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Fertilizers: How To Use Them On Seedlings and Plant Starts

So, you have planted your own seeds and they have sprouted. Now what? Seedlings are no different than their fully grown counterparts, they need to fertilizers just the same.

When it comes to seedlings, you should begin fertilizing as soon as they have their first set of true leaves. What are true leaves? When seedlings sprout, the first leaves you see are Cotyledons. You will notice that these leaves all look the same regardless of what you are growing. These leaves provide the plant with all the nutrients necessary until the true leaves appear. True leaves are the second set of leaves to appear and actually look like the adult leaves will look on a plant. Once these leaves appear, the plant can now begin the photosynthesis process. Once you see these leaves, then it is now time to start fertilizing.

Why do we need fertilizers?

When a seed germinates, it has all the nutrients necessary to start its life off. What happens when those nutrients run out? Pests and diseases have the opportunity to move in and destroy the seedlings. Remember, they have a small root system in whatever you are starting them off in. We are supposed to be helping strengthen their immune system, helping them to get off on the right foot. So that when we plant them in the garden, they have everything necessary and are prepared to thrive.

What to use as fertilizers

Anything you would use to fertilize the mature plant can be used for the seedlings, just at a diluted rate. During the seedling stage, plants need an all-purpose fertilizer. That is a fertilizer where the NPK values are close to equal, 4-4-4, or 20-20-20.


Using an all-purpose fertilizer is a way to ensure the plants are receiving a balanced diet of all the necessary macronutrients. Some crops, like tomatoes, benefit from fertilizers that are rich in micronutrients as well. Such as fish fertilizer or kelp meal. If you see that the underside of your tomato leaves are turning purple, apply either one of these. They contain trace minerals that will solve this issue. Honestly, I prefer to feed my plants compost or worm castings teas instead of all-purpose fertilizers.

How to apply fertilizers

Before we talk about how to apply the fertilizer, let’s make one thing clear; DO NOT FEED SEEDLINGS A FULL-STRENGTH DOSE OF FERTILIZER! If the nutrient says to mix 30 ml/ gallon of water, start by using 1/10 of that and mixing 3ml/gal. Why? Well, the seedlings do not have a fully-grown root system, so a full-strength dose can cause harm, burning the leaves or even killing the plant. Nutrient burn is when the fertilizer or nutrient solution is too strong for the plant and actually causes more harm than good. This can look like burnt edges on the leaves or the plant merely dying, so keep that in mind

There is no right or wrong way to apply fertilizer. Spray it on leaves, water it in on top of the soil, or fill the tray underneath your starts and allow the roots and planting medium to absorb what it needs. The choice is up to you. Whatever you decide, just remember these seedlings are depending on us to become successful. Equip them with the necessary tools to thrive.

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