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 add fertilizer, and the best fertilizer for indoor plants and those in containers is the same.
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 actually feed the soil.
Provided that 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. 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?
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.
The other point is that there are many types of house plants. Very few have been studied in such great detail that we know what makes them grow best in every growing condition.
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 ration of nutrients.
It does not have to be exactly this ratio, but something close is the best choice.
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, and 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 or 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 like temperature, and duration of darkness are also correct.
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.
Types of Fertilizer
You can get fertilizer as granular, water soluble powder, liquid, slow release pellets and spikes.
Spikes are a poor choice in both containers and in the garden since they concentrate fertilizer in one spot.
Granular forms are usually used in the garden where they dissolve when they get wet.
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 the former two. They are easy to find in stores, and are easy to use.
Slow release pellets are specially designed to provide a low level of nutrients over several months. They are less work, but it is impossible to know if they have been used up and need to be replaced. The duration of this depends, to some extent, on how much you water. They are easy to use.
Organic vs Synthetic (Inorganic) Fertilizer
There is both organic fertilizer and synthetic fertilizer. But …. there is no such thing as organic nutrients. By the time the nutrients are released by the fertilizer in a form plants can use, it is all inorganic, 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 some of the material in it has not yet decomposed. Until it does decompose, the plants can’t use it. So you can fertilize today, and the plants can’t get any nutrients. One product I looked at contained “Alfalfa Meal, Kelp Meal, Neem Seed Meal, Oyster Shell, Azomite, Fish 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. A good quality product also lists the micronutrients in the product, but organic fertilizer rarely provides detailed information about micronutrients.
Does 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 products, from reputable companies, are of similar quality. I usually buy what is on sale.
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 come extent.
Some fertilizer is labeled as being acidic, but most aren’t. You can look at the ingredients. Fertilizer using ammonium as the nitrogen source tend to be acidic. Sulfate is also acidic.
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.
Fertilizing is mostly a guess. 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 is a problem, adjust your fertilizer amounts.
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.
As a starting point, use 1/2 of whatever the label says. This will work in most cases. Run water through the pot once a month to remove excess fertilizer buildup.
Watering Indoor Plants
Here is a bonus video you might like.