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Macronutrient fertilisers are generally labeled with an
NPK analysis, based on the relative content of the chemical elements
nitrogen (N),
phosphorus (P), and
potassium (K) that are commonly used in
fertilizers. However, numbers used in this labeling scheme do not directly represent the source composition or absolute nutrient content of the fertilizer. The N value is the percentage of elemental nitrogen by weight in the fertilizer.
The values for P and K represent the amount of P2O5and K2O that would be present if all the phosphorus and potassium in the fertilizer appeared in these forms.[3]
For example, the fertilizer
potash is a naturally occurring mineral composed of nearly pure
potassium chloride (
KCl). As such, its composition is 1:1 potassium to chloride or 52% potassium and 48% chlorine
by weight (owing to differences in molecular weight between the elements). Traditional analysis of 100 g of
KCl would yield 60 g K2O. The
percentage yield of K2O from the original 100 g of fertilizer is the number shown on the label. A potash fertilizer would thus be labeled 0-0-60, not 0-0-52.