Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

TABLE 5.4. Effects of Na on foliar crop yield and quality (adapted from DWAF, 1996). Concentration range (mg/  ) Crop yield and quality ≤ 70 Should prevent the accumulation of Na to toxic levels in all, but the most sensitive plants, even when crop foliage is wet. 70-115 (1) Crops sensitive to foliar absorption accumulate toxic levels of Na when crop foliage is wetted and display symptoms of foliar injury and yield decreases. 115-230 Crops moderately sensitive to foliar absorption accumulate toxic levels of Na when crop foliage is wetted and display symptoms of foliar injury and yield decreases. 230-460 Crops moderately tolerant to foliar absorption accumulate toxic levels of Na when crop foliage is wetted and display symptoms of foliar injury and yield decreases. > 460 Crops tolerant to foliar absorption increasingly accumulate toxic levels of Na when crop foliage is wetted and display symptoms of foliar injury and yield decreases. CALCIUM AND MAGNESIUM There are no guidelines for Ca and Mg levels in water. However, it is important that the levels of these elements are determined so that the SAR of the water can be calculated (Myburgh, 2012c). If the Ca to Mg ratio in the water is less than one, the potential negative effect of Na may be exacerbated (Ayers & Westcott, 1985). In addition, crops irrigated with water containing high levels of Mg may produce low yields due to a Mg-induced Ca deficiency. Although there are still insufficient data to make the Ca to Mg ratio an evaluation factor, should this ratio be less than one, or the Ca to total cation ratio less than 0.15, a further evaluation of the water is required. (1) Threshold for acceptable water quality range. 5.3.5 5.3.6 BICARBONATE AND CARBONATE Although high levels of HCO 3 - in irrigation water can affect plants, soils and irrigation equipment, there are no recommended guidelines (DWAF, 1996; ANZECC, 2000). Irrigation water containing high levels of HCO 3 - and CO 3 2- can increase HCO 3 - in the soil solution. Consequently, Ca and Mg can precipitate as insoluble CO 3 2- when the soil dries out (Van Zyl, 1981; McCarthy et al. , 1988). Relative Na increases, and therefore higher SAR levels, may have an impact on soil physical properties (Van Zyl, 1981; McCarthy et al ., 1988; ANZECC, 2000). High soil pH, which can occur when HCO 3 - levels in irrigation water are high, can also be detrimental to plants as uptake of certain ions can be limited (ANZECC, 2000). Although HCO 3 - in water applied by overhead sprinklers may cause unsightly crystalline deposits on grapes and leaves, these are not necessarily toxic to grapevines (McCarthy et al ., 1988).

IRRIGATION OF WINE GRAPES 109