Should You Use Fertilizer With Vegetable Seedlings? A Soil Scientist’s Opinion

I am a soil scientist and I often get questions asking about using fertilizer with seedlings. The only way to answer this question is through the science of soil. The Gardening In Canada website applies science to the garden in order to make your yard look better. A great garden begins with seedlings. the stronger our seedlings are the greater chance we have with transplanting and ultimately end yields. So what is the best way to achieve this and does it involve fertilizing when the seedling is still young.

As a soil scientist I do fertilize seedlings.

A Contested Topic In The Gardening World

This is a hotly contested topic in the gardening world and rightfully so due to the enormous amount of research and money put into the concept. and the quick answer to all of this is yes you should fertilize. however there are some things to consider. These considerations range from what is the soil pH, what fertilizer form are you using ammonium verse nitrate, and does your fertilizer contain phosphorus and how much.

The best way to look at this is through the eyes of scientific journals. These ultimately are going to give us the most nonbiased look at whether or not we should fertilize our seedlings. The best part about scientific journals is that they are used in the context of productivity and saving money while increasing yields. Meaning they do not want to get you to buy anything they simply want green houses and producers to produce more.

Seedlings That Are Healthy Transplant Better.

Seedling that are healthy transplant is a simple but true statement and classically abnormal growth is a sign of deficiency. For example, with brassica species there is a high desire for calcium and sulfate. Without calcium or other nutrients, the first true leaves can be deformed. I am currently going through this with my Brussels sprouts.

It is important to note that plants can have deformed leaves caused by things other than nutrient deficiencies. However, if you are using fresh soil that is sterilized it is unlikely to be the case. Currently my Brussels sprouts look as though they have been attacked by thrips. This is not the case and I actually believe this is due to chlorosis. The chlorosis has been brought on by the fact that my soil was too moist for too long. This did result in a nutrient deficiency however fertilizing would not have fixed it. The solution was prevention from overwatering with trays that contain holes.

Other things that can cause shoots and young leaf abnormalities is excess amounts of soluble salts which are commonly found in fertilizers. This means that you have to ensure the concentration of fertilizer you are using is within reason.

Organic Seed Starting Fertilizers
Fish Emulsion
Gaia Green

Conventional Seed Starting Fertilizers
Liquid Seedling & Transplant Mix
Liquid Quick Start

How Much fertilizer Seedlings?

A good rule of thumb is to mix it at about 1/4 strength of what is recommended on the package for adult plants. You can continue at this quarter strength until you begin to see the formation of a true plant. I would say once you have over 5 leaves you can then increase this to half strength and then go up from there overtime. When you begin hardening off the seeds for transplant time consider changing the fertilizer to something with higher levels of potassium. We will talk about why this is important later on in this psot.

When Not to Fertilize Seedlings?

Believe it or not there is a time to fertilize and a time not to fertilize. Regardless of the plants age you should not fertilize while the soil is dry. The reason for this is it will cause an imbalance in salts within the soil system. the end result will be a hot zone which will cause burn. This burn effect will show up typically in the leaves as crisp edges. I have a video on why salt shows up in the leaves of the plant over on YouTube

With seedlings it is important that we do not fertilize if the leaves are touching the soil’s surface. Because seedling leaves are so thin are outer layer tends to be sensitive to salt. the end result may pull moisture from the leaves into the salt solution which would be the fertilizer. The outer cuticle on seedlings is not well developed so processes such as osmosis and diffusion can heavily affect the plant.

Organic vs Inorganic Fertilizer For Seedlings

Now you are probably thinking why do you not just use an organic fertilizer if you’re worried about salt burn. And the answer is organic fertilizers still contain a base line of salt. This is because plant nutrients in some cases are taken into the plant via the salt gradient through passive transport systems.

Liquid vs Granular Fertilizer For Seedlings

One thing to note is regardless of if it is organic or inorganic fertilizer it is important to use a liquid form. Granular forms of fertilizers do not work well with seedlings. The reason for this is granular fertilizers have to breakdown before becoming bioavailable. This breakdown will happen outside of the timeline needed for the seedling’s growth.

N-P-K Needed For Seedlings

When we look at fertilizer, we see the numbers 24-15-10 as an example. These numbers we know stand for nitrogen – phosphorus -potassium (N-P-K). The three building blocks of plants but not necessarily all the macro nutrients a plant needs to thrive. Some base elements such as boron and calcium are missing from this numbering system. However, most fertilizers organic or otherwise contain the base elements in some form.

Nitrogen For Seedlings

Nitrogen is important because it helps assimilate amino acids which are the building blocks for protein in the plant. These proteins are components of chlorophyll. Chlorophyll as we know is important to photosynthesis which is the energy source for plant growth. These proteins are also important for several enzyme reactions.

Think of enzymes in plants as the brains behind what the plant needs to do. Enzymes are released based on external environmental factors that trigger them. without enzymes our plants would not be able to interact with the world around it.  

This is why nitrogen is the number one nutrient for plants. It is what makes a plant green for lack of a better term. A nitrogen deficiency in any form for any plant is ultimately a death sentence. There are two popular forms of nitrogen in the gardening community. They are ammonium and nitrate. Each one of these being a bioavailable form of nitrogen. Bioavailable meaning available to the plant at the time of application.

Phosphorus For Seedlings

The second number phosphorus and it is a major component in the plant’s DNA and RNA. Phosphorus is critical for crop maturity and for seed production. Without phosphorus we do not have flowers and we do not have seeds. Without seeds we no longer have plants. Phosphorus is also critical for root development. Lack of phosphorus results in poor roots and therefore poor upper biomass development. This is why phosphorus in a seed starting fertilizer is so important without it we would have weak seedlings.

Potassium For Seedlings

The third nutrient is potassium. Potassium is an indirect nutrient meaning it does not make up anyone part of the plant. We do not have the ability to say potassium helps build roots or that potassium helps build flowers. What we can say though is that potassium is required in the activation of over 80 enzymes throughout the entire plant. another way to look at this is that potassium is responsible for 80 individual functions that help keep the plant alive. For lack of a better term potassium is the brain of the plant.

Oddly enough this potassium is also important to helping the plant withstand extreme colds, extreme heat and even droughts. This is why potassium is often found in seedling mixes or in transplant mixes. The use of potassium when transplanting is important to reduce transplant shock. Transplant shock is reduced when using potassium because it actually helps increase the water use efficiency in the plant despites outside pressures.

What Does The Soil Science Say About Fertilizing Seedlings?

One of my favorite studies on fertilizing seedlings was done by UMass Amherst University. This was done in the context of a greenhouse starting seeds for selling. They set up the study so that some seedlings were left unfertilized others were fertilized at predetermined rates and even some plants were fertilized using specific forms of nitrogen. In this study they used ammonium and nitrate two different forms of nitrogen fertilizer.

This is a table taken from the Amherst study showing the stages of plant growth definations

They broke the study down nicely, looking at different stages of growth and how fertilizer affected them. It is a known fact that during stages two and three nutrient disorders are common. These nutrient disorders can pop up in the form of shoot tip distortions or even death. You also may have misshapen cotyledons or misshapen true leaves. Also, common but not necessarily a fertilizer deficiency is chlorosis which we talked about earlier in this article.

In stages three and 4B program showed that there are significant effects on the height of the ceiling and the overall upper biomass. When fertilizer was used in stages three and four plants fertilized using ammonium, which contains phosphorus showed taller and greener development. The phosphorus rate, while low at only 2%, did make the difference for the root mass below the soil surface.

The Results Of Higher Fertilizer Usage With Seedlings

This abundance in root biomass resulted in higher levels of upper biomass in there for a bigger greener plant. This sounds like a good thing, but what they did not touch on in the study was how these plants tend to transplant. My worry is that with a larger plant such as what was found in this study, we may have significant levels of transplant shock. As plants get larger and develop, they tend to dislike being manipulated out of their soil bed.

My Preference When Choosing Plants From A Greenhouse

This is why in a greenhouse I tend to choose younger plants without flowers. The less mature the plant, the higher rate of survival in my garden. Truly it also means that I will have flowers later into the season. Maybe this is personal preference, and you want flowers earlier in the season only to have less by the time fall rolls around.

Results From Low Or No Fertilizer With Seedlings

In the plants that had low or no fertilizer the shoots were short and had lots of roots. This goes back to my earlier comment do you want more roots, or do you want more shoots when you go to transplants. I would argue you would want more roots because that is what is taking the pounding when you decide to transplant into the garden. The upper biomass and any wilting that may occur from transplanting is just a side effect of too much root damage in some cases.

That means if you have more roots you arguably will have an easier time transplanting. But again, this is up to personal preference and what you are willing to work with. Some people are OK sacrificing a little bit of transplant shock for beautiful blooms earlier in the season.

If that is the case, you are in luck medium to high rates of fertilizer use within reason resulted in higher amounts of biomass above ground and lower root volume. But again, this study did not touch on the ability for the plant to transplant and whether or not this lack of root system would affect the results.

Soil pH An Seedlings

Interestingly enough the soil pH had a huge effect on the use of fertilizer with seedlings. If you remember my YouTube video talking about peatmoss based soils you will understand my interest in the pH affecting fertilizer usage. Peatmoss based soils are notoriously acidic which works in our benefits when starting seeds because it actually helps prevent mold buildup.

However, that acidic soil is out of the ideal range of 5.4 to 6.8 that plants enjoy. This means if you fertilize in a peat-based soil starter mix you may not have full nutrient utilization. Meaning you’re wasting your hard-earned cash fertilizing a seedling that is unable to capture the nutrients. This study went even further into the idea of adjusting the soil pH in a seed starting setup.

Using Lime Additives With Seedling

In this study they used lime as a neutralizer with the peat-based soil. The use of lime is common in acidic soils around the world as it does raise that pH and place it in a more neutral position. This may be something you are interested in especially if you have some special seeds that are difficult to germinate. Lime has no salt and is a completely natural organic (inorganic technically) amendment for the soil.

Using Different Fertilizers to Alter Soil pH

In this study the fertilizers ammonium and nitrate were actually used to alter that soil pH . This is because ammonium is acidic, and nitrate is alkaline. Switching off between the two fertilizer types resulted in a neutral or balanced pH. This neutralization resulted in better nutrient uptake by the seedlings themselves.

For us gardeners we may be interested in using both forms of nitrogen, ammonium, and nitrate in order to neutralize that pH without having to add things such as lime.

In Summary: What I Personally Think

I personally do fertilize at a low rate when the plants are seedlings. However, I use an inorganic form of fertilizer because I personally believe that organic fertilizers will not yield the best results in this scenario. The nutrient cycling or nutrient turnover is much too slow, and you are throwing away your hard-earned cash by simply dumping your organic fertilizer into your seedling tray.

I have even gone years without fertilizing any seedlings and quite honestly didn’t result in any notable differences in and yield or even in the transplant process. One thing I will say is that the use of a potassium fertilizer is important when transplanting to reduce transplant shock. And when transplanting or fertilizing seedlings it is important to use a liquid fertilizer rather than a granular fertilizer due to the excess ability of nutrients.

Your Thoughts And Experiences?

Let me know in the comments down below what your preferred form of fertilizer is and whether or not you fertilize your seedlings. Include any tips and tricks for your garden and be sure to mention your zone because it will help other commenters. As always be sure to share an I will talk to you guys next time.

Ashley The Soil Scientist
Ashley The Soil Scientist

Using science in the garden to help you grow.

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