Handbook for Irrigation of Wine Grapes in South Africa

common practice, water pH can be adjusted by the addition of an alkali such as agricultural lime, or with acidic reagents such as CO 2 , S or H 2 SO 4 (DWAF, 1996). These chemicals are hazardous and special precautions should be taken when using them. Furthermore, changes in soil pH caused by irrigation water will take place slowly (Ayers & Westcott, 1985). Therefore, it is often easier to treat the soil pH problem that develops rather than the irrigation water. Steps that growers can take to minimise the effects of extreme pH are to (i) use an irrigation method that will not wet foliage or products, (ii) apply agricultural lime to maintain soil pH when irrigating with low pH water and (iii) use soil acidifiers such as S and reduced nitrogenous fertilizers when irrigating with high pH water (DWAF, 1996). NITROGEN Nitrogen refers to all inorganic N forms present in the water, i.e. NO 3 - , NO 2 - , NH 4 + and NH 3 (DWAF, 1996). All of these must be added together to quantify water N. Nitrate is the usual form found in natural waters (Ayers & Westcott, 1985; DWAF, 1996; ANZECC, 2000), whereas NH 4 + commonly occurs in wastewater (ANZECC, 2000). High levels of irrigation water N can promote plant growth, leach out, and contaminate groundwater resources as well as stimulate algae and aquatic plant growth in irrigation structures (DWAF, 1996). Permissible N levels for sensitive crops, such as grapevines, are less than 5 mg/  (Table 5.2). TABLE 5.2. Effects of N on crop yield and quality and groundwater contamination (adapted from DWAF, 1996). Concentration range (mg/  ) Crop yield and quality Groundwater contamination ≤ 5 (1) 5.3.2

The N application should, at normal irrigation applications, be low enough not to affect even sensitive crops such as grapes and most fruit trees. Sensitive crops increasingly likely to be affected, depending on magnitude of irrigation. Other crops remain largely unaffected in the lower concentration range, but are increasingly affected as concentration increases. Most crops are affected and only a limited range of crops can utilise the N applied. There are severe restrictions on utilisation of these waters.

The N application should, at normal irrigation applications, be low enough so that most of it would be utilised by the irrigated crop and little be available for leaching to groundwater. Likelihood of ground water contamination increases, depending on volume of irrigation water applied and uptake by irrigated crop.

5-30

> 30

Increasingly serious likelihood of groundwater contamination.

(1) Optimum water quality range.

106 CHAPTER 5 – IRRIGATION WATER QUALITY