This is what I’m doing now. Soil from last year + pumice + compost + fertilizer + river sand + beneficial microbial life. I’m probably doing overkill, the scale is certainly different but I’m going for BIG pots. I need a cubic yard of soil.
Regarding the roots to leave, I pulled out the stumps and big pieces, the rest all is kinda breaking up into the rest of the dirt.
I got the compost/pumice in bulk (a truckload) for about $100, which was enough for my veggie gardens and weed grow bags. I should have used the same organic soilyard for all my dirt last season, it wouldn’t have been such an intensive operation this year. I’d check if you have something similar.
How much dirt are you trying to make/buy? I see at least 15 beans poppin’ and know there are going to be more. 5 gallons each? 75 gallons is 12 cuft, or 6 big bags of soil.