FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Magnesium (Mg) It is the only metal that forms part of chlorophyll and is therefore essential for photosynthesis. The specific function of Mg is relatively unknown, but it does act as activator of various enzymes, through which it plays a role in carbohy drate synthesis, nitrogen metabolism, fat and oil synthesis, etc. In contrast to Ca, Mg is highly mobile in the plant and is readily mobilised from old to young leaves at growing tips. Like Ca, Mg is found in a large variety of minerals, the most important of which are primary minerals olivine, serpentine, talc, pyroxenes and amphyboles, but especially in the secondary mineral dolomite, as well as magnesite, chlorite, vermiculite, and montmorillonite. It is accepted that a Ca:Mg ratio in soil be tween 4:1 and 6:1 is desirable for most plants. In many South African soils it is often much narrower in the subsoil, even 1:1, especially where the parent material is marine sediments or where strongly developed structure is found (see under Ca, remarks about the merits of correction of low Ca:Mg-ratios). Magnesium deficiency symptoms are very characteristic, welldefined, deep interveinal yellowing (chlorosis) on older leaves of white cultivars and red co louration in red cultivars (Fig. 14). It has however, more recently been observed that colouration is also yellow in virus-tested red cultivars (Fig. 15). For some rootstocks the interveinal symptom takes the form of necrotic specks (Fig. 16).

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FIGURES 13 & 14: Clearly defined interveinal yellowing of older leaves in white cultivars (Chasselas, Switzerland) and red colouration in red cultivars (Grenache).

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