yellowing leaves (PICS)

I have some problems with my plant, lower leave are yellowing, you be the judge of my problem.

Magnesium deficiency? What kind of soil is she in?

Its regular soil, bought from flowershop, packed-in-bag-soil. I gave some nutes this morning, i ll see what will happen tomorrow.

Still vegging, maybe 3 weeks old.

What kind of soil is it in? Someone asked but you were really vague.
Are you giving it nutes (if so STOP)
Have you tried flushing the soil?
how long has it been in this soil?

I wasnt giving any nutes till yesterday, coz i thought its maybe deficiency. The plant is in that soil from seed, 3-4 weeks. And about the soil, back here we dont have miracle grow, coco and stuff. I bought regular soil, which worked fine for the other plants. I thought its maybe deficiency coz i already grew few plants before, and now i am using the same soil again.