Fertiliser Blending Calculator
Works out the mix of three fertilisers that makes a chosen grade.
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The formula
solve: Σ(amount × ingredient grade) = batch × target grade, for each of N, P₂O₅ and K₂O filler = batch − Σ amounts
Three nutrients and three ingredients give three equations with one solution. The starting ingredients are urea, DAP and muriate of potash, whose standard grades are 46-0-0, 18-46-0 and 0-0-60.
What to watch out for
DAP supplies both phosphate and nitrogen, so the nitrogen source only makes up what is still short, which is why the amounts are solved together. Filler is any inert carrier, such as sand. Your supplier can say what suits the blend. It can be left out if you adjust your spreading rate instead.
Frequently Asked Questions
How do I make 15-15-15 from urea, DAP and potash?
Per tonne: 198 kg urea, 326 kg DAP, 250 kg muriate of potash and 225 kg filler.
Why does it say the target cannot be reached?
A negative amount has come out. For example, DAP has more N relative to P₂O₅ than a low-nitrogen target allows, and no ingredient can remove nitrogen.