Common Tomato Nutrient Deficiencies and How to Fix Them

June 3, 2024

Tomato plants are a favorite among home gardeners, but they can be particularly sensitive to nutrient deficiencies. Today I will be going into crop science in continuation of our previous tomato guide. If you have not read that one yet, you can find it linked here.

Identifying and correcting these deficiencies is crucial for a healthy, productive crop. All of the nutrient deficiencies that tomato growers in Texas encounter fall into two categories: Macronutrients and Micronutrients. Most of these can be tackled with a good fertilization schedule and patience. Do not expect the problem to be resolved within a week and remember that soil is the most forgiving media on Earth.

How to Diagnose Your Plant

Nutrient deficiencies are the most common hindrance to plant growth and fruit development. When plants run out of essential nutrients, they throw up their own warning flags to help the grower diagnose the issue. Let's go back to biology and chemistry class for a moment.

The big takeaway is that most crops will exhibit these signs at some point due to stress, a virus, or lack of nutrients. Because of this, we must eliminate the possibility of a pathogen issue, like the mosaic virus, prior to assuming we have a nutrient-related issue. The plant will translate this into either chlorosis (yellowing) or necrosis (dying) of leaves.

The chlorosis and necrosis can occur in either the leaf tissue or the leaf veins themselves. Pay close attention during the early stage of an undiagnosed deficiency to how it progresses. These things cannot be corrected overnight.

The pH Factor in Central Texas

Another often overlooked cause of "deficiency" is soil pH, which results in some nutrients becoming non-bioavailable to plants. In acidic soils, essential macronutrients like nitrogen, phosphorus, and potassium (NPK) become less available. However, in our Central Texas alkaline soils (high pH), micronutrients such as iron, manganese, and zinc are less available.

PRO TIP: When using liquid fertilizer, it is important to balance the pH after mixing up your solution. Adding any fertilizer to soil will alter the pH, however soil has an incredible ability to "buffer" the pH to more neutral levels.

Understanding Mobile vs. Immobile Nutrients

Here is a breakdown of what nutrients all plants need. More importantly, we need to know if they are mobile or not. This tells you if the deficiency will show up in old growth or new growth first.

Macro Nutrients (Mobile except Calcium): Nitrogen (N), Phosphorus (P), Potassium (K), Magnesium (Mg), Sulfur (S), Calcium (Ca).
Micro Nutrients: Iron (Fe), Boron (B), Copper (Cu), Manganese (Mn), Zinc (Zn), Molybdenum (Mo).
Want more in depth on secret ferilizter tricks? Read our full guide >>

1. Nitrogen (N) Deficiency

This is the most common deficiency. It starts with yellowing on the lower, older leaves and progresses up the plant. Growth will appear stunted and fruit production will drop.

Quick Fix: Foliar spray application of Seaweed or Kelp at dusk.
Fertilization: Use a nitrogen-rich organic option like blood meal or composted manure.

2. Blossom-End Rot (Calcium)

This appears as dark, sunken spots on the bottom of the fruit. This is primarily caused by calcium deficiency, often exacerbated by irregular watering in the Texas heat.

Consistent Watering: Avoid letting the soil dry out completely.
Supplementation: Use calcium sprays or a CalMag product to bridge the gap.

3. Phosphorus (P) Deficiency

Watch for lower leaves turning dark green to purple, especially along the veins. Phosphorus often becomes "locked" in the soil due to temperature or pH issues.

Solutions: Add bone meal or rock phosphate. Ensure soil pH is between 6.0 and 6.8 for optimal uptake.

The Analogies Block: The Plant's Checkbook

Think of mobile nutrients (like Nitrogen) as cash in your wallet. When the plant gets "broke" (deficient), it can move that cash from its old accounts (bottom leaves) to its new expenses (top growth). Immobile nutrients (like Calcium) are like a certificate of deposit. Once they are put into a leaf, they stay there. That is why Calcium issues always show up in the newest fruits or leaves first.

4. Potassium (K) Deficiency

Older leaves will show yellowing or browning specifically at the edges. You may also notice weak, brittle stems and poor fruit flavor.

Solutions: Use potassium sulfate or wood ash (potash). Improve soil structure with organic matter to prevent leaching.

5. Magnesium (Mg) Deficiency

Look for yellowing between leaf veins (interveinal chlorosis) while the veins stay green. Leaf tips may curl upward, often called "praying leaves."

Quick Fix: Dissolve Epsom salt (magnesium sulfate) in water and apply as a foliar spray.

6. Iron Deficiency

This shows up as yellowing between veins on new leaves. This is extremely common in high-pH Texas soils where iron is present but "locked" away from the plant.

Solutions: Apply iron chelate to the soil or lower pH with sulfur.

FAQ - Troubleshooting Your Tomato Garden

1. Why are my tomato leaves turning yellow at the bottom?

Yellowing at the bottom of the plant is usually a sign of Nitrogen deficiency. Because Nitrogen is a mobile nutrient, the plant moves it from old leaves to support new growth when soil levels are low.

2. What causes the black spot on the bottom of my tomatoes?

This is Blossom-End Rot, caused by a lack of Calcium in the developing fruit. This is often triggered by inconsistent watering, which prevents the plant from transporting Calcium effectively through the stem.

3. Is my soil pH affecting my tomato harvest?

Yes. In Central Texas, our alkaline soil often locks out micronutrients like Iron and Manganese. Maintaining a pH between 6.0 and 7.0 ensures that these nutrients stay bioavailable for your plants.

4. Can I use Epsom salt on my tomato plants?

Epsom salt provides Magnesium and Sulfur. It is an excellent remedy for interveinal chlorosis (yellowing between