FERTILISER GUIDELINES FOR THE WINE INDUSTRY

LEAF ANALYSES Leaf analyses can be implemented as additional means to control the N-nutrient status. As already stated in Chapter 2, large variations do however occur with regard to cultivars, cultivation areas, time of sampling, sampling position, seasonal climatic differences and soil types. A general norm that makes provision for all conditions, therefore, is inevitably very wide-ranging. According to Table 10, it stretches from 1.6% at pea berry stage (deficiencies are expected at lower contents) to 2.7% (growth deviations are expected at higher values) for leaf blades. This wide range therefore has little value for the establishment of a fertilisation programme. If analyses are performed on an annual basis, it may be possible however, to observe trends over time and/or establish unique norms for a specific block. TABLE 10: Norms for the N-content (%) of grapevine leaf blades and petioles from fruit set to berry stage. As already stated in Chapter 2, leaf analysis can be useful however, in case studies. Leaves are sampled in a specific way from vines that display deviations and in the same 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 necessarily 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 indicator for nitrogen status. Although guide lines are not very precise with values ranging widely and being very dependent on whether it rained or irrigation was applied. In practice the total nitrogen content of petioles (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 petiole nitrate N-values exceed 1 000 mg kg -1 . Leaf blades Petioles Minimum Maximum Minimum Maximum 1.6 2.7 0.6 1.0

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