SOIL PREPARATION

CHAPTER 9

FIGURE 9.1: Uneven growth in a young vineyard, typical of compacted soil. In this case vehicle traffic, after the soil had been deep-tilled, caused re-compaction, poor root development and uneven vegetative growth (Photo: J.E. Hoffman, Stellenbosch University).

Prior to the 1980s, clean cultivation was standard practice in most South African vineyards. It is small wonder that Schulte-Karring (1976), after a visit to South Africa, reported the general presence of a 20-25 cm thick compact layer under the loose topsoil as a result of implement action in vineyards. Since then, the grapevine industries moved away from clean cultivation to a system of cover crops and minimum tillage. This system entails the planting of an annual cover crop. A planter is the preferred implement for establishing a cover crop between vine rows, but in practice discs and harrows are still used to prepare seed beds. Disc-harrows are known to cause soil compaction below the disc (Figure 9.2). Harrows on the other hand tend to get blocked when large volumes of cover crop residue still remain on the soil surface during seed bed preparation. Wheel tractors normally used in vineyards can have a marked effect on soil compaction (Van Huyssteen, 1983) and in vineyards on slopes this compaction is most severe under the lower track within the working row (Schulte-Karring, 1976). Van Huyssteen (1983) showed that the first passage of tractor wheels over loose Clovelly soil of granitic origin caused a 15 % drop in the soil surface under the tracks, as well as a significant increase in soil strength down to 45 cm (Figure 9.3). A second passage of the tractor wheels significantly increased the soil strength in the 0-15 cm depth only.

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