FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

After harvest : Approximately 34% of the annual uptake occurs during this period (Table 10). Most of this N accumulates in the perennial organs, while N also translocates from the leaves and shoots to perennial organs. This N is extremely important at the start of the next growing season. Post-harvest fertilisation is therefore essential, but the grapevines must not still be growing actively. The latter is specifically relevant for the early areas with long post-harvest periods (Mpumalanga and Orange River), where excessive regrowth during the warm summer months is easily induced by large N instalments (Conradie, 2005a). In all areas, post-harvest fertilisation should preferably not be applied before end February, and where vines are still growing actively it may even be applied at a later stage. Relative importance of fertilisation at different stages : Fertilisers should not necessarily be applied in all vineyards at all of the above-mentioned growth stages. When applications must be reduced, it will firstly be done for the application between fruit set and veraison, secondly at the application between bud break and fruit set and lastly at the post-harvest application. This aspect will be discussed in more detail in the next section, dealing with different approaches that may be used for the implementation of fertilisation programmes. Fertilisation according to production It is accepted that approximately 4 kg ha –1 N is required per season for the production of 1 ton of grapes, thus amounting to an annual consumption of 120 kg ha –1 for a vineyard with a production of 30 ton ha –1 (Table 8). For vineyards that are managed according to hydroponic systems, i.e. vineyards with small root systems, irrigated and fertilised daily, fertilisation according to production may be used as point of departure. Furthermore, it may be used as a guideline for soils inducing low to moderate vigour. This principle was affirmed for a vineyard on sandy soil with a low organic material content of 0.3% (Conradie & Myburgh, 2000). Where 120 kg ha –1 N was applied, it resulted in a production of 29.5 ton ha –1 , while production was significantly lower (21.0 ton ha –1 ) where only 80 kg ha –1 N was applied. In practice, however, this approach (fertilisation according to production) cannot be generally applied, due to the possible effect thereof on Botrytis rot, as illustrated in Figure 5 for a vineyard that produced approximately 25 ton ha –1 (Conradie, 1998, 2001). On the sandy soil (0.43% C) 120 kg ha –1 N could be applied, without storage ability (occurrence of Botrytis rot after cold storage) being negatively affected. For the loamy soil (1.15% C) Botrytis rot was already relatively high where no N was applied, while it was significantly enhanced for an application of only 60 kg ha –1 N. Above-mentioned examples show that production cannot be used as a general guideline for the calculation of fertiliser requirements, specifically because soil type should also be taken into consideration.

44 | MAINTENANCE FERTILISATION