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How to Fertilize Seedlings

Learning how to fertilize seedlings is an important part of starting a garden from seed. Fertilizer is so beneficial for growing strong, healthy, and prolific plants, but not all plants have the same fertilizer needs.

Seedlings are much smaller than full-grown plants so they have very different fertilizer needs than larger plants. To ensure healthy plants, it’s important to fertilize seedlings the right way.

If you’re not sure what the best way to fertilize seedlings is, you’ve come to the right place! I’ll go over everything you need to know to learn how to fertilize seedlings effectively, including when to start fertilizing, what kind to use, and how often you should do it.

Closeup of watering or fertilizing seedlings. Knowing how to fertilize seedlings will make sure they stay healthy and strong.

Do Seedlings Really Need Fertilizer?

Before we get started, you may be wondering if fertilizer is really necessary for seedlings. The short answer is yes, most of the time, seedlings need fertilizer in order to meet their full potential.

Seeds have all the nutrients required to sprout within themselves but after they grow their first set of leaves, plants have to get their nutrients from the soil.

Unfortunately, many soils don’t have enough nutrients to give your plants all that they need. That’s where fertilizer comes in.

Feeding seedlings with fertilizer gives your plants an added boost of nutrition to make sure they have all the essential nutrients they need to grow and thrive.

Even if you have rich soil, garden plants use up some of those nutrients every year, especially heavy feeders like tomatoes. Using fertilizer correctly adds nutrients back to the soil, increasing your chances for strong, healthy, and robust plants.

Watering or fertilizing seedlings.

You can get by without fertilizer if you have healthy soil, but your plants may not grow as well as they would have with a boost of fertilizer.

How to Fertilize Seedlings

The way you fertilize seedlings has a lot to do with how effective it is, so it’s important to get it right!

Learning how to use fertilizer for seedlings the right way pays off big time when it comes time to plant in the garden. Proper fertilizing gives your seedlings a much-needed nutrient boost and helps them thrive, ensuring they’re strong and healthy when it comes time to move them outside to the garden.

What Type of Fertilizer is Best for Seedlings

The first step in how to fertilize seedlings is to choose the right fertilizer.

For young seedlings, liquid and water-soluble fertilizers are more effective than granular fertilizers. Use a balanced fertilizer with equal parts Nitrogen (N), Phosphorus (P), and Potassium (K).

Measuring liquid fertilizer. It's important to know how to fertilize seedlings so you don't harm your plant seedlings.
Fertilizing seedlings in a hydroponics garden setting.

The amount of each nutrient is easy to find on most fertilizer labels. It’s usually on the front with a set of three numbers showing the concentration of nutrients. In a balanced fertilizer, the numbers are fairly equal, like 10-10-10 or 6-5-5.

This organic fertilizer from AgroThrive has an N-P-K ratio of 3-3-5. It’s specially formulated for fruit, flowers, and berries.

At the seedling stage, I recommend organic fertilizers over chemical fertilizers because they usually work better and are less likely to burn delicate young seedlings.

Bloom City CleanKelp is a good liquid fertilizer. Plant Magic is a good water-soluble option.

You can use synthetic fertilizers if you like, just make sure to apply the right amount to avoid burning plants. Use no more than 1/4 the suggested dose and apply it to the soil only, making sure not to touch the leaves, stems, or roots of the plant.

Check out our article on the Best Fertilizer for Seedlings to see more fertilizer recommendations.

When to Fertilize Seedlings

Wait to start fertilizing until your baby seedlings have at least one set of true leaves. The first leaves that sprout from a seed are called cotyledon leaves. These often look much different than the true leaves of the plant.

Closeup of cucumber cotyledon leaves.
Cotyledon leaves.

Before the true leaves emerge, a young plant already has all the nutrients it needs to grow. Fertilizing at this point is completely unnecessary and can even be harmful.

Once the true leaves appear, plants benefit from a boost of added nutrients.

Some articles will tell you to start fertilizing when plants reach a certain height, but there really isn’t a standard height requirement to start fertilizing seedlings. Different plants grow at different rates and have different-sized leaves. The best rule of thumb is to wait until you see the first true leaves.

A starter tray of cucumber seedlings.
First set of true leaves on cucumber seedlings.

Is There a Time When You Shouldn’t Fertilize Seedlings?

It’s not necessary to add fertilizer when planting seeds. Since seeds naturally contain all the nutrients needed to sprout, It’s best to wait until seedlings have their first set of true leaves before fertilizing.

You shouldn’t fertilize seedlings when they’re stressed. Stressed seedlings are those that are showing signs of problems like discoloration, wilting, leggy stems, or dry leaves.

If your seedlings show any signs of stress, work on getting them healthy before adding fertilizer. We have an article all about seedling problems to help you identify and treat what’s wrong.

Applying Fertilizer to Seedlings

The best way to fertilize seedlings is to dilute the fertilizer in water and apply it when watering.

Follow the instructions on your container of fertilizer to dilute it with plain water but only use 1/4 of the suggested dose. Young seedlings don’t need as much food as larger plants, and over-feeding does more harm than good.

Watering or fertilizing seedlings.

Use the diluted mixture to water your seedlings. You can use a watering can to apply fertilizer on top of the soil or use bottom watering methods.

To bottom water, pour water into the bottom of your seed tray. The soil will soak up the water, bringing it to the roots of your plants.

How Often to Fertilize Seedlings

Knowing how often to fertilize is a big part of learning how to fertilize seedlings! A slow and steady supply of fertilizer is more effective than one or two large doses. Fertilize seedlings about once a week.

Continue fertilizing seedlings the same way each time, by watering with diluted fertilizer. You can start increasing the dose very slowly after the first couple of weeks.

Can You Overfertilize Seedlings?

An unhealthy tomato seedling.

Yes, it’s definitely possible to overfertilize. Little seedlings don’t need a lot of fertilizer. A little goes a long way. Start with 1/4 of the usual dose and increase it slowly as the plants grow. By the time your plants are ready to go outside, they should be up to no more than 1/2 of the regular dose.

How to Tell If You’re Overfertilizing

Brown tips, yellowing leaves (especially at the bottom of plants), wilting, and leggy stems are all signs of overfertilizing. If you notice some of these problems and can rule out other causes like a lack of light or water, too much fertilizer could be the problem.

What Else Do Seedlings Need?

Fertilizing seedlings is just one part of helping them grow into strong and healthy plants. To make sure your seedlings stay healthy, here are some more things you should do.


Keep your seedlings moist at all times. They’ll need water about once a day. If the seed trays are very warm or in front of a sunny window, the soil will dry out sooner.

Using a spray bottle to keep seedlings moist.


Seedlings need adequate light to grow. If they don’t get enough light, plants will try to find it. They reach and stretch, resulting in long, thin stems that are weak and spindly.

If you don’t have a place indoors where seedlings can get enough natural light, use a grow light. Grow lights are a simple way to ensure your plants have enough light to thrive.

Air Circulation

Proper air circulation keeps plants healthy and cuts down on issues like mold and fungal disease. To provide good air circulation, make sure your plants aren’t too crowded and rotate the trays once every few days.

A fan in the room helps with air circulation as well. Use a ceiling fan or a smaller fan and let it blow on your seedlings for a few hours a day.

Wrapping Up How to Fertilize Seedlings

Isn’t it fun watching those little seedlings grow? Now that you’ve learned how to fertilize seedlings, your plants will be happy and healthy as they mature.

A drop of water above seedlings.

To learn more about seedling care, visit our Seed Starting page. We have tons of tips and tricks, how-to guides, product recommendations, and much more to help your plants get off to the best possible start.

Whether you’re a beginner or an advanced gardener, you’re sure to find some good ideas to help you grow healthy seedlings.