One can estimate the parts per million in solution (ppm) produced by a quantity of fertilizer in a gallon of water, based on the fertilizer's N-P-K rating.
For each number in the rating apply the following formula:
> one teaspoon (5ml) of a given fertilizer in a gallon of water
(10xN) 0.768 = ppm
> one tablespoon (15ml) of fertilizer in a gallon of water:
(10xN) 0.256 = ppm
Example: Peter's Professional All-Purpose Fertilizer with N-P-K rating of 20-20-20.
Therefore, one teaspoon (5ml) of Peter's Professional All-Purpose 20-20-20 in one gallon of water will produce a solution that contains approximately 260.4 ppm of Nitrogen, 260.4 ppm of P (P2O5), and 260.4 ppm of K (K2O) for a TOTAL ppm of approximately 781.3 .
It's important to understand that with the term "parts per million," we are expressing a measurement in terms of a RATIO, which, like miles per hour or pounds per square inch, is a relation in degree or number between two separate measurements. "Parts per million" is a common way of expressing the measurement of the dilution of something in solution, which for our purposes here, is the dilution of fertilizer components in water. It just so happens that fertilizer N-P-K ratings are also ratios, as is the measurement of teaspoons per gallon.
Parts per million is most often alternately expressed as milligrams per liter because it's so easy to convert due to the efficiency of the metric system: 1000 milligrams = 1 gram and 1000 grams of water = 1 liter; 1 thousand x 1 thousand = 1 million; voil- parts per million.
Fortunately, the "parts" in "parts per million" are generic. It can be parts of a teaspoon just as easily as it can be parts of a gram. N-P-K ratios are also generic expressions of percentage, or parts per hundred. Unfortunately, 1 gallon does not equal 1 thousand teaspoons; it takes 768 teaspoons (or 256 tablespoons) to equal one gallon. But by knowing this, one ratio can be converted into the terms of another ratio, which is what the formulas above do.
Before relying on these formulas to fertilize your grow, you should be absolutely aware that N-P-K ratings don't necessarily precisely describe the amount of each nutrient in the fertilizer, but instead describe the minimum amount of nutrient to be found, as explained in Mr.Ito's excellent FAQ on the subject.
The only way to know for certain the ppm of your fert mix solution is to measure it's EC, with an electronic TDS meter as you mix it. If this isn't possible, at the very least try to use only high-quality fertilizer brands with reputations for their accuracy and consistency of their analyses and products.
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