Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

Grapevines irrigated with high levels of N may have extended vegetative growth with delayed poor yield of grapes with low sugar (Ayers & Westcott, 1985). When grapevines experience N toxicities, the leaf surface becomes shiny and the leaf edges become necrotic (Fig. 5.4). Although most other crops are unaffected until N levels are higher than 30 mg/  , levels lower than 5 mg/  can still stimulate algae growth. To reduce the effects of high levels of N in irrigation water, growers can (i) reduce N fertilizer by the amount of N added through the water, (ii) dilute water if there is an additional water source, (iii) use a water source that is low in N when crop requirements are low, (iv) limit leaching and (v) control algae growth in irrigation equipment with CuSO 4 (DWAF, 1996). Figur 5.4

PHOTO: D. SAAYMAN

5.3.3 PHOSPHORUS Although there are no guidelines for P levels in irrigation water, there is a long term critical value recommended by ANZECC (2000) of 0.05 mg/  . This norm has been established to minimise the risk of algal blooms developing in storage facilities and to reduce the likelihood of bio-fouling in irrigation equipment. 5.3.4 SODIUM This element commonly accumulates in saline soils, particularly in low rainfall areas where leaching might be limited. Low flow can also cause increased levels of Na in river water (Myburgh & Howell, 2014b). Sampling water at three places along the course of the Holsloot River during the 2012/13 season showed that the Na content Figure 5.5 FIGURE 5.4. An example of grapevine leaf N toxicity symptoms where a vineyard was drip irrigated near Tesslaarsdal.

IRRIGATION OF WINE GRAPES 107