Handbook for Irrigation of Wine Grapes in South Africa

On the day before the grapes were picked, the soil water content was just slightly below the refill line, whereas the midday stem water was -1.41 MPa. It must be noted that midday Ψ s measurements were terminated when the first irrigation was applied. The last measurement before harvest was only made to confirm that the midday Ψ s was consistently related to soil water content throughout the growing season. The grapevines yielded only 10 t/ha. However, this was because the estate managed the VSP canopy meticulously in their effort to produce high quality wine. All excessive shoots and suckers were removed on time. At harvest on 14 March, the juice sugar content, total titratable acidity and pH were 25.9°B, 5.74 g/  and 3.52, respectively. CONCLUSIVE REMARKS The choice of irrigation scheduling methodology lies primarily with individual growers. However, the on-farm conditions may dictate the best approach. For example, in shallow, sandy soils where frequent irrigations are unavoidable, devices that log soil water content will be more suitable than manual measurements every second day. If carried out manually, it is essential that measurements are made regularly to obtain a more complete picture of changes in soil water status over time. Furthermore, soil water status must be determined before and after irrigations to see if the refill line was reached, and to ascertain if the total root depth was wetted. The minimum measurement requirements are readings at two depths. Firstly, one measurement should be carried out where most roots are concentrated to see when an irrigation is required. The second measurement must be at the lower end of the root zone to see if the total root depth was wetted. The soil water content at the lower end of the root zone should also decline between irrigations. If not, it means the vineyard is most likely being over-irrigated. Correct placement of access tubes or sensors with respect to the grapevines is important. In the case of full surface or strip wetting, measurements must be taken at a quarter of the in-row plant spacing from a grapevine. Where drip irrigation does not wet the soil in a continuous band, soil water status should be measured ca. 10 cm from a dripper. Preferably, this dripper should be a quarter of the in-row plant spacing from a grapevine. This might not be the ideal placing, but at least it will show if the dripper wetted the soil over the total root depth. Correct data interpretation is important. In other words, growers must interpret the data in terms of grapevine responses. Growers must also allow time for data interpretation in order to make the right decisions, preferably in liaison with an experienced viticultural advisor. In many cases, growers depend on the advice of probe consultants who do not necessarily have appropriate knowledge regarding soil physical properties and/or grapevine responses. Ideally, probe consultants should measure Ψ S to set credible refill lines according to grapevine water requirements. It is advisable to standardize measurements in order to allow comparison between different soils, vineyards and cultivars, as well as grapevine 8.6

282 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING