupy mess. The closest thing to an "organic" substrate would be mineralized soil but even then, the whole point of the "mineralization" process is to remove everything organic from the soil.
Planted tank substrates are good for your plants not because they have organic matter, but because they have iron and other trace nutrients that plants need. Among the most popular planted tank substrates (that are pH neutral) are Eco-Complete, Flourite and Flourite Sand, and Flora-Max. Then if you want to use a planted substrate to also buffer your pH up or down, there is ADA Amazonia (to lower pH and soften water) and Onyx Sand (to slightly raise pH). Note that Amazonia eventually breaks down and needs to be replaced, which for some people is a dis-incentive to use that one.
