Are you looking for something to give newly rooted cuttings a boost or something to create a good soil environment for ongoing root development?
You would use a high phosphate starter fertilizer to help newly rooted cuttings get a good start.
You want a healthy soil biome for strong roots throughout a grow. You can get that using fresh compost and/or worm castings (not the sterilized stuff you buy in big box stores and most nurseries), or you can go with an additive from your local hydro store.
I would personally go with what ma nature makes for you because you will get greater diversity, but I'm sure what you get in the hydro store will work.