FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

The annual element uptake during the post-harvest period can vary from 15% (K and Mg) to 34% (N). Most of the nutrients that are absorbed during this period, are stored in the permanent organs, while translocation from leaves and shoots to the permanent organs, may also occur. Nutrients that are absorbed after harvest play an essential role at the start of the next growing season, thus pointing towards the importance of post- harvest fertilisation. In order to obtain optimum grape quality, meaningful fertilisation practices should take these uptake patterns into account, because both insufficient and excessive fertilisation can have negative effects. MACRO-E LEMENTS Nitrogen (N) The role of N was only established at the beginning of the 19th century. It is the basic element for cell division and the development of vegetative organs as it forms part of proteins, nucleic acids, auxins and chlorophyll. The plant hormone auxin inhibits growth in length but stimulates branching, and there is a direct correlation between N supply and auxin production. Consequently, N nutrition results in thicker and more branched root growth, while roots are long, thin and sparsely branched where N deficiencies exist, like in bleached sands. Nitrogen is also stored as reserves in cells in the form of alkaloid granules. Compared to the other essential nutrients, nitrogen is more frequently the growth restricting nutrient implying that nitrogen has a positive effect on growth. In contrast maximum fruit yield is obtained at a lower N nutrient level than that at which maximum vegetative growth is obtained. At suboptimal N supply, nodes are initially adversely affected by low N nutrient levels, resulting in low yields in the next season. Excessive applications may lead to water berry ( grape stem necrosis / pallo negro, dessèchement de la rafle ) and to reduced carbohydrate content. Carbohydrates, produced from photosynthesis, are used to produce protoplasm and are consequently not available for accumulation, like sugar in the case of grapes, which leads to delayed ripening. When N supply is high, cells tend to be larger, which can have a negative effect on storage ability and disease resistance. An indirect result is that larger leaves and longer shoots may lead to dense canopies and thus create favourable conditions for diseases (especially Botrytis rot), as well as decreased sunlight penetration, with negative effects, particularly on colour, but also on overall quality. Low carbohydrate reserves and large cells can also contribute to the frost sensitivity of grapevines. Nitrogen is considered mobile, implying that N compounds in older leaves, which no longer function very actively, hydrolyse and that the fragments (probably amino acids) translocate to younger parts of the plant. Deficiency symptoms, namely general yellowing, are consequently mostly present on older leaves (Figure 2). It only becomes visible when there has already been a negative impact on crop load, making it prudent not to wait for the appearance of such symptoms before intervening. Excessive N nutrition results in the rapid enlargement of leaves, which induces a temporary K

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