SOIL PREPARATION

FIGURE 5.17: Round plant holes made by a soil auger easily induce a potting effect. In addition to the smeared sides, roots will be deflected from the sides due to their angle of incidence and consequently grow in a circular fashion (Louw & Van Huysteen, 1993) (Photos: ARC Infruitec-Nietvoorbij). A further disadvantage of a round plant hole is the angle of incidence of a growing root tip on the side of the hole (Nicolosi & Fretz, 1980). The larger the angle of incidence of a root on a soil layer, the easier such a root will grow along the layer instead of penetrating it. According to Louw & Van Huyssteen (1993), square plant holes with uneven sides are best suited to minimise the angle of incidence phenomenon. The size of plant holes is important in promoting proper grapevine root distribution. Albertse & Saayman (1989) recommended square plant holes of 30 x 30 x 30 cm, very similar to Van der Westhuizen (1981) who recommended a square plant hole of 30 x 30 cm and slightly deeper than the length of the grapevine itself. Plant holes that are too small force the grapevine roots upwards or result in the roots being pressed flat onto the bottom of the hole. This causes poor root growth initially and weak root distribution later on (Archer & Hunter, 2010). Small plant holes may also force growers to cut back the grapevine root system before planting. Pruning of roots at planting should be kept to a minimum because roots are important storage organs of, especially, carbohydrate reserves. Such a practice will decrease nutrient reserves of the young vines and the pruning wounds may serve as entry points for soil pathogens (Albertse & Saayman, 1989; Archer & Hunter, 2010). Loosened soil is very susceptible to re-compaction. Practices such as levelling and ploughing of loose soil can cause wheel compaction and a plough sole just below the ploughing depth (Figure 5.18). Such physical restrictions can be overcome by a suitable plant hole (Figure 5.19) i.e. one that is deeper

80 | SPECIAL SOIL PREPARATION STRUCTURES