Calculating the PPM of Nitrogen in Your Fertilizer

How much fertilizer do you use? In North America, it is common to reply with something like “1/8 tsp of a 20-6-12 fertilizer. The problem with this answer is that it is difficult to compare the amount to someone using, for example, 1/4 tsp of 7-3-5 fertilizer.

Are either of these close to the recommended amount of 100 ppm nitrogen?

It is important to compare apples with apples and the best way to do this is to report and discuss fertilizer in ppm units. It is easy to do, it will make gardening communication much more useful and it will give you better insight into your fertilizer use.

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Why Use PPM Values

Fertilizer is available in a variety of NPK values and it is difficult for most of us to mentally convert one formula to another. For example, which dilution contains more nitrogen 1/8 tsp of a 20-6-12 fertilizer added to a gallon of water or 1/4 tsp of a 7-3-5 fertilizer added to 2 quarts? How much more does it have?

Now try answering the same question using ppm. Which dilution contains more nitrogen, a 40 ppm mixture or a 75 ppm mixture? How much more does it have?

The second reason gardeners should use ppm is that it is the standard in horticulture, laboratories, and research reports. Once you know how much nitrogen you use in ppm units, you can compare it to commercial operations and mimic their performance.

Here is an example. Commercial orchids are grown at 150 – 200 ppm nitrogen. How does your 1/8 tsp of a 20-6-12 fertilizer compare to that? Are you using too much nitrogen? Too little? These questions are so much easier to answer if you know your fertilizer ppm value.

What is ppm? Ppm is short for parts per million which is usually based on weight. One ppm = one mg/l.

Calculating PPM Using Imperial Units

The following values are used.

Tsp = number of teaspoons (tsp) of fertilizer added to one gallon of water.

%N = the amount of nitrogen in your fertilizer. This is the “N” value in the NPK. The %N value for an NPK of 20-6-12 is 20.

The ppm value of your mixed fertilizer solution is = Tsp * %N * 15.

(“*” is a multiplication sign, “/” is a division sign).

Example:

A solution made by adding 1/8 tsp of a 20-6-12 fertilizer to 1 gal of water has 37.5 ppm of nitrogen.

ppm = 0.125 * 20 * 15 = 37.5

Note: you can work with fractions in this formula or you can convert fractions to decimal numbers (1/8 = 0.125, 1/4 = 0.25, 1/2 = 0.5, 3/4 = 0.75)

Other Units: Use these values to convert your method of measurement to tsp and then use the above formula.

• Tablespoon: tbs = 3 tsp
• Once: Oz = 6 tsp
• Cup: 1/8 cup = 6 tsp
• Mililiters: 5 ml = 1 tsp

Calculating PPM Using Metric Units

The following values are used.

Wg = weight of fertilizer in grams. You can assume that 1 ml = 1 g

%N = the amount of nitrogen in your fertilizer. This is the “N” value in the NPK. The %N value for an NPK of 20-6-12 is 20.

#L = number of liters of water used to mix the fertilizer.

The ppm value of your mixed fertilizer solution is = Wg * %N * 10 / #L.

Example:

A solution made by adding 5 grams of a 20-6-12 fertilizer to 10 liters of water contains 100 ppm of nitrogen.

ppm = 5 * 20 * 10 / 10 = 100

How Much Fertilizer Should You Use?

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.

Plants only grow well at 200 ppm nitrogen if they also get excellent environmental conditions such as good lighting and perfect watering. Home culture is rarely that good. Our homes usually don’t provide enough light for maximum growth and watering is not as precise.

Part of the reason for these limitations is that homes use windows or low-intensity lights compared to growing in a greenhouse. 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 condition for each type of plant in the way that production greenhouses do.

The MSU orchid collection is grown at 125 ppm. This same fertilizer is used for most of their large collection of houseplants.

For these reasons, houseplants don’t need as high a level of nitrogen as commercial facilities. I’d recommend homeowners use about 100 ppm of nitrogen.

Commercial Fertilizer Products for Houseplants

What do commercial fertilizers suggest for houseplants? I used the suggestions on the label and the above formulas to calculate the ppm nitrogen for each product.

Fertilizer Type	Instructions	PPM of Nitrogen
Miracle-Gro 24-8-16 Plant Food	1/2 tsp / gal	180*
Jack’s Classic 20-20-20	1/4 tsp / gal	75
Dyna-Gro Foliage Pro 9-3-6	1/4 tsp / gal	34
FoxFarm Big Bloom Plant Food, 0.01-0.03-0.7	4 tbs / gal	1.8
Bonide Houseplant Liquid, 10-10-10	1/2 tsp / gal	75

You will notice a wide range of ppm values. The FoxFarm 1.8 ppm essentially adds no nitrogen and is very pricy. Anything under 75 ppm is too low for actively growing plants.

Based on the above table, gardeners ‘ common recommendation to use 1/2 strength or even 1/4 strength of the recommended label amount is wrong in most cases. That is not enough nitrogen for plants to grow well.

Plants that have slowed down growth in winter or after flowering can be given less nitrogen (30-50 ppm).

The Cost of Fertilizer

There is a wide range of fertilizer costs. My recent review of this found that a pound of nitrogen can cost anywhere from $33 to over $6,000. Anything over $75/lb is a waste of your money.