FERTILISATION GUIDELINES FOR THE TABLE GRAPE 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. A characteristic of Mg deficiency is that the symptoms can be observed a long time before a negative effect on vine performance sets in, thus providing ample time for corrective actions. Although berry stem necrosis, as discussed under Ca, is controlled with varying success by Mg sprays, it is not a Mg deficiency symptom. Due to the greater mobility of Mg within the plant, it probably substitutes the cell wall strengthening role of the immobile Ca, for which spraying is ineffective, to some extent. In South Africa it has been found that berry stem necrosis symptoms become visible earlier than in Europe and thus the practice to apply bunch directed Mg sprays of up to 15% at 10 mm berry size. The success of this practice, however, is precarious and under suspicion. As a rule, magnesium is not intentionally fertilised, and the approach should be ensure that the soil contains sufficient Mg. In soils with a low pH it should be adequate to use dolomitic lime during soil preparation, if soil analyses indicate Ca:Mg ratios of > 4 – 6. The minimum norm for Mg, which will be sufficient to prevent deficiencies is 120 mg kg –1 (1.00 cmol kg –1 ) in soils with > 6% clay. In sandy soils, levels of between 50 and 80 mg kg –1 (0.42 to 0.67 cmol kg –1 ) should be sufficient, if K fertilisation is not excessive. Cases of Mg deficiency symptoms do, however, occur occasionally in existing vineyards with soil pH KCl values of ≥ 5.5. Foliar sprays with a 3 – 10% Mg sulphate solution can be applied but is not a permanent solution. Soil applications of Mg sulphate is advised in such cases, but quantities required for correction to the desirable Mg saturation percentage of 15 – 20%, are substantial and the cost of the 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 affect the entire soil pH significantly, and the grapevine is furthermore not sensitive to high pH values, while a source of Mg is now present in the maximum root zone. The leaf analysis norms for Mg are indicated in Table 21, but once again, the precautions as explained for N, are applicable. TABLE 21: Norms for the Mg contents (%) of leaf blades and petioles of grapevines. Fruit set Veraison Leaf blades Petioles Leaf blades Petioles Minimum High Minimum High Minimum High Minimum High 0.16 0.55 0.30 0.80 0.20 0.60 0.40 1.15

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