SOIL PREPARATION

2.7 Weathered rock in the subsoil Rock, in varying degrees of weathering, restricts root, water and air penetration more than the overlying material. The extent of weathering and the original structure of the rock will, however, determine the degree of its impeding effect. Litho-cutanic B horizons (Figure 2.10) on folded fillites are normally open, loose and porous in contrast to horizontally-layered shale that is often extremely dense. The shallower, less weathered and harder the rock, the more negative its effect on root growth and development.

FIGURE 2.10: Weathered shale as part of the litho-cutanic B horizon of a shallow Glenrosa soil that will require deep ripping if it is to be considered for planting of grapevines (Photo: ARC Infruitec-Nietvoorbij). A further inhibiting factor associated with weathered rock is the release of soluble salts, especially in clayey sedimentary and metamorphic rock. Hard, weathered rock that has been loosened during soil preparation has a high permeability and free water moves freely through such a profile from higher to lower positions in the landscape. In such cases cut-off drains will be essential to remove excess water and salts from the land. 2.8 Coarse fragments in the soil The presence of coarse fragments in soil is not an impediment to root development, but justifies a short discussion because stony soils are so common in the Western Cape of South Africa and are often used for the growing of table grapes and wine grapes.

30 | SOIL IMPEDIMENTS TO ROOT PENETRATION