It Could Be Nothing To Worry AboutI will begin by saying that tomato leaves that are yellowing on the bottom are not necessarily a bad thing.
It Could Be Just A Big Bushy PlantSome tomato plant leaves yellow due to a lack of sunlight on mature plants. When the plants become large, bushy, and heavy with fruit, the top portions of the plant can block the lower portions from getting sunlight. This can cause the leaves to yellow, and is really nothing to worry about.
How Are You Watering?Leaves can turn yellow on the bottom of the tomato plant if the plant is not receiving adequate water. Tomatoes need watering the most after transplanting into the garden, or when they are very young seedlings. Tomatoes also need watering during very hot temperatures, especially if the plants are bearing fruit. Usually one good, deep watering a day will suffice in hot temperatures or when bearing fruit. Also tomato leaves can yellow at the bottom due to over watering. Sometimes this can not be helped due to Mother Nature. This is why it is important to have well-draining soil, and to use a good mulch around the plants. Using compost, or a transplant mix, in your garden soil will help if drainage is an issue.
Check The Soil For Lack Of NitrogenYou may need to also check your soil for a lack of nitrogen. Nitrogen will give a plant (everything from grass to vegetable plants) its dark green color, and good growth. A lack of nitrogen in the soil can cause the yellow leaves on tomatoes as well. Test your soil to check for nitrogen levels and other nutrients. Be careful though, do not add too much nitrogen, this can actually "burn" the plant. Higher nitrogen levels can also cause your tomato plants to become beautiful and bushy, but bear no fruit.
Is It A Disease or Pest?If you notice yellow leaves in other areas of the tomato plant, or the yellowing at the bottom is spreading upward, this could be a sign of disease, such as curly top virus (usually curly top virus will show signs of yellowing leaves that are curling up as well), Ringtop Virus, or other diseases. Make sure to check for any pests that may be present in or around your tomato plants.
Ask The ProsIf you are unsure of what is going on with your tomatoes, snip off a limb that is yellowing and take it to your local gardening center or local co-op office, along with a soil sample from around the plants. They might be able to diagnose what the issue is, and the best methods for treating it.
Try Some Heirloom Tomatoes In Your Garden This Year
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