SOIL PREPARATION

as well as number of bunches per vine decreased with reduced soil volume, despite the fact that adequate water and nutrition were supplied in the confined rooting soil volume. Increasing the available soil volume from 0.8 m 3 to 1.2 m 3 , however, did not further increase fertility or vine growth significantly, suggesting that the optimum soil volume had been reached for the one-year-old Shiraz vines. The restriction of root systems can also be positive in situations where excessive vegetative growth, poor berry set and inadequate ripening occurs. Such unbalanced conditions between vegetative and reproductive growth have been reported with Kyoho grapes in certain areas of China and Japan, but similar problems will undoubtedly also exist in other areas or with other cultivars. In an attempt to address this problem of too extensive shoot growth and uncontrolled uptake of water and nutrients, Kyoho grapevines were planted in buried beds to restrict root volumes (Wang et al ., 2001). Allocated soil volumes (0.05 m 3 , 0.1 m 3 and 0.15 m 3 ) were quite small. Smaller root volumes tended to more rapid depletion of soil water and caused shorter shoots, decrease in trunk circumferences, smaller leaf areas and lower photosynthetic rate. Berry set and berry composition were, however, improved. In conclusion, based on practical observations and field experiments, soil preparation in South Africa aims to loosen the soil to a depth of at least 80 cm, but preferably to one meter. Normally no further benefit will arise from loosening the soil deeper than one meter. Considering all the environmental threats to viticulture, such as higher temperatures and lower rainfall due to climate change, a well-developed root system buffers grapevine performance against adverse climatic conditions.

156 | ABOVEGROUND GRAPEVINE RESPONSE