Why do many of my tomato leaves turn yellow

Question

Large individual pots, southern exposure, backyard.

Answer 1

This article discusses a number of possible causes: https://www.gardeningknowhow.com/edible/vegetables/tomato/yellow-tomato-leaves.htm
You can also take leaf and soil samples to the county extension service where they can analyze the problem and give you advice on treating it. This article will help you locate the nearest one:
https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm