Table of Contents


See also the “Soils”, “Compost and organic fertilisers”, “Earthworms and vermicompost” and “Speciality fertilisers” chapters.


1. Overview

Fertilisation is a method of improving the nutritional status of the soils, and can be tailored to provide the correct nutritional requirements at the most appropriate time.

Fertilisers are food for plants; they contain plant nutrients (nourishing substances), which all plants need to grow and stay healthy.

There are 13 elements which plants take from the soil. Nitrogen (N), phosphorus (P), and potassium (K) are used in relatively large amounts. Sulphur (S), calcium (Ca), carbon (C) and magnesium (Mg) are also often required. Zinc (Zn), iron (Fe), manganese (Mn), Boron (B), copper (Cu) and molybdenum (Mo) are other elements.

The soil provides most of the nutrients needed, and shortages can be overcome by using carefully chosen fertilisers. It would be wasteful to apply a nutrient if your soil already has sufficient.

In general, macro elements such as nitrogen (N), phosphate (P) and potassium (K) are the nutrients most likely added to the soil.

Photo used courtesy of Helen Gordon, WWF SA


Nitrogen (N) Applying nitrogen (N) improves overall crop quality. It helps plants develop the green colour they need to take food from the sun, and to take the necessary food from the soil. N increases the number of branches, leaves, seeds and fruits, and accelerates the number of plant cells in the plant. A shortage of nitrogen will result in yellow leaves and poor plant growth.
Phosphorous (P)
Adding phosphorous (P) to the soil leads to better root development, helps with grain and seed development. Plants are assisted to ripen early and mature quickly.
Potassium (K) The addition of potassium (K) to the soil improves crop yields and quality, strengthens plants and helps them resist drought and disease. It helps the plant to breathe and plays a major role in the plant’s use of water (stomata) and its build up of starches, sugars, fats and protein.

Although NPK volumes are the highest, it is of utmost importance to emphasise the necessity of all other elements too.

Magnesium (Mg) Magnesium (Mg) helps plants to breathe and phosphorous to get into the plant. Mg is a vital element in photosynthesis. Too much will slow down the plant’s ability to absorb potassium.
Calcium (Ca) Calcium (Ca) forms the ‘building blocks’ in plant cells which ensure firmness, shelf life and quality produce. Calcium (Ca) strengthens plants and reduces/neutralises toxicity in the soil.
Sulphur (S) Sulphur (S) is essential for uptake of Nitrogen (N). S helps in leaf development and increases the quality of grain and fruit. It is responsible for flavour attributes in crops such as onions and garlic.
Zinc (Zn) A key constituent of many enzymes and proteins


“Straight” and “multi-nutrient” fertilisers

Fertilisers are either “straight” or “multi-nutrient”. Straight fertilisers are products containing one of the main plant nutrients. Some examples are given below:

Plant nutrient  Fertiliser
Nitrogen (N)


Calcium ammonium nitrate (CAN)

Limestone ammonium nitrate (LAN )

Phosphate (P)

Single superphosphate (SSP)

Triple superphosphate (TSP)

Potassium (K) Muriate of potash (MOP)

P and K are shortened forms: P = P2O5 x 0.44 and K = K2O x 0.83