Handbook for Irrigation of Wine Grapes in South Africa

40

Fine sand Coarse sand

35

30

25

20

RAW = 16 mm RAW = 28 mm

15

SWC (mm/30 cm)

10

5

0

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

Ȍ m

(-kPa)

FIGURE 3.12. Soil water characteristic curves i.e . the relationship between SWC and Ψ m , for two sandy soils. For both soils, RAW is between -5 and -100 kPa.

Water holding capacity can also be estimated from soil texture (http://passel.unl.edu/ pages/informationmodule.php?idinformationmodule=1130447039&topicorder=10). Medium textured soils, i.e. fine sandy loam, silt loam and silty clay loam, have the highest water holding capacity, while coarse soils, i.e. sand, loamy sand and sandy loam, have the lowest water holding capacity (Table 3.1). Medium textured soils with a blend of silt, clay and sand particles, as well as strong aggregation, create a large number of fine pores that can hold water by means of capillary forces. Due to the high sand content and very little silt and clay, coarse soils have almost no aggregation or fine pores that can hold water. Fine textured, clayey soils have many fine pores that can hold water. However, the water is held tightly by capillary forces in the fine pores making it difficult for grapevines to absorb it. Since soil texture may vary with depth, so will water holding capacity. A duplex soil may have a sandy surface layer with a low water holding capacity, and clayey B horizon with a higher water holding capacity.

64 CHAPTER 3 – WATER RELATED SOIL PROPERTIES