Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

uptake increases with soil acidity, salinity and total Cd content. Conservative limits are recommended due to its potential to accumulate in plants and soils to concentrations that are harmful to humans (Ayers & Westcott, 1985). Permissible Cd levels are presented in Table 5.5. 5.3.15 CHROMIUM Chromium is not essential for plant growth, but is toxic at high concentrations (DWAF, 1996). Acceptable levels are presented in Table 5.5. Recommended limits are conservative as there is a lack of knowledge on its toxicity to plants (Ayers & Westcott, 1985). 5.3.16 LEAD Lead has many industrial applications which can give rise to Pb contamination in water resources (DWAF, 1996). Furthermore, atmospheric sources, fertilisers, manures, sludges and agricultural chemicals can cause Pb deposits in soils (ANZECC, 2000). Soil pH has a major effect on Pb in soil solution and its solubility decreases with increasing pH (DWAF, 1996; ANZECC, 2000). The permissible Pb levels proposed for grapevines (Table 5.5) are considerably higher, compared to the DWAF (1996) thresholds of 0.2 mg/  for continuous irrigation on all soils and 2.0 mg/  for fine textured soils over a 20-year period. 5.3.17 MERCURY Recommended levels for mercury are less than 0.002 mg/  (ANZECC, 2000). The Hg dissolves other metals forming amalgams and is strongly retained by soils, especially those high in organic matter. 5.3.18 MOLYBDENUM The occurrence of Mo toxicity in plants is rare. High levels of Mo will cause accumulation of anthocyanins in tomato and cauliflower leaves causing them to turn purple (Kaiser et al. , 2005 and references therein). In contrast, legume leaves turn yellow in response to excessive Mo. The permissible Mo levels proposed for grapevines are presented in Table 5.5. It must be noted that this norm only applies to fine textured acidic soil, or acidic soil with a high content of iron oxide.

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