What it looks like?

Looking at it my answer would be very similar to HighRoller909 in terms of the cause. Although corrections are a lot slower in organic vs hydroponic fertilization, you could try applications of compost teas, top dressing of earthworm castings, and so on. I used to make my own organic soil but it would always start running out of nutrition by early-mid flower. Doing the above can refresh and extend the soil through the end.

hard to answer without complete plant photo but to me it looks like it pulled nutes from that leaf,meaning it is in search of nutrients.one other thing that got me to this conclusion is that other leaves look pale as well and branches are too "young green". some genetics want more. but since it is an organic grow,I don't know what you could do.