Yellowing leaves.

Hey, over all your doing good, 1st pic looks like light burn, it appears to be beached white, and that looks to be on an upper leaf vs the bottom. Could have happened from the Sun being to bright and the plants not used to it yet. It will get there you just might loose some leaves but that's ok. look into hardening off plants to get them prepared for outside. I went and looked at your diary, growdiaries.com/diaries/124668-grow-journal-by-pinkscissors19 the rest of the plant looks good though if this is the same plant. I don't think you should be worrying to much, sometimes lower leaves get damaged and die. My most recent grow I got a base case of something on the one seedlings and its already recovering just fine. As for magnesium and Iron, I don't think its any of them either. Magnesium tends to make the leaves yellow a bit with out burning or crisping and then you can see green veins sometimes in the leave structure. Iron tends to make the new growth lemon lime green/yellow and I don't see that as an issue with your plants either. Plus it's harder to get Iron definiteness in soil. Regarding the powdery mildew comment, I don't see any on them leaves, but if it was there pluck them right away and DO NOT water before lights out. That can spike that stuff like no body's business. Its more of an issues in the fall when temps drop and everything is wet. Happy Growing and Best of Luck