I've planted dahlias in roughly the same holes for years. They just keep getting better each time you amend the soil.
No need to rotate crops, you just need to decide which of the 50,000 varieties are worth the space.

Thank you, Poochella for your tip. You are an angel. I started to grow about half of a dozen varieties this spring and still have a great deal to learn.

`I read this somewhere and it works well for me. After digging your tubers in the fall take time to replenish the soil with whatever amendments you need. when spring comes you are ready to plant. Where I live you don't have to dig dahlias, but I can never leave them more than 2 years or they get so big I can't even lift them out of the ground.