Handbook for Irrigation of Wine Grapes in South Africa

Chapter 3

3.7 WATER HOLDING CAPACITY Similar to a sponge, the soil can absorb and retain water. Water holding capacity designates the ability of a soil to hold water that is available to plants. It is useful information for irrigation scheduling and to determine when grapevines will begin to experience water constraints. The water holding capacity, or RAW, is calculated as the difference between SWC at FC and -100 kPa, respectively (Fig. 3.11). Experience has shown that FC is ca. -5 kPa for sandy and -10 kPa for heavier soils in the wine growing regions. To determine the water holding capacity for the soil profile, the available water in each horizon must be determined separately, and then added together.

140

Sandy loam Coarse sand

120

100

RAW = 26 mm

80

60

SWC (mm/30 cm)

40

20

RAW = 16 mm

0

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

Ȍ m (-kPa)

FIGURE 3.11. Soil water characteristic curves, i.e. the relationship between SWC and Ψ m , for two differently textured soils. For the sandy loam soil, RAW is between -10 and -100 kPa, whereas RAW for the coarse sand is between -5 and -100 kPa.

In the case of sandy soils, the water holding capacity depends on the sand grade, i.e. whether it consists of fine, medium or coarse sand. Fine sand usually has higher water holding capacity than coarse sand (Fig. 3.12). The fine, red sand, which occurs in the Lower Olifants River region, is wind-blown material, whereas the coarse, light coloured sand is alluvial material near De Doorns. It is interesting to note that the water holding capacity of the fine sand is comparable to the sandy loam soil shown in Figure 3.11.

40

Fine sand Coarse sand

35

30

IRRIGATION OF WINE GRAPES 63

25

m/30 cm)