Is this a magnesium deficiency?

[PhoenixFlower]

I got yellowing in between the veins of my plants, mostly on the lower leaves. GWE says this can be due to one of six different issues, magnesium deficiency seems the most likely culprit. Reasons are because I've got red stems in addition to the yellowing on my lower leaves (this could be due to genetics, as my plant does sprout purple leaves in flower). Plants are grown in 5 gallon grow bags of the PROMIX herb & vegetable mix, I used PROMIX's granular fertilizer (4tbsp of the 4 1/2 tbsp recommended dose) and I have watered the plants twice since transplanting on July 5 (using approximately a gallon of water until runoff). Lights WERE too close, I've moved them up a few inches, highest plant sits about 18" from the light. I have not added or done anything else to the plants. Anything else you need to know about products used or growing habits, please ask!

What do you folks think? Is this a magnesium deficiency? Or is it something else? If it is a magnesium deficiency, should I use dolomite lime or epsom salts to correct the issue?

[SoOrbudgal]

Could be but if you notice on the top of chart, then go from there with possible diagnosis. Hope this helps.

[LemonBud]

Or this may help.