The formula
kg of compound = need ÷ grade × 100, taking the smallest of the three
Where it comes from
A compound carries the three nutrients in a fixed ratio, and a crop’s needs are rarely in that same ratio. So the compound cannot cover all three at once: it either falls short on one or goes over on another. Going over is not harmless. Excess nitrogen leaches towards the aquifer and is money buried; excess phosphorus locks up in the soil and nobody gets it. So the rule is to apply the rate that goes over on none of them — the smallest of the three — and top up the rest with straights, which carry one nutrient and can be dosed separately.
How to work it out by hand
- For each nutrient, divide what the crop needs by the grade of the compound and multiply by a hundred
- Take the smallest of the three figures: that is the compound rate
- Work out what that rate supplies of each nutrient and subtract it from the need
- Cover the gap with urea, superphosphate or muriate of potash, each by its own grade
What is worth knowing
The crop’s needs do not come from this calculator: they come from removal per tonne of harvest and from a soil test, which says what is already there. A soil rich in potassium needs no potassium even if the crop removes it. And phosphorus and potassium are declared as oxides — P₂O₅ and K₂O — because that is how bags are labelled; if the test comes as the element, convert first.