How to Meet the Nutrient Needs of Your Indoor Plant

image of soil and plant in pot — High angle view of unrecognizable woman, holding trowel and taking care of the plants.

We know a healthy human diet consists of mostly plants, but what does a healthy plant diet look like? It turns out, it’s quite similar to our own.

Plants need many of the same nutrients we do. They use these nutrients for growth, disease resistance, and the production of healthy flowers, fruits, roots, and seeds. Indoor plants are especially needy because of just how much they rely on human beings to survive.
But what exactly are the nutrients an indoor plant needs, and what is the best way to go about providing them? How can you meet the nutrient needs of your indoor plants without overdoing it on one hand or being stingy on the other?
We answer these questions in the following guide. Consider this the manual for indoor plant nutrition, and read on to find out how to do right by your leafy green friends.

The Distinction Between Watering and Fertilizing

It goes without saying that you need to water your plants. Even those of us without an ounce of green in our thumbs know that. Watering is not the only element of plant nutrition, but it is an important place to start.
Learning to water your plants properly can be good preparation for giving your plants the balance of nutrients they need in fertilizer. It is possible to underwater and overwater plants. You want to avoid both.
Underwatering occurs when you let your plant’s potting soil completely dry out in between waterings. Without water, your plants will starve and die.
Overwatering can be even more dangerous. It’s actually the number one killer of houseplants. That’s because it forces air out of the soil, making way for deadly fungi and bacteria.
It’s possible to overdo and underdo fertilizer as well, and we’ll talk more about that later.
The important thing to remember is that both of these processes are as important to your plant’s health as your plants are to your health. There are nutrients your plant needs that water doesn’t provide, and yet water helps your plant activate those nutrients after you have added them via fertilizer.
When performed with the proper frequency and in the correct amounts, these two processes exist in a harmony that keeps your plants thriving.

Nutrients in Fertilizer

So what are those nutrients that plants need and which water does not provide?
The short answer is that there are quite a few. But three of them are so important that they determine how fertilizers market themselves. The rest are secondary to these three, but we’ll talk about all of them just for good measure.

The Big Three

The big three nutrients found in fertilizer go by the acronym NPK, for nitrogen, phosphorus, and potassium (K).
Nitrogen’s role in the life cycle of a plant is encouraging the production of chlorophyll and plant proteins, which makes photosynthesis possible. Since photosynthesis is how plants stay alive, you can see the importance of nitrogen to a plant’s diet. Another thing to note about nitrogen is that it is mobile within soil, which means it has a tendency to get washed away in watering.
Phosphorus is essential for a plant’s healthy root growth. It also helps aid the production of flowers, seeds, and fruits, all elements in a plant’s reproductive cycle.
Unlike nitrogen, phosphorus is immobile in soil. However, it can still be unavailable to a plant if the soil is too cool or has the improper pH.
Potassium helps with fruiting too, and it also gives plants their abilities to resist disease. Plant owners need to watch out for over-fertilizing potassium because too much of this nutrient can block other nutrients from getting to plants.

Secondary Nutrients

The secondary nutrients most plants require actually fall into two categories, the larger secondary nutrients and micronutrients. But for simplicity’s sake, we’ll lump all of them into our “secondary” category, since they all play smaller parts in plant production than the big three.
The true secondary nutrients are magnesium, sulfur, and calcium. Most garden soils already contain these nutrients, but mixes without soil and mixes with only a few ingredients may lack them.
Mixes that don’t include soil also lack many of the micronutrients. The list of micronutrients includes copper, boron, iron, chlorine, zinc, molybdenum, and manganese. The effects of these micronutrients are not fully known, as researchers are still studying them.

When Do Indoor Plants Need Fertilizing?

Now that we’ve answered the “what” and “why” of fertilizer, it’s time to examine the “when.”
There are several different stages of plant growth at which fertilizer can aid the process.
Nitrogen provides great assistance to plants in their young growth phase, though really these plants need a proper balance of all the essential nutrients.
Plants you have just repotted or transplanted require the nutrients that support root growth, especially potassium and phosphorus.
When plants are blossoming or fruiting, phosphorus and potassium are helpful additions to the surrounding soil. And you want to make sure not to over-fertilize nitrogen during these times.

How to Use Fertilizer

If you need a rule of thumb, you should only fertilize your plants once every month to three months during periods when they are actively growing. This means during winter you can give them a break, as plants tend to rest during this season and require very little fertilizer. If you over-fertilize your indoor plants, they can outgrow their pots and suffer root damage.
To ensure your fertilizer doesn’t cause too much buildup of salt within your indoor plants’ soil, leach your pots two to three times per year. You can do this by pouring a great deal of water fully through the pot and letting it drain completely.
As for which fertilizer to choose, nutrient ratios provide guidance in the fertilizer shopping process. There are many commercially available fertilizers that are labeled as specifically helpful to indoor plants. Most of these will suffice for the typical plant owners’ needs.
Nutrient ratios that work well for indoor plants are either balanced ratios (e.g. 6-6-6 or 10-10-10) or 1:2:1 ratios (e.g. 10-20-10 or 5-10-5). These ratios refer to the relative amounts of nitrogen, phosphorus, and potassium, respectively.
Always follow the directions on the label of the fertilizer you choose, whether it comes in tablet, powder, or liquid form. And never use a stronger fertilizer than recommended.

Diagnosing Sick Plants

There are ways to tell if your indoor plants are lacking in specific nutrients, which can help you seek out and treat their specific ailments. Here are a few of the signs of common plant afflictions and the nutrients that can treat them.
Yellowed old growth leaves point to a lack of the proper amount of nitrogen. An anemic, light green appearance to foliage that is supposed to be dark green is another symptom of the same problem.
Old growth leaves with yellow edges is a sign of magnesium deficiency.
New leaves that are warped and misshapen mean your plant needs more calcium.
Plants that do not flower, have dropped flowers, or feature leaves that are unnaturally purple or reddish in color are all signs of phosphorus deficiency. There are several other symptoms of a lack of phosphorus, including abnormally dark green old foliage and leaves with a burnt appearance at their tips.
Potassium deficiency manifests in a couple different ways, including wilted old growth and leaves that are scorched or dark black in appearance.

Nutrient Needs of Specific Plants

Let’s move on from the general nutrient needs of indoor plants to pointers on how to fertilize a couple of specific plants that are common to indoor plant owners. We’ll start with the easiest ones.

Succulents

Perhaps you’ve been reading this entire article asking, “What about succulents? Aren’t they supposed to be almost self-sustaining?”
Yes, actually! These ever-popular plants are an exception to the fertilizer rule. They need very little to no fertilizer, as they can receive most of the nourishment they need from water and a balance fertilization no more than once a month.

Calathea

Calathea misto is no exception to the rule of fertilizing indoor plants only when they are growing. In the case of this indoor favourite, that means monthly fertilization in the spring, summer, and fall.
One of the unique instructions for calathea is that you should dilute the basic fertilizer you use to feed it to half of the recommended strength.

Ficus

Ah, the mighty ficus, the royalty of the indoor plant world. Like the calathea, the ficus requires general-purpose fertilizer diluted to half strength. But you can feed your ficus a little more often than the calathea, around every three weeks, until the plant is no longer actively growing.
You can also make your ficus a double agent by placing it outdoors when the weather is warm.

Take Care of Your Plants and They’ll Take Care of You

Indoor plants require specific care, from watering to fertilization. We hope after reading this guide you understand a bit more about the difference between these essential process as well as how to proceed to deliver your plant’s nutritional needs.