Handbook for Irrigation of Wine Grapes in South Africa

remained almost constant in the mountain and the beginning of the floodplain, respectively (Table 5.3). However, the level of Na increased substantially along the floodplain up to Rawsonville during summer. In contrast, the Na concentrations were comparable along the course of the river when the flow was high in winter. Since no definite point of contamination could be identified, the Na probably became more concentrated when the river flow was reduced along the floodplain as water was being abstracted for vineyard irrigation. TABLE 5.3. Temporal and spatial variation in Na content in water of the Holsloot River, a tributary of the Breede River. Sampling date Locality In mountain Beginning of floodplain Near Rawsonville 4 December 2012 2.3 3.0 16.6 28 March 2013 1.8 2.2 11.6 30 September 2013 2.3 2.4 3.0 Sodium accumulates in leaves and can be toxic to plants. Root uptake and leaf burn are therefore important considerations (DWAF, 1996). Root uptake is mainly determined by the concentrations of Na in the soil solution. Plants irrigated with water containing high levels of Na are exposed to the root zone Na as well as absorption by the leaves, i.e. in the case of overhead irrigation. Accumulation of Na in the soil can cause Ca and Mg deficiencies in plants (ANZECC, 2000), as well as affect soil physical conditions. Excessive Na in irrigation water promotes soil dispersion and structural break-down when the Na levels are more than three times the Ca levels (Ayers & Westcott, 1985). Sodium toxicity is reduced if there is sufficient Ca in the soil. An important consideration for growers is the legal limit of Na for South African wine, which is 100 mg/  (DWAF, 1996). Grapevines are moderately sensitive to foliar injury. Therefore, a Na concentration of 115 mg/  is the upper threshold for overhead irrigation (Table 5.4). In order to minimise Na effects on grapevines, growers should (i) use irrigation systems that don’t wet leaves, (ii) ameliorate the water and/or soil with Ca and Mg and (iii) reduce irrigation frequency by using long irrigation cycles (DWAF, 1996).

108 CHAPTER 5 – IRRIGATION WATER QUALITY