South Africa Wine Technical Yearbook 2025

How should a soil with a high pH be handled? Although it is a time-consuming process, a high pH in soil can be lowered, except when the soil contains free lime from the parent material (Photo 3). If soil drainage is improved and soil samples are taken to monitor the cation ratio, address the deficiencies and improve the balance to rehabilitate the soil, the pH can be lowered. Elementary sulphur can also be used to lower the pH, but good drainage is again essential. The sulphur must convert to sulphate, and the presence of bacteria in the soil is an important requisite for this to happen. In poorly drained soil, bacteria responsible for this conversion will die. Another way to lower the pH is to increase the amount of root growth – more roots result in more organic acids being released, which lowers the pH. Why is magnesium important for the crop? Magnesium is a secondary nutrient essential for optimal photosynthesis, and acts as an activator for various enzymes that play a role in carbohydrate synthesis, nitrogen metabolism, etc. Photosynthesis has to be very effective in order to produce energy to ultimately realise the crop and farm profitably. Magnesium is highly mobile in the plant and is readily mobilised from old to younger leaves. Why are high magnesium (Mg) levels in the soil detrimental? High levels of magnesium can lead to higher soil pH, poor aeration, delayed water infiltration rate, and so forth (Photo 4). To lower the magnesium in relation to the other cations, the calcium and/or potassium levels in the soil should be increased. If there is already sufficient calcium and potassium in the soil, the following steps can be followed. Attempting to wash magnesium out of the soil will not be successful. Magnesium is a positively charged element and clings to a negatively charged soil particle. Magnesium, therefore, has to be converted to a salt in order to leach. We normally recommend a negatively charged element like sulphur, which will bind with the magnesium. Magnesium sulphate is then formed (Epsom salt), which is mobile and can leach. This will only be successful though, if the soil has good drainage. How does lime work in the soil? In general, calcium is associated with soil pH, but magnesium and sodium can increase the pH even faster than calcium. Agricultural lime consists of calcium or magnesium carbonate (Photo 5). If the soil pH is low, it indicates a high presence of free hydrogen ions. When calcium or magnesium carbonate (or both) are added to the soil, a chemical reaction occurs with the hydrogen, forming carbon dioxide and water, and the calcium and/ or magnesium is then left behind. We use lime to increase pH, but the conversion takes time, and the process can even sometimes take up to four years. The speed of the

PHOTO 2. Soil with high sodium levels.

PHOTO 3. Soil with a high pH containing free lime from parent material.

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TECHNICAL YEARBOOK 2025