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Overview of Fertilisers

What are Fertilisers?

Fertiliser through hands

Fertilisers are nutrients added to plants to ensure that crop yields are maximised and plant health is preserved as much as possible. Although nutrients exist naturally in the soil, atmosphere, and in animal manure, they are not always available in the forms that plants can use, or in the quantities needed.

Fertilisers can be classified into two categories: organic or inorganic.

Organic fertilisers are derived from living or once-living material, including animal wastes, crop residues, compost and numerous other by-products of living organisms.

Inorganic fertilisers are derived from non-living sources and include most of our man-made, commercial fertilisers. Man-made and natural fertilisers contain the same elements, but act more quickly.

For commercial purposes, the technical definition of a fertiliser is any natural or manufactured material that contains at least 5% of one or more of the three primary nutrients – nitrogen (N), phosphorous (P), or potassium (K).

Fertiliser production entails gathering raw materials from nature; treating them in order to purify them or increase their concentration; converting them into plant-available forms; and often combining them into products that contain more than one nutrient.

The Role of Nutrients

nutrition role

Soils may be naturally low in nutrients, or they may become deficient due to nutrient removal by crops over the years without replenishment – or when high-yielding varieties are grown that have higher nutrient requirements than local varieties.

There are 2 main types of nutrients

  • Macronutrients are needed by plants in large quantities. The “primary nutrients” are nitrogen, phosphorus, and potassium. Today, sulphur is also considered a key macronutrient. Macronutrients include both primary and secondary nutrients.
  • Micronutrients (or “trace elements”) are required in very small amounts for correct plant growth. They need to be added in small quantities when they are not provided by the soil.

Every plant nutrient, whether required in large or small amounts, has a specific role in plant growth and food production. One nutrient cannot be substituted for another.

Potassium K
Potassium activates more than 60 enzymes, (the chemical substances that govern life and play a vital part in carbohydrate and protein synthesis). It improves a plant’s water regime and increases tolerance to drought, frost and salinity. Plants that are well supplied with potassium are less affected by disease.

Sulphur S
Sulphur is an essential constituent of protein. It is also involved in the formation of chlorophyll. Sulphur is as important in plant growth as phosphorous and magnesium, but its role has often been underestimated.

Magnesium Mg
Magnesium is the central constituent of chlorophyll, the green pigment in leaves that functions as an acceptor of the energy supplied by the sun: 15-20% of the magnesium in a plant is found in the green parts. Magnesium is also involved in enzyme reactions related to energy transfer.

Calcium Ca
Calcium is required for root growth and as a constituent of cell wall materials. Most soils contain sufficient plant-available calcium. Deficiencies may occur in strongly calcium-depleted tropical soils. Calcium is usually applied to limit or reduce soil acidity.

Nitrogen N
Nitrogen is the motor of plant growth. It is taken up from the soil in the form of nitrates or ammonium. As the essential constituent of proteins, nitrogen is involved in all the major processes of plant development and yield formation.

Phosphorus P
Phosphorous performs a key role in the transfer of energy. It is essential for photosynthesis and other chemico-physiological. Phosphorous is indispensable for cell differentiation, as well as for the development of the tissues that form a plant’s growing points. Most natural and agricultural soils are phosphorus deficient. When there are problems with phosphorous fixation, this also limits its availability.

How are Fertilisers Manufactured?

Below is a brief summary of how fertilisers are manufactured

Potassium  used in fertilisers is usually found in various salt forms in deposits derived from evaporated sea water. They occur in beds of sediment in only a few places in the world. Extraction of the potash bearing minerals takes a number of forms. The cheapest, most reliable and safest extraction method is conventional open cut mining which has the significant advantage of nearly 100% resource utilisation, but this is limited to only the shallowest of deposits. Underground mining is typically used for deposits ranging from 400m-1000m and solution mining, where hot water is injected into the deposit to dissolve it before recovering the potash from the subsequent brine in evaporation ponds, is used for deposits that are too deep for conventional underground mining methods. In some cases potash minerals are found dissolved in salt lakes and are extracted though solar evaporation in large swathes of evaporation ponds.

The largest deposit, in Saskatchewan, Canada is 2.7 to 23.5 metres (9 to 77.6 feet) thick and found at depths of over 1000metres (3,200 feet). Solution mining methods are used to extract potash at greater depths. Conventional underground dry-shaft mining methods are used in mines as great as 1100 metres (3500 feet.). The ore is extracted from potash deposits by electrically operated mining machines and conveyed to the surface, where it is crushed. Using a flotation process, salt and clay particles are removed, the brine solution is dried, and the potash is sized by screening. The resultant coarse grade product is then ready for distribution. Fine particles remaining from the screening process are compacted into sheets that are crushed and screened to particle sizes suitable for blending. Processing varies depending on the minerals present and the type of potash product being produced but typically begins with cleaning the extracted material of insoluble clays and deleterious materials such as normal salt (halite or sodium chloride; NaCl) then refining further in various ways, depending on the desired product.

Most of the sulphur used by the fertiliser industry is a by-product of other industrial processes. However, sulphur is also sold in potassium sulphate magnesium sulphate or potassium magnesium sulphate.


78% of the earth’s atmosphere is nitrogen. However, the nitrogen we breathe is in a chemically inert form that plants (except legumes) cannot use. Large amounts of energy are required to convert this nitrogen to a form that can be used by plants. The production of ammonia from atmospheric nitrogen was made possible in the first part of the 20th century by the development of the Haber-Bosch process. The most important nitrogen-based fertilisers are urea and ammonium nitrate.

Phosphorus, in the form of phosphate (a salt of phosphoric acid) is mined from naturally occurring mineral deposits (phosphate rock) that were once sediments at the bottom of ancient seas. Rock phosphate is the raw material used in the manufacture of most commercial phosphate fertilisers. Ground rock phosphate was ounce applied directly to acid soils. However, due to low availability of phosphorous, high transport costs, and low crop responses, very little rock phosphate is currently used in agriculture. Phosphate rock processing consists in the separation of phosphate from the mix of sand, clay and phosphate that makes up the matrix layer.

Most of the sulphur used by the fertiliser industry is a by-product of other industrial processes. However, sulphur is also sold in potassium sulphate magnesium sulphate or potassium magnesium sulphate.

Why do we Need Fertilisers?

Fertilisers increase crop yields

Plants need sun, water and nutrients to grow. The nutrients can be taken from air or soil. If there is an ample supply of nutrients in the soil, crops are more likely to grow well and produce high yields. If even one of the nutrients needed is in short supply, plant growth is limited and crop yields are reduced.

Fertilisers are needed to obtain high yields because they supply crops with the nutrients the soil lacks. By adding fertilisers, crop yields can often be doubled or even tripled. The UN Food and Agriculture Organization (FAO) Fertilizer Programme undertook extensive demonstrations and trials in 40 countries over a period of 25 years. The weighted average increase resulting from the best fertiliser treatment for wheat was about 60%.

Fertilisers contribute to efficient use of land and water and ensure the most effective use of both land and water. Where rainfall is low or crops are irrigated, the yield per unit of water used may be more than doubled and the rooting depth of the crop increased through fertiliser application.


The World’s Growing Need for Fertilisers

The equation for increasing demand for fertilisers in the future is driven by three key variables:

  1. Increasing global population
  2. Reduction in arable land
  3. Changing dietary preferences

The Rising Population

The global population is currently rising at approximately 75 million every year. By 2050, food production will have to increase by almost 50% to satisfy food requirements. To meet this demand, farmers increasingly need to apply more fertilisers to keep pace.

Changing Dietary Preferences

In the context of global development, rising incomes and increased urbanisation means changing dietary preferences. People with rising incomes want to feed their families better diets with high quality fruits and vegetables.

Reduction in Arable Land

As the population continues to increase, the amount of arable land per head of capita will continue to decline. This means crop yields need to improve to maintain food production.



  • DNK presenting at Africa Fertilizer Agribusiness Conference

    Monday, 14 September 2020

  • Colluli Project Update

    Thursday, 03 September 2020