What's ailing this plant????

outdoor grow
Leaves turning yellow after 3 days of trying 12/12 and stopping .
Plant had been groing well until this but I notice another is starting to do the same.

Tried some iron. Originally planted in Miracle Grow garden soil. No problem for 2 months. Just fed it some 8-4-4 bat quano. Trying to fix.
It's just the new growth so far.

Help!!!

I would say magnesium(Mn) deficiency, mix about 1/2 a teaspoon of epsom salt to a gallon of water. if that doesnt help within a few days you may want to do a flush, but its not recommended.

Thanks, ace. I'll try ti.

my leaves r dry at the tips where if u touch it it will crumble .
what should i do?

c.l.