Understanding N-P-K

The three numbers

Every fertiliser bag carries the same three numbers, always in the same order: nitrogen, phosphorus, potassium. A bag marked 15-5-10 is 15% nitrogen, 5% phosphorus and 10% potassium by weight. The rest is filler and trace minerals.

What each one does

Nitrogen (N) drives leaf growth, colour and density. It's the nutrient couch grass responds to fastest, for better and worse: too much nitrogen through a Queensland summer produces a soft, lush flush that mows poorly and invites disease.

Phosphorus (P) supports root development. It matters most at establishment and after renovation. On a well-established court it needs far less attention than nitrogen; turf researchers at the University of Florida's IFAS Extension note that mature warm-season turf often tests high enough in phosphorus already that no extra is needed.

Potassium (K), sometimes called the "toughening" nutrient, builds drought resistance, wear hardiness and disease resistance. It's the one worth banking before the stress of a dry Queensland summer, which is why it gets emphasis in the autumn application.

Why the ratio changes through the year

A spring application leans on nitrogen to trigger recovery growth after dormancy. An autumn application leans on potassium to build resilience before the grass slows down again. Incitec Pivot's turf maintenance guidance for Queensland golf courses follows the same pattern, naming three separate applications across the year rather than one all-purpose mix, because nutrient needs genuinely shift with the seasons.

See also

• Getting the Lawn Through a Hot, Dry Summer
• Soil Testing