FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Zinc (Zn) Plants absorb zinc as Zn 2+ , but it can also be assimilated as chelates by the roots, leaves and bark of vines. It is the metal component and activator of various enzyme systems. A variety of dehydrogenase, anhydrase, proteinase and peptidase systems have been found to be Zn-dependent. It is essential with regard to carbohydrate, protein and fat metabolism and also plays a role in the formation of auxins. Zinc is found in various rocks and Zn-containing minerals. Zn 2+ is released through weathering, and is then adsorbed onto the soil colloids where it exists in equilibrium with soil solution ions. There is no correlation however, between the total Zn-content of soil and the accessibility thereof for plants. The acces sibility of Zn decreases with an increase in soil pH, excessive phosphate appli cations and also on account of an absolute deficiency in sandy soils as a result of leaching. Zndeficiencies has also occasionally been found on the highly weathered granitic acidic mountain slope soils in the Stellenbosch region. Zinc, like Mn, is extracted from soil samples using EDTA, and for soils with a pH KCl of between 5.0 and 6.5, the Zn-content should be at least 0.5 mg.kg -1 (see Table 5 in Chapter 3). For Zn-supplementation in existing vineyards, a 15 kg ha -1 Zn-sulphate solution can be applied on the berms or via the irrigation system and for young vine yards it can be applied evenly on the future vine row zone. A Zndeficiency is manifested by abnormal growth and severe effects on bunch development. Deficiency symptoms first appear on apical leaves as general interveinal chlorosis, comparable to that of Mn. Leaves remain small (known as small leaf symptom), the petiole sinus angle enlarges and internodia are short, which lead to a rosette appearance (known as witches’ broom, Fig. 23). Berries shatter and subsequently bunches are straggly, with berry size varying due to it containing few or no pips and also tending to remain hard and green. Certain cultivars, like i.a. the muscat types, are very sensitive to Zndeficiencies and bunches can be affected before leaf symptoms become visible. The norms for the Zn-contents of grapevine leaf blades and petioles are indicated in Table 16.

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