FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

It is accepted (Table 8 in Chapter 5) that approximately 4 kg ha –1 N per season is required to produce 1 ton of grapes, thus coming down to an annual N utilisation of 120 kg ha –1 . However, as indicated in Chapter 5, production cannot be used as a general criterion for calculation of fertiliser requirements, specifically because soil type should also be considered. On sandy soils, 120 kg ha –1 N may be applied, without storage ability (occurrence of Botrytis after cold storage) being affected, while Botrytis may already be relatively high for soils with high organic matter content, even where no N is applied (note Figure 5 in Chapter 5). However, the N nutritional status of a vineyard is not only determined by the organic matter content of the soil, because other factors, like soil characteristics, irrigation management, scion/rootstock combination and climate, also play a role. Vegetative growth is thus regarded as the most dependable indication of a vineyard’s N nutritional status , because it includes all the above-mentioned factors. The norms that are used to evaluate vigour, together with the appropriate fertiliser guidelines have been discussed in Chapter 5. For vineyards with poor vigour, N fertiliser is applied based on seasonal requirement in a minimum of three instalments, namely approximately three weeks after bud break, shortly before flowering and/or at pea size stage and lastly shortly after harvest. For vineyards with ideal growth, the application at fruit set may largely be eliminated, if the canopy is already fully developed at flowering and active growth is still occurring. For vineyards that are growing more vigorously, the application after bud break may also be eliminated. Post-harvest fertilisation will only be eliminated for overly vigorous vineyards. PHOSPHOROUS Very little is known about the direct role of P on grape composition and grape quality, even though it is accepted that excessively high P levels may affect K uptake detrimentally (Conradie and Saayman, 1989). Due to the role that P plays in the utilisation of carbohydrates and the provision of energy during growth, a P deficiency may be injurious to the grapevine’s capability to maintain active growth during berry enlargement. Consequently, cell division and reaction to enlargement sprays are negatively affected, possibly resulting in smaller berries. POTASS I UM The role of K in cell growth within the berry, has been highlighted by Mpelasoka et al. (2003), with specific reference to the role that it plays in the process where cell walls in the skin become detached from each other, as soon as veraison commences and the cells of the berry start to expand rapidly. The entire process is initiated by enzymes that are activated under the influence of K and that are responsible for the cell walls separating from each other. The role of K in the expansion of berry cells and in berry growth is therefore critical. Furthermore, a distinct correlation between the K content of berries and the accumulation of sugar and dry mass has also been found. Where K deficiencies occur, uneven ripening of berries in the same bunch (Figure 28) becomes problematical (Mullins et al ., 1992).

98 | THE RELATIONSHIP BETWEEN FERTILISATION, THE NUTRITIONAL STATUS OF A VINEYARD AND GRAPE QUALITY