Handbook for Irrigation of Wine Grapes in South Africa

Chapter 3

the water holding capacity up to PWP is a time consuming procedure, the water holding capacity is only determined to 100 kPa in commercial laboratories. For many vineyard soils, it is the range where most of the plant available water occurs. Since water can be readily absorbed by grapevine roots in this range, it is referred to as readily available water. A soil water characteristic curve is then calculated by plotting soil water content against pressure (Fig. 3.9).

A

B

FIGURE 3.8. Undisturbed soil cores in brass rings (A) and high pressure pots (B) used to determine soil water characteristic curves in the laboratory.

140

Sandy loam Coarse sand

120

100

80

60

40

Soil water content (mm/30 cm)

20

0

0 10 20 30 40 50 60 70 80 90 100 110

Ȍ m (-kPa)

FIGURE 3.9. Examples of laboratory determined soil water characteristic curves for two differently textured soils.

It is sometimes impractical to extract undisturbed soil cores from gravelly or loose soil. As an alternative, soil water characteristic curves can be determined in situ . Soil water characteristic curves can be obtained by measuring soil water content

45 50 Figure 3.10

IRRIGATION OF WINE GRAPES 61