3 weeks old and yellowing, please help me out =[

I'll have pictures in 1 hour, but till then i can only describe it.

There is a yellowing of the lower leaves on my biggest plant (still not that big 3 weeks old), And now my other two smaller ones are slowly developing it. Do you think its lack of nutes? are they old enough for them? i checked soil ph and it was light green which means under 7 above 5 (seriously such a bad gap lol).

What do you guys think

Short version: my plants have white spots on upper leaves, yellowing on lower.

i was reading the guide on how to diagnose.

Two came to mind Nitrogen deficiency and Magnesium deficiency

Will my 20-20-20 nutes take care of the Magnesium deficiency? Or do i need something else?

PICTURES

How Close Is Ur Lighting, If Its Darker Brown Than Its More Than Likely Caused By Heat Stress, If The Plant Is Young Wqatch To Not Over Water Or Over Fertilize And Make Sure Your Plant Is Getting Plenty Of Fresh Air Circualtion, If U Check All This And The Problem Still Occurs Do Not Prune The Leaves Let Them Die Natuarally And Fall Off But I Dont See It Neccessary To Come To That If All Of The Above Is In The Correct Condition

You have the same peoblem I seem to be looking up now. I believe it is as jtl567 say a Magnesium deficiency. I am trying to find out how much epson salts. Good luck I'll be checking back!