SOIL PREPARATION

the sides using at least four passes of a two-furrow mouldboard plough. This is, however, not an ideal method since the tractor wheels compact the shoulders of the ridge during construction. In a field experiment, Myburgh & Moolman (1991a) successfully constructed ridges of 400 mm and even 600 mm high with an articulated grader without wheel traffic on the loose soil of the ridge. In all of these construction methods, ridges usually have to trimmed by hand, especially when a flat top is needed on which two vine rows can be planted. An excavator is the preferred implement for the construction of ridges (Figure 5.2). By fitting a scoop of 1.5 m to the excavator, one load of topsoil on each side of the ridge can create a ridge of 40 cm high in rows of 4.5 m wide (distance between the tops of ridges). This is a fast process and the ridge can be shaped well enough that no further trimming is necessary. The use of an excavator also gives an operator good control over the depth of topsoil removal and prevents the placement of subsoil clay on the ridge.

FIGURE 5.2: Excavator constructing ridges (left) and well-constructed ridges made by an excavator (right) (Photos: J. Nolte).

Compared to an excavator, the use of an offset disc-harrow is a cheaper and equally effective method of constructing ridges. The tractor only moves in the ditch between ridges and consequently does not compact the soil under or on the ridge while the plough rolls the soil from the ditch onto the ridge. Ridging is a form of surface drainage in contrast to subsurface drainage. Ridges must consequently have a row direction that allows run-off water to flow from the vineyard. The wetter and more level the land is, the more important the row direction of the ridges becomes. Run-off must not pond in parts of the ditches between rows (Figure 5.3), but be able to flow freely out of the vineyard. Ridges on slopes should have a row direction that is not straight down-hill, but at a gradient (between 2⁰ and a maximum of 10⁰) to ensure that surface water

62 | SPECIAL SOIL PREPARATION STRUCTURES