Answer: If they are already up to a foot tall, I wonder whether they have been getting sufficient light. Light intensity drops quickly with distance: try to keep the light just an inch or two above the tops of the plants--any higher and the plants will stretch toward the light, the plant will be weak, and leaves may become pale. (Also, it's considered best to leave the light on about 16 hours a day, rather than 24. But this shouldn't be causing the problem.)<br><br>If you've been keeping the lights close to the plants, then the yellow color must be caused by something else. Have you fertilized regularly? (Plants grown in small pots need frequent, light fertilizing. I use a balanced fertilizer mixed at half strength weekly.) Could you have over-fertilized? (Fertilizer in too high a concentration can damage plants.) Do you see signs of insects or disease? Is the temperature in the room fairly steady? Does it get very cold or drafty? These are the variables you need to control. to grow tomato seedlings successfully. I can't pinpoint what the exact problem is.
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