I like a granular fertilizer, that is easy to spread, when it comes to amending the soil around my ornamentals. Crepe Myrtles don't need much, in my opinion, but I'll toss them some general purpose flower & veggie fert. if I'm out spreading it.

This season I've been using Fox Farm's "Marine Cuisine" and some various types of guano depending on what I'm fertilizing. This way I have a well balanced base to which I can add extra N, P, or K in the form of bat or sea bird guano. I think it's best to get some fertilizer that you can use on more than one plant and the Crepes are not that demanding or fickle. They can have a splash of whatever you are using on your veggies or they'd be happy with just a thick top-dressing of fresh organic compost and a restored mulch layer. As with many plants grown primarily for their flowers, too much Nitrogen can be a detriment so don't fertilize too heavily and at all costs avoid the "lawn" type fertilizers which are full of bioavailable nitrates that will just serve to make your Myrtle a big bushy bloomless monster (- nitrates which will also rapidly leech into the nearest creek or river as a major pollutant).

If you really want to baby something, I like the OMRI-listed Earth Juice line of products. They have seperate formulations for "Grow" (high N) and "Bloom" (high P & K, no N), as well as a micronutrient solution and one that's purely for boosting the Potassium levels (called Meta-K). This can get quite expensive when used for large applications and I mostly use it to keep container plants as happy as possible, but if all you are growing is Crepe Myrtles and you want to go all out "organically" then get a soil test and spoil your babies with a precisely formulated Earth Juice regimen.