Is this really a potassium defiency? Any suggestions how to deal with it?

Magnesium deficiency... the interveinal chlorosis coinciding with rust spots is more likely Mg-deficiency. It's very possibly to lock it out with too much p or k -- which often is instructed by the peanut gallery. it's not foie gras. we cannot force feed it to increase its metabolism. if you changd formulas over last 2-3 weeks consider those changes as likely part of the cause. this stuff takes time to present veisibl symtoms. if p an k are not too high and pH is proper, then i'd wager it is Mg deficiency. After looking at diary.. your organic soil isn't going to make it till the end. better give it more than calmag. < 2 mL per gal? what is that 5-10ppm? if your amended soil is starting to run out of stuff, be reasy to supplement it. take notes on timeframe of how it plays out. if you stick to same soil next time can preemptively supplement befoer symptoms occur.