Fertilizing Tomatoes: How and When to Fertilize Your Tomato Plants

Tomatoes are the quintessential summer vegetable, topping everyone’s wishlist, but they do have particular needs if you want an abundant harvest.

Some might consider tomatoes easy to grow, but they are picky when it comes to soil conditions, the amount of water provided, and fertilizing frequency. That means you need to know how often to fertilize tomatoes.

So, when and how often should you fertilize tomato plants?

Tomatoes need to be fertilized as seedlings as they grow inside, and you also need to fertilize when you transplant the seedling outside. Wait until flowers begin to develop, and fertilize again, as well as when fruits start to develop. Once fruits form on the plant, add light fertilizer once every two weeks until the end of the growing season.

To provide the best outcome for your tomato plants, you need to know when and how to fertilize. It can make the difference between having an abundant harvest and weak growth. Let’s find out the answers together.

What Nutrients Do Tomato Plants Need?

Tomatoes are heavy feeders, and they need a lot of nutrients to grow properly. If you fail to fertilize, your plants will have a small harvest.

Tomatoes need the three primary nutrients – nitrogen, phosphorous, potassium, and a host of other nutrients. Each one provides a vital function for the plants. Let’s take a look at a few!

  • Nitrogen: This nutrient helps take care of the foliage on the plant, but if you have too much nitrogen, it’ll create a bushy plant with little to no fruit. That would be a serious bummer.
  • Phosphorous: Your plant needs phosphorus for the growth and development of the roots and fruits. It’s an essential nutrient in the earlier stages and at the final stage of growth.
  • Potassium: This nutrient helps the plant grow rapidly and produce the flowers that eventually turn into the fruits. Potassium is essential when it comes to photosynthesis and disease resistance.
  • What about the little nutrients? NPK are the big macronutrients that all plants need in larger quantities, but plants also need smaller nutrients and elements to grow. A few that tomato plants need include:
  • Calcium: This is needed for proper root and leaf growth. It also helps to produce firm tomatoes.
  • Magnesium: Without magnesium, your plant won’t stay green. It also helps to improve flowering and fruit quality.
  • Zinc and Boron: These elements help your plant with the flowering process and ripening of the fruits.

How Do I Know If My Plants Need Nitrogen?

One of the most vital nutrients is nitrogen, and it’s also quickly depleted if you don’t prepare your soil correctly in the early stages. Good soil with plenty of rich humus usually has plenty of nitrogen.

But how do you know if your soil needs extra nitrogen?

If you notice the bottom leaves of your tomato plant turning yellow, it’s often a sign that your plant needs additional nitrogen. Watch your leaves; they are a reliable indicator of problems taking place at the soil level.

What Is The Best Fertilizer For Tomato Plants

The Best Tomato Fertilizer

You want to find a fertilizer that contains all of the macronutrients needed by your plants, including nitrogen, phosphorous, and potassium. It also needs to have the essential micronutrients, including magnesium, calcium, boron, and zinc.

The problem is that tomatoes need all of these nutrients at different ratios at different periods throughout their growth cycle. That can make picking the right one a daunting task.

Here is what you should remember.

No one single fertilizer works for your plants at all points of the growing season. You have to buy multiple fertilizers; that’s just part of gardening.

When you look at commercial fertilizers, they’ll have a number series present on the container, such as 10-10-10. That stands for NPK – nitrogen, phosphorous, and potassium. A 10-10-10 is 10% nitrogen, 10% phosphorus, and 10% potassium, with the rest being filler materials.

It’s also a smart idea to get your soil tested before you fertilize.

  • If your soil is balanced or high in nitrogen, you’ll want to select a fertilizer that is lower in nitrogen and higher in phosphorous.
  • If your soil lacks nitrogen, a balanced fertilizer will get the job done correctly, such as a 10-10-10.
  • As a general rule, if you’re using aged manure or compost, you don’t need to add nitrogen to your soil. You’ll only want to add phosphorous at the initial stage of growth, which is after transplanting the seedling.

The Best Phosphorus Fertilizers For Tomatoes

The Best Phosphorus Fertilizers for Tomatoes

At times, depending on how you prepared your garden beds, you might only need to add phosphorous.

This happens when you add plenty of compost, ensuring your soil is full of nitrogen already. Phosphorous is still necessary for the boost in growth needed for tomato growth.

When you want to add phosphorous, it’s typically suggested that you use bonemeal and organic fertilizer spikes. Some people don’t like to use bonemeal because it’s an animal by-product.

If you feel that way, you can buy traditional phosphorous fertilizer that is synthetic but not made from animals.

Bone Meal

Despite being an animal by-product, bonemeal is an organic fertilizer that you can use when you plant your seedlings to encourage strong root growth. As you might guess from the name, this fertilizer is made of ground-up animal bones, typically beef bones, but sometimes other bones are used.

Most bonemeal you buy in the store has a ratio of 3-15-0, but it will vary from brand to brand.

You should know that bonemeal is a slow-release fertilizer; it can take up to four months to fully break down in the soil. Testing your soil is advised to help determine the level of phosphorous already present, but on average, one pound per 10 square feet is sufficient for an entire growing season.

Fertilizer Spikes

If you head to your local garden stores, you’ll find an array of fertilizer spikes available. All you need to do is place a spike about six inches away from the base of your stem, including container-grown tomatoes.

Look for a fertilizer spike that has a higher amount of phosphorous and average nitrogen or potassium. A fertilizer spike typically lasts about two months, making it a cost-effective, easy way to fertilize your plants.

Compost Tea – A Fantastic Option

Another fertilizer for tomato plants that they love is compost tea. You can add more of what you need, so if you need more potassium, you can add additional banana peels, which is a great source. 

You can also chop up banana peels and bury them in the soil around your plants for extra potassium boosts!

To make compost tea, scope some up, and put it in a cheesecloth bag. Add a bunch of banana peels as well! Then, put the cheesecloth bag in a 5-gallon bucket of what and let it soak for several days. 

When you’re ready, use the compost tea to water around your plants. This is a simple way to get your plants a leg up. 

When and How Often to Fertilize Tomatoes

When and How Often to Fertilize Tomatoes

Tomato plants need to be fertilized at several different times. Each stage is vital, and without proper nutrients, the plants won’t successfully grow into the next step.

Here is when you need to fertilize your tomatoes.

1. Add Compost When You Prepare Garden Beds

Before you plant everything, you have to prepare the garden beds. This should be done in the spring as you prepare for the upcoming season.

Before you plant, you should add a few inches of compost or aged manure into your garden beds. Compost is gold, and it’s rich in nitrogen, which is an essential nutrient needed by tomatoes and all vegetables. You can never go wrong if you add four to six inches of compost to your garden beds.

Now your garden beds are ready for planting. Huge tomato plants are in your future!

2. Fertilize Your Seedlings

Assuming that you started your tomato seeds at home, you’ll need to fertilize once the tomato seedlings germinate and sprout. Tomato seedlings grow very quickly, sometimes surprisingly quick.

From the time that tomato plants sprout to when they bear fruit, the length of time is typically four months. To help your plants keep up with this initial burst in growth, fertilize your seedlings. 

3. Fertilize When You Plant Your Seedlings

It’s been a few weeks since you fertilized your seedlings, and it’s time for these babies to go outside into the big garden bed. They’re about to hit a major growth spurt, so they need plenty of nutrients to help them through it.

If you filled your garden bed with compost or aged manure, you don’t need to add any nitrogen at this point. Adding more nitrogen could be hazardous for your plants, leading to burning. Instead, you want to add just phosphorous at this initial stage of growth.

Pick one of the best phosphorus fertilizers for tomatoes and use that when you plant your tomato seedlings.

If you made your compost and used a lot of banana peels and bones, you might not need to use any fertilizer. You would need to be intentional and ensure you added a lot of those items.

4. Fertilizing When Flowering Starts

During this flowering stage, your plants need plenty of nitrogen and potassium. Potassium is essential if you want to encourage strong, healthy growth and more flowering. Right now, your potassium levels should be at least double that of your nitrogen.

At this stage, you can use either an 8-32-16 or a 6-24-24 fertilizer. Look at the instructions on the package and follow what it suggests.

5. Watch For Fruit Growth And Add Extra Fertilizer 

Now you wait! Watch your plants and wait until you see the first fruits develop into the size of a golf ball. Once that time appears, it’s time to apply the next dose of fertilizer. This will help encourage more fruit production.

During this stage, phosphorous is an essential nutrient, along with potassium. Continue to add compost tea with extra banana peels to be sure it doesn’t lack potassium, but the star of the show should be phosphorous and nitrogen.

We continue to assume that your compost’s addition was sufficient and will provide all of the nitrogen needed for proper growth.

In most cases, your garden soil should have sufficient phosphorous levels, but if you notice that your fruit isn’t developing correctly, you will need to add some additional fertilizer. At this stage, you can select an 8-32-16, which should be sufficient.

6. Light Fertilizing Until The End Of The Season

From now until the end of the harvest period, you can lightly fertilize every two to three weeks. You don’t want to feed too much, so avoid putting too much on at one time.

During this stage, phosphorus still plays a vital role, along with calcium, to produce fruit. If you’re growing indeterminate tomato plants, I would suggest fertilizing more often. Determinate plants focus their fruit growth all at one time, so you don’t need to feed as much. Your phosphorous fertilizer should be sufficient.

One Size Doesn’t Fit All

Listed above are general fertilizing recommendations, but what works best for your garden comes from trial and error. It’s impossible to pinpoint one fertilizing routine that works for everyone. Here are a few considerations.

  • If you have sandy soil, you’ll need to use more fertilizers than if you have sticky or clay-like soil. Sandy soil doesn’t hold nutrients well, so they wash right out.
  • When you plant tomatoes in containers, the plant will use up the nutrients more quickly — plan to fertilize more often than you would for in-ground tomatoes.
  • Never get the fertilizer on the leaves!

The Right Way to Fertilize Tomatoes 

The Right Way to Fertilize Tomatoes

There is a right and a wrong way to feed tomato plants. Doing so improperly could cause your plants to burn or not absorb the nutrients in the right way.

There are a few rules that you should remember about fertilizing your tomato plants.

Add Organic Matter First

It’s always best to add a few inches of organic matter, such as compost or aged manure, to the top of your soil before planting. This is especially true if you don’t plan to use synthetic fertilizer. Adding 8-12 inches of organic matter improves drainage while also providing many of the vital nutrients your plants need to grow.

Compost is like gold!

Watch Where You Fertilize

It’s not safe to apply fertilizer over the entire plant. Fertilizing too close to the plant could cause it to run off onto the stem, burning the plant. You should never apply fertilizer on leaves either; it can have the same burning result. 

Mix The Fertilizer

When you’re fertilizing tomatoes, you want to mix the tomato fertilizer into the soil at the bottom of the planting hole. It should be about six inches away from the plant because you don’t want the fertilizer to burn it. Don’t worry; it’ll still easily access the nutrients.

So, dig a small trench around the base of the plant, sprinkle in the fertilizer, and put the unfertilized soil back over it. 

Understand Natural Vs. Synthetic Fertilizers

As you probably guess, natural fertilizers are organic products, either animal or plant-derived. Synthetic fertilizers are made of potassium, nitrogen, and phosphorous.

Typically, if your soil has plenty of organic matter, you don’t need synthetic fertilizers. However, using a balanced fertilizer won’t hurt your plants in small applications.

Water Correctly

In the end, it won’t matter how you fertilize or when you do if your plants cannot absorb the nutrients you add to the soil. You have to be sure you’re watering your plants correctly for proper root health.

You should always water slowly and deeply at the stem of the plant. Give enough time for the water to enter the soil and absorb. It’s best if you can water early in the day or later in the evening when the sun isn’t so hot.

Tomato plants need 1-2 inches of water each week.

You must water BEFORE, not after fertilizing. Water around the base of the plant deeply. Then, spread the fertilizer on the ground. The nutrients will eventually spread into the plants. 

How Do I Know If I Fertilized Too Much?

There is such a thing as too much of a good thing, so yes, you can add too much fertilizer to your plants. Too much fertilization is worse than a lack of nutrients; it could lead to the plant’s death rather quickly.

But how do you know if that happened? Most importantly, if you did over-fertilize, is there a way to fix your mistake?

Signs of Overfertilization

  • Yellowing Leaves
  • Bushy Leaves
  • Delayed Flowers
  • Sediment Build Up on Soil Surface
  • Yellowing & Wilting Lower Leaves
  • A Sudden Loss of Leaves

Fixing Overfertilization

So, if you have a few of these signs, it’s usually a good indicator that you added too much fertilizer – OOPS! Your intentions were good, and luckily, it’s often a fixable problem. Here is what you need to do.

1. Add Mulch

You can’t just add any mulch; you need an organic mulch that will decay over time. The decomposing process requires nitrogen, so if you have extra nitrogen in your soil, adding a layer of something like sawdust that decomposes quickly helps. Try mixing the sawdust into the ground for even better results.

2. Scrape Off The Sediment

When you add too much fertilizer, you might end up with a layer of sediments or white salts over the top of the soil. You can scrape off this layer and dispose of it to help heal your soil.

3. Soak The Soil

If you have raised beds or grow in containers, soaking the soil thoroughly can be helpful. It lets the water drain out. For those doing in-ground gardening, soak your plants thoroughly several times. This process is calling flushing or leaching of the soil.

Fertilizing Is Essential

Without proper fertilization, your tomato plants will never reach their maximum potential. Gardeners need to know how often to fertilize tomatoes and the nutrients that the plants need at each stage to grow correctly. Remember to keep the fertilizer off the leaves and water deeply before planting, and that compost is your best friend.

Updated on by Amber Noyes

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