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Best Fertilizer for Tomatoes at Different Stages - What NPK Ratio, When & How Often

From his early days, Brandon helped his grandmother in her garden. He has always been passionate about tomatoes.

The Best Way to Fertilize Tomatoes

The best fertilizer for tomato plants has macronutrients such as nitrogen, phosphorous, and potassium, as well as essential micronutrients such as magnesium, calcium, boron, and zinc. However, tomatoes need different ratios of these nutrients during each growth cycle. If you're new to gardening, fertilizing tomatoes properly may seem like a daunting task. Whether you're growing from seeds or from seedlings, soil nourishment is a topic you don't want to be ignorant of.

With guidance, you're going to have an amazing crop from your tomato plants, year after year. But before we proceed, let me get one thing clear: There is no single tomato fertilizer that works best for all gardens at all times of year. If there were, gardening would be a breeze (and arguably also boring). Let's take a look at some facts and fertilizer options to help you pick the right one, based on the plant's stage of growth.

The best fertilizer for tomatoes aims for a higher fruit yield and not just a lot of foliage.

The best fertilizer for tomatoes aims for a higher fruit yield and not just a lot of foliage.

Tomato Plants Fertilizing Basics

  • The fertilizer requirement of tomato plants depends on their stage of growth. It is important that every nutrient be present at all times; however, the suggested ratio of nutrients changes with each stage of growth.
  • In general, a tomato plant's roots aren't deeper than 6-7". Keep this in mind when mixing fertilizer into the soil. An important topic, I've covered this in-depth in my article titled "how deep do tomato roots grow."
  • Commercial fertilizers have a number series, such as 10-8-10, which basically stands for nitrogen-phosphorous-potassium (or elements N-P-K). 10-8-10 means the fertilizer contains 10% nitrogen, 8% phosphorous, 10% potassium, with the rest being filler material.

Which fertilizer nutrients does a tomato plant need?

Phosphorous is vital for the growth of roots, potassium helps with flowering and general growth, and nitrogen helps with foliage.

  • Phosphorous is crucial for the growth and development of roots as well as fruit. It is therefore an important nutrient in the initial stage and then again in the final stage.
  • Nitrogen takes care of the foliage, but too much nitrogen leads to bushy plants with little or no fruit.
  • Potassium helps the plant grow rapidly and produce flowers and fruit. But it's very important when it comes to photosynthesis and tolerance to some diseases.

Do I need fertilizer to start my tomato seeds?

Tomato seeds do not need any added nutrients to germinate, as the seed contains sufficient amounts to put up the first true leaves. If you're planning on planting from seed, you may be interested in my guide on planting tomato seeds to help you get the first stage right. If you are at the stage where your seedlings are ready to be transplanted or if you've got some store-bought plants, you should check out my guide on transplanting tomatoes.

Do my tomato seedlings need fertilizer?

Yes, they do. Once the tomato seedlings have germinated, they are going to grow very quickly, with an initial burst in growth just prior to their flowering. As a rule of thumb, they usually bear fruit within four months of being planted.

Full-grown plants will need fertilizer, too.

For this period, you will be watching the plant carefully for cues about what fertilizer it needs (we'll discuss this later). Of course, requirements depend on many factors such as the type of tomato, the soil, and the environmental conditions.

If these steps are not done right, no amount of tomato fertilizer will help. Keep in mind that plants are very resilient. You don't need to be a pro to get through this stage. But knowing exactly what needs to be done sure does help.

Required Micro and Macro Nutrients for Tomatoes

A list of the most essential macro and micro-nutrients for growing tomatoes. Please note that the list is not exhaustive in any sense.

Nitrogen

For continuous growth of foliage.

Phosphorous

For root and fruit development, and it also helps fight stress.

Potassium

For continuous growth; aids in photosynthesis and makes the plant less susceptible to some diseases.

Calcium

For root and leaf growth and to help produce firm tomatoes.

Magnesium

Helps keep the plant green; improves flowering and fruit quality.

Boron and Zinc

Flowering and even ripening of the fruit.

Phosphorous is vital for the growth of roots, potassium helps with flowering and general growth, and nitrogen helps with foliage.

Tomato Fertilizing Timetable

As a rule of thumb, tomatoes usually bear fruit within four months of being planted.

When the plant is growing rootsStarting SeedsSeedlingsPlants

Type of nutrition needed

Phosphorus

The seed itself should provide all needed nutrients

Nitrogen helps foliage; potassium helps plant growth, and flower and fruit production

Watch the plant carefully for cues about what fertilizer it needs: phosphorous needed for fruit; nitrogen helps foliage (but too much lessens fruit production)

Why Your Tomato Plant's Roots Matter

Your choice of fertilizer for tomatoes won't matter unless your tomato plant is able to absorb the nutrients you've incorporated into the soil. To ensure root health, you're going to need to make sure that your watering techniques encourage deep rooting. The plants aren't just going to be growing in height and girth, but they are also going to branch out, causing them to be top-heavy. You may want to provide external support to prevent the plants from drooping and falling over. Deep roots also help in this regard.

Secondly, it is best to place your tomato plant in the ground or in a big container, since small containers lead to the clumping of roots, and if a plant's roots are too crowded, it struggles to take up the nutrients it needs.

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