Second this, except the part about promix, you have to be careful about which one you get if you go that route.
One type of promix (not sure which, I've never used it) is basically peat and perlite, which will have no nutrients. It's a soilless medium, like what you have.
Promix herb & vegetable mix (what I use) will let you go about a month without feeding, and it is organic. If you pot up as your plant grows you can sometimes get away without feeding until you're starting flower. Some lighter feeding strains will react a bit to the nitrogen level, but should grow out of it if they're otherwise healthy.
I would recommend going with hydro nutes in a soilless medium as your plants will need higher levels of micronutrients (iron, zinc, etc) which are absent in soilless mixes, and are at lower levels in most nutes.
If you really want to stay organic, you should use soil, imo.