I will take a shot, I am no expert but I would look at a Magnesium def. You say it is on the bottom leaves. Calcium would be a guess but if calcium it would be higher on the plant due to it's struggling mobility inside the plant. Magnesium is more mobile meaning it can reach the top of the plant without help so you see deficiencies lower in the plant tissue. I would tell you how much to increase but I am in water medium not a soil type medium. . Hopefully someone will chime in with more experience like Rick, Chas,King , Vince, Rabbit. 

Are these under the leds? Leds burn cooler so less transpiration so less mobility in the calcium and mag.

Just taking a shot at this so wait till somebody else chimes in with more experience. I am no expert,still green and learning fyi.

Also I would be working on bringing that light down a little at a time if you are in the 12/12 stage of lighting. In flowering I would lower them down to 18"-12" for flowering. Do this a little at a time