SOIL PREPARATION

4.2 Root pruning The deep tillage of soil in an existing vineyard accomplishes two things, namely removal of compacted layers and, inadvertently, also root pruning. The results of grapevine root pruning pertaining to well-established vineyards have not been thoroughly investigated. Most root cutting experiments were done under conditions where impediments to root growth was not a factor at all, e.g. in water culture (Buttrose & Mullins, 1968), in containers (Poni et al ., 1992; Ferree et al ., 1999; McArtney & Ferree, 1999) and in all these studies cited, the grapevines were young plants. Generally, removal of part of the root system led to a decrease in transpiration rate, photosynthesis, and shoot growth, and the reductions were proportional to the severity of root pruning. Ferree et al . (1999) reported different plant responses depending on the phenological stage (bloom or veraison) when the treatments were applied. Removing 25-30 % of the cross- sectional area of framework roots of 5-year-old drip-irrigated Shiraz (Syrah) on 101-14 Mgt rootstock changed the grapevine water relations, even though the vines were well-watered and grew in a deep sandy clay loam soil (Smart et al ., 2006b). These effects occurred immediately after root severance and continued throughout the season. Their results suggested that the grapevines had limited stem and root hydraulic capacity and were consequently unable to supply the water requirements under the conditions of high evaporative demand that prevailed at the experimental site. Most studies on root pruning of fruit and grapevines generally aimed to reduce excessive vegetative growth and consequently shading, thereby improving fruit quality (Schupp & Ferree, 1987, 1988; Dry et al ., 1998; Ferree, 1992; Smart et al ., 2006b; Giese et al ., 2015). In contrast, root pruning experiments in South Africa have always aimed at improving the growth and yield of declining vineyards when the cause of such a decline was due to soil compaction. The alleviation of soil compaction in existing vineyards and the cutting of their roots always go hand in hand; the one without the other is not possible. This not only increases the soil volume, but the regrowth that occurs at the severed roots also rejuvenates the root system. Root pruning stimulates the formation of new roots behind the severed ends (Figure 4.6). Van Zyl & Van Huyssteen (1987) reported as many as 34 new roots which formed on a single Sultanina root after it had been pruned. They also observed that thick roots had better regeneration ability than thin roots and that regrowth occurred close to severed root ends. Younger plants usually also have a better regenerative ability than older ones (Geissler & Ferree, 1984), although Litvinov (1967) recommended that deep cultivation should not be done in vineyards younger than six years and that it should only be repeated after two to three years.

54 | ROOT RESPONSE TO SOIL CONDITIONS