Treating acidic soils

Most crops yield better in soil that is not too acidic.

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Acid soils are generally unproductive and need to be improved by neutralising – or counteracting – the acidity with lime. To measure the acidity of your soil, you need to have the soil pH tested. Consult with your agricultural extension officer about this or speak to your fertiliser supplier who will generally do the test free of charge. The pH is measured on a logarithmic scale of 0 to 14, where 7 is ‘neutral’, 7 to 14 is alkaline (‘sweet’) and 0 to 7 is acidic. A logarithmic scale means that a pH of 5 is 10 times more acidic than a pH of 6 and 100 times more acidic than a pH of 7.

When plant nutrients such as calcium, magnesium and potassium are replaced by hydrogen and aluminium, the soil becomes acidic. This ‘replacement’ is often caused by leaching after heavy rain. Acid soil is bad for crops because:

  • Plants can’t easily absorb natural or applied phosphates and molybdenum.
  • It hampers bacterial and biological activity in the soil, which feeds the plants.
  • It could lead to nitrogen, phosphate and sulphur deficiencies, while there’s so much manganese, aluminium, iron, copper and zinc that the plant could be poisoned.
  • Fertiliser applications aren’t effective and this leads to low yields.

Most crops grow better in soil with a pH of 6,5 to 6,8, although some, such as potatoes, can handle a pH of 6 or even 5,5.
However, acidity usually becomes a problem as soon as the pH is about 5,5, and successful crop production is impossible in the long term without lime (see Table 1).

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In the case of heavy soils, apply once every three to four years, on average. The effect of the lime will only really become apparent two to three years after application. Test the pH again after three years to see if more lime is needed. Smaller quantities of lime applied more often will eliminate the risk of too much lime in sandy soil. Ideally, lime should be applied before ploughing, at least six to eight weeks prior to planting or sowing. Unlike fertiliser, it reacts slowly in the soil. Ploughing in is preferable, as thorough mixing to a ploughing depth is essential. Also, lime worked in with a disk plough is often better utilised in the first year.

In crop production, agricultural and dolomitic lime are normally used. Slaked lime or building lime can be cheaper, but is caustic and absorbs moisture, which makes handling it unpleasant and could delay germination. Agricultural and dolomitic lime are concentrated and smaller quantities can be applied. Agricultural lime is calcium carbonate, while dolomitic lime is a mix of calcium and magnesium carbonate. It should be used on acidic soils where magnesium deficiencies occur.

Dolomitic lime is slightly less soluble than agricultural lime, but its neutralising effect is about 10% more effective. This means the quantity used can be reduced by 10% to achieve similar results. Agricultural lime with a minimum carbonate content of 80% should be used.

Sources: The Directorate Communication, department of agriculture, and the Fertiliser Society of South Africa. For more information, contact the society on 012 349 1450, email [email protected] or visit