Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
A material for holding the roots stable so the plant does not fall over or drift around the tank.
It also holds fertilizer in a way that plants can get it when they want.
Various elements and simple molecules that plants use to grow.
These are not the same things, so no, substrate is not fertilizer.
Use any substrate you want. Use fertilizer.
Some substrates come pre-loaded with fertilizer. Before that fertilizer is used up you need to add more. It is overkill to keep replacing the substrate just because the original fertilizer it came with is gone. All you need to do is add more fertilizer.
Some substrates break down over time, especially high organic material products. These probably should not be used. Look for a mineral based substrate like one of the special clay based products that have a high cationic exchange capacity. These materials will hold the fertilizers in the best way so they are available to the plants.
One way to make a substrate more fertile is to use slow release fertilizer deep in the substrate. You need to add more periodically, but nothing like daily or weekly. Every few months, depending on the fertilizer you choose. You can replenish the substrate via water column dosing if you want, but this needs to be done more frequently.