Handbook for Irrigation of Wine Grapes in South Africa

Soil auger

Drop hammer

Metal cans

FIGURE 8.18. Equipment required to collect gravimetric soil samples in the field.

The cans containing the soil samples are weighed to obtain the wet mass (M w ). Following this, the lids are removed and the samples placed in a special oven with a fan fitted to extract the moisture. The samples need to be dried for at least 16 hours at 105°C. After drying, the lids are replaced and the samples placed in a desiccator to allow cooling. The dry mass (M d ) is then determined by weighing. The mass fraction of water in the soil is calculated as follows: Θ m =[(M w -M d )÷(M d -M c )] Eq. 8.3 The mass fraction water is converted to the volumetric soil water fraction as follows: Θ v = Θ m x ρ b Eq. 8.4 where ρ b is the bulk density of the soil (g/cm 3 ). The soil water content in mm water per depth layer is calculated as follows: SWC= Θ m x ρ b x d x 100 Eq. 8.5 where d is depth of the layer in decimeter (dm) and multiplication by 100 is required to convert the mass fraction to percentage. For example, if the gravimetrically determined Θ m in a 20 cm deep layer is 0.15, and the bulk density is 1.6 g/cm 3 ,

252 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING