SOIL PREPARATION

CHAPTER 10

A third soil preparation trial comparing different implements and working depths on a granitic Glenrosa soil under dryland conditions commenced at Stellenbosch in 1980 (Van Huysteen, 1988). Natural soil compaction showing bulk density values of 1 750 kg/m 3 occurred at 450 mm to 1 050 mm depth. The land was planted to Pinotage/99R and Pinotage/101-14 Mgt after deep tilling. However, despite significant differences in penetrometer soil resistances, soil preparation techniques and depths had no effect on either root growth or grapevine performance. Occasional root penetration through cracks to at least 750 mm on the control (shallow ploughing) was considered to be the cause of this lack in vine response. In a further attempt to exactly determine the soil depth requirements of grapevines under both dryland and irrigated conditions, Myburgh et al . (1996) conducted an experiment with Pinot noir/99R on a granitic Glenrosa soil. In this experiment an excavator was used to create trenches of exact depth and width, the bottoms of the trenches were mechanically compacted to prevent any root penetration and their side walls sealed off by plastic sheeting (Figure 10.2). In the case of irrigated treatments, the entire trench was lined with thick plastic sheeting and equipped with drain pipes to create a type of a field lysimeter. The position of the vineyard on a 15° slope allowed the catching of all subsurface drainage water from the irrigated treatments and subsequently the calculation of an accurate water balance for the vines. The full range of treatments, including their available soil volume, is described in Table 10.2.

FIGURE 10.2: Stages in the construction of soil volumes for a trial with Pinot noir grafted on 99 Richter; trenches were lined with plastic and provided with drainage pipes before the soil was back-filled (Photos: ARC Infruitec-Nietvoorbij).

Pruning mass increased with soil depth among the dryland group of treatments. Ridging, as well as trenches of 400 mm depth, showed no benefit over the control in the second and third seasons after planting. A significantly higher vegetative growth compared to the control occurred in treatment plots of 800 mm and deeper in the second season and at 1 000 mm and deeper in

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