SOIL PREPARATION

Terracing is a very expensive practice and should only be considered when table grapes or high quality wine grapes are planted. Terraces should be wide enough to accommodate at least two vineyard rows per terrace (Figure 5.9). In order to curb costs these structures should however, become narrower the steeper the slope. The cost-benefit ratio of terracing on land which is so steep that only one vine row per terrace can be planted, is probably not justified since erosion and the control of run-off then become serious problems. The first step is to determine and mark the position of the terrace along the contour. Terraces should have a lateral slope of approximately 0.5 % to prevent ponding of surface water, have a uniform width over their entire length and be limited to a maximum length of 150 m (Saayman & Van Huyssteen, 1981a).

FIGURE 5.8: Normal cultivation in a vineyard on sloping land gradually results in sideways displacement of soil and a fairly level area between vine rows (Photo: J.E. Hoffman, Stellenbosch University). The width of a terrace will be determined by the slope of the land and the soil depth. Terraces can be approximately 6 m wide at a 20 % slope if the soil is more than 1 m deep. For a similar soil depth, 4 m wide terraces can be constructed on a 30-40 % slope while the terrace width should further decrease to 3 m on still steeper slopes (Saayman & Van Huyssteen, 1981a). According to these authors, unused mountain soils in South Africa normally have slopes of 35-45 % and more, which will allow terraces of 3-4 m wide that can easily accommodate two vine rows per terrace. A technique for the construction of terraces that has been used in the past, and that is still used today in South Africa, employs the blade of a crawler

70 | SPECIAL SOIL PREPARATION STRUCTURES