If I have to dig my holes 3' deep and 3' wide, how much bagged soil would it take to fill one of these holes? Im just trying to figure out how much soil im going to need to pick up this week as I wont have the ability to make it back into town to buy more once im out there im out there for the season. Im new to the outdoor in-ground gig, ive done it in pots but never into dug out holes. so any help is much appreciated guys!!!