FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Nitrogen

• The nitrogen status of a vineyard can have a direct or indirect influence on grape composition and ultimately wine sensory characteristics. In terms of the direct influence, various researchers have found that increased Nappli cations in the vineyard lead to increased levels of the Nfixing compounds like total N, amino acids, arginine, proline, ammonium and FAN (total assimilable N) in the grapes and must. These N-compounds are important components, precursors and nutrition for yeast metabolism for flavourant and aroma formation. • There must be sufficient amounts of nitrogen to ensure good growth, flower ing and fruit set, and for optimum fermentation there should be a minimum of 250 to 300 mg N/L FAN available in the juice. • Low N-levels in grapes and must give rise to low FAN-levels which lower yeast metabolism and can lead to e.g. stuck fermentation or to the formation of H 2 S (hydrogen sulphide), which naturally is detrimental for wine quality and wine sensory characteristics. • In various countries, research projects are currently underway where N-ap plications in the vineyard is done later in the season as foliar and/or soil applications, in some cases as late as a few weeks before harvest, in order to determine the effect on grape composition/wine style/flavourant spectrum. A lot of varying results have been reported regarding the cultivars’ reaction to the different treatments. • Jordaan (2005) found that additional N-fertilisation during the set and verai son periods had a negative effect on the FANcontent of the must, but that it did however increase berry mass, which is positive for yield objectives but not for quality wine objectives. It should be taken into consideration that the soil on which the trial was conducted, had a carbon content of 1.4% – the Ncontent in the soil should be sufficient at this norm. Conradie (2001) indicated that nitrogen fertilisation on soil with a carbon content of 1.14%, has no effect on FANcontent. In contrast, nitrogen fertilisation does have an effect on soil with a lower carbon content (0.50%C). On such soils the FAN-content was increased where fertilisation was applied by fruit set, while application at veraison proved to be less effective. It should be considered however, that the water status of a soil can also play an important role. On soils that are relatively dry by veraison, no effect on nitrogen fertilisation can be expected.

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