According to Gardening Know How and Home Guides, tomato leaves turn yellow for many reasons, including: a lack of nitrogen in the soil, lack of water, pest problems or magnesium deficiency. Minor yellowing during the growing season or on the lower parts of the plant is normal ? it does not call for grave concern. However, a bacterial, fungus or other tomato disease typically requires treatment.
The first step to alleviating extensive yellowing of tomato leaves is to verify that the plant is being watered frequently. Gardening Know How recommends a drip hose to ensure adequate watering of the tomato plant. Next, check the nitrogen in the soil. If the soil is lacking in nitrogen then apply nitrogen rich fertilizer by following the manufacturer's directions on the container. Pest problems, normally having to do with aphids, can be treated with insecticide or by more natural means, such as the introduction of ladybugs.
A magnesium deficiency can be identified by yellowing that occurs between the individual veins of the leaves. This focused yellowing can also be coupled with a significant decrease in the amount of tomatoes produced by the plant. If these symptoms occur, first test the soil to discern the amount of magnesium in it. Some plants exhibit symptoms that suggest that magnesium is lacking in the soil when it is not. However, if magnesium levels are low, then dilute one tablespoon of Epsom salt in a gallon of water. Spray the plant with the solution when the plant is flowering and when tomatoes are beginning to appear.