Plant problem help

Hey guys was wondering if anyone could help me figure out what the problem is here....pic attached

It's about 3.5 weeks into flower and I just can't quite figure out what is causing these brown spots/dieing leaves

looks like phosphorous deficiency or lockout what nutes are you using? how often? soil or hydro?

I'm using FFOF soil with jr peters/jacks classic nutes

I'm thinking you may be right about the phosphorus deficiency....I'm giving her a healthy feeding of the blossom booster 10-30-20