Yanno, what's too close because I have got a 1000W HPS on the two of those plants and my way to measure it was putting my hand down there on top of my plant and if my hand wasn't getting hot, then that wasn't too close for my plant. Is that wrong?
Depends. How long have they been under the hps light? How long in the MG soil with no feed?
If it was N deficiency, the plant would be pulling food from the lower leaves.
various micro nutrient deficiencies can make tops yellow. iron and sulfur i think. sulfur is rare. iron def makes super yellow, sickly looking new growth. yours look more like the light is just a little intense for the plant. Especially suspect this since it appears as though the other, shorter pic in the plant does not look like it has it as bad.
ive never used straight miracle grow before, but i am guessing it is not packed with micro nutes.
I mix it into the soil when planting ornamentals and such, or top dress in an outdoor garden.