As has been mentioned, it depends upon the plant species, and numbers, as well as your aquarium. Nutrients come from fish foods (as fish waste in the substrate) and water changes (the hard minerals here, like calcium, magnesium, some others). Some people find this sufficient, but again it depends upon the plants.
If fertilization is required, the first to use is a complete liquid. Some nutrients can only be taken up via the leaves (potassium, nitrogen, carbon, oxygen, etc) so adding nutrients into the water column will cover all bases to some extent. The water circulates down through the substrate, so liquid nutrients get to the roots of substrate-rooted plants as well as the leaves where this is needed.
With heavier feeders like swords, also using substrate tabs will improve growth. I use the tabs for the larger sword plants and the Tiger Lotus and Aponogeton. But I also have very soft water, near zero GH, and this makes a difference too.