50% promix, 30% castings, 20% compost would do the job IMO. Hell you can change castings and soil around however you want, I know a fella that grows in pure worm castings and has great results. The shrimp compost contains some great nutes as well, but i have no experience with it and couldn't give an opinion. all of it is organic, so I would think you could really mix it the way you want.