When I was learning to grow, and old timer told me to dig my holes as big as a # 2 washtub. That's 15 gallons. I haven't always dug them that big, but they sure do seem to do better when I do. Something else he said, was to never plant closer together than 4 1/2-5 feet. You want to give them the room to get big if that's what they want to do.
This year I'm using 8 or 9 gallons of soil per hole, and will mix in dirt to finish filling the hole. For years all I used was a traditional farm fertilizer like 13-13-13. But I was having to walk a lot further back then.
I don't water very much. Just too easy to get complacent and get sloppy. I do pray for rain though. I'm growing in real sandy land, so I look for signs of moisture when picking a spot. If you are in hills, you can build a berm on the downhill side of the plants to catch run-off.
Good luck.