I use Jobe's tomato food spikes. They love it. Two spikes per plant, every two months or so. The spikes, being designed for tomatoes, are formulate to give maximum flowering and thus fruit....riiiight??
So that means more flowers for the dahlias.

I have read that high nitrogen content in the fertilizer will give you soft tubers that will not keep in storage. In my experience, that was the case. I planted some near my roses, which get a lot of high nitrogen fertilizer. These tubers all became nothing but mush in my basement this winter, while the others all did fine. Lesson learned for me - plant only commonly available dahlias amongst my roses, and treat them as annuals!

The spikes are 8-24-8. I used them two years ago and my dahlias went gangbusters and my tubers were heatlthy and firm. I had to skip a year but am now growing them again.

Thanks for the spikes suggestion. I prefer to use organic fertilizers in the garden, especially with edibles, but perhaps the spikes would be worth a shot.

Yes, the 8-24-8 would be a good formulation for dahlias, I would think. Quite low on the nitrogen side, with a good emphasis for growing the tuber.

I think my problem with fertilzers is that I spend the winter reading all sorts of stuff about growing dahlias and than go on line and see all sorts of things that say they are the one and only thing for dahlias and buy eveerything and throw it in the garden. It will be interesting if I get away with it this year. Next year tomato spikes