How to Fertilize Your Seedlings for Healthier Plants

Have you ever started seedlings and it gets to a point where nothing is happening? You water them, you give them light, maybe you even sing to them, but you’re still experiencing growth stalls? They just don’t seem to grow no matter what you do!

The solution? You need some fertilizer!

In this post, I answer some common questions about fertilizing and take you through my own fertilizing process so you can have success all the way from seed starting to main season gardening!

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WHERE TO START WITH FERTILIZING

When I first started seed starting, I just used the same kind of fertilizer I would use outside in summer. While it did work, and I did get results, I later learned that it is better for your plants to use a seed-starting fertilizer. This is because seed-starting fertilizers promote root development while all-purpose fertilizers promote all-around plant health.

So what kind of fertilizer should you use for seed starting? The number you want to look for on seed starting fertilizer is 10-52-10.

WHAT DO THE 3 NUMBERS ON MY FERTILIZER MEAN?

Fertilizers have these three numbers to tell you the ratio of the main nutrients: nitrogen (N), phosphorous (P), and potassium (K). This is known as the NPK ratio or the NPK number. This means 10-52-10 fertilizers are comprised of 10 parts nitrogen, 52 parts phosphorus, and 10 parts potassium. These macronutrients are the most important nutrients to ensure your plants grow healthy, full, and strong and they each promote different aspects of plant growth. Nitrogen promotes the growth of healthy green foliage, phosphorus promotes root development and stronger blooms, and potassium promotes healthy overall plant growth and hardiness.

This is grossly oversimplified, but I just wanted to give you the Coles Notes to get you started. Knowing more than this isn’t crucial unless you are a more advanced gardener (5+ years). Then you might want to start getting into more specific fertilizers for whatever your gardening niche is.

In your first year, there is really no need to have more than an all-purpose fertilizer for main season gardening and a seed-starting fertilizer for seed starting. All-purpose fertilizers typically have three of the same numbers (e.g. 10-10-10 or 4-4-4), meaning the main nutrients are included in equal amounts.

CHEMICAL FERTILIZER VS ORGANIC FERTILIZER

I have used both chemical fertilizers and organic fertilizers with success and am not going to shame or guilt anyone into using one or the other. This can be a hot-button issue, but I do want you to know a bit about which fertilizers have worked for me, so you can choose what will work best for you!

There are a variety of reasons why a gardener might not want to use water-soluble or chemical fertilizers that are found in the big box sores. If you want to go organic or more natural, you can use a fish hydrolysate. This is typically used for seed starting but can also be used throughout the year if you wish (my guess is because the middle number is a bit lower, so it is less potent).

Be warned: fish fertilizers are really stinky.

I personally don’t use them for indoor seed starting for this reason. But I will use them outdoors once my seedlings are planted in the garden. If you want a seed-starting alternative for fish fertilizer, find an organic fertilizer with as high of a middle number as you can get. Again, 10-52-10 is the golden ratio for seed-starting fertilizers, so try to find something as close to that NPK ratio as you can.

WHICH FERTILIZER IS BEST FOR YOU?

In terms of organic options, there is a kelp meal fertilizer I’ve used in the past that worked really well! If you are local to Saskatoon, SK or area, Earth Medicine Fertilizer is a fantastic organic fertilizer. Another option I’ve had a positive experience with is Evolve Fertilizer, also made in Canada! For non-organic options, RapidGrow is a tried and true brand that I’ve bought locally that carries all-purpose fertilizer and water-soluble seed-starting fertilizer.

For cut flowers, I suggest using the Gaia Green Power Bloom fertilizer. It is a more natural product and it encourages blooms. One thing to note about these fancier fertilizers: These are not meant to be mixed with water and then poured on top of the soil. They’re meant to be pre-mixed with the soil ahead of time for slow release. So, when you go to plant your cut-flower seedlings, plant them into a mixture of soil and fertilizer to slowly nurture your cut flowers!

WHEN SHOULD I START FERTILIZING?

When you are seed starting, you can start fertilizing when you see four leaves (i.e. two sets of leaves or one set of “true leaves”) on your seedlings! Below are the quick and easy step-by-step instructions to do so (or you can check out this short reel on IG for the same info!):

Step-by-step instructions for fertilizing seedlings:

Grab a water-soluble fertilizer (or fish emulsion) and a watering can
Scoop the appropriate amount of fertilizer into the watering can
Fill with water (according to the directions on the fertilizer you’ve chosen)
Water your seedlings (from the bottom) that have four or more leaves!
That’s it!

Do the above steps every Friday, and your seedlings will thank you!

Side note: Were you wondering why you should fertilize on Friday? You can easily fertilize any day of the week, but fertilizer and Friday both start with F, so it’s an easy way to remember that it’s time to fertilize!

WANT TO KNOW A FERTILIZING SECRET?

More is not more. You might think that if your seedlings are stalling out or not growing very well, adding more fertilizer than is recommended will help them grow.

Wrong.

This can kill your plants. It is always best to ere on the side of caution and start with a half-strength mixture (i.e. using half of the amount of fertilizer that the package suggests with the same amount of water). For example, if the package suggests a full scoop of fertilizer per measurement of water, you can start with half a scoop and work your way up from there!

DO ALL PLANTS NEED FERTILIZER?

You don’t necessarily have to fertilize your perennials. Shrubs and trees need specific fertilizers (if you even need them), so don’t throw your all-purpose fertilizer on them. They don’t even necessarily need fertilizer in their first year. So, in terms of perennials, shrubs, trees, or anything that is staying in place for a long long time, you only have to worry about fertilizer requirements when you first plant it or if somewhere down the line if it is looking unhealthy.

New to seed starting? Check out these blog posts:

STILL CONFUSED ABOUT FERTILIZING YOUR SEEDLINGS AND PLANTS?

Fertilizer is one of those gardening things that seems really intimidating, and you will find a lot of conflicting advice on the Internet about it. So if you’ve read all of this and are still struggling, here are the four steps I would follow to ensure your success:

Wait until you can count at least four leaves on your seedlings
Buy a fertilizer that has a 10-52-10 NPK ratio on it
Use it at half-strength every week on Friday (because fertilizer and Friday both start with F)
Then switch to an all-purpose fertilizer once the plants are in the ground

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