Fertilisation: basic principles that every farmer should know

Achieving optimal yields is greatly dependent on providing crops with the right nutrition at the right time. Understanding the role of fertilisers and ensuring the correct application can make the difference between profit and loss.

Fertilisation: basic principles that every farmer should know
Samples for soil analysis should be taken from various locations in the land.
Photo: Lindi Botha
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Fertilisation is an exact science, and no two lands can be treated the same. It is therefore paramount that a farmer understands the different types of fertiliser, application levels, and timing of application before embarking on a fertiliser programme.

Crops continuously extract nutrients from the soil, and over time this can lead to severe depletion of soil fertility and land degradation.

This in turn destroys the productive ‘capital’ of the soil and reduces the farm’s ability to produce a crop. If optimal yields cannot be achieved, it becomes uneconomical to continue buying and planting seed.

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The right mix
For optimal fertiliser usage, the soil type, nutrient levels and yield potential of the crop and area must first be determined. Soil that is well looked after normally has the capacity to provide most nutrients needed, and shortages can be overcome by using carefully chosen fertilisers.

It is wasteful to apply a nutrient if there is enough of it in the soil.

In general, macroelements are the nutrients most likely to be added to the soil. Micro- and secondary elements are also applied in small doses, as they play a crucial role in assisting plants to absorb macroelements.

The most important elements can be summarised as follows:


  • Nitrogen (N) improves overall crop quality. It helps a plant develop chlorophyll, the green pigment that enables the plant to absorb energy from sunlight to fuel its activities. Nitrogen is also absorbed from the soil and used by the plant to form proteins for growth. Simply put, N increases the number of plant cells, branches, leaves, seeds and fruit. A shortage of N results in yellow leaves and poor plant growth.
  • Phosphorous (P) in the soil leads to better root development, and helps with grain and seed development. Plants are assisted to ripen early and mature quickly.
  • Potassium (K) in the soil improves crop yield and quality, strengthens the plant, and helps it resist drought and disease. Potassium also helps the plant breathe and plays a major role in its use of water and its build-up of starches, sugars, fats and protein.

Secondary elements:

  • Calcium (Ca) forms the ‘building blocks’ in plant cells that ensure firmness, shelf life and quality produce. It strengthens the plant and reduces or neutralises toxicity in the soil.Magnesium (Mg) helps a plant breathe and aids in its absorption of phosphorous. It is a vital element in photosynthesis. However, too much will slow down the plant’s ability to absorb potassium.
  • Sulphur (S) is essential for N uptake. It is a key factor in leaf development and increases the quality of grain and fruit. It is also responsible for the intense flavour of crops such as onions and garlic.


  • Boron (B) is critical to the health of the plant. It is essential for cell division and the creation of new plant cells.
  • Although zinc (Zn) is needed only in small amounts, it is crucial to plant development as it plays a key role in growth hormone production.

Lime is an inorganic (non- living) mineral that helps reduce acidity in the soil. Too much acid decreases the availability of P, inhibits the uptake of water and fertiliser, makes herbicides less effective, and suppresses the effectiveness of microorganisms in the soil.

If conditions are highly acidic, it may be more economical and beneficial to lime rather than increasing fertiliser application.

Speciality fertilisers
Hugo Opperman, head chemist at Microbial Biological Fertilizers International (MBFi), notes that due to diminishing agricultural land and a growing population, there is a need for more specialised agricultural practices to get the most out every piece of land.

“This has resulted in a rise in the use of speciality fertilisers, which can significantly increase crop yields by using a more targeted approach.”

Opperman says that the biostimulants market has grown exponentially over the past few years, with an estimated global market value of over US$2 billion (about R26 billion) in 2018.

Speciality fertilisers optimise specific growth stages, metabolic functions and the ability of a plant to cope with certain stress conditions. The three main classes of speciality fertilisers include:

  • Nutrient speciality fertilisers.These are versatile and can be applied either via seed treatments, preplanting or foliar fertiliser.
  • Biostimulants. These are substances or microorganisms applied to seeds, plants or the soil to enhance nutrition efficiency, crop quality traits, and abiotic stress tolerance to guard against, for example, drought, waterlogging or extreme temperature. Biostimulants can be applied regardless of the soil’s nutrient content. One of the most popular growth-enhancing biostimulants is seaweed extract; it is also effective in relieving plant stress, as it contains a high level of antioxidants.
  • Biological fertilisers. These contain bacteria or fungi, and act upon plant growth through symbiosis. They improve plant growth by improving soil structure, increasing nutrient availability and uptake, and by producing compounds that stimulate growth, disease resistance and control of pathogenic organisms and pests. The most widely used organisms in this group are nitrogen-fixing rhizobium bacteria.

How much fertiliser?
Fertiliser applications depend largely on soil type, nutrient levels, and the requirements of the crop. For this reason, it is best to have soil and leaf analyses done to determine
any shortages. The analyses must be soil- and crop-specific and taken from several locations to get a true picture.

Once a fertiliser programme has been worked out for the farm, it is important to keep to the application schedule. If the fertiliser is applied at the wrong time, it will be ineffective.

It is usual to apply a basal dressing at planting, and a top dressing before the plant flowers. The following are best practices in fertiliser application, according to the International Fertilizer Industry Association:

  • Fertiliser requirement: Over- or under-fertilising can both affect your profit. Too much will increase your costs and have an adverse effect on the environment; too little will decrease the yield and thus income.
  • Biological life: Do not neglect the biological life in the soil; a healthy micro-organism population makes more plant nutrients available.
  • Product choice: Compare the different products. Products that are more concentrated can reduce costs (there is less to transport), but normally do not contain the same quantity of secondary elements.
  • Application: Although band placement of fertiliser is generally the most effective method, broadcast application and foliar sprays have their place. How you apply fertiliser will affect its efficiency.
  • Precision farming: This ensures that the entire land is fertilised according to the soil analysis and expected yield. Money is saved, yields are increased, and risk is reduced.
  • Weeds; Weeds absorb water and plant nutrients, so it is important to eradicate them. Proper soil cultivation enables more water and fertiliser to infiltrate the soil.
  • Plant sap analysis: Take regular plant sap samples to help you address nutrient deficiencies in time. A sap analysis will also help you avoid applying fertiliser unnecessarily.

How much is too much?
While correct fertiliser applications can significantly increase crop yield, there is a maximum obtainable amount of crop produced for any given amount of fertiliser and other farm inputs used.

It is therefore important to establish what the maximum amount of fertiliser requirement would be and the level of profitability.

Dr Pieter Haumann, CEO of the Fertiliser Association of South Africa, says that at high levels of input use, the nutrients applied to the soil are not taken up completely by the growing crop even under the best conditions.

“While analysing the economics of fertiliser use, the principal considerations are the production increase attributed to fertiliser, and the relationships between the cost of fertilisers and the price of produce.

“At some point, an increase in fertiliser will cost more money, but won’t result in a greater yield.”

The environment
Over-fertilising has a negative impact on the environment. “Nitrate and, to a lesser extent, sulphate and boron, are not held strongly by the soil and can leach down with percolating waters and contribute to undesirable water enrichment,” says Haumann.

“Phosphate generally moves only a short distance from the application site, mainly through soil erosion or surface run-off. Over years, phosphate applied through fertilisers or organic manures can move to deeper layers of coarse-textured soils in high-rainfall areas.

“If it exits the soil profile and moves into water bodies, its concentration increases and it can lead to excessive growth of algae and result in eutrophication, to the detriment of other organisms.”

Email Hugo Opperman at [email protected], or Dr Pieter Haumann at [email protected]