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Fertilizing accurately using PPM

© Kenneth Joergensen

You may be diluting your fertilizer to "1/4 strength" for seedlings but how do you allow for plants which require more fertilizer, or less? and how much more fertilizer do petunias require compared to pelargonium, and to impatiens?

Introducing: "PPM"
To get around this problem, the greenhouse industry uses a universal term commonly referred to as PPM: "Parts Per Million". It allows greenhouse and seed companies to recommend fertilizer levels for their specific plants during the various growth stages, and it permits gardeners to mix their own fertilizer to these specifications irrespective the type of fertilizer used.

Example: "feed impatiens 100 ppm every two weeks and then drop to 50 ppm two weeks before setting out".

Luckily, the information required to calculate PPM is available directly from the N-P-K numbers on the fertilizer labels.

By using the universal measurement PPM we can move on from the imprecise guidelines such as "use a 1/4 strength houseplant fertilizer", and vastly improve the quality and performance of our bedding plants.

For the purpose of this article, we can define PPM as: number of grams of a dissolved substance in 1,000 liters of water.

PPM ("Parts Per Million") is named so because there is 1,000 grams in one liter of water and by dissolving in 1,000 liters of water we get one million parts (1,000 liters x 1,000 gram per liter = 1,000,000 parts).

Salt Water Solution
Using salt water as an example, let's look at an example of how to calculate PPM:

If 200 grams of sodium chloride (table salt) were dissolved in 1,000 liters of water you would produce a 200 ppm salt water solution. If you had 500 liters of water you would need only 100 grams of sodium chloride to produce the same 200 ppm saltwater solution.

The formular is:
ppm = grams dissolved divided by liters in mixing tank multiplied by 1,000.

Fertilizer Solutions.
As gardeners we are usually not interested in salt water but instead in nitrogen, phosphorus, potassium, etc. Fertilizer labels list the relative strength of these, such as 28-14-24, indicating that the fertilizer in question is 'not pure' but contains 28% Nitrogen, 14% Phosphorus, and 24% Potassium.

We must account for the "dilution" in our fertilizer calculation which can then be changed as follows:

For example, to create a 200 ppm nitrogen solution we would have to increase the number of grams used by dividing the ppm figure by the percentage of nitrogen in the fertilizer (in this case 28%).

seedling flat

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