You're plant looks fine, stop over worrying it man, If this is a first time growing then you are doing better then 95% of new growers . let it do what she is going to do , smoke a joint and relax BB
i do hydro, but here is a link for ph in soil it could help you out it looks like a nitrogen issue but your already half done with the flowering cycle, i dont know if i would worry to much about it but thats just me and like i said, i havent done soil in years, someone else im sure will chime in and be more help, my NL start showing yellow around the same time too, honestly i think your worrying to much but just my opinion mate http://www.kindgreenbuds.com/marijuana-grow-guide/raising-or-lowering-the-ph-in-the-soil-mix/