I find that a self-watering container is quite convenient for growing a container garden. It’s easy to set up and saves your time watering the plant. But you should be aware of how to fertilizer such self-watering containers.

You can fertilizer self-watering containers similar to a regular container by using a granular fertilizer or liquid fertilizer. But you do need to be aware that the fertilizer will deposit mineral salts in the potting soil over time. So you will need to flush these salts out every year.

I’ve written a lot more details on how you can fertilize your self-watering containers and how you can get rid of the mineral salts that deposit in the potting soil. I’ve also suggested a few organic fertilizers that you may find beneficial for your container plants.

Add fertilizer into the potting mix

The easiest way to add fertilizer to your self-watering container is to mix it in the potting mix. You do this when you’re preparing the container for planting.

You can use a granular fertilizer or a slow-release fertilizer that will release nutrients in the soil when it absorbs water from the reservoir.

The problem of mixing in the fertilizer with the potting mix is that it will cause a buildup of the mineral salts over a period of time. So you will need to clear up these salts from the soil before it becomes toxic for the plant.

Add fertilizer on top of the potting mix

The other option is to add the granular or slow-release fertilizer on top of the potting mix once you have the plant growing in it. The potting soil will absorb the nutrients from the fertilizer when it wicks water from the reservoir.

You need to add such fertilizer once every month during the growing season of the plant. But it’s best to check the manufacturer’s instructions on the amount of fertilizer to use and how often to add it on top of the potting mix.

This method also has the same issue that it will cause a buildup of the salts over some time. These salts can become toxic and kill your plant so you will need to either flush them out or avoid using this method of fertilization.

Add liquid fertilizer to the water reservoir

You can also add liquid fertilizer to the water reservoir when fertilizing the self-watering container. This is a fast-release fertilizer that gets absorbed by the potting soil when it wicks the water from the reservoir.

You can add it to the watering can when watering the plant. You can check the manufacturer’s instructions on the quantity of liquid fertilizer to add to the water and how often you should use it for the plant.

The liquid fertilizer will also cause the mineral salts to deposit in the potting soil over a period of time. You will need to either flush out the soil or replace the potting mix to get rid of this. Otherwise, the plants can die due to the toxicity.

How to prevent salt buildup due to fertilizer

The problem with using a slow-release or liquid fertilizer in a self-watering container is that the mineral salts collect in the potting soil. There is no water flowing from the top to flush it out of the drainage holes as in a traditional container. So you can use some of the below ideas to prevent such a buildup of salt.

Use compost in the potting soil

The easiest and best thing you can do is completely avoid using any kind of fertilizer in the self-watering container. Instead, you can mix up organic compost when you prepare the container and potting soil for planting.

The organic compost will add essential nutrients to the potting soil and provide it with good texture. It will also introduce beneficial microorganisms that will improve the potting soil.

You can also add a layer of organic compost every month on top of the potting soil so it keeps getting a regular supply of the required nutrients.

compost packet
Packet of compost I used for my plants

Flush out the mineral salts

In a regular container, you water the potting soil from the top and the excess salts will get flushed out from the drainage holes at the bottom. But we know that in a self-watering container, the water is absorbed due to wicking from the reservoir at the bottom.

In a self-watering container, you can flush out the excess salt after every growing season. You can water the container from the top till the reservoir starts to overflow and the water comes out of the provided outlet.

You need to keep doing this for a while to ensure that as much salt as possible is flushed out of the potting soil. You should also tip the container and let the water drain out of the reservoir to prevent any mineral salt still remaining in the reservoir. Otherwise, it will go back into the potting soil due to wicking.

You won’t be able to use this method of flushing if the self-watering container does not have an outlet to drain the excess water in the reservoir.

Replace the potting mix every season

If you don’t have organic compost or the self-watering container does not have an outlet to drain water, you can consider replacing the potting mix every year before you plant. You can dispose of the potting mix and introduce some new material in the self-watering container.

You can even put the potting mix in a regular container with drainage holes and flush it well with water using the method mentioned above. This will help remove the excess mineral salts from the soil. You will need to add fertilizer again in this potting soil after it has dried out. And it will be ready to go back in the self-watering container.

What fertilizer should you use for a self-watering container?

If you are making your own compost that is the best nutrient source you can add to your self-watering container. But if you don’t have access to such compost you can make use of slow-release fertilizer or liquid fertilizer in the potting soil. I recommend that you use organic fertilizer in your container.

organic fertilizer
Organic fertilizer I use for my plants

It will give your plants the required nutrients without affecting the beneficial organisms in the soil. And it won’t leach into the ground and cause potential harm to the wildlife. Organic fertilizer also has a lower chance of burning the plant’s roots due to overfertilization.

It’s best to go with a balanced fertilizer when you’re starting the plant in the self-watering container either using a seed or a seedling. A balanced fertilizer is one that has an equal distribution of the three macronutrients N-P-K. Nitrogen, Phosphorus, and Potassium. So a balanced fertilizer could have an N-P-K such as 5-5-5 or 10-10-10.

I would recommend using the Jobe’s Organic Granular Fertilizer if you’re looking for granular, organic fertilizer. This is a balanced, all-purpose fertilizer that has an N-P-K value of 4-4-4.

If you want a liquid fertilizer, you can go for the Triple 10 All Purpose Liquid Fertilizer. This is a balanced all-purpose liquid fertilizer that has an N-P-K value of 10-10-10.

Once the seedling has grown to a few inches, you can use a fertilizer that is specific for the needs of the plant.

The nitrogen is a nutrient that is beneficial to the foliage of the plant. So if you’re growing a plant for its leaves such as a houseplant, a leafy vegetable, or a herb, it’s good to use a fertilizer rich in nitrogen.

The phosphorus is beneficial to the roots, flowers, and fruits of the plant. So if you’re growing the plant for its roots such as carrots, radishes, and turnips or fruit such as tomatoes, cucumbers, eggplant, it’s good to use a fertilizer high in phosphorus.

The potassium in the soil is beneficial to the overall development of the plant and to develop a strong stem and root system.

If you’re looking for an organic fertilizer that is rich in nitrogen, I would recommend the Down To Earth Organic Neem Seed Meal Fertilizer. If you need a fertilizer that is rich in phosphorus, you can choose the Down To Earth Organic Bone Meal Fertilizer.

And if you’re looking for a fertilizer that is rich in potassium, you will find that the Down To Earth Organic Langbeinite is a good option.

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