My guess is heat/light related. Look at how it's curling in. It's trying to cover itself up from something. Usually its from excessive heat/light. Do you have an air-cooled reflector? If not I would recommend to raising it up to 16"-18" from the tops. The rusty spots look like it's getting hot in the soil from too many nutes. Flush, flush, flush, and flush again. If your going to be adding nutes you need your growing medium to be as "inert" as possible (much like coco in a relative sense). FF has pretty hot nutes that don't really work the greatest for cannabis, imo. I'm sure some growers will disagree and I'm sure they've had bomb ass dankity-dank buds come off of their plants. I just believe part of the "less is more" mentality. It's not bad to use though if you mix it with some FF Light Warrior to kinda level the nutes out. Also, I've found that if feeding additional nutrients to you do a "feed, water, feed, water, ect." feeding schedule every or every other day depending on how much she's drinking.
Ultimately, my favorite soil that I would recommend to any one is Super Soil created by Subcool. I've just seen amazing results with it. Of Organic quality and growth rate of a hydroponic plant when you get it dialed in. It's changed my mind from using hydroponics to it. You can find the recipe online. Coco is also pretty awesome. That's my second recommendation. DWC being the third.