Handbook for Irrigation of Wine Grapes in South Africa

E s

= 12 x 15 mm = 180 mm Preparation depth

Potential root depth

Soil water content

Time

E s

= 3 x 15 mm = 45 mm

Potential root depth

Preparation depth

Soil water content

Time

FIGURE 7.28. Schematic illustration of Phase 1 E s losses in the case of shallow soil preparation (top) and where the soil preparation depth is equal to the potential soil depth (bottom), i.e. if it is assumed that the soils are similar, and that Phase 1 E S is 15 mm.

In many cases, grapevines are planted on ridges that were constructed as a means of soil preparation (Myburgh, 1994). Since ridges increase the soil surface (Fig. 7.29), the additional evaporation will cause the soil to be drier, compared to unridged soil throughout the growing season (Fig. 7.30). Therefore, ridged soils are usually drier and warmer than unridged soil (Myburgh & Moolman, 1991; Myburgh & Moolman, 1993). Consequently, grapevines on ridges will require more irrigation, compared to unridged soil. Vineyard soils should only be ridged where (i) a high water table occurs during early spring, (ii) heavy subsoil does not allow conventional drainage or (iii) the topsoil is shallow and the subsoil cannot be loosened by means of conventional deep soil preparation.

216 CHAPTER 7 – IRRIGATION STRATEGIES