Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

TABLE 5.5. Recommended maximum concentrations of trace elements and heavy metals in irrigation water as proposed by Van Zyl (1981). Element Continuous irrigation on all soils (mg/  ) Irrigation of fine textured soil for 20 years (mg/  ) Boron 0.75 2.00-10.00 Cadmium 0.01 0.05 Chromium (VI) 0.10 1.00 Copper 0.20 5.00 Fluoride 1.00 15.00 Iron 5.00 20.00 Lead 5.00 10.00 Manganese 0.20 10.00 Molybdenum 0.01 0.05 Zinc 2.00 10.00 5.3.9 COPPER Copper is an essential micro-nutrient, as well as an important component of several plant enzymes (DWAF, 1996; ANZECC, 2000). The solubility of Cu decreases with increasing soil pH (DWAF, 1996). Recommended levels are presented in Table 5.5. There are no known negative effects of Cu on irrigation systems (DWAF, 1996). 5.3.10 BORON Boron is essential for plant growth and is required for the correct functioning of meristems, cell division and enlargement, and is thought to play a role in the synthesis of hormones such as auxins (Saayman, 1981). However, B is toxic to plants at low concentrations (DWAF, 1996). Grapevines range from very sensitive (Van Zyl, 1981) to sensitive (Ayers & Westcott, 1985; DWAF, 1996; ANZECC, 2000) to B toxicity. Excess B accumulates in grapevine leaf edges and older leaves develop necrotic spots (Saayman, 1981). Leaf edges do not grow, whereas the middle leaf part continues to grow normally and the leaf curls to the bottom (Fig. 5.5). Boron concentrations are generally low in South African waters (Van Zyl, 1981). Although surface waters rarely contain B, well water and springs occasionally contain toxic amounts (Ayers & Westcott, 1985). Recommended levels are presented in Table 5.5. In general, irrigation water containing B levels less than 0.50 mg/  are considered to be suitable for grapevines (McCarthy et al ., 1988), whereas levels lower than 0.75 mg/  are recommended by Ayers and Westcott (1985).

IRRIGATION OF WINE GRAPES 111