Answer: Yellowing of the lower (oldest) leaves isn't all that unusual with tomato and zucchini plants. As long as your plants are putting out healthy new growth, they are just fine. Snip off the yellowing leaves and water thoroughly when the top two inches of potting soil is dry. (Dig your finger down into the soil - if it is still moist two inches down, you won't need to water; if it's dry, it's time to water).
Good luck with your garden!
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