Choosing The Right Fertiliser

Shoots, Roots, Fruits = NPK. Say it out loud... and repeat. This is your gardening mantra to understand what helps plants grow. If you have ever grown huge tomato vines with very few fruit or planted flowers and ended up with plain green bushes, this mantra may help you master your garden.

In this article, we’ll explain what NPK fertiliser is, why it’s important, and help you understand what helps plants grow in Australia. Let’s kick things off with a definition.

What Does NPK Stand For?

There are three main nutrients your plants need to grow and fruit, they are Nitrogen (N), Phosphorus (P) and Potassium (K), which is what NPK stands for.

What is NPK Fertiliser and What Does it Do?

Your plants need different ratios of these nutrients based on what you want them to do:

• If you want vegetation growth (shoots), you need Nitrogen which promotes leaf growth.
• If you want to fuel root growth, Phosphorus is what you’re after.
• To promote good fruit, vegetable and flower growth, you need Potassium.

In theory, soil should contain all the nutrients your plants need to thrive but in reality, differences in pH, soil type, and previous use of the soil cause deficiencies, which is where fertiliser comes in handy. Packaged fertilisers outline their NPK ration on the pack, but making sense of what’s best for your needs can be difficult.

Different Fertilisers for Your Garden

Let’s take a look at a general-purpose liquid fertiliser: 12: 1.4: 7.

This means there is 12% Nitrogen (N), 1.4% Phosphorus (P) and 7% Potassium (K) and a total of 20.4% in the mix. The remainder of the solution is most likely made of trace elements and other beneficial soil conditioning elements.

Don’t be fooled into thinking bigger numbers mean a better fertiliser though because when it comes to fertiliser, there is such a thing as too much of a good thing. Pumping nitrogen into your soil may result in plants growing leggy and weak rather than standard-sized and strong.

Soil pH

Adding nutrients to soil isn’t done in isolation, soil pH influences how nutrients can be absorbed by plants.

See below for a table displaying the availability of the key nutrients by pH. The thicker the bar, the more available the nutrient is at that pH. A soil pH between 6 and 7.5 is optimal for most plants to thrive, but there are always exceptions, so check what your specific plant variety prefers. Before adding anything to your soil we suggest testing your pH so you know what you're working with.

Information about Australian Natives

Australian native seeds, for the most part, prefer well-draining acidic soils and don't like fertilisers high in Phosphorus. We recommend a specific native fertiliser for natives.

What Helps Plants Grow: A Summary

N = Nitrogen = Shoots (Vegetation, Leaves)

P = Phosphorus = Roots

K = Potassium = Fruits (Fruit, Vegetables & Flowers)

Know your soil pH before you add anything to the soil.
Know what pH and nutrients your specific plant varieties prefer so you can select an appropriate fertiliser.

Your local garden centre should be able to assist you with selecting an appropriate fertiliser for your specific needs.

Further Gardening Advice from Mr Fothergill’s Seeds

For a deeper dive into the technicalities of growing in your own home garden, check out our garden advice blog.