Handbook for Irrigation of Wine Grapes in South Africa

TABLE 5.7. Effects of Mn on drip irrigation systems (DWAF, 1996). Concentration range (mg/  ) Clogging of irrigation equipment < 0.1 Only minor problems encountered. 0.1-1.5 Moderate problems encountered. > 1.5 Severe problems encountered.

5.3.13 ZINC Zinc is an essential plant nutrient that is required in small amounts (DWAF, 1996; ANZECC, 2000). At higher concentrations, Zn induces Fe deficiencies in plants (DWAF, 1996). The low solubility of Zn under alkaline soil conditions can also induce plant Zn deficiencies. Toxicity symptoms are illustrated in Figure 5.7. Zinc is more readily available to plants in acid, light textured soils (ANZECC, 2000). Guidelines are presented in Table 5.5. Figure 5.7

Z n

Z n

Z n

PHOTO: D. SAAYMAN

N

Z n

Z n

FIGURE 5.7. An example of grapevine leaf Zn and N toxicity symptoms of grapevines growing in Gauteng.

5.3.14 CADMIUM Cadmium is readily taken up by plants even though it is not an essential nutrient (DWAF, 1996). Because of its chemical similarity to Zn, it can interfere with plant metabolic processes since it may block Zn binding sites (DWAF, 1996; ANZECC, 2000). Soils can contain high Cd levels due to the addition of P-fertilisers, manures, composts or bio-solids which contain Cd as an impurity (ANZECC, 2000). Cadmium

114 CHAPTER 5 – IRRIGATION WATER QUALITY