Some of my little tomatoes have had yellow leaves that look kind-of dried out. I decided to investigate the possible reasons and solutions for this. Reasons that tomato leaves turn yellow can range from simply not enough water or sun to plant disease. I investigated photos of the possible diseases and decided that mine were definitely not diseased.
I still do not know exactly what the problem is, though. So I decided yesterday afternoon that I would go ahead and put my little plants in the super sunny part of my garden (they were still in their planters because they're still so small) and give them a good drink when the strength of the sun weakened (to keep the sun from scorching my leaves). That solves my water and sun problem and they should green up again fine.
As far as the nutrients in the soil go, I am going to get myself a soil kit sometime this summer and do some learning about how to increase desired levels of NPK in the soil naturally depending on what plant that I want to grow in a certain area. But that is for another day.