Nutrient units: N, P2O5, K2O

Fertiliser units denote the oxidised forms conventionally used to quantify the three primary macronutrients contained in fertilisers and taken up by crops. Nitrogen is expressed directly as a percentage of elemental nitrogen (N), whilst phosphorus is converted to diphosphorus pentoxide (P₂O₅) and potassium to potassium oxide (K₂O). This convention, inherited from historical chemical analyses, enables standardisation of declarations on labels, application recommendations and commercial transactions. On a fertiliser bag, the N-P₂O₅-K₂O sequence indicates the legal guarantee of each available nutrient^[1]. Thus, a 12-32-16 grade corresponds to 12% nitrogen, 32% P₂O₅ and 16% K₂O^[1]. Compound fertilisers engrais-npk typically display a total N + P₂O₅ + K₂O between 30% and 60%^[2].

Conversion between elemental and oxidised forms

The shift from one expression to the other rests on conversion factors established by oxide chemistry. To obtain elemental phosphorus content from P₂O₅, multiply by 0.436; conversely, elemental phosphorus converts to P₂O₅ by a factor of 2.29^[3][4]. For potassium, the elemental-to-K₂O factor is 1.20, and the inverse K₂O-to-elemental potassium factor is 0.830^[3][4]. European fertiliser product legislation explicitly incorporates these coefficients in its technical annex, permitting manufacturers to declare contents in elemental form in addition to — or in substitution for — the oxidised form^[4]. Calcium, magnesium and sodium follow a similar logic with respective factors of 0.715 (CaO to Ca), 0.603 (MgO to Mg) and 0.742 (Na₂O to Na)^[5]. Sulphur may be expressed as sulphate (SO₄) with a factor of 0.33 to elemental S, or as sulphur trioxide (SO₃) with a factor of 0.400^[3][5].

Agronomic uses and nutrient balances

N, P₂O₅ and K₂O units structure fertiliser recommendations and crop offtake forecast balances exportations-des-cultures. The FAO reports that average nutrient uptake generally splits into 35% nitrogen, 17% P₂O₅ and 48% K₂O, giving a ratio of 1.0 : 0.5 : 1.4^[6]. Each tonne of nitrogen removed is thus accompanied by 0.5 tonne of P₂O₅ and 1.4 tonne of K₂O on average^[6]. Specific requirements vary considerably by species: sweet potato extracts 51.6 kg/ha nitrogen, 17.2 kg/ha P₂O₅ and 71.0 kg/ha K₂O for a yield of 10 tonnes of tubers per hectare^[7]. Integrated nutrient management programmes in India commonly prescribe 120 kg/ha nitrogen, 60 kg/ha P₂O₅ and 60 kg/ha K₂O for rice in the Upper Ganges plain^[8].

Market and regulation

World production statistics are framed in nutrient tonnes, that is, in pure N, P₂O₅ or K₂O equivalent. In 2021, global fertiliser production reached 259.1 Mt of nutrient, of which Europe accounted for 17.3 Mt^[9]. Nitrogen dominates total consumption with 58% of the total^[10]. In the European Union, imports covered 46% of nitrogen requirements, 58% of P₂O₅ and 45% of K₂O in 2022^[11]. International trade is mostly negotiated in US dollars per tonne of product, yet cost comparisons per fertiliser unit require bringing each material back to its effective N, P₂O₅ or K₂O content tonne-de-produit-et-tonne-d-element. European legislation enshrines the N-P-K order as the mandatory sequence for label declarations^[4]. Minimum thresholds apply: 2.5% N, 2% P₂O₅ or 2% K₂O for solid organo-mineral fertilisers containing a single declared primary nutrient^[5]; 3% N, 1% P₂O₅ or 1% K₂O for solid organic fertilisers with a single primary nutrient^[5]. Regulatory tolerances govern the deviation between declared value and actual content: ±50% relative deviation for P₂O₅ and K₂O, within an absolute limit of 1.0 percentage point^[4].