I found the leaves on my tomato seedlings turning yellow and wanted to know how to fix this. I did some research that helped me find the source of the problem and it can help you too.

Your tomato seedlings are turning yellow because they’re not getting sufficient sunlight. You can move them to a location that is getting better sunlight. Some other issues that can cause yellowing are lack of water, nutrients, or pest and disease attacks.

You might be able to fix the problem of your seedling leaves turning yellow. But first, you need to know what are the exact reasons causing it.

1. Tomato seedling has sprouted first leaves

When your tomato seeds germinate, they will produce their first set of leaves known as cotyledons.

As the plant keeps growing, it will produce new leaves while the cotyledons will turn yellow and fall off.

You need to check if your seedlings are at this stage of growth where they’re shedding the first set of leaves. This is completely normal as the leaves turn yellow. And you don’t need to do anything about it.

2. Lack of sufficient light

You need to provide at least 8-10 hours of direct sunlight for the tomato seedlings to grow well.

A lack of sunlight will prevent the seedlings from using photosynthesis to create their food. And a lack of food will cause the leaves to turn yellow.

Sometimes the tomato plant can be bushy and prevent the sunlight from reaching the lower leaves. It is fine to do nothing as long as the plant is growing well.

If many of the leaves are turning yellow, you need to remove the obstructions causing a lack of sunlight. This may involve pruning the tomato plant or other plants near it.

If the plant is obstructed by a wall or large tree, you may need to move the tomato seedling to another location. This will be easier if you’re growing the plant in a container.

If the seedling is in the ground, you need to carefully remove the root ball making sure the roots are not getting damaged. Then carry them over to the new planting location and plant the roots in the soil.

Cover the roots and lower part of the stem with soil so it can get a strong base and develop even more roots.

Water the seedling well so that the roots can settle and grow well in the new spot.

3. Too much or too little water

A common problem when growing tomato plants is giving them too little or too much water. This will happen whether you grow the seedlings in the soil or a pot.

Both of these conditions will not allow nutrients to reach the tomato plant through the roots. And this will cause the leaves to turn yellow and fall off.

The lack of water will dry out the roots and they will not be able to extract the nutrients from the soil and transport it to the leaves.

Excess water will drown the roots and they will again not be able to send the required nutrients to the leaves.

The solution to this problem is figuring out how much water your tomato plants need. The thumb rule is to give your tomato plants 2 inches of water every week.

But I don’t find that measurement quite useful and have a better solution. Every day you need to monitor the soil near your tomato plant.

Stick your finger in the soil till it’s 1-2 inches inside it. If the tip of your finger does not feel moisture, you need to water the tomato seedlings.

You need to water well making sure the water is reaching through the soil to all the roots. Keep watering slowly till the soil is soaking with water.

If you’re growing the tomato seedlings in a container, you need to pour the water till it drains out from the drainage holes at the bottom.

Once you’ve watered the tomato plants in this manner, you don’t need to worry about a lack of water. Just follow the process of checking the soil every day and you’ll be giving them just the required quantity when needed.

4. Poor garden soil or potting soil

Another reason that the leaves of your tomato seedlings may be turning yellow is due to poor soil. This is a more common problem with garden soil than potting soil.

The main reason is that the garden soil may contain too much clay causing the water to pool and drown the roots.

Or it may contain too much sand that causes the water to drain out fast without the roots getting tie to absorb the required nutrients and moisture.

The garden soil may also contain harmful pests and diseases that reach the roots of your tomato seedlings and infects them.

Or the garden soil may contain harmful chemicals that can be toxic to your tomato seedlings causing their leaves to turn yellow.

5. Too much or too little fertilizer

You don’t need to add fertilizer when starting tomato seeds. But once the seedlings have grown a bit you’ll need to add a little bit of fertilizer. This will help the foliage and fruits of the tomato plants to grow healthy and fast.

If the tomato plants are growing in-ground, the roots might be able to spread and get some nutrients. But if you’re growing the plants in a container, you need to be careful of providing sufficient nutrients.

If the tomato seedlings don’t get the required amount of nutrients from the soil, the leaves will start to turn yellow.

The solution is to either add a slow-release fertilizer to the soil or some fast-action liquid fertilizer every couple of weeks.

You do need to be careful not to over-fertilize the tomato seedlings. The excess fertilizer will burn through the plant’s roots and damage them. This will prevent nutrients from reaching the leaves and turn them yellow.

When growing the seedlings, you need to provide nutrients for the foliage. A nitrogen-rich fertilizer like cottonseed meal or fish emulsion is a good choice.

Once the fruits start developing on the tomato plants, you want to provide a fertilizer that is rich in potassium and phosphorus but low in nitrogen. This allows the plant to focus on growing the tomatoes rather than the foliage.

6. Seedlings suffering from pests and diseases

If your tomato seedlings get attacked by pests and diseases, there’s a high chance of the leaves turning yellow.

The pests like aphids and mealybugs like to suck the sap from the leaves or chew them. A few of them won’t cause much harm but when there’s a huge infestation, the plant will get damaged.

Ants farming aphids on my pepper plants
Ants farming aphids on my pepper plants

The leaves may start to turn yellow or brown and wilt. And if proper care is not taken, they will start to fall off.

There are several fungal and bacterial diseases like fusarium rot, blight, and septoria leaf spot that can affect your tomato seedlings. A symptom of these diseases is brown spots developing on the leaves and they turn yellow.

The best solution to these diseases is to choose tomato seeds that are resistant to them.

You need to be careful when watering the plant so that the soil does not splash on the leaves. Many diseases reach the plant foliage from the soil. Or due to the humid environment that may get created.

You need to be careful to water only the base of the plant. You can use a drip-irrigation or soaker hose for the watering. You can also water in the morning so the sun will evaporate excess water from the foliage.

You can support the tomato plant with a cage or a stake to keep the leaves and fruits away from the soil.

If there are some leaves that have touched the soil, it’s best to cut them off even if the leaves seem healthy.

The next best option is to pluck the leaves as soon as you find the disease harming the plant.

In some types of diseases, this will help the plant to recover and grow back the lost leaves. But some of the diseases like fusarium or verticillium wilt cannot be reversed.

The plant will get fully infected and eventually die. It’s best to take the plant out and dispose of it. These diseases can remain in the soil for several years so you should not use the same location to grow tomatoes again for a few years.

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