Gardeners normally talk about “teaspoons” of fertilizer when they make up fertilizer solutions, which leads to all kinds of confusion and misuse of fertilizer. Some of you are using too little, while others use way too much. On the other hand, horticultural professionals don’t make these mistakes because they talk about PPM values instead.
It’s time gardeners change their habits. Start using Parts Per Million (PPM) units instead of teaspoons in your discussions. This will improve online information and reduce your fertilizer mistakes. I discussed the reasons for doing this, along with several examples, in Calculating the PPM of Nitrogen in Your Fertilizer.
To help you use ppm values, I have created this simple-to-use Fertilizer PPM Calculator, which shows you how much fertilizer to use to make up a required ppm solution. It works for both metric and imperial units, and can be used for both solid and liquid fertilizers.
PPM Calculator
Step 1: Enter your fertilizer information:
Target nitrogen PPM: ppm
% Nitrogen in fertilizer: % (first number in the NPK)
Quantity to make: gallons or liters
Step 2: Make up your fertilizer as follows:
Add this amount of fertilizer if you are using gallons:
tsp
tbls
US fluid oz
ml
Add this amount of fertilizer if you are using liters:
ml
g
tsp
tbls
Note: 1: The above calculation is based on the nitrogen level. In order to get the right PPM for other nutrients, use the preferred NPK ratio of 3-1-2.
Note: 2: The amounts have been rounded off to the nearest usable numbers and they assume that 1 g of fertilizer is equal to 1 ml, and that a teaspoon (tsp) contains 5 grams. This will work for most fertilizers.
What is ppm? PPM is short for parts per million, which is usually based on weight. One ppm = one mg/l.
The Right PPM for Your Plants?
I have looked at several horticultural recommendations and research papers that determine the optimum nitrogen level for growing potted plants. The nitrogen levels are in the range of 100 to 200 ppm nitrogen. For maximum productivity, the values tend to be around 200 ppm.
However, plants can only utilize 200 ppm nitrogen if they’re growing in excellent conditions, including lighting and watering. Since home culture is rarely that good, gardeners should stay below the highest level, and I recommend 100 ppm nitrogen.
Part of the reason for this lower value is the common use of windows or lower-intensity lights compared to greenhouse culture. Another important point is that most gardeners have a collection of different plants that all have to grow under one set of conditions. We don’t maximize the growing conditions for each type of plant in the way that production greenhouses do.
The MSU orchid collection is grown at 125 ppm. This same fertilizer level is also used for most of their large collection of houseplants. I find that 100 ppm grows streptocarpus that flower all year long. Commercial growers of African violets use 100 to 125 ppm nitrogen.
How does this compare to commercial fertilizers? Surprisingly, their recommendations vary widely from 165 ppm every two weeks to a low of 30 ppm every week.
Plants that have slowed growth in winter or after flowering can be given less nitrogen (30-50 ppm).
Seedlings should be fertilized as soon as you see true leaves forming, and they should also be given 100 ppm nitrogen.
Best NPK Ratio to Use?
The best NPK ratio for most houseplants and containers is a ratio of 3-1-2 or 3-1-3. This does not have to be a perfect ratio – something close to that will work.
When using the 100 ppm solution what is the frequency of application? Monthly, weekly, with every watering? The Miracle-Gro all purpose water-soluble 24-8-16 label instructs to use 0.5 teaspoon per gallon (140 ppm solution) every two weeks for house plants.
According to the calculator, 1/3 teaspoon per gallon makes a 100 ppm solution.
Thanks, John A.
Every watering.
Thank you so much for this! It is an excellent presentation that explains the reasoning and science behind using this method instead of the more common ‘guestimate’ measurements given on product labels and is very helpful for calculating more accurate percentages of fertilizer for optimal results.
Your recommendation of 3-1-2 is interesting as a lot of gardeners recommend higher phosphorus for higher number of flowers in vegetables in the garden
High phosphorus does not produce more flowers – common myth.
https://www.gardenmyths.com/nitrogen-leaves-phosphorus-flowers-potassium-health/
Thanks Robert another great report. With our window boxes and many containers we use a ‘Miracle Grow’ diluter once a week. I tell people that plants need food just like people and if you want plants to perform and flourish they need food too. Thanks
Would this be appropriate to use for outdoor containerized plants (fruit trees, vegetables, ornamentals)?
The application does not matter – it calculate dilutions based on nitrogen.
So the oft cited advice to use more fertilizer in outdoor container plants due to increased leaching caused by more frequent watering holds no water?
I don’t think your statement is often cited. What is said is that any container that is watered a lot need to have the nutrients replaced more often.
So you don’t want to use “more fertilizer” in sense of higher amounts, but you do need to keep the level in the contain at the level plants want. Watering leaches out nutrients, so you have to fertilize more often.
Interesting. The recommended dosage printed on the packages of many fertilizers even differentiate between outdoor and indoor use with the former being higher.
Recommendations only apply to containers using soilless media. Outside you should fertilize to replace missing nutrients – no fertilizer company knows anything about your soil.
As always your blogs are very helpful. I used your PPM formula from a previous post and my Tomato seedlings never looked so good. I have been adding nothing but Nitrogen to my outdoor, raised bed filled with soil garden since a 2019 soils test showed high levels of Phosphorus (140 mg/L) and Potassium (1,200 mg/L).