Most plants need fertilizer to grow healthy. This is especially true for houseplants in soilless potting mixes.
However, choosing the best fertilizer indoor plants, knowing how much to use and when to apply it are not as straightforward as they may seem.
In this article, I’ll go through all the details every plant owner needs to know about fertilizers for houseplants.
Fertilizer plays a bigger role in houseplants compared to those outdoors or in your garden. That’s because of a few reasons.
- The potting mix doesn’t contain any nutrients. Even if you do buy those with fertilizer, the nutrients get used up by the end of the first month or so.
- The size of your container limits the ability of your plants’ roots to grow and search for nutrients. This is less of a problem outdoors.
As such, it’s always important to think of potting mix and fertilizer together.
That said, the biggest problem with fertilizer isn’t the lack of it. Instead, it overusing it.
More often than not people tend to add too much fertilizer thinking that it will speed up their plants’ growth.
While the concept may make sense, it doesn’t work that way.
Too much fertilizer can “burn” your plants. This damages them.
Also, it’s important to understand that every time you water your plants, nutrients leach out with the water. As such, frequent watering also means more feeding.
Primary Nutrients for Plants
Whenever you see a bag or container or fertilizer, 3 numbers will often stand out. These represent the three macro-nutrients your plants get, namely:
- Nitrogen – helps your plant grow healthy green leaves
- Phosphorus – is important to fruit and flower production
- Potassium – is essential for developing healthy stems and root system
The numbers you see in the label refers to the percentage of each element in that particular fertilizer. They are likewise arranged in a specific order, Nitrogen-Phosphorus-Potassium.
So, if you see a canister that’s labeled 7-10-7, that means it contains 7% nitrogen, 10% phosphorus and 7% potassium.
Because the phosphorus is the higher level, something like this is likely used to help flowering plants bloom.
It’s also worth mentioning that there are 3 other nutrients plants need that you won’t find in fertilizer. These are:
That’s because the 3 are present in air and water.
Below is a chart of plant macronutrients N-P-K (nitrogen, phosphorus, potassium) and the function of each nutrient.
Also included in the chart are the signs of deficiency and signs of excess for each nutrient.
In addition to the three main nutrients, there are 3 more secondary nutrients that plants need. These are:
- Calcium – is to your bones as it is to a plant’s cell wall. As such, they need enough calcium to provide stability to their structure.
- Magnesium – is essential for chlorophyll production and growth
- Sulfur – plays a role in nitrogen metabolism. Also, your plant also needs to produce proteins and chlorophyll.
These 3 nutrients are often found in the soil. As such, they’re not necessarily included in fertilizer. But, since you’re using potting mix for your houseplants, you’ll need them.
That said, these three are called secondary nutrients because plants don’t need as much of them as they do the primary nutrients.
But, they do require more than the micronutrients below.
Finally, there are other nutrients that plants need in trace amounts. This is why they’re called micronutrients. These are.
Below is a chart of the different micronutrients including their functions, signs of deficiency and signs of excess.
This will help you diagnose any possible problems related to nutrients.
Kinds of Fertilizer Formulations
Like your vitamins, the amount of each of these nutrients can vary per fertilizer product. Different manufacturers will also have their own variations.
But, you’ll see a few terms that are often used to describe fertilizer products. Here are a few popular ones.
- Balanced. This means that the fertilizer contains an equal balance of the 3 primary nutrients. For example, you may have something that says 6-6-6 in the label. This is a balanced fertilizer that contains 6% nitrogen, 6% phosphorus and 6% potassium.
- High Nitrogen. If your plants need more nitrogen, this is the one to get. Some plants will require more or less of different nutrients. As such, choosing the right kind goes a long way in getting the best results. Since nitrogen promotes foliage growth, it’s something you can use for leaf-heavy plants. But, do beware that using it too much or applying it too often can cause a plant’s leaves to grow more than its flowers.
- Bloom Booster. For flowering plants that aren’t blooming as they should, giving them a bloom booster helps. It is high in phosphorus (the middle number) which promotes fruit and flower growth.
- Organic Fertilizer – is fertilizer made from organic sources. This means they’re made from animals and plants. And, there are no synthetics in them. Thus, if you’d like to avoid chemicals, this is a good choice. In general, organic fertilizer often has lower concentrations of macronutrients compared with synthetic fertilizers. But, this doesn’t mean they’re less effective. Instead, it’s because the law requires that you can only list readily available nutrients. Since the rest are released over time (slow release), they aren’t included in the label’s numbers.
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Types of Fertilizer
In addition to the different formulations, you’ll also have the option to choose the form in which the fertilizer comes in.
Here, you’ll see liquid, tablets, sticks and many more. Among them, liquid and slow-release fertilizers are ideal for indoor use.
Meanwhile, granular fertilizer works best for your outdoor plants.
That said, tablets and sticks are probably the most convenient to use. But, they aren’t as efficient as the others in terms of distributing the nutrients throughout the soil.
So, we’ll focus the discussion on these 3 kinds.
Liquid fertilizer is a good choice for your houseplants because it’s easy for you to control. You get to decide how much to add each time.
They’re added to water in the can. So, you’re essentially fertilizing while watering.
But, depending on the fertilizer your use, you may not need to include them every time you water. To make sure, do read the label.
In addition to controlling when and how much, this kind of fertilizer also provides a steady supply of nutrients to your plants.
Granular fertilizer comes in pellet forms. They’re somewhat like the tablet you drink with water.
You place them in the soil or container by hand. You can likewise sprinkle them over the area or pot. And, once you water them, the pellet dissolves to release the nutrients.
Unlike liquid fertilizer, this method “dumps” all the nutrients into the spot where the pellets are placed. You also don’t have control over how much goes into which area.
However, its biggest draw is that it’s very cheap.
Slow Release Fertilizer
Continuing on the tablet analogy above, this is the controlled-release, timed-release or slow-release version of your medications.
Thus, instead of instantly dumping all the nutrients, they’re coated such that they release the nutrients at different intervals.
This slower, gradual release allows one application to last between 2-9 months. It also gives your plants a consistent supply of nutrients without overdoing it.
The downside to this kind of fertilizer is its cost. But, because of the longer duration, you may want to do a little bit of math to figure out whether it’s cheaper or more expensive than the other methods.
How Much Fertilizer Should You Use?
The interesting thing about fertilizer is that it’s worse to overfeed your plants than to underfeed them.
Just like overwatering, too much fertilizer can kill your plants.
And in most cases, the recommended amount of fertilizer is higher than needed. Thus, I use half the suggested amount on the label, which from experience produces good results.
Here, it’s a good idea to be more conservative than aggressive.
If your plants’ color is lighter than they should be, increase the amount of fertilizer by a bit until you get the right concentration.
Unfortunately, doing the opposite and starting from too much fertilizer and reducing it slowly doesn’t work. By the time you realize it, your plant could already be damaged from fertilizer burn.
Finally, for most plants, growing season starts when spring comes and continues until fall. It is during this period of active growth when they need the extra boost.
In contrast, most plants don’t need to be fed during the winter.
How to Choose a Fertilizer for Your Houseplants
Most of the time, a balanced fertilizer (5-5-5) contains all the nutrients your houseplant needs to be healthy. Since you’re using potting soil, you don’t have to deal with soil issues.
In contrast, choosing fertilizer in your garden or if you decide to use regular soil will depend on the kind of soil you have. Thus, soil testing is key.
This way you’ll be able to buy a fertilizer that makes up for any deficiencies in the soil, in case there are some.
That said, there are also specialty fertilizers for certain kinds of plants. This ensures that they contain the right formulation for that specific plant.
Natural vs. Synthetic Fertilizers
Below is a chart showing the differences between natural and synthetic fertilizers (organic vs inorganic/chemical fertilizers).
Below is a chart listing common organic fertilizers used for plants. For each, you can see the amount of nitrogen, phosphorus and potassium.
Note that the amounts can vary significantly depending on the kind of product you get based on the manufacturer.