N-P-K grade

The N-P-K formula denotes the mass composition of primary nutrients in a fertiliser, in the fixed order nitrogen (N) — phosphorus as phosphorus pentoxide (P₂O₅) — potassium as potassium oxide (K₂O). A 12-32-16 designation equates to 12% N, 32% P₂O₅ and 16% potash (K₂O)^[1]. The European regulation on fertilising products stipulates this N-P-K sequence for labelling organic and organo-mineral fertilisers, with secondary elements designated by their symbols Ca-Mg-Na-S and their contents expressed as oxides in brackets^[2].

The formula may incorporate secondary elements and trace elements shown in brackets: 20-6-12(3-20) 0.1B indicates 3% MgO, 20% SO₃ and 0.1% boron^[3]. The combined N + P₂O₅ + K₂O total for compound fertilisers typically falls between 30% and 60%^[3]. This coding applies to binary compound fertilisers (NP, NK, PK) as well as ternary (NPK)^[3].