I read a grow journal where the guy did an experiment with 3 gallon containers vs. 5 gallon containers. Everything else exactly the same. The plants in the 5's out yielded the 3's every time. Said there was a direct correlation between root volume and yield. And this guy was an old timer that had grown med's for years so he was looking specifically for the most efficient cost effective method you could use...for soil. He was getting like 1.5 pounds per plant with the 3's and over 2 pounds with the 5's.

And the extra cost is really nothing. If you could get a half a pound more would you spend the extra few dollars for dirt?