Handbook for Irrigation of Wine Grapes in South Africa

Chapter 6

Figure 6.18

Ȍ m

(kPa)

-500

-400

-300

-200

-100

0

-2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 0 0

Stellenbosch - Sandy loam/Loamy sand Swartland - Sand Swartland - Sandy loam Lower Olifants River - Sand Lower Olifants River - Sandy loam/Loamy sand Lower Orange River - Sand Lower Orange River - Sandy loam

Ȍ S

(MPa)

FIGURE 6.18. The relationship between midday Ψ S in Cabernet Sauvignon and Ψ m in differently textured soils in various grape growing regions in South Africa (Myburgh, 2011l).

6.3.4 SOIL SALINITY The level of dissolved salts in the soil solution determines how strong the osmotic potential will be. In addition to Ψ m , Ψ o controls the rate of water uptake by grapevine roots. In nature, water will flow from a solution with a low Ψ o to one with a higher Ψ o via the process of osmosis. Consequently, water will begin to flow from the root cells into the soil solution if the Ψ o in the soil solution becomes higher than the Ψ o in the root cells. When this happens, soil salinity can have the same devastating effect on grapevine water uptake as severe soil water deficits (Fig. 6.19). In fact, soils can be wet, but if the Ψ o is excessively high, grapevines can begin to die back due to insufficient water uptake.

IRRIGATION OF WINE GRAPES 161