Most aquatic plants draw their nutrients from the water column as opposed to from the substrate - amazon sword being one exception, and this can be remedied by adding root tabs around the base of the plant.
You could add some soil if you really believe in the benefit - so, only place soil (under the the layer of sand) where your plants will be. If for example in the middle and front of your tank there will be no plants, then no soil is needed there.
IMO though, you're better off using liquid ferts and a good light (depending on the lighting requirements of the plants you choose)
Also, like fish, plants have different requirements when it comes to GH, PH and temperature, so its worth checking your parameters against the data thats available for the plants