Yellowing Tomatoes

Question:

My tomato seedlings have yellowing leaves, starting at the oldest leaves. What's wrong?

Minnesota Master Gardeners say:

 Your tomato seedlings are showing signs of nitrogen deficiency. The nutrients stored in the tomato seed and the potting soil are now all used up, and you need to begin fertilizing your seedlings.

Fertilize your tomato seedlings sparingly once a week. (For future reference, begin to fertilize after the first true leaves appear.) Use a water-soluble fertilizer mixed one-fourth strength to avoid burning the seedlings. This fertilizer should be applied only to soil that is already slightly moist.

In addition, I would suggest that you avoid spray-watering your seedlings as this will encourage leaf diseases. Water only the soil, avoiding wetting the leaves as much as possible.

For further reading,
Tomato time - well, almost: http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Mar0101.html#tomato
Starting seeds indoors: http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/H236startseeds.html