This to me still looks be a Mag deficiency, this is what i found in the plant problems forum. I have read other people using Fox Farm that have suffered Mag deficiency's, does anyone agree with me? I have had this problem in my prior couple attempts, but never tried to compensate with Cal/Mag, I have in fact done a res change every week in prior grows, and still had the problem, that is why I jumped on this soon as I seen leaf color change, I kind of expected it. I really want to get my pump out of the tank, but I think I will wait one more week.
I know I have read other threads of guys saying they add Cal/Mag to the res, but I cant recall at what dosage. I think I'm going to add a couple TSP to the reservoir now. I will bring the EC up a little, I think the plants are large enough to handle a little more.
I'm using a Hanna continuous PH/EC/PPM/Temp meter, so I know exactly where my EC is.
Magnesium is a component of the chlorophyll molecule and serves as a cofactor in most enzymes.
Magnesium (Mg) deficiency.
Magnesium deficiency will exhibit a yellowing (which may turn brown) and interveinal chlorosis beginning in the older leaves. The older leaves will be the first to develop interveinal chlorosis. Starting at leaf margin or tip and progressing inward between the veins. Notice how the veins remain somewhat green though as can be seen in figure 15.
Notice how in Figure 16 and 17 the leaves curl upwards like they're praying? They're praying for Mg! The tips may also twist.
This can be quickly resolved by watering with 1 tablespoon Epsom salts/gallon of water. Until you can correct nutrient lockout, try foliar feeding. That way the plants get all the nitrogen and Mg they need. The plants can be foliar feed at ½ teaspoon/quart of Epsom salts (first powdered and dissolved in some hot water). When mixing up soil, use 2 teaspoon dolomite lime per gallon of soil.
If the starting water is above 200 ppm, that is pretty hard water, that will lock out mg with all of the calcium in the water. Either add a 1/4 teaspoon per gallon of epsom salts or lime (both will effectively reduce the lockout or invest into a reverse osmosis water filter.
Mg can get locked-up by too much Ca, Cl or ammonium nitrogen. Don't overdo Mg or you'll lock up other nutrients.