Labeling of fertilizer

The labeling of fertilizers varies by country in terms of analysis methodology, nutrient labeling, and minimum nutrient requirements. The most common labeling convention shows the amounts of nitrogen, phosphorus, and potassium in the fertilizer.

Labeling of macronutrient fertilizers

Macronutrient fertilizers 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 value for P is the fraction by weight of P₂O₅ in a fertilizer with the same amount of phosphorus that gets all of its phosphorus from P₂O₅. The value for K is analogous, based on a fertilizer with K₂O.

For example, the fertilizer potash is a naturally occurring mineral consisting of nearly pure potassium chloride (KCl). As such, it contains one potassium atom for every chlorine atom, and is 52% potassium and 48% chlorine by weight (because of the difference in atomic weights of the elements). K₂O is similarly 83% potassium. To have 52% potassium, therefore, a fertilizer that gets all its potassium from K₂O would have to be 63% K₂O (.52/.83 is .63). Pure KCl fertilizer would thus be labeled 0-0-63; because potash is less than pure (it contains other compounds that contain no potassium), potash is labeled 0-0-60.