FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 5

Fertilisation according to leaf analyses Leaf analyses can be used as an additional means to manage the N nutrient status. However, as already stated in Chapter 2, analytical values may vary significantly, due to large variations as a consequence of cultivar, cultivation area, time of sampling, sampling position, seasonal climatic differences and soil type. A general norm that makes provision for all conditions is, therefore, inevitably very wide-ranging. According to Table 13 it ranges, for leaf blades at fruit set, from 2.0% (deficiencies are expected at lower values) to 3.2% (growth deviations are expected at higher values). This wide range has therefore little value for the establishment of a fertilisation programme. However, if analyses are performed on an annual basis, it may be possible to observe trends over time and/or establish unique norms for a specific block. Leaf blades Petioles Minimum Maximum Minimum Maximum Minimum Maximum Minimum Maximum 2.0 3.2 0.9 1.2 2.8 3.4 0.6 0.95 * Veraison analysis could be unreliable, because leaves that are traditionally sampled in the bunch zone may be aged (senescent), broken out or mechanically damaged, thus not giving a reliable indication of the nutritional status of the vine anymore. Leaf analysis, however, can be useful in case studies by sampling leaves in a specific way from vines that display deviations and sample in an identical manner from healthy or better performing vines, in closest possible proximity to the deviating vines. The leaves of the healthy/better performing vines then serve as direct control, rendering the time of sampling and all the other variables less critical. No norm is now used, but rather the relative differences between the two samples as indication of possible elemental deficiencies or imbalances. The nitrate content of petioles, sampled at the flowering stage, is quite widely used in the USA and Australia as an indicator of nitrogen status. Even though guidelines are also not very precise, and very dependent on whether it rained or whether irrigation was applied, values cover a much wider range, in comparison to total N. In practice the total nitrogen content of leaf blades (for example) can vary as little as 10% between under and over fertilised vineyards, while the nitrate content of petioles can vary with 100%. It can tentatively be accepted that nitrogen fertilisation at fruit set will be unnecessary if the nitrate- N values of petioles exceed 1 000 mg kg –1 (Conradie, 1998). Phosphorus (P) Phosphorus is an essential macro-element, being an ingredient of phospholipids and nucleic acids. It plays an essential role in energy transfer through adenosine Petioles Leaf blades TABLE 13: Norms for the N content (%) of grapevine leaf blades and petioles from fruit set to pea size berry stage, as well as veraison. Fruit set to pea size berry stage Veraison*

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