FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

Ca nutrition should be adequate. Vineyards with restricted root systems (for example some drip irrigated vineyards) may develop Ca deficiencies if the pH of the soil drops to sub-optimal levels (pH water < 6.5 of pH KCl < 5.5) in the root zone. In such cases the pH in the root zone should be adjusted and inclusion of Ca in the fertilisation programme can be justified. The leaf analysis norms for Ca are shown in Table 20.

TABLE 20: Norms for the Ca contents (%) of leaf blades and petioles of grapevines.

Fruit set

Veraison

Leaf blade Petiole Minimum High Minimum High Minimum High Minimum High 1.50 2.50 1.00 2.00 1.50 2.40 1.20 2.10 Petiole Leaf blade

Magnesium (Mg) It is the only metal cation 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 carbohydrate 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. Like Ca, Mg is found in a large variety of minerals, the most important of which are primary minerals olivine, serpentine, talc, pyroxenes and amphiboles, but especially in the secondary mineral dolomite, as well as magnesite, chlorite, vermiculite, and montmorillonite. It is accepted that a Ca:Mg ratio of between 4:1 and 6:1 is desirable for most plants. In many South African soils, the ratio is often much lower in the subsoil, even 1:1, especially where the parent material is marine sediments or where a strongly developed structure occurs. Magnesium deficiency symptoms are very characteristic, well defined, deep interveinal yellowing (chlorosis) on older leaves of white cultivars and red colouration in red cultivars (Figure 12). It has however, more recently been observed that colouration is also yellow in virus tested (clean) red cultivars (Figure 13). For some rootstocks the interveinal symptom take the form of necrotic specks.

60 | MAINTENANCE FERTILISATION