FERTILISER GUIDELINES FOR THE WINE INDUSTRY

CALCIUM (Ca) AND MAGNESIUM (Mg) Both calcium (Ca) and magnesium (Mg) are essential nutrients, required for optimal grapevine performance. Generally they are so abundant in soils that have a pH within the optimal range that fertilisation with these nutrients is not required. Through proper lime applications, to raise the soil pH and R-value to 10, sufficient Ca and Mg are applied to the soil to satisfy the nutritional re quirements of the grapevine (Eksteen, 1969). Nevertheless, some sandy soils with an excessive volume of stones, might be deficient in Ca and Mg even though the soil pH is optimal. Likewise, when the wrong lime (e.g. calcitic lime) is used in Mgdeficient soil, applications of Mg might be required. Plant-available Ca and Mg can also be extracted using NH 4 Ac or Mehlich III. Similar results are obtained between the two extractants for soil with pH KCl < 6.0 (Nathan, 2005). The following general norms may be used as guidelines for minimum Ca and Mg concentrations. The Ca:Mg ratio must preferably also not exceed a value of 6.

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TABLE 3. Minimum norms for calcium and magnesium concentration in soil, as deter mined using ammonium acetate, to ensure optimal grapevine performance.

Sandy soil

Clayey soil

Nutrient

mg/kg

cmol(+)/kg

mg/kg

cmol(+)/kg

Calcium

360

1.80 0.30

500 120

2.50 1.00

Magnesium

40

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