Fertilizer PPM Calculator

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.

What About Foliar Feeding?

Foliar fertilizer is applied at a higher concentration because very little ends up in the leaf. Lorent Espinoza from California State University, Fresno, suggested the following amounts for fertilizers that are mainly used to supply nitrogen..

• Urea: 2.3 to 9.2 g/l nitrogen or 2,300 to 9,200 ppm nitrogen
• Ammonium nitrate: 0.35 to 1.7 g/l nitrogen or 350 to 1,700 ppm nitrogen
• Calcium nitrate: 0.8 to 2.4 g/l nitrogen or 800 to 2,400 ppm nitrogen

Other sources recommend that for fertilizer that contains multiple macronutrients, apply nitrogen at 1 to 2%, or 10,000 to 20,000 ppm nitrogen. This is for agricultural fields.

My recommendation for home gardeners is not to use foliar feeding on potted plants. See Foliar Feeding – Should Gardeners Use It?