SOIL PREPARATION

FIGURE 2.3: Shallow grapevine rooting in a Wasbank soil due to the presence of a traffic pan. The vineyard on this soil had to be uprooted because of poor performance (Photo: ARC Infruitec-Nietvoorbij). Impediment of grapevine root growth was successfully characterised by Van Huyssteen (1983) in terms of bulk density and penetrometer resistance. Penetrometer resistance, which is an indicator of soil strength, was measured by a constant-speed penetrometer. In a wide variety of soil types, a critical, albeit poorly defined, penetrometer resistance of 2 000-2 500 kPa was found above which root penetration of several crops other than grapevines are drastically impeded (Zimmerman & Kardos, 1961; Taylor & Gardner, 1963; Taylor & Burnett, 1964; Greacen et al., 1969; Bar-Yosef & Lambert, 1981). This critical penetrometer resistance can be reached either by increasing soil compaction or by decreasing water content of the soil, i.e. by drying of the soil (see “3.2 Soil physical parameters”). Several studies showed the relationship between indicators of soil compaction and grapevine root distribution. In a pot experiment with Chenin blanc/99R Van Huyssteen (1988) clearly found a gradual decrease in root penetration with increasing soil compaction (see “3.2 Soil physical parameters” for more information on the experiment). This result was obtained with several soil types.

22 | SOIL IMPEDIMENTS TO ROOT PENETRATION