Brown/burnt spots on 1 plant only

Hello mate, looking your situation you might have more a lockout than a nute burns but you might have also a pH fluctuation caused by a wrong Ca/Mg ratio because you're using a organic products to feed and if the ratio is away from 3:1/4:1 Ca/Mg so you might have a lot of problems. I would try with a big flush and then feed again but checking the Ca/Mg ratio in your tap water to fix it before. Good luck and happy harvest!

NK def for sure, could be also P,Ca,Mg they're starving

Hi bro, I'll start with your soil mix. Nutrients are typically used at the beginning for light-mix soil. If your soil is already rich in nutrients, you can harm the plant. In the picture, it looks like you also have light stress. Raise the lamp higher or reduce its intensity. Water the plant with pH-balanced clear water a few times before returning to the nutrient mix. Good luck, buddy.