Auto northern lights crispy yellow leaves ?

Hello fellow Growers I have 4 auto northern light plants all on day 55 out of 84 as noted by breeder... But about a week ago two of my plants started to yellow and actually get crispy ! My light was a little close... Maybe a foot above the canopy but now I have it around 15 inches from the tallest bud,with my air-cooled 600 watt hps. The two that are yellowing are in happy frog soil running fox farm trio. The other two are planted in a mix of compost i made myself along with worm castings perlite and peat moss. Those are not yellowing what so ever but also seem to be taking longer to grow. All 4 seeds were planted on may 2nd and sprouted from rockwool may 6th. I would love to finish strong with this grow Any help would be greatly appreciated!! Thank you fellow Growers so much !

Underfed plants don't go crispy like that. Fox Farm Trio will build up salt content in soil which will pull water out of your plant if you are not doing some leaching of your pots every now and then, so that would be my guess.