SOIL PREPARATION

CHAPTER 3

were filled with soil compacted to bulk densities between 1 300-1 700 kg/m 3 while the upper half was loosely filled with the same soil and allowed to consolidate naturally. Root penetration into the subsoil decreased with increasing compaction, but no bulk density or penetrometer resistance were found at which root growth stopped abruptly (Figure 3.3). This experiment clearly demonstrated the favourable effect of loose soil with regard to root growth and also aboveground grapevine performance.

70

60

50

40

30

20

G1 r = -0.62 G2 r = -0.91 G3 r = -0.98 G4 r = -0.97 G1K r = -0.96

Dry Root Mass in Subsoil (g/vine) 10

0

1200

1300

1400

1500

1600

1700

1800

Bulk density (kg/m 3 )

FIGURE 3.3: Relationship between dry root mass and bulk density established for five soil types: G1-Glenrosa, G2-calcareous Hutton, G3-Estcourt, G4-Hutton, and G1K-Limed Glenrosa (redrawn after Van Huyssteen, 1988). Soil compaction can also be measured by several other tools and methods, although none of these methods has been calibrated against root growth to give a critical value, i.e . a value at which all grapevine root growth stops completely. Soil strength or mechanical resistance is a physical parameter that has proved especially useful to evaluate the suitability of soil for root growth and functioning. In many studies of root penetration into compact media, soil resistance has been measured with penetrometers (Hillel, 1980). Due to the fact that soil offers a higher resistance to a rigid penetrometer cone than to roots,

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