What is NPK?

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What do the numbers on NPK fertiliser mean?

When looking at fertilisers, you may be faced with a series of three numbers. For example, our General Purpose (17-9-9) and our Balanced (11-11-11) fertilisers. The numbers represent the value of three macronutrients used by plants – Nitrogen (N), phosphorus (P) and potassium (K); also known as NPK. Nitrogen is expressed as N, phosphorus as P2O5 and potassium as K2O. So our General Purpose (17-9-9) fertiliser has a ratio of 17% nitrogen, 9% phosphorus and 9% potassium.

Why is NPK important?

All plants need nitrogen, phosphorus and potassium to allow them to grow. Each one of these nutrients does something a little bit different for the plant. And without enough of any of these nutrients, the plant will not grow healthily and risks dying.

Nitrogen (N)

Nitrogen plays a big role in helping plants to photosynthesize – allowing plants to convert energy from the sun into food for them to use. Nitrogen is responsible for the growth of green leaves you see on the plant.

Phosphorus (P)

Phosphorus mainly stimulates root development, which is necessary for the plant to gather nutrients from the soil as well as supporting the plant’s weight. Phosphorus also aids flowering and fruit development and fuels the energy processes in the plant.

Potassium (K)

Potassium helps with the movement of water, nutrients and carbohydrates throughout the plant. It also helps the overall functions of the plant perform optimally. It aids in photosynthesis by regulating CO2 uptake, as well as activating enzymes for the production of Adenosine Triphosphate (ATP). ATP is an important energy source for many chemical processes that occur within plants.

Perhaps the greatest role of potassium is in maintaining a healthy water balance in the plant.

Knowing and understanding the NPK values of a fertiliser can help you make a decision on which type of fertiliser would be best for the type of plant you’re growing. If you’re growing leafy vegetables it may be beneficial to apply a fertiliser which is high in nitrogen, as nitrogen promotes growth of leaves. A fertiliser high in Phosphorus is important for establishment of a strong root system and products higher in potassium are important for flowering and for crops with high amounts of water uptake, such as potatoes.