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 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.
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:
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.
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:
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:
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.”
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.”
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