Handbook for Irrigation of Wine Grapes in South Africa

Chapter 4

A

% B

Dripper

Dripper

F c

F c

F c

F c

F c

F c

F g

Grapevine root system development tends to follow the wetted pattern under drippers. Restricted root development is more likely to occur in sandy or gravelly than in heavier soils (Fig. 4.6). However, in some sandy soils a restrictive layer, e.g. Dorbank, causes lateral water distribution in the lower part of the root depth. If there are no other limitations, the deeper roots will also spread laterally (Fig. 4.7A). In drip irrigated vineyards on slopes, gravitational water flow causes the water distribution pattern to be lopsided towards the lower side of the rows. In some cases, this may concentrate root development to the lower side (Fig. 4.7B). Due to leaching, acidification under drippers may occur if the pH of the soil is not well-buffered, particularly in sandy soils. Excessively high acidity can be avoided by lime application based on frequent soil sampling (Fig. 4.8). In really poorly buffered sandy soils, liming could become an annual practice to restore the soil pH in the root zone above the threshold value for grapevines (Fig. 4.9). FIGURE 4.5. Diagrams illustrating the water distribution under drippers (A) in sandy or gravelly soil and (B) in loamy or clayey soils. Solid arrows indicate capillary (F c ) and gravitational (F g ) flow, respectively. F g

Figure 4.6 igur 4.

A

B

FIGURE 4.6. Grapevine root distribution under drip in (A) a red, sandy soil near Vredendal and (B) in a sandy loam soil near Robertson.

IRRIGATION OF WINE GRAPES 77