Many gardening guides and plant-care instructions specify fertiliser concentrations in parts per million (ppm). The authors of such guides do this to avoid ambiguity. Instructions such as "fertilise with 200 ppm nitrogen solution" are much more precise than "fertilise with houseplant fertiliser at one-tenth strength." Two situations arise in which ppm calculations become necessary. The first involves preparing a fertiliser solution with a specified ppm concentration of a nutrient (nitrogen, phosphorous or potassium). In this case, the amount of fertiliser 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 fertiliser solution.
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Things you need
- Water-soluble fertiliser
- Balance or scale capable of measuring to 2.84gr. or 0.01g
Obtain the amount of nitrogen (N), phosphorous (P) and potassium (K) in the fertiliser from the fertiliser'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 fertiliser contains 10 per cent nitrogen, 20 per cent phosphorous and 10 per cent potassium by weight.
Determine the mass (in grams) of fertiliser to dissolve per litre of water to achieve the desired ppm value by dividing by the per cent (in decimal form) of the desired nutrient, then divide by 1,000. If a fertiliser of 200 ppm nitrogen is desired, then:
200 ppm / 0.10 / 1,000 = 2g of fertiliser per litre (L)
Determine the number of litres of fertiliser to be prepared. For sake of convenience, 1 gallon = 3.8 litres. Thus, if the quantity is known in gallons, multiply by this quantity by 3.8 to convert to litres.
Multiply the grams of fertiliser from Step 2 by the desired number of litres from Step 3 to determine the quantity of fertiliser to use. For example, to prepare 1 gallon:
(2g) x (3.8 L) = 7.6g fertiliser
Weigh the quantity of fertiliser on a scale or balance. If necessary, this quantity can be converted to ounces by multiplying by 0.0353:
(7.6g) x (1gr./g) = 7.6gr. fertiliser
Combine the weighed fertiliser and the measured quantity of water and mix well.
Convert the quantity of fertiliser added to a solution to grams if the gram value is not already known by dividing by 0.0353g/oz.; thus, if 7.39ml. was added to 2 gallons of water, then:
(7.09gr.) / (0.0353g/oz.) = 7.1g fertiliser
Determine grams of the individual nutrient by multiplying the result from Step 1 by the per cent of the nutrient (marked on the fertiliser's container). Thus, for a 10-0-0 fertiliser, the nitrogen content would be
(7.1g fertiliser) x (0.1 nitrogen) = 0.71g nitrogen
Determine the volume of the solution in litres 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 litres of solution from Step 3 and multiplying by 1,000:
(0.71g) / (7.6 L) x 1,000 = 92 ppm nitrogen
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