Sup guys, sorry took a bit to reply, I'm at work. Sorry i missed the info about where that dead leaf came from. This plant could easily be showing signs of more then one deficiency/lock-out. When P gets locked out, the leaf kind if dries up and shrivels but holds a flat green color,dead purple/blue/brown/black splotches can confirm that, eventually that leaf WILL fall from the plant without yellowing, very much like N deficiency. When you have K deficiency you will for sure notice the leaf having a two tone kind of color, the darker end by the petiole and yellow end at the end followed by severe dead necrotic spots. Its hard to diagnose somebody' else plant without actually getting a feel for myself. But to the OP, if you watch you're growth take note of the beginning of any spots/colors/ briddle stems/dry spots ect in order.