Those bags are for small pots, and lazy people.
Get yourself a tiller and till in mulch, leaves, whatever rabbit or chicken **** or aged manure you have.. mix in different soil types you have on the properly. . Test your soil and add in any nitrogen, magnesium, etc... as needed.
If you don't have any equipment to mix your own and want to buy dirt. Get a dump truck load for about $300
How do you go about testing soil without having to mail it to a lab?