Things You'll Need
Balance or scale capable of measuring to 0.1 oz. or 0.01 g
Many gardening guides and plant-care instructions specify fertilizer concentrations in parts per million (ppm). The authors of such guides do this to avoid ambiguity. Instructions such as "fertilize with 200 ppm nitrogen solution" are much more precise than "fertilize with house-plant fertilizer at one-tenth strength." Two situations arise in which ppm calculations become necessary. The first involves preparing a fertilizer solution with a specified ppm concentration of a nutrient (nitrogen, phosphorous or potassium). In this case, the amount of fertilizer that needs to be added to a given volume of water must be calculated. The second situation involves calculating the ppm of a specific nutrient in an already-prepared fertilizer solution.
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Calculating the Amount of Fertilizer to Achieve a Given PPM
Obtain the amount of nitrogen (N), phosphorous (P) and potassium (K) in the fertilizer from the fertilizer's label. These are the so-called N-P-K values, such as 10-20-10. The numbers refer to percentages. A 10-20-10 fertilizer contains 10 percent nitrogen, 20 percent phosphorous and 10 percent potassium by weight.
Determine the mass (in grams) of fertilizer to dissolve per liter of water to achieve the desired ppm value by dividing by the percent (in decimal form) of the desired nutrient, then divide by 1,000. If a fertilizer of 200 ppm nitrogen is desired, then:
200 ppm / 0.10 / 1,000 = 2 g of fertilizer per liter (L)
Determine the number of liters of fertilizer to be prepared. For sake of convenience, 1 gallon = 3.8 liters. Thus, if the quantity is known in gallons, multiply by this quantity by 3.8 to convert to liters.
Multiply the grams of fertilizer from Step 2 by the desired number of liters from Step 3 to determine the quantity of fertilizer to use. For example, to prepare 1 gallon:
(2 g) x (3.8 L) = 7.6 g fertilizer
Weigh the quantity of fertilizer on a scale or balance. If necessary, this quantity can be converted to ounces by multiplying by 0.0353:
(7.6 g) x (0.0353 oz./g) = 0.268 oz. fertilizer
Combine the weighed fertilizer and the measured quantity of water and mix well.
Calculating the PPM of an Already-Prepared Fertilizer Solution
Convert the quantity of fertilizer added to a solution to grams if the gram value is not already known by dividing by 0.0353 g/oz.; thus, if 0.25 oz. was added to 2 gallons of water, then:
(0.25 oz.) / (0.0353 g/oz.) = 7.1 g fertilizer
Determine grams of the individual nutrient by multiplying the result from Step 1 by the percent of the nutrient (marked on the fertilizer's container). Thus, for a 10-0-0 fertilizer, the nitrogen content would be
(7.1 g fertilizer) x (0.1 nitrogen) = 0.71 g nitrogen
Determine the volume of the solution in liters by multiplying gallons by 3.8 L/gallon. Thus, for the example in Step 1,
(2 gallons) x (3.8 L/gallon) = 7.6 L
Determine ppm by dividing grams of nutrient from Step 2 by liters of solution from Step 3 and multiplying by 1,000:
(0.71 g) / (7.6 L) x 1,000 = 92 ppm nitrogen