Are you annoyed by your tomato plant’s bottom leaves turning yellow? If so, you are probably wondering why it happens, and how to fix it.
So, why do a tomato plant’s bottom leaves turn yellow? A tomato plant’s bottom leaves will turn yellow due to age, nutrient deficiencies, uneven watering, or diseases. Nitrogen deficiency and early blight are common causes of yellow leaves on the bottom of a tomato plant.
Of course, there are some steps you can take to prevent yellow leaves on your tomato plants (assuming age is not the problem!).
In this article, we’ll look at the reasons your bottom tomato plant leaves are turning yellow and what you can do to prevent it.
Let’s get started.
Why Are My Tomato Plant’s Bottom Leaves Turning Yellow?
There are many reasons that a plant’s leaves may turn yellow (chlorosis). Sometimes, the bottom leaves will be affected first.
Some common causes of yellow leaves on tomato plants include:
- Nutrient deficiencies
- Uneven watering
- Lack of Sunlight
- Transplant shock
We’ll start with age, since this is common at the end of the growing season.
As tomato plants age, their leaves will eventually turn yellow. This happens after the plant finishes fruit production for the season.
The time when yellow leaves appear will differ, depending on the type of tomato. There are two basic types of tomatoes:
- Determinate – these tomatoes are shorter, and they produce fruit just once during the growing season.
- Indeterminate – these tomatoes are taller, and they produce fruit throughout the growing season, until frost stops them.
Since indeterminate tomato varieties produce fruit until frost, their leaves will turn yellow later than determinate varieties.
Also, as a tomato plant ages, it gets more leaves at the top. These top leaves shade the lower leaves from sunlight.
As a result, the lower leaves are not able to produce as much energy from photosynthesis. The plant will then draw nutrients out of the bottom leaves and use them for growth of higher leaves.
The bottom leaves will then turn yellow and eventually fall off. According to the Mississippi State University, yellow leaves caused by age are totally normal and not a cause for concern.
Of course, if your plants are young and it is early in the season, then yellow leaves may indicate that there is a problem at hand. One possible cause of yellow leaves on younger plants is a nutrient deficiency.
Nutrient deficiencies can cause all kinds of problems for plants. However, one of the common ones is yellow leaves.
It may seem surprising, but a deficiency of certain nutrients will cause the bottom leaves of a plant to turn yellow first. This is due to the way that plants use and transport those nutrients.
There are two basic categories of nutrients that plants use:
- Mobile nutrients – the plant can move these nutrients around within its tissues.
- Immobile nutrients – the plant cannot move these nutrients around within its tissues. After immobile nutrients are used for growth in the plant, they are stuck where they are.
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- Magnesium (Mg)
- Chlorine (Cl)
- Molybdenum (Mo)
- Nickel (Ni)
The following nutrients are immobile:
- Calcium (Ca)
- Sulfur (S)
- Boron (B)
- Copper (Cu)
- Iron (Fe)
- Manganese (Mn)
- Zinc (Zn)
As it turns out, a deficiency of a mobile nutrient (such as nitrogen) will cause the bottom leaves of the plant to turn yellow first. Here’s why.
First, the plant experiences a deficiency of a mobile nutrient, such as nitrogen.
Next, the plant prioritizes growth and survival of its upper leaves. This is because the top leaves will get more sunlight and help the plant to produce more energy.
Then, the plant moves nitrogen from the lower leaves into the upper leaves. This is possible because nitrogen is a mobile nutrient.
Finally, the bottom leaves turn yellow when they start to run low on nitrogen. They will eventually turn brown and fall off if the nutrient deficiency persists.
If the nutrient deficiency is severe enough, yellow leaves will start appearing higher and higher on the plant. Eventually, even the top leaves will turn yellow.
At that point, there are no other leaves to take nitrogen from, and the plant will begin to fail.
Now we know why a mobile nutrient deficiency causes the bottom leaves of your tomato plant to turn yellow. Next, we need to know what causes the problem, and how we can treat it.
The first step is to get a soil test, which will tell you exactly where the problem lies. You can learn more about how to do a soil test in my article here.
A soil test may reveal several potential problems:
- Soil pH Imbalance (if the soil is too acidic or too basic)
- Lack of a Nutrient (if there is not enough of a nutrient for plants to uptake through roots)
- Excessive Nutrients (if there is too much of one nutrient that blocks uptake of another nutrient)
Soil pH Imbalance
If your soil pH is too low (acidic) or too high (basic), then you will see nutrient deficiencies in your plants. This is because soil pH has a huge effect on the availability of nutrients in soil, as you can see in this chart from Research Gate.
As you can see from the chart, the availability of phosphorus (a mobile nutrient) drops off rapidly when pH goes below 6.0. The availability of nitrogen, potassium, and magnesium (all mobile nutrients) also drop off as pH decreases.
There are several ways to increase soil pH, including:
- Lime – calcium carbonate will raise the pH of your soil. However, it also adds calcium, so be careful about adding too much, or you could end up with a magnesium deficiency (more on this below).
- Dolomitic lime – calcium and magnesium carbonate will raise the pH of your soil, while also providing both calcium and magnesium.
It is also possible to see nutrient deficiencies if your soil pH is too high. You can add elemental sulfur or sulfates (iron or aluminum) to lower soil pH.
However, don’t add anything until you do a soil test. Otherwise, you could be trying to solve a problem you do not have.
Lack of Nutrients in Soil
Even if the soil pH is in the proper range (6.0 to 6.5 for tomato plants), there may still be a lack of nutrients in the soil.
A deficiency of any mobile nutrient in the soil will prevent a tomato plant from absorbing enough of that nutrient through its roots. This will cause the bottom leaves of the plant to turn yellow.
To fix a deficiency of nitrogen, phosphorus, or potassium, you can use an NPK fertilizer. I explain what the numbers on a bag of fertilizer mean in this article.
You can also use supplements that are meant to provide a dose of a specific nutrient:
- You can learn more about high-phosphorus fertilizers here.
Excessive Nutrients in Soil
There is yet another way to end up with a nutrient deficiency in your tomato plants. If you add too much of one nutrient, it could prevent a plant from absorbing another nutrient in the soil.
For example, if you add too much lime (calcium carbonate) to your garden, there will be excessive calcium in your soil.
Calcium and magnesium “compete” for uptake by a plant’s roots, since they are chemically similar and behave similarly.
With too much calcium in soil, plants cannot absorb enough magnesium. This will lead to the bottom leaves of the plant turning yellow.
If a soil test reveals that the soil pH and nutrient levels are correct, then there could be another problem. Watering too much or too little can also cause yellow leaves on your tomato plant.
When you over water a tomato plant (or any plant), the soil stays too wet for too long. This keeps the roots wet and prevents them from getting enough air.
Eventually, this will cause root rot. After enough damage has been done, your plants will not be able to absorb water and nutrients through their roots.
The result will be yellow leaves on your plant. In the case of a mobile nutrient deficiency, the bottom leaves will turn yellow first.
Remember that over watering can spell the end for your plants. Basically, you are giving them too much of a good thing.
Lack of Water
A lack of water due to drought (or forgetting to water) can also lead to nutrient deficiencies. According to the Michigan State University Extension, plant nutrients must be dissolved in water for plants to absorb them through their roots.
Without enough water in the soil, the nutrients will not dissolve, and plants cannot absorb them. This will lead to mobile nutrient deficiencies and yellow leaves on the bottom of the plant.
Pests are another possible reason that the bottom leaves of your plants are turning yellow.
Some pests damage the roots of plants. This will prevent them from absorbing the water and nutrients they need from the soil, leading to yellow leaves.
Other pests, such as aphids, will drain the liquids from the stems and leaves of plants. This will also take away precious water from plants, and can lead to yellow leaves.
There are many plant diseases that can affect your garden. Early blight and Septoria leaf spot are two common ones that can cause the lower leaves of tomato plants to turn yellow.
Early blight is cause by Alternaria tomatophila or Alternaria solani, both of which are fungi. Early blight spreads faster in wet and humid conditions.
The fungi that cause early blight can survive in soil over the winter and infect plants in the spring. These fungi can also get into your garden from infected seeds and seedlings.
Septoria Leaf Spot
Septoria leaf spot is a disease that is also caused by a fungus. It affects the leaves on tomato plants, but not the fruit.
According to the Michigan State University Extension, the bottom leaves of the plant are often affected by Septoria leaf spot. At first, you will see small dark spots, but with enough of these spots, the leaves will turn yellow.
Your best bet is to prevent Septoria leaf spot, since it cannot be cured. You can learn more about Septoria leaf spot in this article from the Michigan State University Extension.
Lack of Sunlight
A lack of sunlight can be another cause for yellow leaves on the bottom of a tomato plant. Without enough sunlight, plants cannot create energy by photosynthesis.
Since chlorophyll is involved in photosynthesis, a plant making less energy will not need as much chlorophyll. Chlorophyll is what makes plants green, so less chlorophyll means faded green or yellow leaves on plants.
Finally, transplant shock can cause the bottom leaves on your tomato plant to turn yellow. Often, the roots of a plant are damaged during transplant.
If the damage is severe enough, the plant won’t be able to absorb water and nutrients from the soil. This will cause water or nutrient deficiencies, even if there is plenty of water and nutrients in the soil.
Should I Remove Yellow Leaves from My Tomato Plant?
You should not necessarily remove yellow leaves from your tomato plant.
For one thing, a plant often knows when to drop its leaves. This will happen naturally when a leaf is using more energy than it is producing.
In addition, a yellow leaf may contain some mobile nutrients. The plant can still move these mobile nutrients from a yellow leaf to other parts of the plant.
If you suspect that the yellow leaves on your tomato plant are caused by disease, then you could remove them to prevent the disease from spreading.
However, some plant diseases have no cure. In those cases, your best bet is to remove the entire plant (not just the leaves) to prevent the disease from spreading to the rest of your garden.
Now you have an idea of what might be causing the bottom leaves of your tomato plant to turn yellow. You also have some steps you can take to prevent or fix the problem.
I hope you found this article helpful. If so, please share it with someone who can use the information.