Answer: Yellowing leaves on tomato plants are quite common and can be caused by a variety of things, including transplant shock, over- or underwatering, and lack of nitrogen. Tomatoes like rich, moist soil that has plenty of organic matter and drains well. Tomatoes need nitrogen at the start of their growing for green healthy leaves.
Tomatoes are moisture sensitive and need a regular supply of water to thrive and produce fruit. How often to water depends on your particular soil's characteristics. You want to keep it uniformly moist, but not soaking wet, to a depth of 12 to 18 inches.
If only the older (lower) leaves are yellowing, it might not be reason for concern and the best treatment may be to simply pinch off the affected leaves.
Q&A Library Searching Tips