NUTRITION and FERTILITY
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Calculating Parts Per Million
Greenhouse growers frequently express the concentration of fertilizers, in terms of parts per million (ppm). This unit of measure is relatively unique to the greenhouse industry and often there is some confusion on how ppm is calculated. The following is a "simplified" formula suitable for most greenhouse applications.
I. To calculate the ppm contained in 1 ounce of material first solve for B:
A x 75 = B
A = the % active ingredient (AI) in the fertilizer
B = ppm contained in 1 ounce of the material in 100 gallons of water
Example: Calcium nitrate contains 15% N (0.15 x 75 = 11.25). If 1 ounce of calcium nitrate is dissolved in 100 gallons of water the solution will contain approximately 11.25 ppm N.
II. To calculate the number of ounces of material required to make up a desired ppm concentration solve for C:
C = Desired ppm conc. / BWhere:
B = ppm contained in 1 ounce of the material in 100 gallons of water (from above).
C = number of ounces of material to add to 100 gallons of water to achieve the desired concentration.
Example: To make up a 250 ppm solution of calcium nitrate first multiply the AI x 75 (.15 x 75 = 11.25). Next divide the desired concentration by 11.25 (250/11.25 = 22). To make up a 250 ppm solution of calcium nitrate you would add 22 ounces to 100 gallons of water.
I. Make up a nutrient solution of 300 ppm N. Half of the N to be supplied by ammonium nitrate, half by calcium nitrate. (Ammonium nitrate = 33% N, Calcium nitrate = 15% N).
.33 x 75 = 29.75
.15 x 75 = 11.25
150/29.75 = 5 150/11.25 = 13
To make up a 300 ppm nutrient solution of 1/2 ammonium nitrate and 1/2 calcium nitrate dilute 5 and 13 ounces respectively in 100 gallons of water.
II. Make up a 200 ppm solution of ammonium nitrate for use with a 1:100 proportioner.
.33 x 75 = 29.75
200/29.75= 7 ounces of ammonium
7 x 100 = 700 to adjust for proportioner
To make up a 200 ppm solution of ammonium nitrate for use with a 1:100 proportion dilute 700 ounces of material in 100 gallons of water.