05. March 2021

How Much Fertilizer Should I Use?

Having issues with your indoor plants and seedlings? The problem might be overfertilization. Small plants are especially sensitive to strong fertilizers!

Close-up of cabbage plants.

I sowed this cabbage in early December. I transplanted it once and fertilized it twice since. But how do you know when enough is enough? This article is about overfertilizing.


Spring is just around the corner, which of course means that a lot of us want to start pre-cultivating vegetables and flowers for the season. Then, we take pictures and share them on social media. Isn't that the perfect motivation? Well, I've seen a lot of pictures like this these past few days, and noticed that many seedlings indoors seem to be damaged. The people growing them are oblivious to what might have happened. What went wrong? My first thought is about fertilization. How much fertilizer is too much, and what happens if we overdo it?


Read more: Tips on starting seeds indoors


We fertilize our plants indoors mainly in two different ways: By transplanting to new, nutrient-rich soil or using some kind of product.

A good rule of thumb is that small plants need a little bit extra nutrition, and larger plants need more. If we overfertilize our small seedlings, then they might get damaged or even die, while the larger ones are a bit more resilient. In other words: Be careful about overfertilizing your seedlings. They are vulnerable and can't handle the excess energy that comes with a hefty load of fertilizer in the soil.


How much fertilizer is too much? Fertilized broccoli.

Here we have the Baby Matsuri broccoli plants. Their leaves are slightly more yellow than the pointed cabbage growing behind them. The new leaves on the broccoli plants look nice though, so I don't have to worry too much about it. I fertilized them with diluted bokashi leachate, but they basically have everything they need in the soil already. They were transplanted around two weeks ago.



We usually sow our seeds in a little trough, perhaps we do broadcast sowing. It doesn't really matter what kind of soil you use, since you plan on transplanting the seedlings anyway. Do this when they grow a little larger and put them in individual pots or in a trough with new (and a bit more) soil than before. This time though, we want to go for soil with added fertilizer. The nutrients in the soil should last around 3-4 weeks for most vegetables.

This is a great way to give your new plants a boost and allow them to refill their stocks. The plants need to grow, develop their roots, leaves, stalks... Well everything really! The growth rate is going to be stable as long as you keep transplanting them to larger pots with more nutrients as they get bigger.


Read more: 4 Organic fertilizers for your garden



We can for some reason decide not to transplant at the right time. If this happens, then you might want to provide your plants with some extra nutrients in their first little trough, so that you can delay transplanting a bit longer. We often use some type of liquid fertilizer that we dilute with water.

Since the plants are still so small, you need to be very careful with the amount of fertilizer. A very small dose is better than overfertilizing. The liquid fertilizers are after all so easy to absorb.

When you transplant, you actually only need to add more nutrients to the soil when the plants start to show signs that they need it (or preferably just before it starts showing.) However, some soil is less fertile and might need some extra help. And then we have the more demanding plants that need a bit more to grow and function properly too of course.


The middle plant has a damaged leaf, and I think it's because of overfertilizing. It might react to too much leachate in the soil, or perhaps some splashed on the leaves that then got a bit burned under the grow light. The other cabbage plants look nice, but I noticed similar damages on my lettuce too.



I had to use some fertilizer for the trough of plants I'm bringing to the hotbed later, since the transplanting got delayed. Bad timing. The plants are more sensitive when they grow in a plug tray with very little soil. I fertilized them with diluted bokashi leachate. The leachate smells a bit which is why I wanted to put it right on the soil instead of watering from underneath. As you can see in the pictures, some of the plants are damaged. This is exactly what happens when they get too much of a good thing. The leaves get very dry. But I still think that the plant is going to make it. The entire trough got enough of water to sort of dilute the fertilizer. And I'm fine with losing one or two plants anyway, as long as the rest of them are ok.

Just make sure to be extra careful if you use bokashi leachate, since it also affects the pH of the soil. A small seedling can die very quickly if the leachate fertilizer is too strong.


Avoiding overfertilizing
  • use a very small dose of fertilizer
  • fertilize seldom, maybe once a week or so
  • fertilize when the plants show signs that they need more nutrients
If you added too much fertilizer...
  • water the plants with regular water to rinse some of it out
  • change the soil
  • get started on a new sowing if you think that your current one is going to die


Read more: Fertilize with diluted urine indoors


Don't forget that your plants can get quite severely damaged by absorbing too much fertilizer and they might have a hard time recovering. It's actually better to do the fertilizing later rather than overdo it from the beginning.

It might seem strange, but I actually think it's a good idea to sacrifice a plant to experiment on. This way, you get to know more about how the plants act when you use too much fertilizer. You might get different results depending on the type of fertilizer you use of course.


Using too much fertilizer, a cabbage plant in a hand.

This pointed cabbage really seems to be enjoying itself! It was transplanted two weeks ago and I added just a little bit of fertilizer to the water a bit later.


One thing I really want to emphasize today is to stay cool and not get overly attached to the details. A LOT of things can go wrong (or will go wrong, that is) for anyone who tries growing indoors. It's just a risky process. But on the other hand, it's a learning experience. What didn't work this time might just work the next. Maybe you use exactly the same amount for plants around that very same size next time, with great results? So many things factor into the results. Heat, light, pests, humidity, type of fertilizer, your time and attention are just a few examples. Sometimes, we just need to accept that we can't foresee the future.

More: Starting a new year in the garden


If it looks like things are going south, just get started on a new sowing! Just allow the disappointment, swear and start over. It usually works out just fine in the end!

Just remember not to use too much fertilizer in the beginning. Small amounts for small plants. If you feel insecure, just use even less than you planned from the start. Or, just wait for the signs. Yellow leaves usually means that the plants need some extra help. Good luck!
/Sara Bäckmo

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