What is this curling?

your plants need more nitrogen. im not sure what stage your at but if youre using a 13/14 right now im assumimg you're in flower. plants still need normal amounts of nitrogen for the first few weeks of flowering. but they also need the p and k more during that time as well

but, p deficiency looks nothing like that and you can look at any number of leaf symptom charts to confirm that this is not an opinion. If you have a blueish tint to some leaves or dark copper spots, that'd by symptoms for a p-deficiency. dark bronze/metallic is further progression of p-deficiency. paling bottom-up on plant and tip-in on leaf is 99.99% nitrogen deficiency symptom. either inhibited or low concentrations.. commmon sense will dictate.

low N that has advanced beyond chlorosis, or you've inhibited it's availability with recent formula change or pH swing. Could also just be the plant shedding a lower, shielded leaf. common sense should allow you to eliminate the possibilities. it's a myth that boosting p or k in flower increases yield or potency. If avoiding toxicities and deficiencies, you are in a good spot. Amping up and causing a toxicity does no good. if a toxicity does show up, it's evidence that the plant is receiving WAY more than it can use, which obviously doesn't help to treat it like foie gras.

Good answer organoman ferry scientific 👍

Phosphorous deficiency. Remember, what is showing in the leaves now, is a direct symptom of what was happening in the root zone 2 weeks ago. The fact you just gave 13/14 is pure co-incidence.