FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Magnesium deficiencies can be found in acidic, leached soils, especially sand, but also in soils containing free lime due to the antagonism between Ca and Mg as far as uptake is concerned, as commonly observed in Europe (Fig. 13). A characteristic of Mgdeficiency is that the symptoms are observable a long time before a negative effect sets in on vine performance, providing ample time for corrective actions. Although berry stem necrosis, as discussed under Ca, is controlled by Mg sprays with varying success, it is not a Mgdeficiency symptom. Due to the greater mobility of Mg in the plant, it probably substitutes the cell wall strength ening role of the immobile Ca for which spraying is not effective to some extent. In South Africa it has been found that berry stem necrosis symptoms become visible earlier than in Europe and thus bunch-directed Mg-sprays of up to 15% can be applied prior to bunch closure. As a rule, magnesium is not intentionally fertilised and provision should be sufficient if dolomitic lime is used in soil preparation, where analyses indicate lime requirement and Ca:Mg-ratios of > 4-6. Cases of Mgdeficiency symptoms do however, occasionally occur in existing vineyards with soil pHs (KCl) of ≥ 5.5. Foliar sprays with a 310% Mgsulphate solution can be applied, but isn’t a permanent solution. Soil applications of Mg-sulphate is appropriate in such cases, but amounts required for correction to a desirable Mg-saturation percentage of 15-20% are substantial and the cost of material is high. A deep placement of about 1 ton ha -1 dolomitic lime between the vine row and tractor track is an option. It will not significantly influence the soil pH as a whole, as the grapevine isn’t sensitive to high pHs, while it will ensure that a source of Mg is present in the maximum root zone. Sulphur (S) The uptake of sulphur from the soil occurs as sulphate ion, which is then re duced in the plant to serve as component of albumen, amino acids and certain vitamins and enzymes. It is also an enzyme activator and involved in the con version of light to chemical energy. It is furthermore a component of flavourants and, through its conversion to protein, helps to prevent nitrate accumulation. Sulphur is found in a large variety of minerals, like sulphides and sulphates, of which iron pyrite (FeS 2 ) and gypsum (CaSO 4 ) are the most important con cerning soil. The sulphides erode by means of oxidation to sulphates, which precipitates as gypsum in arid regions. The solubility of gypsum is sufficient to meet the S-requirements of plants. It is found near the coast in S-contain ing spray and in industrial areas in emissions that end up in the soil. These

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