The Best Fertilizer for Tomato Plants

The best fertilizer for tomato plants is not what you would think. If you’ve ever gone down the aisle at a garden center, there are a lot of formulations of fertilizers specifically for tomatoes. However, there are only rare instances when the premade fertilizer mixes are the right thing to add to the soil. 

Let’s dive into the details of what exactly you need for healthy tomato plants that produce loads of delicious tomatoes!

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Test the Soil

The number one most important thing to do before adding ANYTHING to your soil is to test it. I will explain why with a personal story.

When I first started a garden, I would add an all purpose fertilizer to the soil every year. An all purpose fertilizer for a vegetable garden, generally includes equal parts nitrogen, phosphorus, and potassium or NPK. Little did I know, adding an all purpose fertilizer was a huge mistake!

About 3 years into gardening in this location, I decided to send my soil to a professional lab to be tested. When the test came back the phosphorus levels were over 1000 ppm. To put this into perspective, the normal range is 42-68 ppm. I had about 20 times the level that should be present.

Can you have too much of the required nutrients for plants? 

The answer is yes. And to make matters worse, my own ignorance and actions had caused this problem.

Having too much phosphorus in the soil is a major issue. Excessive phosphorus reduces the ability of the plant to take up necessary micronutrients like iron and zinc. 

It is also water soluble and drains into surface water like streams and lakes causing problems for the water quality and messes with the delicate balance of bacteria present in water.

The only way to correct the problem of excess phosphorus is time and not adding any more!

So this long way of saying I HIGHLY recommend testing your soil prior to adding anything. They make all kinds of tests now and some of them are so easy to do and to read. They take all the work off your plate.

I did an entire article on how to test your soil and what is in my opinion the best all around soil test.

What Necessary Nutrients Do Tomatoes Need to Thrive?

The needed nutrients for  tomatoes are more complicated than meets the eye. Even though a tomato plant can live on only nitrogen, phosphorus, and potassium, it doesn’t thrive unless it has the right balance and levels of the major NPK and micronutrients.

This includes micronutrients of zinc, calcium, magnesium, sulfur, boron, copper, iron, and manganese. Tomatoes also need trace minerals.

An example of a tomato grown with all the nutrients it “needs” but still ends up absolutely tasteless is a hydroponically grown tomato. 

If you want tomatoes with out of this world flavor, they need to be grown in soil. They also need to have all the right nutrients mentioned in the right balance.

Is Organic or Conventional Fertilizer Better for Tomatoes?

Research is continually being done on organic fertilizer vs. conventional and the pros and cons of each. Based on my own research, I have the opinion that organic fertilizer is superior in the long run. 

However, you can also do your own deep dive into this and decide for yourself. Conventional also known as synthetic fertilizers go through a manufacturing process. The main benefit is they are inexpensive.

On the other hand, organic fertilizers come directly from plant or animal sources. They also can come from rock minerals.

Additionally, organic fertilizers often have a wide range of nutrients. For example kelp meal contains potassium but also many trace minerals.

An organic fertilizer feeds beneficial soil microbes, which boost plant health. A conventional fertilizer does not. In fact, research shows synthetic fertilizers have a negative effect on soil biology.

Therefore, the best fertilizer for tomato plants is derived from organic sources and that is what you will find in my recommendations below.

What is the Best Fertilizer for Tomato Plants?

The best fertilizer for tomato plants is one that meets the unique needs of your soil. However, prior to tomatoes being put in the soil, they are started from seed. Let’s dive into seed starting fertilizer first.

An important note: Any recommendations of fertilizers in this article come from positive results and personal experience in the garden using them. I link to brands I use, for example, Down to Earth’s Blood Meal. However, other fertilizer companies also make blood meal. Feel free to use any brand you like.

Tomato Fertilizer Mix for Seed Starting

If you start your own seeds in a seed starting mix, fertilizer is essential. Usually a seed starting mix has no soil at all, so what added fertilizers are not based on a soil test.

I have my own seed starting soil block mix recipe. It uses blood meal, bone meal, and kelp meal for fertilizer, in addition to the actual soil medium.

One thing that really boosts my plants is not just considering the essential nutrients but also the soil biology. I add a soil prebiotic called Ultra to the seed starting mix prior to making soil blocks. 

I add 1 oz Ultra into a container filled with about 10 quarts of seed starting medium. The results are big, beautiful plants that are healthy at transplanting. 

What Tomato Fertilizer to add at Transplanting (An example)

When transplanting tomatoes, they go into the soil of the garden. This is where the soil test becomes important. I will give you an example of what I add to the planting hole of my tomatoes, based on my soil test.

Above you will see the soil test results in the area of the garden where I have tomatoes growing this year. Each nutrient is optimum or above optimum with the exception of boron, which is only just barely below where it should be.

Compost

The first thing to add is compost. Compost will boost organic matter in the soil. Not all compost is created equal or has the same nutrient profile. For the most part compost alone will not supply the tomatoes what they need for additional fertilizer and nutrients. 

Add 1 inch of compost to the soil surface.

Blood Meal (Nitrogen)

On a soil test, no nitrogen is listed because it disappears so quickly. Nitrogen is definitely necessary for tomato plants so I add this every year. 

To supply the nitrogen the plant needs, in each planting hole I add 2 tablespoons of blood meal and mix it with the soil.

Kelp Meal (Potassium and Trace Minerals)

Kelp meal adds a very small amount of potassium. In my soil test, I have enough potassium. So the reason I add this is because it is loaded with trace minerals that tomato plants love!

The kelp meal will correct the small boron deficiency in my soil. It will also add additional calcium and magnesium which is really important to tomatoes. A fertilizer company had their kelp tested and you can see the results in this kelp meal nutritional analysis.

Kelp meal has close to 60 trace minerals. This equates to healthier plants and delicious flavor!

For the kelp meal, apply at the same rate of 2 tablespoons in each planting hole. 

What about phosphorus?

I add no phosphorus to the tomatoes at planting! My soil test shows my soil has abundant phosphorus and therefore no phosphorus should be added.

I have never added any phosphorus fertilizer to my soil since starting my garden in this location five years ago. For a few years, I added a compost made with chicken manure which naturally added some phosphorus even though I don’t need it.

If your soil does need phosphorus, I recommend bone meal or fish bone meal. Apply at the same rate of 2 tablespoons in each planting hole. 

Soil Biology Booster

In addition to the fertilizers, increasing the beneficial soil microorganisms will help the plants combat stress, reduce disease, and with a stronger plant comes reduced pest pressure as well. 

If you have the time, you can make your own! The JADAM Organic Farming book has recipes for increasing beneficial soil microorganisms.

On the other hand, if you don’t have time to make it from scratch or would rather buy something instead, there are a lot of premade options, probably hundreds!

I personally only have experience with a few. A friend of mine who is a fantastic small farmer told me about Ultra (mentioned above) and that is what I use. 

I add 1 oz of Ultra or about 2 tablespoons per gallon of water and water it into a 300 square foot garden area. A little bit goes a long way and it’s very affordable considering how much coverage you get.

Water this every two weeks.

When to Side Dress Tomato Plants with Additional Fertilizer

After tomatoes have been growing for several weeks, you may need to add additional fertilizer to the plants. If the plants look healthy, likely you will only need to do this once about a month after transplanting.

Again, what you add will be based on what the soil needs. If you have a ton of green growth, dark green leaves, and no tomatoes, that is a sign you have too much nitrogen and shouldn’t add more.

For me, I add one of two options: 

1. A liquid fertilizer with the soil prebiotic diluted in water
2. OR ¼ cup blood meal and ¼ kelp meal around each plant.

If you don’t get much rain and you are drip irrigating I don’t recommend adding dry fertilizer. It needs a lot of water to break it down. Instead use a liquid fertilizer. This can be added through a fertigation setup or a watering can.

But if you have an overhead sprinkler, side dressing the plants with a dry fertilizer will work well.

Best Fertilizer for Growing Tomatoes in Pots

The only time I would recommend getting a formulated tomato fertilizer is if you are growing in pots. As you water them throughout the season they lose a lot of nutrients. Since plants grown in pots do not have access to deeper soil and all the minerals, a steady addition of fertilizer is essential.

Therefore, it is important to be adding fertilizer every few weeks during the entire growing season.

Conclusion

The best fertilizer for tomato plants is an organic formulation. The type of fertilizer added to the soil should be customized based on a soil test.

As a result, it’s better to add single ingredient fertilizers rather than blends. The only exception being if you are growing tomatoes in pots. 

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