There are several reasons for yellow foliage on tomatoes. Most common this time of year (mid May) is cold night weather. Tomatoes love warm nights and hot days and you did not mention where you live but if night temps are below 50 degrees the cold nights could be yellowing the leaves. The second reason is too much water or poor drainage. Make sure the water runs from the drainage holes in your container and remember that tomatoes like to be on the dry side. This means poke your finger down one inch into the soil and do not water if the soil feels damp. Lack of nutrients also causes yellow leaves and if you are growing in potting soil your plant really needs fertilizer. I use Osmocote on my plants following the directions on the label and using a capful for a 2 gallon container, so double that for a five gallon pot. If only the lowest leaves turn yellow that could be normal leaf shedding and with warmer nights, less water and some fertilizer I suspect you will get a great harvest this summer. Keep growing, Marianne Binetti