While it's going to depend some on which dahlia, what size container, etc., soils for dahlia should tend toward the very light. Smaller dahlia in containers seem unconcerned about their soils.

(I've simply used 1/2 Miracle Gro Moisture Control with 1/2 sand mixed in and gotten away with it perfectly for pots where I don't pre-decide what will go into them until May, and don't change soils. Contrary to a lot of negative opinion, it works well and I've raised Harlequin and Figaro dahlia in pots with no issues, and even larger dahlia that require staking or tomato cages. The mix is light enough).

This works in a place where summers tend toward the hot and dry and where these plants will need watering daily from mid-July onward regardless of what I plant them in.

I'd suggest purchasing an already-stabilized potting mix as buying this by pieces and parts means it arrives without the pH stabilized. Dahlia are particular about wanting a relatively neutral pH, which is not easy to do at home without good measurement.

So to replicate mine:

25% Peat, 25% Bark Fines (as 1/2 Miracle Gro Moisture Control Potting Mix)

20% Perlite (available at the store, doesn't need adjustment)

30% Sand (available at the store, doesn't need adjustment)

(Dust some bone meal up top when you plant; the stuff is a fertilizer, and very slow).

Others may recommend more bark, less peat, and no sand. That's OK, I guess, but for dahlia, the soil should retain some water while being well-draining. That mix will drain fast, but not retain water. Assuming an outside dahlia, that will dry out very, very fast.

If you find my mix dries out in your area too fast, replace half of it next year with more potting mix to increase the water retention. There are other ways to do that...but in the case of dahlia, we don't want to make the soil too wet or heavy. Bark at least breathes well.

As stated by the previous poster, there are a lot of additional factors that will contribute to the answer for you.

I live in Colorado front range Zone 5. It is very dry here. Hot in the Summer. My dahlias are in very large containers (the ones I grow take up a lot of space) that sit out in my yard. The containers are about 2' wide and about 18" deep or more. They receive lawn sprinkler over-spray and I hand water them with a hose every single day (sometimes twice a day in the really hot period).

I use this raised bed soil. It really smells when you open the bag so I think it has a lot of compost in it which the dahlias seem to like. I do also use a water soluable fertilizer once per week.

https://www.whitneyfarms.com/product/craft-soils-and-mulches/organic-raised-bed-mix

(disclaimer : I'm not saying this is what you should use. I am only replying to what I do and it works for me. I think you really need to experiment to find out what works for you. It may be beneficial to ask neighbors in your local area.)

Yep, don't be afraid to experiment. And what works for me in PA, and popmama in CO might not work for you.

I confess that some pots might be closer to pure Miracle Gro Potting Soil without adjustment because I was probably lazy that year, and haven't changed it in fifteen years except to re-mix. As it decays, I just add more with some sand or whatnot, whatever's handy.

This is not an exact science. :-)

WOW!! Reusing Miracle Gro potting soil for 15 years with only an addition of sand or more MG mix!! Not a process many container gardeners would endorse :-)

LOL!! Organic "feedings" do nothing to refresh used potting soil. It is the deterioration of the particle size of the media and lack of aeration due to compression and collapse over time and fertilizer of any type will not change that. Fertilizer cannot reconstruct pore spaces.

" incredibly skilled" and "talented" are not terms I would use to describe this methodology. More like just dumb luck. But to each their own gardening peculiarities.