The Best Fertilizer for Tomatoes at Different Stages of Growth
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 about.
With guidance, you're going to have an amazing crop, year after year. But before we proceed, let me get one thing clear: There is no single 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.
Tomato 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.
- Commercial fertilizers have a number series, such as 10-8-10, which basically stands for nitrogen-phosphorous-potassium (or elements N-P-K). This 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 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
For continuous growth of foliage.
For root and fruit development, and it also helps fight stress.
For continuous growth; aids in photosynthesis and makes the plant less susceptible to some diseases.
For root and leaf growth and to help produce firm tomatoes.
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
When the plant is growing roots
Type of nutrition needed
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
Fertilizer won't matter unless your tomato plant is be 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.
How to Fertilize While Transplanting Tomato Seedlings
All plants need nitrogen to grow. Most naturally occurring soil contains sufficient nitrogen for plant growth, but if you're using a large proportion of coconut husk or some other kind of filler material, you're not going to have enough nitrogen in there.
How do I know if I need to add nitrogen to my soil?
You can only know for sure if you've tested your soil. However, as a general rule of thumb, if you are using fresh compost, you do not need to add any nitrogen, just phosphorous at the initial stage of growth (that is, soon after transplanting the seedlings). You may not even have to add phosphorous if your compost contains a lot of banana peels and bones.
Two Ways to Add Phosphorous While Transplanting Tomato Seedlings
For phosphorous, it's best to use a combination of bone meal and organic fertilizer spikes, but it's totally understandable if you don't want to use bone meal because it's an animal product (in which case, measure your soil and add a bit of traditional phosphorous fertilizer).
1. Bone Meal
Bone meal is a great way to use organic fertilizers while at the same time ensure that your tomato plants have sufficient phosphorous to grow strong roots (which aid the production of fruit).
Bone meal, as the name suggests, is a fertilizer made up of ground animal bones: usually beef, but other animal bones (like fish bones) may be used. Today, most commercial bone meal products are in the ratio of 3-15-0. One of my favorites is the with a 2-14-0 ratio. The essential micronutrient calcium in this mix also prevents Jobe's Organic bone meal fertilizertomato blossom end rot, a common tomato plant disease. Additionally, it helps strengthen cell walls and therefore aids in helping prevent your tomatoes from cracking and splitting. My article on why do tomatoes split is something you would want to read before you get your first fruit because this is a common problem and one that is easily avoidable.
How do I use bone meal?
Before adding bone meal, it is advisable that you check the pH level of the soil. If it is above 7, you should work on reducing this number before adding the fertilizer.
Bone meal is a slow-release fertilizer, and it takes up to 4 months to completely break down in the soil. This is why one application of 1 pound bone meal to 10 square feet (1 kg to 2 square meters) of soil is all you may need. Notice that I have used the word "may."
2. Fertilizer Spikes
I definitely recommend you place 3 to 6 inches away from the stem of each plant just after transplanting. This is even more important if you're growing in a container, as potted plants use up the soil's nutrients quickly. Jobe's 6-18-6 organic fertilizer spikes
You don't have to get Jobe's fertilizers—all you need is a spike that contains a higher percentage of phosphorous and a decent amount of nitrogen and potassium. Fertilizer spikes just need to be pushed into the soil. In general, they last around 2 months before being used up.
For phosphorous, it's best to use a combination of bone meal and organic fertilizer spikes.
How to Use Fertilizer Spikes
Tomato Fertilizer Needs During the Growth Phase (Pre - Flowering)
Indeterminate vs. Determinate Plants
For determinate tomato plants, there is a clear distinction between the growth phase and the fruiting phase, so you can cater to those specific nutrition needs. This is not the case with indeterminate kinds. For indeterminate plants, you're going to need constant fertilization with fertilizers of 8-32-16 or 6-24-24. Basically, the nitrogen content is 1/4 of phosphorous and at least 1/2 that of potassium. This guide deals with determinate varieties.
Nitrogen needed for pre-flowering plants:
Nitrogen is an essential component during the growth phase of your determinate varieties because it helps form many structures—like chlorophyll, which is essentially the green part of the plant that aids photosynthesis—and aid the formation of genetic material.
If you have good soil and use a lot of compost, you will not have to bother adding any nitrogen. But if you notice the bottom leaves of your tomato plants turning yellow, this is a sign that the plant needs more nitrogen. (However, there are also other causes of yellow leaves aside from nitrogen deficiencies. Not to scare you, but yellow leaves are also an indication of excess nitrogen, but more often than not it's due to a lack of it. To learn more, read The Causes & Cures of Yellow Leaves on Tomato Plants.)
I personally choose to use compost at the start, and then 2 weeks after the first month, I add in some compost tea. This provides sufficient nitrogen and also replenishes some of the other macro and micronutrients.
When to Add Potassium: Just Before Flowering Until the End of the Season
If you want your plant to produce a lot of flowers, it's going to need that constant source of nitrogen—not too much, though, because overfeeding promotes the growth of leaves and not flowers.
But more importantly, this is the time your plant needs some good potassium fertilizer. Potassium is an important nutrient that helps promote strong growth. As stated, your potassium levels should be at least double that of your nitrogen if you add fertilizer to your garden. Go ahead and buy yourself some 8-32-16 or 6-24-24 and add it as per the instructions on the pack—or you could do what I do, make a compost tea, as explained below.
How to Make a Potassium-Rich Compost Fertilizer Tea
Banana peels are a great source of potassium and, when you bury them, they release it slowly, which aids plant growth throughout the stages.
- Collect a few banana peels,
- chop them into pieces (preferably as seen in the image below)
- and bury them in the soil.
While your plant is growing, keep collecting banana peels and preparing the tea in a big container—something that you can cover up so that the smell of rot does not spread or attract creatures. Keep adding to this with new peels as the season progresses.
How deep should I bury the banana?
Some I push it deep near the base of the roots, just an inch below the soil. But that's not deep enough if you want a lot of flowers.
How to Fertilize a Tomato, From Flower to Ripe Fruit
During this phase, the plant still needs nitrogen, but not in large amounts. If your plant has grown well until now, there's no need to add any nitrogen into the soil.
Phosphorous is one of the most crucial nutrients for good fruit. Potassium is also important, so continue adding some of that potassium tea but maybe once a week, not more. Most garden soil does, in fact, have sufficient phosphorous, and by this stage your plant would, if planted in-ground, have developed symbiotic relations to get phosphorous. You could always add some if you notice that your fruit is not developing well, this is something you only learn through the experience of the first crop or by testing your soil. In the first year, there is no harm if you add in some fertilizer that is high in phosphorous. If you choose a fertilizer, maybe go for something on the lines of 8-32-16 at this stage.
Just before flowering:
Next, find out how many days on average it takes for your specific variety of tomato plant to flower (not fruit). Up to two weeks before this date, start pouring in some of that water you've been collecting. Depending on how much you've got and how concentrated it is, you may have to dilute it or just use a little every other day.
I cannot really give you a deeper explanation on this as this is something you'd figure out from experience. You're going to have to wing it by keeping an eye on your plant. If you notice that growth is slowing down stop adding any more. You can't really go wrong if you keep an eye on the plant.
Use Organic Tomato Fertilizers to Encourage Symbiosis
If you are planting in-ground (as opposed to in-container), you may want to consider Organic all-purpose fertilizer because it has beneficial soil microbes and fungi that help plants grow through a process of symbiosis. Dr. Earths
Why is symbiosis important for in-ground tomatoes?
For example, the fungi Myccorhizal reaches far into the earth and transports phosphorous to the roots of the tomato plants in exchange for sugar and starch. I first learned about this from Grow It Organically and I'd recommend that you read the page to learn more about other phosphorous sources.
If you have a well-performing garden, you may already have a high level of activity and symbiotic systems in place, and you may not need to artificially induce them into the environment. I'd only recommend you do so if you have a garden that does not get green on its own, or if you have added a lot of new, inert topsoil.
Four Reasons to Use Organic Tomato Fertilizers:
- They are easier to use.
- It's better for the environment and your local water table.
- They are less potentially harmful to the environment.
- Inexperienced gardeners who use factory-produced, concentrated chemical fertilizers (instead of natural, powdered rock) tend to overuse, which leads to "lockout," a plant's defense mechanism which causes it to completely stop absorbing nutrients.
Fertilizing Indeterminate Tomato Plants
I have had a few people reach out asking me about the best fertilizing techniques for indeterminate varieties. In short, you do the same thing as you would for determinate varieties, but you would have to feed the plants more frequently.
After some outreach and looking up reviews online, I've found a fertilizer mix by that seems like a good choice as it's got the usual macro and micro-nutrients listed earlier in the article in addition to some other ingredients that Dr. JimZ has tested out over decades while he grew massive tomato plants. It is my understanding that these additions apart from breaking down and eventually being directly consumed by the plant add diversity to the soil, thereby harboring a healthy soil microbiome. Dr. JimZ
I've contacted the team at Dr.Jimz and found out that these are the ingredients they use in their organic fertilizer: Organic alfalfa meal, non-GMO soy protein, soft rock phosphate, calcium, magnesium, sulfur, potassium, iron, zinc, manganese, copper, cobalt, boron, Norwegian kelp meal, lactobacillus acidophilus extract, magic mineral blend, Dyna gold mineral blend mushroom mycelia extract.
One Size Does Not Fit All Tomatoes
These are general fertilizer requirements: What kind of fertilizer mix you need for your garden depends on the nutrients present in the microenvironment of your tomato bed. You can either learn this from experience or by testing your soil.
Other points to keep in mind:
- Sandy soil need fertilizers more often than sticky, clay-like soils, as sandy soils do not hold nutrients as well.
- Container gardens need more fertilizer since the plant uses up the soil's nutrients more quickly.
- Do not get fertilizer onto the leaves of the plant—only the roots. If you dissolve fertilizer in water, make sure that you water the ground and not the leaves, no matter what time of day it is.
Over-Fertilization of Tomato Plants
A lack of nutrients results in stunted growth, increased likelihood of disease, and inhibited fruit. But excess fertilization can be even worse than a lack of nutrients in the soil.
Your tomato plant could struggle and grow and produce some tomatoes if the soil is infertile, but if you give it too much fertilizer, it could die on the spot. Do not let this sentence scare you. It's not easy to over-fertilize, and fixing it is always an option.
Signs of Tomato Over-Fertilization
- Yellowing leaves. One cause of yellow of your tomato leaves could be an excess of nitrogen in the soil. Under a condition of excess nitrogen, tomato plants do not absorb a sufficient amount of water, which results in some of the older leaves yellowing prematurely.
- Bushy leaves and delayed flowers. Excess nitrogen would also lead to a lot of leaves and flowering is put off. Only experienced gardeners who know when to expect blossoms would be able to detect this problem.
- A skin of stuff on the surface: You may see a heavy build-up of sediment and fungi growing on the top surface of the soil.
- Yellowing and wilting of lower leaves. There are so many issues that lead to yellowing leaves that you need to be an expert to tell the difference.
- A sudden loss of leaves.
How to Fix Over-Fertilization
- The decay process uses nitrogen, so one way to treat excess nitrogen in the soil is to spread mulch—or for even quicker results, sawdust—on to the soil. Mixing in some sawdust and not just placing it on the top would be a better way to use it.
- If you notice fertilizer build-up through white salts on the top of the soil, scrape off this top layer and dispose of it.
- If it's in a container, soak the soil thoroughly with water and then let it drain out. If planted in-ground, just soak the plants thoroughly. Repeat the soaking a few times,—it's known as "flushing" or "leeching" the soil.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.