Mixes composed of perlite, vermiculite, and rockwool and other inert media should be treated with a mild application, 300-400ppm, of fertilizer prior to seed introduction to provide available nutrients and buffer the pH.
Adding Hormex, Superthrive or some other auxin/vitamin based supplement will accelerate early plant development. It is not beneficial to apply additional fertilizer to seedlings in rockwool or other inert media until the first set of true leaves appear, at which point a 1/4 to 1/2 strength application is made.
Excessively rich organic soil mixes are best avoided until the tender, young plants are well established. As such it is possible to feed young seedlings in soil with a 1/4 to 1/2 strength solution of fetiliser after two to three weeks or after the first set of true leaves appear but only if the soil is not super hot in terms of nutrients.