Free NPK Requirement calculator with clear step-by-step results.
Compound fertilisers never match a crop's nutrient ratio exactly. Setting the rate to satisfy nitrogen - usually the limiting and most valuable nutrient - then reports what phosphate and potash that rate happens to supply and what remains to be made up with straight products.
Compound rate
kg/ha = nitrogen target / nitrogen percentage of the product
Nutrient supplied
Supplied = compound rate x nutrient percentage for each of P and K
Nutrient application limits, closed periods and buffer zones near water are legally enforced in many regions. Base rates on a current soil analysis and comply with any nutrient management regulation.
Nitrogen drives yield response most directly and cannot be stored in soil between seasons. Phosphate and potash are buffered by soil reserves, so a small shortfall is less immediately damaging.
The shortfall shows zero and you are applying more than the target. Repeated over-application of phosphate is an environmental and regulatory concern - pick a lower-P grade.