SOIL PREPARATION

can be attributed to the alleviation of soil compaction and the concomitant better root development or to the nutrient and chemical improvement of the soil as a result of compost application. Depth of loosening between vine rows is also important because that will determine the number of roots that are cut and what soil volume will be available for exploitation by the new roots. The aim should be to loosen the soil to a depth of at least 50-60 cm. The availability of a suitable implement that can fit in between rows may be a limiting factor. A small excavator is probably the ideal machine to use, especially when compost or other ameliorants are to be incorporated into the soil. A ripper will also serve the purpose of alleviating soil compaction, but cannot incorporate compost or other ameliorants effectively into the soil. No evidence exists in South Africa to support the view of the French viticulturist Champagnol (Archer & Saayman, 2018) that root renewal through deep tillage holds the risk of spreading virus diseases (Carstens, 2018). It seems more possible that certain fungal diseases such as blackfoot, pythium and phytophthora will be enhanced by root wounds. This risk will probably increase the wetter the soil and the younger the grapevines are, but no research has been done to support this theory (Halleen, 2018). Weighing of all the current scientific evidence suggests that intentional root pruning per se is not a practice that can be generally recommended to improve grapevine yield and shoot growth. Deep soil tillage in existing vineyards should only be applied where a thorough soil investigation shows root-impeding compaction. The goal should therefore be to create a more favourable root environment with some root pruning as an unintentional consequence. More research is, however, needed before a reliable prediction can be made regarding grapevine performance to ripping of the inter-row area in existing vineyards. Such research must take into account soil type, soil water conditions, root distribution and rate of re-compaction, among other factors.

58 | ROOT RESPONSE TO SOIL CONDITIONS