SOIL PREPARATION

2.3 Soil stratification Soil texture can significantly affect grapevine performance, but does not limit root distribution per se. A gradual increase in clay content with depth is not limiting to root growth. Abrupt (sharp) transitions between texturally different soil layers will, however, prevent root penetration. Such layering within soils, where layers differ profoundly in texture, and abrupt transitions between layers, are a characteristic of Dundee soils (Figure 2.4). In South Africa these soils are young and occur in many viticulture areas along rivers such as the Olifants, Orange and Breede Rivers; in other words they are alluvial in origin. The layering prevents a homogeneous root distribution and may even prevent roots from exploiting the full depth of these potentially deep soils.

FIGURE 2.4: Dundee soil showing stratification due to alluvial deposition of texturally different layers (Photo: J. de Jager, Vinpro).

Layered soils should be deep-tilled before planting to remove the layers and obtain good mixing. Depending on the lithological origin of the mother material, climate, and the environment, stratified alluvium can vary from well-drained to wet, acid to alkaline, and non-saline to saline and even sodic. Care should therefore be taken that any brackish and/or clayey soil layers are not brought to the surface (Saayman & Van Huyssteen, 1981a).

24 | SOIL IMPEDIMENTS TO ROOT PENETRATION