Fertilizing Tomato Seedlings (Common Questions)

Tomato seedlings are kind of like children. We want to give them the best head start so they can get established and grow into strong and healthy plants that hopefully don’t need much babying from us anymore. Applying some fertilizer to your tomato seedlings can give them the head start they need to grow past that vulnerable stage. But is it necessary? What kind of fertilizer can you use? And how much should you add? Below are some answers to these common questions.

Should You Fertilize Tomato Seedlings?

If you are starting tomato seedlings in a medium that already has nutrients, like soil or potting mix with added fertilizer, you do not need to fertilize your tomato seedlings. However, if you are using only coco coir, peat moss, or other seed-starting mixes without any nutrients, you should apply fertilizer.

Most potting mixes or seed-starting mixes have small amounts of fertilizer added, which is more than enough to support the growth of tomato seedlings. So, you don’t need to use any additional fertilizer, and you do risk over-fertilizing if you do. Naturally, using soil or even well-rotted compost to start your tomato seedlings will also contain enough nutrients for them as well. 

However, if you are sowing tomato seeds into a growing medium like plain coco coir or peat moss, or you’ve made your own potting/seed-starting mix without adding any fertilizer, it’s recommended you fertilize your seedlings. Still, you should use a very light application of fertilizer, and you don’t need to use a frequent fertilizing schedule, either.


When Should You Feed Tomato Seedlings?

The optimal time to start feeding your tomato seedlings is after one or two sets of true leaves begin to grow. True leaves are the leaves that look like regular tomato leaves, not the initial two seed leaves when a seed first sprouts.

Seeds have enough stored starch, protein, and minerals to supply all the energy and nutrients a seedling needs to germinate, emerge from the soil and grow its first few leaves. So when your tomato seedlings poke out through the soil and unravel their first two narrow seed leaves, there is no need to fertilize them.

Once your tomato seedlings start growing their true leaves, the leaves that look like tomato leaves, they need to start taking up their own nutrients to continue their development. At this point you should start lightly feeding your tomato seedlings.

Seed leaves (red arrows), also called cotyledons, are the first leaves that emerge from a seed. It’s recommended to not fertilize seedlings until at least one or two true leaves (blue arrows) appear. True tomato leaves look like typical tomato leaves, whereas seed leaves are narrow and smooth.


What Is the Best Fertilizer for Tomato Seedlings?

The best type of fertilizers for tomato seedlings are fast-acting liquid fertilizers with a balanced NPK ratio or with slightly more nitrogen to promote leaf and stem growth. However, you should use any fertilizer at quarter strength to half strength to avoid burning your tomato seedlings. 

Fast-acting liquid fertilizers have all plant-available nutrients ready to be taken up by your tomato seedlings roots. A slow-release fertilizer or an organic fertilizer (even liquid ones) will take time to break down and release nutrients for your tomato seedlings. They can still be used, but not in the same way as fast-acting liquid fertilizers.


Can I Use Slow-Release or Organic Fertilizers for Tomato Seedlings?

Using slow-release fertilizers like granules or organic fertilizers are not recommended for seedlings unless you are mixing them into your seed-starting mix. Moreover, organic fertilizers work best with active soil microbes which slowly break them down, releasing plant-available nutrients for your tomato seedlings, meaning they are ideally used in soil-based or compost-based mixes. If you do this, then you don’t need to fertilize your seedlings until after you transplant them.


Examples of How to Use Different Fertilizers for Seedlings

Using a Liquid Fertilizer:

You have many options for liquid fertilizers, and you don’t need a specific one for tomato seedlings. One widely available product is Miracle-Gro’s water soluble plant food, with an NPK of 24-6-16. This is perfectly fine to use, but is too strong to use at full strength. Either mix with half or quarter amount of fertilizer in water (quarter is a safer choice for tiny seedlings) or follow the directions to make full strength liquid fertilizer, then dilute it to quarter to half strength before applying. To err on the side of caution, I generally fertilize at quarter strength when the seedlings are small, then half strength after the tomato seedlings are several inches tall with multiple true leaves. 


Using a Granular Slow-Release Fertilizer:

Granular fertilizers are ideal if you want to make or amend a potting/seed-starting mix and not have to fertilize your seedlings directly. Mix it in with your seed-starting mix at half the amount it says on the package. It’s better to underfertilize than overfertilize, since seedlings have low nutrient requirements, anyway.


Using an Organic Fertilizer

If you want to use organic fertilizer, you should mix it into a soil-based or compost-based seed-starting mix, so you know there are beneficial bacteria and fungi that can break it down and release the nutrients for your seedlings. It’s harder to overfertilize with organic fertilizers, which tend to have a relatively low NPK while also breaking down slowly, and therefore are much gentler on your seedlings. Granular or meal organic fertilizers are both perfect for making organic seed-starting mixes with soil or compost.



How Often Should I Fertilize Tomato Seedlings?

If using a liquid fertilizer, apply weekly or every two weeks at quarter to half strength. If you’ve premixed granular/meal/powdered fertilizer into your seed-starting mix, or your mix already has fertilizer added (many potting mixes already do) your tomato seedlings don’t need any further fertilizing until after transplanting. 


When Can I Start Using Regular Strength Fertilizer?

There is no hard-and-fast rule on exactly when to start using regular strength fertilizer. To err on the side of caution, you can apply full strength fertilizer after transplanting your tomato seedlings outside or into their final container.


Can You Over-Fertilize Tomato Seedlings?

Tomato seedlings can be overfertilized, and much more easily the smaller they are. Seedlings have much lower nutrient requirements. Always apply diluted fertilizer, quarter to half strength, and only after the first one to two true leaves (the ones that look like tomato leaves) have appeared.