What do NPK numbers mean? Fertiliser labels explained simply

Fertiliser labels look like algebra, but there are only three letters that matter. Once you can read them, choosing a feed for any plant in the garden takes about five seconds.

What each letter does

• N is for nitrogen: the leaf builder. Nitrogen is the engine of green growth. Lawns, hedges, leafy veggies like spinach and lettuce, and any young plant building its frame want plenty of it. Too little and leaves go pale and growth stalls. Too much on a flowering plant and you get a lush green bush with no flowers.
• P is for phosphorus: roots and flowers. Phosphorus powers root development and the switch into budding and blooming. It is why bloom feeds carry more of it. One Australian caveat: many natives (grevilleas, banksias, proteas) evolved in low-phosphorus soils and prefer gentle, dilute feeding rather than heavy phosphorus doses.
• K is for potassium: fruit, flowers and toughness. Potassium moves sugars around the plant, sizes up fruit, deepens flower colour and hardens plants against heat, cold and disease. Tomatoes, citrus and roses are all heavy potassium users.

Reading the numbers

A label reading 10-4-6 is 10% nitrogen, 4% phosphorus and 6% potassium by weight, always in that order. The ratio matters more than the size of the numbers: a nitrogen-heavy ratio is a grow feed, a phosphorus and potassium-heavy ratio is a bloom feed. Concentrates have smaller numbers than granular products because they are diluted in water, so never compare a liquid label to a granular label digit for digit.

Matching NPK to the phase, not just the plant

The same tomato plant wants different food in October than in December. While it is building leaves and a root system, it wants nitrogen. The moment flowers appear, its needs flip toward phosphorus and potassium. That is why we make the garden a two-feed system: Medigrow is the nitrogen-led phase 1 feed for growth, lawns and leaves, and Medibud is the phosphorus and potassium-led phase 2 feed for flowers, fruit and yield. Knowing when to switch between them is most of the skill of feeding.

Common questions

What about all the other nutrients?

Plants also use calcium, magnesium, iron, zinc and other trace minerals in small amounts. Good feeds include them; Medigrow, for instance, carries calcium, magnesium, iron, zinc, copper, manganese, molybdenum and boron alongside its nitrogen.

Is a higher number always better?

No. Concentration only tells you how much to dilute. What the plant experiences is the mixed strength at the roots or leaf, and dilute-and-often beats strong-and-rare every time.

Can the wrong NPK hurt my plants?

The classic mistake is high nitrogen on flowering plants: gorgeous leaves, no blooms. Match the ratio to the phase and the problem disappears.