yellow bottom leaves?

i have about 5, 8 week old plants that are about five and a half feet tall. some of the bottom leaves are turning yellowish and i was wondering what this was due to or what i should do. i water every day and they are outside in the sun. i used miracle grow tomato once in a small concentration. help!!!

Some leaf lose at bottom is normal... plant energy is working up the plant...as long as you don't see it spread up...whats the N in you tomato stuff ?? might need more ..just a possibility..pics help..

i think it was an 18-18-12 concentration but im not positive