SOIL PREPARATION

the third season. In the third and final season of the experiment, shoot growth of the 1 200 mm deep treatment already exceeded the capacity of the trellising system (Figure 10.3). Unfortunately any deductions regarding yield response to soil depth were impossible since the authors limited the crop load to only 12 bunches per vine on all treatment plots. TABLE 10.2: Soil preparation treatments applied in a soil-volume trial at Nietvoorbij (Myburgh et al ., 1996). Soil preparation treatments Available Root Volume (m 3 /vine) Dryland Irrigated Control, ploughed to 200 mm Not known Ridges, 400 mm high Not known Trenches, 400 mm deep Trenches, 400 mm deep 1.62 Trenches, 600 mm deep 2.43 Trenches, 800 mm deep Trenches, 800 mm deep 3.24 Trenches, 1 000 mm deep 4.05 Trenches, 1 200 mm deep Trenches, 1 200 mm deep 4.86 Similar to the dryland treatment plots, pruning mass of the irrigated grapevines increased with soil depth despite the fact that leaf water potentials showed no signs of water stress among treatments. Factors other than water alone determine aboveground growth. Size of the root system, as well as the balance between subterranean and aerial growth, are most probably involved.

FIGURE 10.3: Appearance of Pinot noir grafted on 99R under a) irrigation of shallow soil and b) dryland conditions on the same soil that was loosened to 1.2 m depth (Photos: ARC Infruitec-Nietvoorbij) Soil depth did, however, affect the evapotranspiration of the three irrigated treatments (Table 10.3) as would be expected from the differences in shoot growth. The irrigated 1 200 mm deep treatment showed excessive vegetative

154 | ABOVEGROUND GRAPEVINE RESPONSE