SOIL PREPARATION

such open ditches easily collapse, and weeds cause blockages to water flow. It should furthermore be kept in mind that open ditches are physical obstacles to implement traffic in vineyards and therefore it is not always a practical solution to collect subsurface drainage and surface run-off.

FIGURE 6.4: Open ditch that was dug to drain low-lying wet soil. (Photo: J. de Jager, Vinpro).

6.4.3 Stone drains Stone drains are simply open ditches that are filled by a few layers of stones ( ca. 30 cm diameter) at the bottom to create a subsurface channel for the flow of excess water. An effective channel for water flow can be created by placing two rows of stones at the bottom against the trench walls and by covering them with stones to form a roof (Figure 6.5). Smaller stones, followed by gravel and eventually coarse sand, are placed on top of the large stones before the ditch is filled up by soil (Van Woerkom & Streutker, 1971). This is not a drainage system that is generally used anymore since it requires manual labour and the availability of large volumes of stone in the vicinity. Van Woerkom & Streutker (1971) were, however, of the opinion that stone drains still constituted the major portion of underground draining systems in South Africa in the 1970s. Stone drains are less prone to blocking by plant roots than pipe drains. Its application is, however, restricted to more clayey soils due to the chance of silt and sand blocking the openings between the stones.

92 | SOIL DRAINAGE