My watermelon plant is growing like crazy but the older leaves are turning yellow and dying. The only pics of leaves I could find that looked like mine were said to have ozone damage from air pollution but I live out in the country. All my other plants (peppers, squash, herbs) seem happy. This is in an outdoor greenhouse. I have a small ebb & flow system using organic Iguana Juice (1/2 grow 1/2 bloom) nutrients and it runs for 5 minutes every hour during the daytime. Anyone know what's wrong? Adjusting the ph has had no effect.