Different reasons affect in yellowing tomato leaves. It is great to do a soil test that help to identify pH level and nutrients level in the soil. Adequate watering is essential to prevent from yellowing leaves. Water to the base of plant without leaves and it is good to water in the morning. Feed carefully to the plants and liquid fertilizer help to absorb nutrients to the plant quickly. Be careful to provide adequate sunlight. And if the leaves turning yellow due to bacterial and fungal diseases, the diseased leaves should remove.