The important things in fertilizer are:
When fertilizers are offered for sale, the percentage of N, P, and K must be written on the bags or boxes, but for historcal reasons, P is shown as %P2O5 and K is shown as %K2O.
E.g.:9-23-30 which means: 9% N, 23%P2O5 and 30%K2O.
In Australia the pecent of elemental sulfur must also be shown. In the UK, the elemental composition (in percentages) may also be shown alongside the mandatory traditional system, provided the numbers are put inside square brackets.
A soil test can tell how much N, P, and K is needed.