you will be fine. i use 9 and 11 gallon pots and i grow BIG plants, and super soil works well with just water, remember the bigger the plants the more nutes it will require. Here is a hint, go a bit heavy on the worm castings and that makes a big difference for the bigger plants. some people reuse the supersoil up to 3x, however i do not since my plants are too big.... i just add water. just harvested yesterday and i had 5 plants, all 6 feet and they will be about 4-6 o ea.....