SOIL PREPARATION

CHAPTER 10

growth similar to its dryland counterpart, a factor which promised future problems with unfavourable micro-climate in the canopy and poor wine quality.

TABLE 10.3: Effect of soil depth on average total evapotranspiration for irrigated Pinot noir/99R on a Glenrosa soil at Stellenbosch (Myburgh et al ., 1996). Evapotranspiration (mm) Soil depth First Leaf Second Leaf Third Leaf 400 mm 183.7 264.0 387.1 800 mm 227.2 330.6 521.7 1 200 mm 262.6 400.6 614.4 Considering all indicators of grapevine growth and physiological activity from planting up to the fourth year, optimum shoot growth occurred on the 600 mm to 1 000 mm dryland treatment plots as well as on the 400 mm and 800 mm irrigated treatments. From these results it was evident that irrigation could, within limits, compensate for lack of soil depth, but more importantly, also vice versa, namely that adequate soil depth could compensate for lack of irrigation in the coastal regions of South Africa. This is an important finding against the ever decreasing availability of irrigation water due to population growth and expected drier conditions in the Western Cape as a result of climate change. It should, however, be borne in mind that the results were obtained for young grapevines before they came into full bearing. It is imperative that research be conducted to determine whether grapevines that bear higher grape loads will respond differently. The above-mentioned result pertaining to the use of irrigation to compensate for lack of soil depth should not be confused with findings that frequent irrigation and sufficient fertilization cannot counteract the effects of a small available soil volume in grapes (Wang et al ., 2001; McClymont et al ., 2006) as well as in other crops (Richards & Rowe, 1976; Myers, 1992; Menzel et al ., 1994). It has been suggested that the decrease in growth was induced by chemical signals from the roots. This is probably also true of the soil volume studies by Myburgh et al . (1996), but the results suggest that optimum soil depth for dryland and irrigated conditions were 80-100 cm and 40-80 cm respectively. In Australia, McClymont et al. (2006) compared the performance of one-year- old Shiraz/Teleki on a range of confined available soil volumes, as well as on unconfined ripped soil. Soil volumes used in their experiment (volume range of 0.15 m 3 -1.2 m 3 per vine) were much smaller than those used in South Africa (Table 10.2) by Myburgh et al . (1996). Vegetative growth of these grapevines

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