Do Tomatoes Need Nitrogen? (4 Ways to Provide It)


You might be wondering if tomatoes need nitrogen.  After all, nitrogen is in the air all around us.

So, do tomatoes need nitrogen?  Yes, tomatoes need nitrogen in the soil to grow.  Nitrogen is used to produce chlorophyll, which is a key part of photosynthesis (energy production).  Without enough nitrogen, tomato plants will begin to turn yellow, starting with the bottom leaves.

Of course, it is also possible to give your tomato plants too much nitrogen.

In this article, we’ll talk about how to give your tomato plants nitrogen.  We’ll also get into the proper balance of nitrogen and how to avoid the extremes.

Let’s get started.

Do Tomatoes Need Nitrogen?

Tomatoes do need nitrogen for proper growth.  According to the University of Missouri Extension:

“Tomato plants low in nitrogen appear stunted and spindly with a yellowish cast to the leaves. Too much nitrogen creates excessive vine growth, twisted foliage, delayed flowering and lower yield.”

chlorosis
Chlorosis, or yellow leaves on plants, is often caused by nutrient deficiencies, such as a lack of nitrogen.

A tomato plant that gets enough nitrogen will grow plenty of thick vines to support fruit.  It will also produce lots of big leaves to produce energy.

This energy will go into fruit production later in the season.  That is, unless plant gets too much nitrogen.

According to the University of Arizona Extension, too much nitrogen can cause blossom end rot in tomatoes.

So, how do you know if your tomato plants have too much or too little nitrogen?  We’ll start off with signs and symptoms of nitrogen deficiency and get to excessive nitrogen later.

What are the Signs of Nitrogen Deficiency in Plants?

According to the University of Arizona Extension, one of the main signs of nitrogen deficiency in plants is when the older (bottom) leaves turn yellow.  The bottom leaves turn yellow first because nitrogen is a mobile nutrient.

As such, a plant will pull nitrogen out of lower leaves to provide it to upper leaves, which get more sunlight and can produce more energy.

Often, the rest of plant will turn light green due to a lack of nitrogen.

What are the Causes of Nitrogen Deficiency in Plants?

A nitrogen deficiency in plants has a few possible causes:

  • pH imbalance (you will need to adjust the pH with lime or sulfur)
  • Lack of nitrogen in soil (you will need to add nitrogen)
  • Excess carbon in soil (carbon will tie up nitrogen and make it unavailable to plants)

It is a good idea to confirm this with a soil test.

A soil test will tell you the level of nitrogen (and other nutrients) in your soil.  In addition, a soil test will indicate soil pH, which also affects plant nutrition.

You can learn more about how to do a soil test in my article here.

soil
A soil test will tell you more about the nature of the problem if you suspect a nitrogen deficiency.

Let’s get into the causes of nitrogen deficiency and how to fix them.

pH Imbalance

The ideal soil pH for tomato plants is in the range of 6.0 to 6.5.

If your soil pH is too low or too high, it can lead to a nitrogen deficiency in your plants.  This is true even if there is plenty of nitrogen in the soil!

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 nitrogen (a mobile nutrient) drops off rapidly when pH goes below 6.0.

There are several ways to increase soil pH, including:

  • Lime – lime, or calcium carbonate, raises soil pH.  It also adds calcium, so be careful about adding too much of it.  Otherwise, your plants will get a magnesium deficiency.
  • Dolomitic lime – dolomitic lime, or calcium and magnesium carbonate, raises soil pH and also provides 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.

Lack of Nitrogen in Soil

If a soil test reveals that the pH is in the proper range (6.0 to 6.5 for tomatoes), then there may be a lack of nitrogen in the soil.

A soil test will reveal a nitrogen deficiency in your soil.  Another sign is that plants (other than tomatoes) also have from yellow leaves.

A nitrogen deficiency often occurs when you plant the same crop in the same place for many years.  Eventually, the plants take all the nitrogen out of the soil.

To avoid this problem, use crop rotation or add soil amendments to your garden (more on these methods later).

Excess Carbon in Soil

If there is plenty of nitrogen in your soil, it could be that there is also too much carbon in the soil. 

According to Wikipedia, adding too much carbon to soil will tie up nitrogen.  This makes it unavailable to plants, even though there is plenty of it in the soil.

Adding fresh sawdust to your garden is one possible cause of excess carbon in your soil.  Sawdust has lots of carbon, so putting a high concentration in one place can lead to nitrogen deficiency in your plants.

sawdust
Adding too much sawdust to your soil will cause a nitrogen deficiency, due to high carbon levels.

Instead, add sawdust to a compost pile along with grass clippings, leaves, and food scraps.  This will allow it to break down, and provide a more balanced nutrition for your garden.

You can learn more about how to compost sawdust in my article here.

If you think there is too much carbon in your garden, mix in some new material.  This should be either soil from your yard or compost that does not contain much carbon.

What is a Good Source of Nitrogen for Tomato Plants?

There are many ways to add nitrogen to your soil to help tomato plants grow, including:

  • Compost
  • Manure
  • Cover crops (green manure)
  • Fertilizers

Compost

Compost contains a good mix of nutrients, including nitrogen.  It also adds organic material to soil.

This organic material encourages the growth of beneficial organisms, such as earthworms and bacteria.

Compost
Compost adds nitrogen and other nutrients to soil. It also provides organic matter to improve soil structure.

Organic matter also improves drainage of clay soil and helps sandy soil to retain water.

The best part is that you can make compost from yard waste and kitchen scraps.  You can learn more about how to make your own compost in my article here.

Manure

Manure is simply animal waste and bedding.  Common sources for gardens are from cows, horses, and chickens.

Manure contains a good amount of nitrogen, but it will vary depending on the animal it came from.

manure
Make sure to use aged manure, not fresh manure, in your garden.

Just remember that you want to allow manure to age before putting it in your garden.  This neutralizes pathogens in the manure, and also reduces the chances that you will burn your plants with “hot” manure.

Cover Crops (Green Manure)

As mentioned earlier, a lack of crop rotation can cause a nitrogen deficiency in soil over time.  To avoid this problem, you can plant different crops in the same area in alternating years.

Cover crops (also called green manure) are used to help replace nutrients in soil between plantings.

For example, legumes are crops that move nitrogen from the air into the soil.  This happens because there are nitrogen-fixing bacteria on the roots of legume crops.

Some common cover crops (green manure) include:

  • Alfalfa (Lucerne)
  • Clover
alfalfa
Alfalfa is one common cover crop (green manure) that restores nitrogen to soil.

You can learn more about green manure in my article here.

Fertilizers

If your soil still needs nitrogen after using compost, manure, and crop rotation, then fertilizers are an option.  They have high nitrogen content, but this high concentration comes at a cost.

It is easy to burn your plants by over fertilizing them.  For example, if you forget to water after fertilizing, you can burn your plants.

You can also burn plants if you add too much fertilizer at once.  Always follow the instructions on the fertilizer package and water in the fertilizer after applying it.

Some examples of high-nitrogen fertilizers include:

  • Feather meal
  • Blood meal
  • Fish emulsion
fish
Fish are a good source of nitrogen for plants.

You can find more ideas for high-nitrogen fertilizers in my article here.

Just remember that some fertilizers that are used in farming may not be appropriate for the home garden.  According to the Colorado State University Extension:

“Urea or ammonium nitrate fertilizers are not recommended sources of N.”

https://extension.colostate.edu/topic-areas/yard-garden/vegetable-gardening-nitrogen-recommendations-7-247/

The high concentration of nitrogen and salt make these fertilizers too harsh for home gardens.

Which Fertilizer is Best for Tomatoes?

The best fertilizer for tomatoes is one that gives them a balance of the big 3 NPK nutrients: nitrogen, phosphorus, and potassium.

You can learn more about what the numbers on a fertilizer package mean in my article here.

Unless your soil is deficient in nitrogen, you will want to use a low-nitrogen fertilizer.

Can Tomatoes Get Too Much Nitrogen?

Yes, tomatoes can get too much nitrogen.  This can happen if you:

When tomato plants get too much nitrogen, they will often exhibit stunning green growth of their vines and leaves.

However, this production of green plant matter often comes at the expense of the fruit itself.  The vines and leaves will just keep growing, without producing as many flowers or fruit.

To avoid this, get a soil test before adding anything to your garden.  Follow any directions on fertilizer packages if you use them.

It is easier to add more nitrogen later than to remove it after you add too much!

Conclusion

Now you know how to give tomatoes nitrogen.  You also know that it is a fine line, and that too much or too little can both hurt them.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

Recent Content