Best Fertilizer for Indoor Plants and Containers

There are so many types of fertilizer it’s hard to know which one to use. Which NPK ratio is best? Is one brand better than another? Organic vs synthetic. Soluble vs slow release. This all seems so complicated, but in this post I will simplify the whole process of selecting the best fertilizer.

Fertilizer for Indoor Plants and Containers

Most indoor plants and container plants are grown in a soilless mix that is mostly peat moss, decomposed wood or coir. All of these function about the same way and none of them provide significant nutrients for plants. That is why the gardener needs to provide fertilizer, and the same one can be used for both indoor plants and containers.

It should be noted that we DO NOT feed plants. What we do is add nutrients to the soil mix and let plants absorb it. We add nutrients to soil. We don’t feed plants.

Provided the soil contains enough of each nutrient, plants will grow fine. If the amounts of nutrients get too high, they will damage roots which shows up as damaged leaves. If the nutrient levels are too low, the plant will just not grow properly.

There is No Such Thing as Plant Specific Fertilizer

Use google to look for house plant fertilizer. You will find numerous products with differing nutrient ingredients – the NPK value. When I did this I found 15-30-15, 18-6-12, 10-10-10, 11-11-18 and 10-15-10. These are all made by experts in house plant fertilizer. How can each one be the ‘best’ for houseplants if they are all different?

They can’t!

The reality is that there is no such thing as the right fertilizer for any plant. Remember that when we fertilize we add nutrients to soil. The plant, for the most part, takes what it needs. It does not really care what ratio you use provided that there is no deficiency.

Another important other point is that there are many types of house plants. Very few have been studied in enough detail that we know what makes them grow best in every condition. Fertilizer companies are just guessing.

Don’t be conned by marketing. There is no such thing as specific fertilizer for tomatoes, orchids, house plants, African violets, cactus, or any other type of plant.

Proper NPK Ratio

The NPK ratio is the amount of nitrogen, phosphate and potash (potassium) in the fertilizer. Read more about that here: Fertilizer NPK Ratios – What Do They Really Mean.

When plants are analyzed for nutrients, the average amount is in a ratio of about 3-1-2. That means this would be a good average ratio for providing nutrients. So the fertilizer could be a 3-1-2, or 6-2-4, or 9-3-6, etc. Each of these has the same ratio of nutrients.

It does not have to be exactly this ratio, but something close is a good choice. Some plants do like a bit more potassium, so a 3-1-3 ratio is also good but almost no plant wants a higher P value (middle number).

The nutrient that plants need most is nitrogen so having more P and K than required is a waste of your money and it is a waste of our natural resources. To be honest, I usually buy something that is close to the right ratio and at the same time, low in price.

The total amount of ions in a fertilizer solution can be checked with an EC meter.

Myth About Blooms and Root Growth

A piece of common advice says:

• Nitrogen is used to grow foliage.
• Phosphorous encourages root growth.
• Potassium results in more blooms.

This gives the impression that adding more potassium, for example, will produce more blooms but that is not true unless potassium levels are too low. All of the nutrients are needed to grow leaves and they are all needed to grow roots and flowers. If any one of them is missing, a plant can’t grow, period. If you provide adequate fertilizer, adding more of one of these nutrients will not grow a better plant.

If you are having problems blooming a plant, consider providing less nitrogen. That will slow down vegetative growth and encourage blooming, but this only works if other conditions such as temperature and duration of darkness are also correct.

Types of Fertilizer

You can get fertilizer as granules, water soluble powder, liquid, slow release pellets and spikes.

For most people slow release fertilizers are the easiest to use but are more expensive. I use water soluble fertilizer because it’s cheaper and it gives me more control.

Spikes and Granular

Spikes are a poor choice in both containers and in the garden since they concentrate fertilizer in one spot. They should not be used anywhere.

Granular fertilizer is usually used in the garden where they dissolve when they get wet. Don’t use them for containers.

Soluble Powder and Liquids

The best option for indoor plants and containers is soluble powder, liquid or slow release pellets. If you are new to gardening I suggest using soluble powder or liquids. They are easy to find in stores and easy to use.

Water soluble fertilizer (soluble powder) and liquids are usually diluted before use. This saves shipping water around the country. The diluted fertilizer is then used in place of a watering.

Some people fertilize infrequently. Others with every watering. To some extend this depends on how concentrated you make the mixture, but consider this. Plants do best with a constant stream of nutrients at their roots. Given that fact, why would fertilizing infrequently make any sense? It doesn’t really.

I fertilize with every watering. Once a month or two I might skip the fertilizer and just use water, to help rinse out any excess accumulated salts.

Slow Release Pellets

Slow release fertilizer, also called timed release fertilizer, consist of small plastic balls which contain the nutrients. They are designed to release a bit of fertilizer each time they get wet, over an extended period of time. These work quite well and are used by many nurseries since they save time. You only need to apply them once or twice a season, a good option for those of us that forget to fertilize.

Slow release fertilizers do have some issues. It is impossible for you to know when they are used up. You are left with waiting for the plant to show you it is suffering and needs more fertilizer. Another problem is that nutrients are washed away each time you flush the pot – this is discussed in Container Gardening – Selecting the Right Soil.

Don’t be fooled by the little plastic balls. They remain long after the fertilizer has been emptied from them. Just because you see the little balls does not mean you are still fertilizing the plants.

Organic vs Synthetic (Inorganic) Fertilizer

Both organic fertilizer and synthetic fertilizer is available, but …. there is no such thing as organic nutrients. By the time either fertilizer releases plant available nutrients they are all inorganic/synthetic nutrients and plants can’t tell the difference.

The choice between organic and synthetic is a life choice for you – the plants don’t care.

The one problem with organic fertilizer is that much of the material in it has not yet decomposed. Until it does decompose, the plants can’t use it. So you can fertilize today, but the plants can’t get any nutrients. One product  I looked at contained “Alfalfa Meal, Kelp Meal, Neem Seed Meal, Oyster Shell, Azomite, Fish Emulsion and Bone Meal.” None of these can be used by plants until they decompose, and you can’t tell when that happens.

The advantage of synthetic fertilizer is that plants have immediate access to the nutrients. A good quality product also lists the micronutrients in the product, but organic fertilizer rarely provides detailed information about micronutrients.

Does the Brand Matter?

I am sure that there are better brands. The problem is that you have no idea which they are. Just because a brand is popular does not mean it is a good quality brand, and endorsement on social media means nothing since most gardeners don’t know how to test the quality of the fertilizer. Statements like, “it works for me”, or “my plants love it” are of no help.

Fertilizer technology is fairly simple and you can expect that most synthetic fertilizer from reputable companies are of similar quality. I usually buy what is on sale.

Some brands have been maligned for corporate actions or misinformation. For example, the claim that Miracle Gro should not be used because of the so-called “blue copper” in the product is a myth.

Do You Need Mycorrhizal Fungi?

Mycorrhizal fungi are not usually added to fertilizer, but they are sold along with fertilizer as an important amendment. Are they needed? Do they help?

When plants are fertilized properly they do not need mycorrhizal fungi. In fact, if the phosphorus levels are keep at normal levels, and plants are watered, they provide no benefit, and will just die off.

Micronutrients are Important

Nitrogen, phosphorus and potassium are the most important nutrients, but there are another 20 some minor nutrients that you also have to add to plants. Choose a fertilizer that includes micronutrients and don’t worry too much about the actual amount of each nutrient in the product.

Match Fertilizer to Your Tap Water

What is the pH of your tap water? If you have either high carbonate levels or a high pH, it is a good idea to use an acidic fertilizer which will modulate the high pH to some extent.

Some fertilizer is labeled as being acidic, but most aren’t. Look at the ingredients. Fertilizer using ammonium as the nitrogen source tend to be acidic. Sulfate is also acidic.

Tap water normally has enough calcium and magnesium in it, which is the reason most fertilizer does not contain these macronutrients. If you are using very soft water, distilled water or RO water you should use a fertilizer that contains calcium and magnesium, sometimes called calmag. You can also buy calmag as a separate product.

How Much Should You Use?

This is a difficult question to answer. The goal of fertilizing is to replace the missing nutrients in the soil, but you have no idea which are missing. The amount remaining in the pot or container depends on how much it is watered, how much water flows out of the bottom of the pot, your water chemistry, and how much your plants have used. These are all unknowns and quite variable.

It is important to realize that the amount of fertilizer you add depends very much on your watering habits and the amount of rain you get. That is why it is difficult to give you exact numbers.

Start a regular fertilizing program and watch the plants. If you use too much, they will show burnt leaves. If you don’t fertilize enough you will get small leaves and fewer flowers. If either of these are a problem, adjust your fertilizer amount.

Stop fertilizing when plants are not healthy because fertilizing sick plants only makes them worse.

Use less fertilizer when plants are dormant. For example, cactus need almost no fertilizer in winter since they stop growing.

A simple starting point is to use 1/2 of whatever the label says. This will work reasonably well in most cases. Run water through the pot once a month to remove excess fertilizer buildup.

Most plants do well with about 100-150 ppm nitrogen.

If you are using a slow release fertilizer, follow instructions on the bottle.

Calculating 100 ppm

A teaspoon (tsp) of soluble fertilizer weighs about 5 g (will vary depending on the product).

One tsp of a 10% nitrogen fertilizer added to one gallon of water will produce a 132 ppm nitrogen solution. The same solution will be produced by using 1/2 tsp of a 20% fertilizer or 1/3 tsp of a 30%.

What Happens When a Container Dries Out?

When a container of soil has lots of water, the nutrients, ie the salts in the fertilizer, stay in solution–for the most part. So the phosphate, iron, magnesium, etc exist as ions floating around in the water that surrounds the soil particles. In this form the nutrients are easily absorbed by plant roots.

As the water starts to evaporate, the ions in solution become more and more concentrated. Water escapes into the air, but the nutrient ions can’t go anywhere–they stay in the pot. As the concentration of ions increases, some of the ions start getting too crowded and they move out of the water and form solid salts attached to the soil. You have seen this happen if you have old pots around. The white/yellow deposit on the pot are solid salts that have been deposited as water evaporates. The same thing happens in your water kettle. As you boil water, some salts deposit in the bottom as a white/yellow film that gets thicker over time.

Which nutrients become solids first? It depends on the fertilizer you use, and on the quality of your water. Each ion behaves differently, but calcium, phosphate and magnesium tend to become solids (precipitate) more quickly than other ions.

Once these nutrients have precipitated into a solid, they tend to remain that way even if more water is added. Think of your old pots and the white deposit on them. The white stuff does not easily wash away with fresh water. You usually have to scrape the white stuff off to get rid of it.

Plant roots react negatively to both high concentrations of ions in solution and to the dry salts. This is one reason why over fertilizing in pots is bad for your plants. Too little fertilizer may stunt your plants, but too much can kill them.

Watering Indoor Plants

Here is a bonus video you might like.