Why is Tomato Plant Turning Yellow?
Can't send pics.
Certified GKH Gardening Expert
Without seeing it, and exactly where and how the yellowing is ocurring, I will not be able to tell you what is happening. I will only be able to give you the long list of possible causes. It is very important to know where the yellowing is happening, your watering habits, what you feed the plant, the temperature and humidity and any other information you can provide in order to give you a definite answer.
Yellowing on new growth is almost always a deficiency of nitrogen, but can be attributed to a high soil pH, too much water, not enough light, or a grow lamp that is positioned too close to the plant.
Yellowing at the bottom can be normal with age, or depending on venation can be Magnesium and Calcium deficiencies. It can be a sign of infection if accompanied by black or brown leaf tips.
Yellowing through the middle of the plant can be attributed to lack of light, or bad light penetration. It can also be a sign of over, and underwatering.
The whole plant turning yellow can signify poisoning, soil toxicity, (rarely, sun-bleaching
) fast pH fluctuation, and strange heat waves in the 100 degree range for extended periods.
What I can do, for now, is include an article that will give you a good rundown of the care that tomatoes require in order to be healthy: https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-tomatoes-guide.htm