Has anyone grown prk organically?
Sometimes finicky strains do better in organic soil where they can self-regulate (to a point).
I'm running 5 'half and half' right now and they are also showing signs of sensitivity. No burnt tips, but they are twisting a little.
My first PRK was in organic bagged soil, same as my 3 half and half and showed mutations that the h&h don't have. The other five are in inert calcined clay and three have showed mutations from the first set of true leaves before getting the first fertilization.