FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 5

steps b and c, with b a rapid process (nitrification), which is executed by specialised, autotrophic Nitrosomonas bacteria, and c a very rapid process which is also executed by specialised, autotrophic bacteria, Nitrobacter. In well aerated soils NO 3 – -ions are the most prominent for plant nutrition but may also be toxic in high concentrations ( inter alia growth arrest phenomenon). FERTILISATION PROGRAMME FOR NITROGEN Fertilisation scheduling Bud break to fruit set : As already indicated (Table 10), 25% of the annual N requirement is absorbed from the soil between bud break and fruit set. However, at the start of this period the N requirement of the new growth (leaves, shoots and bunches) exceed the amount absorbed from the soil, implying that a large portion of the required N is supplied through remobilisation from the permanent structure (roots, perennial wood). If insufficient reserves were accumulated during the post-harvest period, it will be impossible to rectify “deficiencies” by means of excessive applications, during the first three to four weeks after bud break. In practice, however, the aim is to have a fully developed canopy at the end of this growth stage already, implying that rapid active growth is required. If more than the natural requirement is applied from three to four weeks after bud break, growth is stimulated. Up to 40% of the annual N requirement may be applied during this period in vineyards that are not excessively vigorous, instead of the 25% that is naturally absorbed. Fruit set to veraison : Approximately 41% (Table 10) of the vine’s annual requirement is absorbed between fruit set and harvest. For vineyards where active growth stops before veraison, practically all of this is absorbed before veraison, where after uptake stops temporarily (Conradie, 1980). In more vigorous vineyards only 30% of the annual requirement may be absorbed between fruit set and veraison, followed by 11% between veraison and harvest (Conradie, 1986). In such vigorously growing vineyards, the leaves still act as a sink for N, that may lead to delayed ripening and insufficient colour development. To prevent grapevines from growing after veraison, the recommendation is to reduce the total amount of N applied during this period to 25% of the annual application, and that this is applied shortly after fruit set. This will prevent grapevines from growing after veraison and bunches from being over supplied with N during ripening, which will make it more susceptible for decay. However, evaluation of of vegetative growth is essential during this period and where growth is below average an additional N application (of approximately 15 kg N ha –1 ) can be done at 10 mm berry size. Veraison to harvest : For vineyards with average to vigorous growth, no fertilisers should be applied after veraison, while at most 10 – 11% of the annual requirement may be applied in vineyards with below average or poor growth. Excessive fertilisation during this period can be very detrimental for quality, because the susceptibility for decay will be increased.

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