If you want to make your own, I'd suggest the 5-1-1 mix. That's what I use on all my containerized plants (except cacti and succulents). Otherwise, any bagged potting soil will work although you will need to be careful about watering as most are excessively water retentive.

And what is the 5-1-1 mix?

5 parts bark fines

1 part peat

1 part perlite

few tablespoons of dolomitic lime to buffer pH.

A zillion discussions about it on the Container Gardening forum. It is extremely popular.....and for good reason.

Thanks for the info on 5-1-1 mixes, Gardengal. I really appreciate it.

When you say "bark fines," would something like this work?

https://www.homedepot.com/p/Kellogg-Garden-Organics-2-cu-ft-Amend-Garden-Soil-for-Flowers-and-Vegetables-672/100427390

And as for the dolomitic lime, this says "dolomite limestone." Look like a good choice?

https://www.lowes.com/pd/Espoma-Garden-6-75-lb-Organic-Lime-PH-Balancer/3806463

"I grow all of my dahlias (which isn't a lot) in BIG containers. Dahlias take up a lot of space."

Depends on the dahlia, really. Thomas A. Edison, absolutely. :-) I love my Harlequin, though, and those will fit in a pretty small pot or a smaller area of garden. There are tons of smaller dahlia for the pot or garden edge that are simply lovely, like Figaro dahlia.

"I use Raised Bed soil made by Whitney Farms."

I just use Miracle Gro Moisture Control potting soil like in other pots. The little baby dahlia love it. :-)

raymondo, if you want to follow the 5-1-1 method, then the Kellogg product is not what you want to use. There is a whole lot more than just bark fines in that and what bark there is may not be a very large percentage of the final product. And it is also not intended to be used as a potting or container soil but mixed with regular soil for inground planting. Same with popmama's raised bed soil. Raised bed or garden soil is not the same as potting/container soil and there are some pretty valid reasons why one cannot be freely substituted for the other without impacting plant health and performance. I can direct you to some excellent and very informative discussions on this topic if you are interested.

Pine bark fines are what you are looking for.......no other additives :-) These are sometimes sold under the label of 'soil conditioner' or just fine bark mulch (and depending on location, the bark fines may not be pine....that's OK). In a pinch, you can use orchid bark or repti-bark (sold as bedding material at pet supply outlets). But look for the pine bark fines if you can :-)

If all this is too much trouble, you can use the bagged commercial potting soil of your choice. But because almost without exception, these are heavily peat based, they are also very water retentive and so finely textured, they lose aeration rapidly. You need to be very hands-on with watering to make sure you are providing enough but not too much.

You can also mix in sand or some gravel or additional bark if the idea of aeration in standard potting soil bothers you, but I personally haven't observed it being a problem in any pot for years. I don't replace my outside soils, it just simply doesn't seem to ever become an issue and roots go so deep I need to let them winter-rot before removing the corpses the following spring. Observably, the plants are healthy.

I've also never had watering issues; in fact, here in PA during our hot and dry summers, the highly-water-retentive structure of the moisture control is the reason why I choose the stuff. Watering twice a day is not an option, and the soil will aerate itself as it dries anyway from shrinkage and normal fungal expansion and contraction, just like any well-kept soil.

So YMMV. I'd be less inclined to do what I do in a cooler, wetter environment.

Thanks for all the input here. I have a bags of vermiculite, perlite, worm castings, and peat moss ready to go, so I was hoping to find a specific recipe to help me find just the right mix. I'll be planting my dahlias in 10-gallon grow bags, which worked very well last season, but not until I started hand-watering every day. This year I'm experimenting with placing the grow bags in some kind of shallow, water-holding tray where they'll wick up water from below. Which is why I want to get just the right potting mix which will keep them moist but not so much as to rot the tubers.

A very rapid search turns up:

"Can I mix perlite and vermiculite?

A basic potting soil mix for plant seedlings can be made by mixing two parts each of compost and peat moss. Add one part vermiculite or perlite. The vermiculite should be moistened before adding it to the potting soil mix."

This would be too wet for a dahlia but might do for seedlings for the short period they're in it. I'd use your worm castiings instead and change it to "extremely large handful" while increasing the vermiculite/perlite to 1.5 parts to increase airiness.

There would be no problem with increasing perlite near the surface to keep the tuber aerated.

I'm a bit surprised the tubers rotted. One thing to try would be to bring them a bit closer to the surface than strictly recommended (the standard is about six inches). I tend to bury mine about 3" deep instead to assure they dry out faster.

While my gardens tend toward the dry, during a wet summer, that can change very, very fast.

Morpheuspa, you'd use the worm castings in place of... what?

And sorry, I didn't mean to imply that the tubers rotted last year. They did quite well. I just want to avoid them rotting in the coming season when I try watering from below. I believe it was an article on the American Dahlia Society's website that suggested adding polymer crystals to the mix, so I went out and got some, but now I'm afraid that might keep things too moist.

Oh, sorry, I get going and give half a thought sometimes when doing two things at once. :-) Use the worm castings in place of the compost, at least in part. The compost is great, but might hold too much water. Although you can preferentially use that at the lower part of the mix to do so and wick it up into the upper part to water the dahlia.

You shouldn't have too much trouble with the polymer crystals! Just a tablespoon or so (unexpanded) will hold a boat-ton of water, but won't overdo it. They last a solid season or so before starting to disintegrate.

Seriously, test one of those puppies in a glass of water. It expands into this pretty little glass-like jewel (but soft). Then just dry it out on the side of the sink back into a solid crystal.

I've never had much trouble with tubers rotting, except in storage over the winter in storage, oddly enough, when kept rather dry... :-)

Thanks for the input, Morph. Yeah, I love those polymer crystals. They're more fun than Silly Putty and Play Doh put together. :D

I haven't had trouble with tubers rotting either, but then, I've never watered grow bags from the bottom. Maybe I'm just worrying too much. We garden, we learn. For example, I learned not to make your peat moss too moist when storing your dahlia tubers in a warm garage. Nearly all my tubers have sent up early shoots. But at least I know my dividing was a success. :)