Answer: Based on your description I am not sure what the problem would be, but it is possibly related to crowding or other stress. Tomatoes are very heavy feeders, so make sure you are fertilizing and topdressing with compost on a regular basis; seaweed based water soluble fertilizers also contain numerous micronutrients that might be missing in the potting soil. You might try foliar or soil applications of Epsom salts (Mix at a rate of one Tablespoon per gallon of water) just in case there is a soil deficiency of magnesium which can cause yellowing of the lower leaves beginning at the bottom of the plant.
Tomatoes also need an evenly moist soil, which can be hard to achieve in a container, especially a crowded one. Next year, you might try adding a moisture holding polymer to the potting soil. Also be sure that the soil has enough material to promote good drainage, such as sand or perlite, and enough organic matter to help it stay aerated and hold water more evenly. Your goal is to keep the soil evenly moist but not soggy or sopping wet. You may need to water twice a day to accomplish this but it will depend on the weather and the size of the plants, too.
If the foliage remains yellow although you have fertilized enough, or if it browns, remove it. Keep a close eye out for developing signs of insect or disease, as well.
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