NPK Explained

by Corinne Mossati

In a previous article, I explained how to amend soil at the start of the growing season for leafy plants, root vegetables, fruiting plants and Australian natives. In this article, I explain what NPK stands for and the role of each nutrient.

NPK Explained

NPK – Photo © The Gourmantic Garden

What is NPK?

In short, NPK stands for nitrogen, phosphorus and potassium, three essential nutrients that are needed for plant growth and overall plant health. You’ll find this on the labels of fertilisers along with a number or a percentage which refers to the ratio of each nutrient in fertilisers, for example, NPK 19: 2 : 7.

NPK: N for Nitrogen

Nitrogen (N) is responsible for leafy growth. A fertiliser with a high nitrogen ratio is crucial for leafy greens such as lettuce, chard, celtuce, Asian greens such as bok choy and herbs. If you’re growing root vegetables such as beetroot, carrots and radishes, avoid using a fertiliser with a nigh nitrogen content. Why? Have you ever grown carrots only to get a healthy mass of carrot tops and a thin spindly vegetable? A fertiliser that is high in nitrogen could be the cause. Too much nitrogen goes into the production of leafy tops at the expense of the root while fruiting and flowering plants can lose their blooms and fruit. Legumes, such as peas and broad beans which have nodules on their roots don’t need a lot of nitrogen as they are nitrogen-fixing plants.

Sources of nitrogen include composted manures, green manure cover crop, nitrogen-fixing plants such as beans and other legumes, and liquid fertilisers with ammonium, nitrate or urea.

To tell if plants have nitrogen deficiency, look for signs such as yellow or discoloured leaves, weak stems, lower quality fruit and smaller flowers.

NPK: P for Phosphorus

Phosphorus (P) is an important nutrient responsible for strong root growth. It also aids the plants in nutrient uptake. Sources of phosphorus include blood and bone and fertilisers with phosphate. As a general rule, when choosing a fertiliser for Australian native plants, avoid one that is high in phosphorus.

For signs of phosphorus deficiency, look for purple colouration on leaves, low fruit yield and stunted growth.

NPK: K for Potassium

Potassium (K) is another essential nutrient that gives energy to the plant and is important for flower and fruit production. Sources of potassium include potash and wood ash.

Signs of potassium deficiency include chlorosis, a condition where the leaves start to turn yellow. Initially, the space between the leaf veins turns yellow or white, and then slowly spreads across the entire leaf. The condition affects older leaves first and as it spreads, younger leaves turn a shade of yellow.

Now that you know what NPK is and what each nutrient is responsible for, always check the NPK ratio of fertilisers and potting mix and use the right ones for your plants.

Want to Know More?

You’ll find more information on gardening basics and common gardening terminology in my 260+ page digital book GROW YOUR OWN COCKTAIL GARDEN available now.


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