How to use this calculator
- Enter or choose target concentration of the nutrient.
- Enter or choose nutrient content on the fertiliser label.
- Enter or choose label states the nutrient as.
- Enter or choose reservoir or tank volume.
- Enter or choose volume in.
- The fertiliser to dissolve updates as you type. Use Copy result or Share to keep it.
Method
Grams = ppm × Litres ÷ (Elemental% × 10); Elemental% = Label% × Factor (P₂O₅ → P: 0.4364; K₂O → K: 0.8301)
- ppm
- mg of the nutrient per litre of solution
- Factor
- mass share of the element in its oxide, from atomic weights
- Stock ratio
- how many times stronger the stock is than the final solution
Worked example
- Nitrogen needed = 200 mg/L × 100 L = 20,000 mg = 20 g.
- Fertiliser = 20 g ÷ 0.155 = 129.03 g.
- For a 100× stock: 1.2903 g/L × 100 = 129.03 g per litre of stock.
To reach 200 ppm nitrogen in 100 L with a 15.5% N fertiliser, dissolve about 129 g.
What the result means
A fertiliser supplies several nutrients at once, so calculate the main one and then check what the others come to (ppm of the other nutrient = grams per litre × its % × 10). Confirm the final solution with an EC meter.
Assumptions and limits
- The fertiliser dissolves fully and the volume change from dissolving it is negligible.
- Nutrients already in the source water are not subtracted.
- Compatibility of fertilisers in one stock tank (calcium with sulphates or phosphates) is not checked.
Questions
Why convert P₂O₅ and K₂O?
Labels in many countries give phosphate and potash as oxides, while recipes give elemental P and K. 1% P₂O₅ is 0.436% P and 1% K₂O is 0.830% K.
What does a 100× stock mean?
The stock is 100 times stronger than the final solution, so an injector set to 1:100 dilutes it to the target.