Quick help

So i was having yellowing leaves on the bottom of my plants and im almost sure it’s because of a nitrogen deficiency, the stems on my plants have turned a reddish/purpleish color i just recently started putting a new nutrient in.. “8-2-2” how long does it take for a plant to overcome a nitrogen deficiency and will my stems ever return back to the normal green or no?

Are you sure it's not a magnesium deficiency?? That will cause stems to change to red/purple, and leaves to yellow.