prob with roots helppppp

just looked at my plants roots today and noticed on 1 of my plants the roots look powdery. ive got a recycling 3 pot system 2 600hps lights, bubblers,e c meter, ph meter,temp 27deg ph6.0 to 6.5 e.c 1.8 to2.0 4 weeks into veg

each pot has...

2 fiffty liter pots
1 sitting on top of each other
bubblers into each bottom pot and 1 into res
100 liter res
lava rocks[hydroton rocks?]
water 30min on 2 1/2 hours off
2 part nutt

the bottom pot of my system holds about 1, 2 inches of nutts and in one of my pots the roots look powdery and there is a dust like shit on the bootom of the pot. roots dont smell but are very fragile the roots sit in the nutts that is why i got bubblers in each pot not sure if watering enough plants nice and bushy had root rot last time changed all hoses,fittings i no still got rot but never seen the roots on 1 plant look like this harvest time last grow 44 oz
also the plants look a little on the light green side any help would be much help cos it got me stuffed....

peace...pot...patience

theratman:confused: :confused: