NPK fertilisers: mix a bag and work out the percentages sim
Scoop fertiliser salts into a bag and watch the percentages of nitrogen, phosphorus and potassium work themselves out from Mr. Hit the target cards, see what missing nutrients do to seedlings, and treat phosphate rock with acid.
Fertilisers: NPK, percentage composition by mass, phosphate rock, and lab against industrial production · covers the GCSE (chemistry only), IGCSE, O-Level (percentage composition only) topic
What the sim shows
Plants need nitrogen (N) for leaves, phosphorus (P) for roots and potassium (K) for flowers and fruit. NPK fertilisers are mixtures of salts that supply all three: ammonium salts and nitrates for nitrogen, phosphates for phosphorus and potassium salts for potassium. The percentage of an element by mass in a salt is (number of atoms × Ar) / Mr × 100, so ammonium nitrate, NH₄NO₃ (Mr 80), is 28/80 = 35 % nitrogen. In a mixture, add up each element's mass and compare it with the total mass. Real bag labels give phosphorus and potassium as P₂O₅ and K₂O; this page uses the elements themselves. Phosphate rock is insoluble, so it is treated with acid to make soluble phosphates. Ammonium sulfate can be made in the lab by titrating ammonia solution with sulfuric acid and crystallising, or in a factory from ammonia (Haber process) and sulfuric acid (Contact process), continuously and on a large scale. Too much fertiliser washes into rivers. Ar from the data booklet. Model limits: the pots' growth and the river are illustrative, kept simple.
- Shelf: seven fertiliser salts and a filler
- Balance and bag: the NPK label is worked out live
- Pots: all three, no N, no P, no K
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