10 gallon pot mixture

Ill be planting in 10 gallon rubbermaid tubs on these high cliffs and ill be able to use some of the native soil from the grasslands below. I figured id use 50% native soil but im a bit stumped on what else i should mix in. Water retention is pretty important. I have access to all the best potting mixes ferts just not sure how to divy them up... I was thinking 50% earth,, 30% pro-mix,, 20% worm castings,,, 5/5% Vermiculite/Perlite,,, then a little lime and a little blood meal. Then feed every other watering with Fox Farm grow big then Big Bloom during flowering. Really need help fellas

what you have in mind sounds pretty good honestly. You don't really need to be too heavy on available nutrients. Fox Farms Grow Big and Big Bloom should help alot.