Our soil here in southern Az is so poor & rock hard that i just do raised beds. The planter bed is made from 6 redwood fence boards 5.5″ wide and 6ft in length. They only cost a few bucks a piece at Lowe’s or Home depot. In the corners I have  1ft long 2×2 posts to anchor the boards to with deck screws. The dimensions are 6×2.5 x 1ft finished size. Using redwood makes them rot & insect resistant. On the bottom you can staple “hardware cloth”–this is fine stout 1/4″ wire mesh that deters moles & gophers from tunneling up & in to your garden. I can fit 7 marijuana plants in the planter

The big bales of Kellogs planting soil (3 cubic feet) is a good “base”. There are online volume calculators so you can be sure to buy enough soil to fill the box. This one here states that it’ll take 18 cu. ft to fill the box. I would buy 14 cu ft of the soil and mix in 1 cu ft each of Vermiculite, Mushroom Compost, Composted Chicken Manure and Earthworm Castings to give it a good seasonal start.

If these are going to be pots in an indoor grow you will have the pot filled 3/4 of the way with the planter soil and a cup each of the vermiculite, mushroom compost, composted chicken manure and the earthworm castings mixed all in.

As a general rule , most bagged container soils will feed a marijuana plant for 4 months. Marijuana doesn’t need MUCH feeding , but does need SOME.  I use commercial fertilizers like Miracle Grow or Fish emulsion but I use low dose–about 1/4 the amount specified for veggies like tomatoes or corn. Too much fertilizer and you’ll get “Newt Burn” or nutrient burn. This manifests itself as dried yellow/brown leaf edges. Your remedy here is to flush the post or planter bed with water to dilute the overfeeding.