Handbook for Irrigation of Wine Grapes in South Africa

Chapter 3

3.3 BULK DENSITY Density refers to the mass of a substance per unit volume, e.g. g/cm 3 or kg/m 3 . In soils, particle density is the mass of solid particles per unit volume. The solid particles do not contain pores and their density is determined by the nature of the dominant mineral. Mean particle density for 71 soil samples collected all over the wine growing regions was 2.65±0.03 g/cm 3 (Van Huyssteen, 1989). The small standard deviation from the mean value implies that the particle density does not vary substantially between these soils. Since quartz has a particle density of 2.65 g/cm 3 , it means that it is the dominant mineral in most vineyard soils. Soil bulk density refers to the soil mass, i.e. including the pores, per unit volume. Therefore, bulk density is lower than particle density for a specific soil. Bulk density is used to describe the compactness of soil. For soils in the wine growing regions, a bulk density of 1.4 g/cm 3 is considered to be loose, whereas 1.8 g/cm 3 means that the soil is severely compact, and may impede root penetration (Van Huyssteen, 1989 and references therein). In fact, it was previously reported that grapevine root development progressively declined as the bulk density in the subsoil increased (Fig. 3.7).

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y = -91 714x + 180 19 R² = 0 9863

Root mass (g/grapevine)

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ȡ b

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FIGURE 3.7. The relationship between root mass of potted Chenin blanc/99 R grapevines and ρ b in the subsoil (after Van Huyssteen, 1989).

3.4 WATER CONTENT Soil water occurs in the pores between the solid particles. If the soil is saturated, all the pores are filled with water. At saturation, the water in the coarse pores drains rapidly due to gravity. The SWC at which the rapid drainage stops is referred to as field capacity. Following this, the soil loses water via evaporation or absorption by roots. As the soil dries out, the water is replaced by air. Since less energy is required, water will first be absorbed from the bigger pores. As the soil becomes

IRRIGATION OF WINE GRAPES 59