Fertiliser facts

Allan Haschick provides an introductory look at fertilisers and the two major categories into which they can be divided - macro and micro elements.

We all have to eat at some point - it is the only way we receive nutrients and vital minerals to grow into healthy human beings. Like us, plants require the necessary food to grow and develop - the saying “you are what you eat” is also true for plants. Poor feeding and low nutrient levels can lead to poor or diminished plant growth and little or no flowering and fruit production. Although plants find most of their nutrients in the soil, we cannot always expect the soil to provide all they require for optimal growth, and additional fertilising is required. Fertilisers are broken into two major categories: macro elements and micro elements.

fertiliser_facts_3Macro elements

These are the main elements required for every basic function of the plant. Most of them are readily available to plants, but sometimes need to be supplemented. These main elements are:

* Oxygen and hydrogen, most commonly found in water.
* Nitrogen and carbon, which are found in the air and also in the soil.
* Potassium and phosphorus, which are found in the soil.

Carbon, oxygen and hydrogen are necessary for building the basic cell structure. As these are the most commonly found elements, we seldom need to provide extra of these for the plants to perform. It is the other three elements - nitrogen, phosphorus and potassium - that are most commonly lacking, and which are most commonly found in most basic fertilisers.

Fertiliser ratio

You will find a fertiliser ratio on any bag, bottle or container of fertiliser available on the market today (this is a legal requirement). This ratio is actually a very helpful tool in understanding what the fertiliser actually does for the plant. To the ordinary person, it may mean nothing at all, but it is actually very simple to understand. A typical example of fertiliser ratio is 3:1:5 (26) SR.


The first number in the ratio above (3) represents nitrogen. This element is needed for the production of the green parts of the plant. The '3' means that 3 parts of the fertiliser is nitrogen.


The second number of the ratio (1) is phosphorus. This element is necessary for root growth and production. The '1' means that there is 1 part phosphorus to 3 parts nitrogen.


The third number in the ratio (5) represents potassium. This element controls the production of flowers and fruit.  The '5' means there are 5 parts potassium to parts to 3 parts of nitrogen and 1 part phosphorus.



The fourth number always found in brackets (26) is the percentage value of the whole product that is pure fertiliser in the said ratio, that is, how concentrated the fertiliser is. In the above ratio, 26% is fertiliser with the remaining 74% being the filler. Most fertiliser is often too strongly concentrated to distribute evenly in the garden, so manufacturers blend the fertilisers with a carrier to allow for more even distribution. So this figure represents how concentrated the fertiliser is. Therefore, 3:1:5 (22) is slightly weaker than 3:1:5 (26).


The letters found after some fertiliser ratios tell you what type of fertiliser it is. In this example, SR stands for Slow Release. This fertiliser is not instantly available to the plant, but is released in even, regular doses to the plant. It lasts longer than other regular fertilisers with the same ratio and needs application at intervals further apart.

Micro elements

These are the elements that are needed in smaller quantities than those of the macro elements. They are often nicknamed the 'multivitamins' for plants. Certain soils, incorrect pH and water quality influence the availability of these elements, sometimes resulting in deficiencies. Often these deficiencies are shown in discoloured foliage, poor leaf maturation or poor fruiting. A long list includes iron, aluminium, manganese, magnesium, copper, boron and more. Deficiencies can often be corrected by dosing the 'patient' plant with trace element solution or fertilisers that contain micro elements.

Organic versus inorganic

All plants will utilise food from any source, be it organic or inorganic. Inorganic fertilisers (chemical) have a set or controlled quantity of elements. This means that we have more control over what our plants will absorb, and thus have more control over how they will perform. For example, a high nitrogen fertiliser promotes taller, bushier, green growth.

Organic fertilisers often have fertiliser ratios, but often they are not as concentrated, so more has to be applied to get the same result. But, organic fertilisers often include micro elements, which result in a more balanced diet for plants. Regular use of organic fertilisers will result in more healthy plants overall.

Did you know

Organics such as soils, composts and manures can yield a fertiliser ratio if they are tested? The concentration of these elements depends on their composition.

fertiliser_facts_4Fertiliser jargon


Nitrogen is a very strong fertiliser and needs to be dissolved in plenty of water for the plant to use it. When not enough water is available after the fertiliser is applied, the fertiliser will absorb water from the plant cells to dissolve the nitrogen. This results in brown leaf edges and wilted soft shoots, or, in severe cases, whole sections of the plant drying out or dying. Remember to always water in fertiliser straight after application.


When there is a shortage of certain nutrients in the soil, plants will show symptoms of deficiency. Poor growth, mottled leaves and deformed flowers and fruit are often symptoms of nutrient shortages. Most deficiencies can be cured with the use of trace element fertiliser.

Ratio rhyme

This easy rhyme will help you remember what the fertiliser ratio stands for:

  • Leaves and shoots.
  • Stems and roots.
  • Flowers and fruits
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