Handbook for Irrigation of Wine Grapes in South Africa

Chapter 3

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FIGURE 3.3. Examples of well-developed grapevine root systems in soils where the texture is more or less homogenous throughout the profile near (A) Stellenbosch and (B) Robertson.

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FIGURE 3.4. Examples where compact subsoils limit grapevine root systems near (A) Lutzville and (B) Groblershoop.

In some cases, soil texture may vary considerably within the profile. This will have a limiting effect on root development and distribution, and subsequently, the availability of water and nutrients. Root penetration, or root depth, will be restricted where light textured topsoil overlies heavy clay (Fig. 3.5). These so-called duplex soils commonly occur in the Coastal region of the Western Cape. In its natural state, the clay is generally too hard, or compact for roots to penetrate. The subsoil must be loosened by deep tillage to allow root penetration. Since the clays are generally unstable, it will slake if the soil is mixed in such a way that the subsoil lands on the surface. The slaked clay will cause poor infiltration and/or runoff. Therefore, the right implement, e.g. a finger plough, should be used to prepare duplex soils. Grapevine roots are unlikely to grow from a heavier textured layer down into one that has a lighter texture, i.e. even if there are no limitations. This occurs merely because the water availability is higher in the heavier layer. These inverted textural layers commonly occur in alluvial soils (Fig. 3.6). In order to allow deep root development, these alluvial soil layers need to be mixed thoroughly before planting.

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