Handbook for Irrigation of Wine Grapes in South Africa

5.3.7 CHLORIDE Toxicity caused by irrigation water containing Cl - is most common (Ayers & Westcott, 1985). Furthermore, NaCl, KCl, CaCl 2 and MgCl 2 are highly soluble in water (DWAF, 1996; ANZECC, 2000). Growers need to consider both root and leaf uptake of Cl - . In the case of root uptake, Cl - levels of less than 700 mg/  in the soil solution are not considered to be harmful to grapevines (Van Zyl; 1981). Since the rate of leaf uptake is greater than that of root uptake, grapevines are more sensitive to foliar injury from direct contact with Cl - (Van Zyl, 1981; DWAF, 1996; ANZECC, 2000). For South African vineyards, Cl - levels of 150 mg/  are considered to be the upper threshold for overhead irrigation (Van Zyl, 1981). However, it has been reported that irrigation water Cl - levels of 70 to 175 mg/  can still cause foliar injury (Van Zyl, 1981). Crops sensitive to foliar absorption accumulate toxic levels of Cl - when crop foliage is wetted when levels range from 100 to 175 mg/  (DWAF, 1996) and display foliar injury and yield can decrease. Recommended guidelines are less than 140 mg/  (DWAF, 1996). Chloride also forms a series of complexes with Cd in the soil solution (ANZECC, 2000). Therefore, as soil Cl - increases above 400 mg/  , CdCl + will be more abundant in solution than Cd 2+ . Due to the increased mobility of Cd in this form, Cd concentrations in plants are increased. The risk of increasing plant Cd concentrations is low when irrigation water Cl - levels are lower than 350 mg/  . However, there is a high risk of increasing plant Cd concentrations when irrigation water Cl - levels are higher than 750 mg/  . Management practices to minimise Cl - uptake as proposed by DWAF (1996) are to (i) use irrigation methods that do not wet leaves, (ii) reduce the number of wetting and drying cycles, i.e. reduce irrigation frequency, (iii) irrigate at night when temperature and evaporation are lower and (iv) apply surplus irrigation in order to leach Cl - from the soil as described by Myburgh (2012c). 5.3.8 FLUORIDE Fluoride is the most reactive halogen, and is only weakly absorbed by the soil (DWAF, 1996). Important factors determining F uptake are soil type, soil pH, Ca and P. In this regard, neutral and alkaline soils deactivate F, which restricts root uptake. Recommended levels are presented in Table 5.5.

110 CHAPTER 5 – IRRIGATION WATER QUALITY