I am raising tomato plants, Amish Paste and Wisconsin 55, from seeds purchased from an online seed order company. I planted the seeds in a standard potting mix that had fertilizer incorporated in it. When small, the seedlings appeared healthy. Now that they are 12" high, some of each variety (but not all) have developed yellowing leaves. These leaves are mostly at the bottom of the plants. I water the soil at the top of the container, not wetting the leaves .I have moved them into larger containers, using the purchased potting mix. They are all still under grow lights. Can you tell me what is wrong with the sickly looking ones? Thanks.
Garrett County Maryland
Hi- the symptoms in the photos are most likely caused by some minor environmental stress. We don't think it's due to too much or too little fertilizer. Some possibilities: the containers are small for the plant size (good that you are potting up to larger containers), grow mix is too wet (allow the top of the mix to dry slightly before watering), temperature fluctuations, plants growing into or above lights.
Remove the lower leaves that are affected. There is a large enough fertilizer "charge" in the grow mix to carry the plants along for 6-8 weeks so no need to fertilize. Let us know if the problem worsens after you move them into larger containers.
You may want to adjust your seeding time next year so that you have 6-7 week old transplants when you are close to your final spring frost date:
Growing transplants: https://extension.umd.edu/hgic/topics/starting-seeds-indoors