Deficiencies with super soil?

Here's a photo of Mess Medical's PuTang about 10 weeks old. Some limited topping has been done to keep her branching out. The medium is Coast of Maine Stonington Grower's Mix, an organic, compost-rich soil that is not supposed to need anything more than water until deep into the flowering stage. And yet I have bright yellow edges on larger, lower fan leaves. I recall Mass Medical saying that LEDs could produce a Cal Mag deficiency in some instances. My lights are a mix of Viparspectra 450s and HLG 360s. The grow is otherwise vigorous. And what would be the best organic source for some appropriate amendments? thanks.

My guess is potassium def, blackstrap molasses unsulphured, or you have a potassium lockout from too much mag, either way bump up potassium.