Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The dirt, as a medium for holding fertilizers, will hold up a very long time.
You will need to keep adding fertilizers.
Low tech: Fish food and some root tablets will probably be enough. Fish food seems to have less K and Fe than the plants need. If the tank is lightly stocked you might not be adding very much fish food, so the root tablets are a good way to add more. Home made clay balls of fertilizer are just fine, I would emphasize K and Fe, but not skip the other minerals.
High tech: I would go the full EI method, or something similar.