Guide to fertilizing tomato plants for ambitious home gardeners

To grow large and tasty tomatoes, you must understand when, why, and how to fertilize your tomato plants.

And to become a master tomato gardener, you must understand that tomato plants have specific needs at different times of the growth cycle.

Quick facts: Give your tomato seedlings half the recommended dosage of liquid fertilizer when you see true leaves. When the seedlings reach 15 cm / 6 inches, start giving your plant the full recommended dosage of fertilizer. 

Giving nutrients and fertilizers at the right time is crucial for root development, healthy foliage, and fruit production. 

You can grow tomatoes in good-quality soil without adding extra compost, side-dressing, or fertilizer, but your yield will suffer.

For a no-hassle approach to home gardening, using a balanced fertilizer with an NPK of 10-10-10 will provide your plants with the right amount of Nitrogen (N), Phosphorus (P), and Potassium (P) to support both growth and fruit production. 

If you want healthy plants producing a bountiful harvest, read on, and I will share what you need to know about fertilizing tomato plants throughout the season.

Understanding the importance of fertilizer for tomato plants

Fertilizing your plants correctly and at the right time is essential to grow healthy and high-yielding tomato plants. 

Let’s dive in and look at the nutrients your tomato plants need and how they affect growth, root development, and yield.

Tomato plant nutrient needs

Growing tomatoes, you can come a long way by understanding the three letters N, P, and K, representing the primary macronutrients tomato plants need. 

You may have seen the abbreviation printed on bags with enriched soil or liquid fertilizers but never paused to think about what they mean.

• Nitrogen (N): vital for green growth as in leaves, branches, and stems. A strong and healthy plant is needed to support the fruit you harvest. But too much Nitrogen will produce bushy, overgrown plants with little to no fruits. Fish emulsion is a popular nitrogen-rich fertilizer, but be careful not to overdose.
• Phosphorus (P): Phosphorus supports root development and flower and fruit production. Phosphorus contributes to a plentiful harvest. Bone meal is a popular soil amendment for adding Phosphorus.  
• Potassium (K): Potassium promotes the overall well-being of the plant by, for example, giving it a stronger resistance to diseases. Potassium also plays a critical role in photosynthesis and overall plant health. 

Then we have secondary nutrients like calcium, magnesium, and sulfur and micronutrients such as iron, zinc, manganese, and copper. 

Calcium is known to prevent blossom-end rot in tomatoes and promotes healthier fruits.  

Magnesium is a component of chlorophyll and aids with photosynthesis, and too low levels of magnesium can lead to yellowing leaves and lackluster plants. 

Related: Why are seedlings turning yellow (Yellowing leaves)?

Micronutrients like iron, zinc, manganese, and copper are needed in tiny doses and play their part in ensuring healthy and high-yielding plants. 

The secondary and micronutrients are not required and are best delivered by buying a quality fertilizer designed for tomatoes. They are needed in minute doses, and you want to avoid overdosing.

I recommend purchasing a quality fertilizer designed for tomatoes to give your plants this extra attention. 

As a hydroponics fan, I always have bottles of liquid fertilizers at home. Most of these packages have instructions on how to give the nutrients depending on the medium you are using. The dosage varies between soil and hydroponics, where water is the medium.

How nutrients impact growth and yield

Speaking from experience, properly fertilizing your tomato plants directly and measurably impacts your plant’s growth and yield. 

At the same time, overdosing can adversely affect your plants’ health and fruit production.

More is not better by default.

Applying suitable fertilizers using the proper dosage supports:

• More robust and healthier plant structure: The correct amount of Nitrogen helps your plants develop strong stems and lush foliage – critical for supporting fruits and synthesizing sunlight into energy for further development. 
• More robust root system: The correct dosage of Phosphorus promotes a strong and extensive root system allowing your plant to access water and the necessary nutrients more efficiently.
• Healthier and more disease-resistant plants: The proper amount of potassium benefits overall health and develops a stronger and more disease-resistant plant.
• Richer fruit production: Correct dosage promotes the development of larger and tastier fruits and a richer harvest.

Remember that the correct dosage and timing are critical for success. Be mindful of both amount given and the frequency. 

Over-fertilizing or overdosing will lead to unbalanced growth, shocked plants, and diminished yield.

Always refer to the instructions printed on the packaging of your fertilizer. 

Tip: Give half the recommended dosage and observe your plants if unsure. You can always give more later, but it is challenging to remove fertilizer already given. 

Different types of tomato fertilizers

There is a wide range of commercial, organic, and even DIY fertilizers for tomatoes. Results may vary, and here I chose to give my view of the types of fertilizers I am familiar with. 

I have divided the fertilizers into four groups: Organic Fertilizers, Slow-Release Fertilizers, Liquid Fertilizers, and DIY fertilizers.

Organic Fertilizers

Organic fertilizers are the obvious choice if you grow organically and take a more natural approach to growing tomatoes.

And the good news is that they work. 

If you still need to start composting, this is the time. Compost is the best organic fertilizer for tomatoes, and you can make it yourself at home. 

But it is not a quick fix, as composting can take as much as a year, depending on your method. 

Your compost is ready for use when the material looks and feels like soil and has a clean and earthy smell.

Another widely-used commercial organic fertilizer is the Black Kow Nitrogen Phosphate Composted Cow Manure.

But again, this is not a quick fix, as the nutrient and goodness will be released over time. 

Slow-Release Fertilizers

Slow-release fertilizers are, as the name suggests, never a quick fix. Still, slow-release fertilizers are an excellent way to improve the nutritional value of your soil.

Here the Osmocote Smart-Release Plant Food is a popular and relatively inexpensive choice. 

Slow-release fertilizers are a great way to provide sufficient nutrients over the entire season without risking overfeeding your plants. The nutrients are there when the plant needs them.

Liquid Fertilizers

If you are looking for a quick fix or a solution, you should look into liquid fertilizers. 

The advantage of liquid nutrients is that they are diluted in water and then fed to the plants for an immediate nutrient boost. Be careful to follow the application instructions as there is a higher risk of over-fertilizing when using liquid fertilizers as the effect is immediate. 

I prefer using my hydroponic fertilizer from Terra Aquatica Tripart, but the FoxFarm Liquid Nutrient Trio Soil Formula offers good value and similar performance. The packaging includes application instructions for growing in soil and water, and I can choose to boost my plants when needed. 

DIY Fertilizers

What about egg shells, wood ashes, banana peel in water, or even using the head of a fish? There are many homemade recipes where the author claims to have cracked the code. 

And I am not here to rain on anyone’s parade, but there is what I like to call the “attention factor.” 

The year you decide to make a DIY fertilizer or add wooden ashes to your tomato plants is also when you have decided to invest more time and effort into your plants. You start to do many things right. Maybe for the first time. 

Now this particular year, your yield has increased significantly. Was it the wooden ashes or the overall care and attention you gave your plants?

I generally avoid most DIY fertilizers as I have found that the correct dosage and frequency are critical for good results. 

How much Potassium does a banana peel soaked in water for two days deliver? Three days? Large peel? You get the picture—too many questions without answers.

My tomatoes are important to me, and the commercial alternatives are relatively inexpensive and much easier to give in appropriate doses. 

A note on tomato fertilizers

Remember, you are never starting from zero. It is vital to have at least an idea of the level of fertilizers already present in your soil.

If you buy soil bags, the information is printed on the bag; if it is quality soil, you rarely have to add extra Nitrogen. 

One common mistake is to add too much Nitrogen, not realizing that the soil you are using already has sufficient levels to sustain the growth of your plants. 

Your plants will grow tall, with strong and dark green leaves. Your plants look impressive. But you later realize you have fewer flowers and fruits than you expected. 

If you buy soil bags, use a fertilizer with lower Nitrogen and higher Phosphorus content, like a 5-10-5 or 5-10-10 mixed fertilizer. 

Fertilizing frequency and techniques

We have already established that it is essential to fertilize tomato plants if we expect a plentiful harvest. We have also touched upon the importance of enough but not too much fertilizer.

In the next section, we will look at how to manage the frequency or how often you fertilize your plants. 

From seed to seedling

The seed holds the energy required for germination and sprouting first leaves. When you see true leaves, add half the recommended dosage of liquid fertilizer and increase to full dosage when the seedling reaches 15 cm / 6 inches.

If you are developing seedlings in, for example, coconut coir, you should add a nitrogen-based fertilizer like 10-5-5. 

But it is important to monitor your plants. Whether you start tomatoes from seed or buy seedlings from a local nursery, it is essential to know what type of soil you use when potting up your seedlings in nursery pots.

Tomato seedlings growing in compost-rich quality soil may not require additional fertilizer added into the soil during the early stages of development. 

Strong growth and a vibrant green color signal healthy seedlings.

I use compost-rich, quality potting soil and rarely have to add extra nutrients when potting up my seedlings into their first nursery pots. 

At this stage, you should focus on the soil where the plants will grow when ready to be moved outdoors. 

Now is a great time to improve the soil with compost. Spread a 10 cm/4-inch layer of compost over the whole planting area and use a spading fork or shovel to turn the compost into the soil gently. 

This is a great routine to incorporate into your gardening schedule. Make it a habit to turn compost into your planting areas and raised garden beds every spring and fall. 

The flowering phase

Your plant needs some but not too much Nitrogen to produce many flowers. Instead, at the pre-flowering stage, you want to boost the potassium level you feed your plants. +

Aim for an NPK where the Potassium level is at least double the Nitrogen level.

From flower to fruit

If your plants look healthy and strong, you do not have to worry about adding Nitrogen at this final stage. 

Now you want to focus on adding Phosphorus and some Potassium. Consider using a fertilizer with an NPK ratio around 1-4-2, like 8-32-16. 

Now you must watch your plants and be prepared to fertilize them as often as every two weeks until the end of the growing season.

Remember to monitor your plants and do not fertilize them too often. Follow the application instructions from the manufacturer and be prepared to adjust watering and fertilizing routines should you see any signs of stress or adverse effects. 

Summary: NPK Ratios for Tomatoes

You want to give your tomato plants a balanced amount of nutrients. And ideally, this includes the primary macronutrients represented by NPK ratio, secondary nutrients, and micronutrients.

Dosage and frequency are critical, as over-fertilized plants will produce a poor yield.

So how on earth can we accomplish this without a degree in chemistry?

Here are my recommendations based on your needs and situation:

You need to boost your plants and want a bumper harvest: A quality liquid fertilizer for tomatoes will give your plants the nutrients they need. Follow the instructions on the packaging.

You have plenty of time and want to prepare the soil for your tomato plants: Work a thick layer of compost into your garden beds before moving your plants outdoors. Buy a quality liquid fertilizer for tomatoes using the NPK ratios below as a guideline depending on your needs.

• NPK 5-10-5: Fertilizer with 5% nitrogen, 10% Phosphorus, and 5% potassium by weight. Promotes root development and fruit production. Use when your plants start to form fruits to encourage fruit production and overall plant health.
• NPK 5-10-10: Fertilizer with 5% nitrogen, 10% Phosphorus, and 10% potassium. Start using it in the pre-flowering stage to help your plants produce a bountiful harvest.
• NPK 10-10-10: A balanced fertilizer with equal parts nitrogen, Phosphorus, and Potassium. Perfect for the home gardener wanting minimum fuss while giving the tomato plant what it needs. NPK 10-10-10 is often used as a general vegetable garden fertilizer but is not tailored to the unique requirements of tomatoes.

How to know if your plants need a boost

Inspect your plants regularly for growth, form, and color changes. Signs of nutrient deficiencies include:

• Yellow leaves: Could indicate nitrogen deficiency. But remember that the first and lower leaves often turn yellow after serving their purpose. 
• Purple or brown leaves: Could indicate Phosphorus deficiency. But leaves can also turn black if exposed to cold weather. 
• Weak plants: This could indicate Potassium deficiency or poor watering habits.

If you notice irregularities, do not be afraid to boost your plants. Use a balanced fertilizer and start by giving half the recommended dosage. 

If you use quality soil, leaf discoloring is often a sign of pests, weather changes, or watering too much or insufficient. 

If nothing helps, I recommend buying a soil test lit or sending a soil sample for testing.

How to avoid overfertilization

Giving your plants too much fertilizer will create imbalances and adversely affect your plants. But is there a way to prevent or minimize the risk of over-fertilization?

Here are some tips and advice to help you play it safe:

1. Know where you are starting: Adding fertilizers blindly can create imbalances, and your plants may suffer.
2. Know your fertilizer: Know what you are feeding your plants. Homemade fertilizers are very popular, but do you know what level of nutrients you are feeding your plants?
3. Follow recommended application rates: Do not feed your tomato plants too often. Follow the instruction on store-bought fertilizers or, if unavailable, stick to a two-weekly schedule from the pre-flowering stage. 
4. Use the correct dosage: Measure carefully and ensure you give the right dosage at the correct time. If anything, start by giving less, as you can always add, but it is difficult to remove.
5. Timing the plant’s needs: Give your plant the nutrient it needs at the correct stage of development. An example would be to avoid nitrogen-heavy fertilizers from the pre-flowering stage.
6. Slow-release and organic fertilizers are easier to use: Synthetic fertilizers give plants an immediate boost making them more prone to over-fertilization. Slow-release fertilizers deliver nutrients gradually and are thereby less likely to cause harm.

Related: Can tomato plants recover from transplant shock? Expert tips and insights.