Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

5.3.11 IRON Iron is an essential micro-nutrient (DWAF, 1996). However, plants can be sensitive to Fe deposits on leaves and fruit. Furthermore, Fe can precipitate in irrigation equipment (DWAF, 1996; ANZECC, 2000). Iron deficiencies can develop in alkaline soils, where Fe(OH) 2 precipitates (ANZECC, 2000). The Fe 2+ dissolved in irrigation water is relatively unavailable to plants as it oxidises and precipitates upon aeration when applied to soil. However, under reducing conditions, precipitated Fe 3+ can be reduced to the more soluble Fe 2+ . Precipitated Fe in soil binds P and Mo rendering them unavailable to plants and contributes to soil acidification (Ayers & Westcott, 1985; ANZECC, 2000). General guidelines are presented in Table 5.5, whereas the effect of Fe on irrigation equipment is given in Table 5.6.

TABLE 5.6. Effects of Fe on drip irrigation systems (DWAF, 1996). Concentration range (mg/  ) Clogging of irrigation equipment < 0.2 Only minor problems encountered. 0.2-1.5 Moderate problems encountered. > 1.5 Severe problems encountered.

5.3.12 MANGANESE Manganese is essential to plants where it is involved in N metabolism and chlorophyll synthesis (DWAF, 1996; ANZECC, 2000). Under waterlogged conditions in association with low pH, soil Mn is reduced and its solubility increased (DWAF, 1996). Consequently, concentrations in the soil solution can accumulate to levels that are toxic to plants. In grapevines, this manifests as dark coloured, raised spots on shoots, tendrils and leaves (Fig. 5.6). Oxides of Mn are known to clog irrigation systems. General guidelines are presented in Table 5.5, whereas the effect of Mn on irrigation equipment is given in Table 5.7. Figur 5.6

PHOTO: D. SAAYMAN

FIGURE 5.6. Manganese toxicity observed in a vineyard growing near Goudmyn.

IRRIGATION OF WINE GRAPES 113