Compound fertilizers deliver two or more nutrients in a single product, declared as a grade in the fixed order N — P₂O₅ — K₂O, with secondary nutrients such as sulfur appended. NPS [CAS?]-4S therefore denotes 23 % nitrogen, 21 % P₂O₅, no potassium and 4 % sulfur. These are formulations rather than chemical substances: they have no single CAS number, and their constituents are declared individually. The grades in this category are zero-potassium basal products, differing in nitrogen-to-phosphorus ratio and in whether sulfur is supplied at maintenance or corrective levels.
Compound NP / NPS grades
| Product | Formula | CAS |
|---|---|---|
| Np 16 20 0 | — | — |
| Np 20 20 0 | — | — |
| Nps 16 20 0 13 | — | — |
| Nps 20 20 0 13s | — | — |
| Nps 23 21 0 4s | — | — |