Handbook for Irrigation of Wine Grapes in South Africa

drier, the water that remains in the smaller pores are held tighter by increasing capillary forces. The capillary force will increase upon further drying until the water is held so tight that roots cannot absorb it. This soil water content is referred to as permanent wilting point. Soil water content can be expressed as mass of water per unit mass of oven dry soil. This ratio is referred to as gravimetric soil water fraction ( ϴ m ). This ratio can be multiplied by 100 to obtain mass% soil water content. Soil water content can also be expressed as volume of water per unit volume of soil, and is referred to as volumetric soil water fraction ( ϴ v ). The latter can be calculated as follows: ϴ v = ( ϴ m ) x ρ b Eq. 3.1 where ρ b is in g/cm 3 . The ϴ v value is multiplied by 100 to obtain volume% soil water content. With respect to irrigation management, the SWC must preferably be expressed as mm per soil depth. This can be calculated as follows: SWC = ϴ v x 100 x d Eq. 3.2 where d is in decimeter (1 dm = 10 cm). For example, if ϴ v is 0.12 in a 30 cm deep soil layer, the SWC = 0.12 x 100 x 3 = 36 mm. Details of measuring soil water content are discussed in Chapter 8. 3.5 MATRIC POTENTIAL The Ψ m refers to the capillary forces that bind soil water in the pores. Therefore, Ψ m is a measure of the energy that grapevine roots must exert in order to absorb water from the pores in the soil. Since the water in the pores is held under suction, or negative pressure, Ψ m has a negative value. Consequently, Ψ m is usually expressed as -kPa or -MPa. Matric potential is commonly measured by means of tensiometers as an indicator of soil water status for irrigation scheduling purposes. Details of measuring Ψ m are discussed in Chapter 8. 3.6 SOIL WATER CHARACTERISTIC CURVES Soil water characteristic curves describe the relationship between soil water content and matric potential. Therefore, it indicates the concomitant decrease in soil water content and matric potential as the soil becomes drier. Soil water characteristic curves determined by means of undisturbed soil cores in high pressure pots are usually more accurate than in situ measurements. Soil cores are extracted from soil profiles in the field using a specifically designed auger so that each core is retained in a brass cylinder (Fig. 3.8A). The soil cores are placed onto ceramic plates inside the pots, and then subjected to a range of increasing pressures (Fig. 3.8B). As soon as the water outflow at a given pressure stops, the soil cores are weighed to determine the water content at that specific pressure. The initial pressure is usually 2.5 kPa, and the process is repeated up to a pressure of 1 500 kPa, which is equivalent to permanent wilting point. Since determining

60 CHAPTER 3 – WATER RELATED SOIL PROPERTIES