it's not magnesium deficiency, you only get magnesium deficiency when the plant has lived an extended life, usually you'll get it if you double or triple harvest.

How old is the plant? it looks to me you're worried about the first leaves yellowing. this is normal, over the course of it's life your plant will lose a majority of it's lower leaves.. there's nothing you can or should do about it. It's pretty normal.

You need to distinguish what are.. and are not actual plant problems.