Problem Solver

> View All Articles

What is NPK ratio mean?


August 29, 2022


Searles Gardening Team

NPK on the labels of fertilisers refers to the macronutrients or the major nutrients fertilisers contain - Nitrogen, Phosphorus and Potassium.

All fertilisers must state the NPK analysis on the bag as a minimum requirement, the trick is understanding what the numbers mean for fertilising plants. The main feature of nitrogen (N) is for growth and greening, phosphorus (P) is for root development, and potassium (K) is for flowering and fruit. A plant in good health can withstand pest and disease attacks better and so a balanced NPK is the basis of the building blocks of overall plant health.

A plant basically can do only one thing well at a time, you don’t often see a plant flowering and fruiting heavily while growing at a massive rate. If a plant is low in nitrogen, it goes yellow and has little to no growth.  A plant low in potassium struggles to flower and then fruit to its full potential.  Roots hold the entire plant firmly upright and allow for the absorption of water and nutrients from the soil with sufficient phosphorus.

Look for a fertiliser where flowering is about to happen, it is high in potassium (K) such as Hibiscus and Bougainvillea Fertiliser - NPK 13.5 : 3.4 : 12.5. For lush green lawns, then Robust Lawns Booster, NPK: 22 : 1.3 : 5.8, with a high nitrogen count is ideal. You also don’t want to push the plant all year with one aim, that would be like you having the same dinner every night, we all need different nutrients to be at an optimal health. Native plants traditionally prefer low phosphorous levels, like Australian soils usually have. Native plant fertilisers and potting mixes have very low phosphorous for this reason or the plants would possibly perish with high levels, Robust Native fertiliser has NPK: 16 : 0.85 : 7.5 or Native Food with NPK: 9.5 : 1 : 8.5.

In overseas writing, NPK is stated differently to Australian NPK ratios. When researching the best NPK for your plant, ensure you are reading written material designed for your country.

Always look at NPK and the times it says to fertilise to determine if a product is a fertiliser or not.  Choose fertilisers high in potash during flowering and fruiting season, low in phosphorous for Australian natives and high in nitrogen for green and growth.  

Trace elements or nutrients in minute quantities, that’s for another day.