I think we all can agree that plants need nutrients in order to grow. Gardening in containers is very popular for various reasons. However, as I have researched lately, I have discovered that many of the myths, tips, and tricks out there have too much false information when it comes to fertilizers – especially for container gardening.
So, does potting soil need fertilizer? Yes, plants require three macronutrients in order to grow – nitrogen, phosphorous, and potassium (N-P-K). Whether the potting soil is homemade or bought from the store, it will almost always require some type of fertilizer to be added to it during the season.
While I like to follow nature as much as possible with my own gardening methods, I also like to back up my claims with scientific research and the practices used by master gardeners. Let’s explore what I have found about potting soil and its fertilizer requirements and best practices.
What Type of Fertilizer Should I Use with My Potting Soil?
First, it is important to know the type of potting soil being used. The bag of soil was likely labeled “potting mix”, “potting soil”, or some other name that is geared towards container gardening. If the bag of soil already has fertilizer in it, it is required to report this information somewhere on the bag. Some companies have fertilizer included in the mix and some do not.
If the bag does not have any fertilizer listed or you made your own potting mix, then nutrients will definitely need to be added in order for plants to grow. Potting soils are technically “soil-less” mixes that most commonly contain peat moss or choir, vermiculite, and perlite. While these components have great qualities that are required for container gardening, none of them have any nutrients for plants; therefore, fertilizer must be added.
A few different options are available when choosing a fertilizer. First, decide if an organic option or synthetic option is preferred. On this site, I always encourage the use of organic methods in order to follow nature. That does not mean that synthetic fertilizers will not work. In fact, they definitely will work, if used correctly. As far as plants are concerned, they cannot tell the difference between organic macronutrients and synthetic macronutrients. In the end, it is exactly the same, chemically.
I prefer to use organic fertilizers as much as possible. In my opinion, this is much more important when gardening in the ground, than when gardening in containers.
In the ground, organic fertilizers feed the soil life that is so critical to building a healthy soil structure for plants. The microbes feed on the organic matter and turn it into nutrients available for plants.
For potting mixes used in containers, building soil is not necessary. The great soil structure is already there, but it just needs nutrients. No difference is evident between nitrogen, phosphorous and potassium whether they came from organic sources or synthetic sources. In the end, they are still nitrogen, phosphorous and potassium!
With that said, for potting mixes, I choose to use organic fertilizers because I have access to them for free! I mix my own homemade compost and worm castings into my potting mix. If you don’t have access to compost and worm castings, here are some other good organic fertilizer choices.
- Blood Meal – A good source of nitrogen for plants which promotes leaf growth. It’s fairly inexpensive and only a small amount is necessary to get positive results.
- Bone Meal – A good source of phosphorous which promotes healthy root growth for plants. Another fairly inexpensive option that should last a good while, depending on the size of the garden.
- Kelp Meal – Provides potassium and other sources of minerals to promote plant growth.
- Wood ash is also another good potassium option that may be accessible for free. Hardwood ash has more minerals than softwood ash. Wood ash can be good if the potting mix has a peat moss base because it can help neutralize the pH.
As I said, I choose to make my own compost and worm castings that way I don’t have to buy fertilizer, but some gardeners want to amend potting mix with nutrients and that is perfectly fine to do by buying some of the choices listed above. It is important to follow the directions provided on the bag.
As I mentioned before, while I will always encourage gardeners to use organic choices when gardening in the ground, it doesn’t make as much of a difference when container gardening as long as the fertilizer is used correctly. Gardeners are not trying to build and feed soil life when container gardening. Container gardening is basically using a soil-less potting mix, so plants still need access to some nutrients. Again, I see no difference between organic N-P-K nutrients and synthetic.
- Dry fertilizers – These are the type of fertilizers that gardeners might use on the lawn and they are generally not recommended for potting mixes. Their nutrient levels tend to be too high and get flushed out too easily with the first watering.
- Slow release fertilizers – This is a good option for potting mixes. This type of fertilizer is contained in little plastic balls and released slowly with each watering. This fertilizer should only need to be used once or twice for the season. The only downside is that it is difficult to tell when the fertilizer is out. The little plastic balls remain even if the fertilizer has been depleted.
- Water soluble fertilizers – This is the type of fertilizer that is dissolved in water and then applied to plants (or it can come in liquid form too). It provides immediate access to the nutrients for plants, which can be great if the plants are in need of a quick boost. This is probably the cheapest option but it is necessary to use the product much more often than the other fertilizers.
One big issue with synthetic fertilizers is over fertilizing. In the next section, we’ll discuss how much fertilizer should be used in potting mix.
How Much Fertilizer Do Potted Plants Need?
The amount of fertilizer used in your containers is very important; especially when using synthetic fertilizers. One of the most common issues with synthetic fertilizers is over fertilizing. This can burn and end up killing plants. Always follow the directions on the product. To play it safe, always choose to under fertilize instead of over fertilize. Some master gardeners suggest using half the recommended doses of synthetic fertilizers.
With organic fertilizers, don’t worry about over fertilizing and burning plants. The nutrients from compost, worm castings, and other organics are released slowly and the plants use the nutrients as they need them.
If you search for homemade potting mixes online, you will undoubtedly find ten or more different recipes, and they all probably work just fine. Here is what I use for my homemade potting mix and it seems to work well for me.
- 4 parts peat moss
- 1 part vermiculite
- 1 part perlite
- 1 part compost
- 1 part worm castings
This is also what I use as my seed starting mix. When I transplant my seedlings into their final destination (in the ground or in a container with potting mix) I sprinkle in about a tablespoon or so of more compost and worm castings.
Here is a full article explaining the process of my homemade potting and seed starting mix: DIY Potting Soil and Seed Starting Mix to Save Money.
Does Watering Wash Fertilizer out of Potting Soil?
It is necessary to make sure to keep containers watered because the soil in containers will dry out much more quickly than soil that is in the ground. If the soil has ever dried out before, white powder/salt residue may have been noticeable in the container. This is the leftover nutrients that existed in the water after it evaporated. Plants are not able to take up these nutrients in this form, so water them before this happens.
Nutrients from fertilizer do potentially get washed out each time containers are watered. If the container seems too dry and fertilizing seems necessary, then first, flush out the container with a lot of water. This will help get rid of any of that salt build up and get the soil nice and moist to be able to accept the new fertilizer. After the soil is wet, then water with fertilizer, just enough to see the water trickling out of the bottom. If a lot of water runs out of the bottom the fertilizer application is being wasted.
If you are using a slow release fertilizer and not applying more fertilizer often when watering, then you should still flush out the container regularly.
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Let your roots grow down into him, and let your lives be built on him. Then your faith will grow strong in the truth you were taught, and you will overflow with thankfulness.Colossians 2:7
What’s the difference between potting soil and potting mix? These terms are normally used interchangeably. In other words, they both refer to soil-less mixes intended to be used in containers. If a product has the word “soil”, just make sure it mentions containers on the bag if that is the intended use. Garden soil should not be used for containers.
What’s the difference between fertilizer and plant food? These terms are often interchanged, but they are not exactly the same. Technically, plant food is the available nutrients in the soil for plants to use, while fertilizer is an amendment to the soil that supplies or creates the plant food.
Can topsoil be used as garden soil? Yes, topsoil can be used in a garden! For example, topsoil can help fill up raised beds. However, topsoil does require other amendments such as compost, peat moss, and vermiculite to make it suitable for plants to grow. Topsoil is not recommended to be used in containers.