SOIL PREPARATION

have two impeding properties in the subsoil, namely natural compaction and low pH. Vineyards on these soils respond well to deep loosening and lime application before planting. The recommended method in this case will be shift delving to a minimum depth of 80 cm. Adequate mixing of lime with the soil can be obtained with single-direction delving provided the correct cutting width (60 cm) and implements are used. A decision to proceed with double delving should only be taken after inspection of the modified soil profile has been done to assess the effectiveness of the first operation. Red and yellow soils also occur in the more arid inland viticulture areas of South Africa. In these low rainfall regions, the red and yellow soils are normally not acid, but they may contain hardpans in the subsoil due to cementation by silica, lime or iron, and manganese accumulation. These hardpans are known as dorbank along the Olifants River and as kalkbank in the Breede River valley. Grapevines respond well to the breaking up of such cemented or massive soil layers (Saayman & Van Huyssteen, 1981a). In the Vaalharts area, limestone hardpans are commonly found at more than 1.0 m depths. It is unnecessary to break up a hardpan at such a depth, but poor irrigation scheduling on such soils can lead to waterlogging and salinisation. Due to its high fine sand fraction (> 70 %), the Vaalharts soils are prone to compaction just below the tillage depths. This compaction occurs at a shallow depth, but must be removed before planting. Soil preparation to a depth of 45 cm using a delve plough should be adequate, but examination of a soil profile will indicate whether deeper tilling will be required. 7.2.4 Duplex soils Duplex soils are characterised by a relatively permeable horizon(s) overlying a slowly permeable subsoil horizon which is not a cemented hardpan. Often a concretion or gravel layer directly overlies the clay. These soils that occur commonly in the Western Cape, experience periodic waterlogging in localised patches as well as lateral water movement on the clay. Wet duplex soils such as Kroonstad, Longlands and Cartref soils must be drained (see chapter 6 on “Soil drainage” for selection of an effective drainage system). Soil preparation must be aimed at breaking up any continuously cemented concretion layers and to improve the leached E horizon by incorporating lime and nutrients that will mostly be necessary. The soil preparation implement can, to a limited extent, be allowed to remove the sharp transition between the E horizon and the clay, but care must be taken not to bring clay to the soil surface (Figure 7.19). Ploughing the clay up to mix with the overlying soil horizons will disturb the internal drainage of the soil and may cause waterlogged areas in the soil. Furthermore, many clays are unstable and will slake when it lands on the

128 | CHOICE OF IMPLEMENT FOR SOIL PREPARATION