If the leaves at the bottom of your plants are showing signs of turning yellow, don't worry!
All the goodness from the roots goes into producing top growth and the lowest leaves are often ignored. If the higher leaves of your plants are turning yellow it is probably because they are struggling to absorb nitrogen.
In this case, a foliar feed with a balanced food like miracle grow will help. Deficiencies are often due to low temperatures are plants that have used up all the food in the pot they are in, in which case move to a bigger pot with more compost.
I usually remove yellow and poorly looking leaves from the bottom of tomato plants.