It could be something as simple as a lack of nitrogen in your soil. If this is the case, you could check the nitrogen level of your soil, or you could just add a little bit of nutritious matter to the soil and watch to see if the yellow tomato leaves don’t just green up for you. Yellow tomato leaves could be a sign of lack of water. A drip hose is an excellent way to water the soil regularly to help prevent yellow leaves on your tomato plants.