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Fertiliser – Getting to know what the words on your pack actually mean.

All plants need food – a combination of nutrients which are required for proper growth. Some plants need a lot of fertiliser, and sometimes, like the case of most Australian natives, they need very little at all. Generally, plants will get these nutrients from the soil and air around them, but when they can’t, we need to help them out a bit with fertilisers – either natural or synthetic.

But what is fertiliser made up of? What does the information on the pack actually mean?
Read on as we outline some basic characteristics of common fertilisers and the differences between them.

Image from page 114 of "Lilly's complete annual : bee supplies spray materiels poultry supplies fertilizers seeds" (1915)

Understanding Fertiliser Ingredients

First of all, let’s make sure that you are familiar with the basic characteristics of any fertiliser, whether it’s a bag of chicken manure or a factory produced synthetic solution. You will almost certainly see the three capital letters N, P and K written somewhere on the packet, with a number as a % next to them.

This stands for nitrogen (N), potassium (K) and phosphorus (P) which are your primary nutrients required for plant growth, and what % of the overall product is made up of that nutrient. The % content of each nutrient will determine what the fertiliser will do for your plants, and therefore help you identify the right product for your garden.


What are each of these nutrients needed for?
By reading the packet, we now know how much of each nutrient exists in our fertiliser, but what do each of them do?

Nitrogen is needed for the growth of the leaves and stems. If you want big quick greening of your plants (like a leafy vegetable or your lawn), using a rich nitrogen fertiliser will do this.

Phosphorus makes plant fibres stronger through promoting new growth, boosts blooming, as well as helps unlock other nutrients inside the plant for use.

Potassium is responsible for fruit and seed production and aids in surviving harsh summery conditions when they happen. Think of potassium as an elixir for your plants. Using it just before flowering will help bud development and fruit growth.

Now it’s not a case of using one over the other, but more so finding the right balance between the three. Each of these works best combined with the others.

These three nutrients are the basic food needs for a plant; they are required if you want your plant to grow strong and healthy, and a deficiency in one will seriously hurt your plant.

What are trace elements.

Like we said, NPK are the basics needed for plant growth, and without them, plants will die. But there are also some other elements used by plants which are needed, but in much less quantities than NPK, and these help your plants go from simply surviving, to flourishing!

In my experience, the absolute most common trace element deficiency I come across is a lack of magnesium (mg). It is responsible for creating chlorophyll (where the green colour in plants comes from), which is necessary for photosynthesis in the plant leaf.

Without magnesium, there is less chlorophyll in the leaf, which leads to yellowing around leaf edges and between veins. This is most often seen in citrus and gardenias.

Other trace elements include Iron, Boron, Sulfur, Calcium, and each of these is responsible for some part of the plants functions, so they need to be considered when fertilising. You’ll find specific ‘Trace Element’ fertilisers, but they’re often also sold as specific plant blends, like ‘Citrus Booster’ or ‘ Rose Feeder’.

It’s important that we keep our plants happy, by providing the right mix and levels of nutrients to maximise growth, but don’t go rushing out to the nursery just yet! Much like taking multi-vitamins for your body in place of a healthy diet, it can sometimes be over kill.

Plants will get most of what they need from the space around them, and if we provide good healthy soil with lots of organic matter and plenty of moisture, you’ll find you won’t need those artificial nutrients very often at all.

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Clinton Anderson

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