FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Molybdenum (Mo) Shale and granite are the rocks with the highest concentrations of Mo, with the best-known minerals in the form of sulphide, oxide, and molybdates. In soil it occurs predominantly as molybdate ions, which are adsorbed by the positive charges at low pHs. The concentration of Mo in plants is much lower than for the other essential micro-nutrients. It is an essential component of nitrate reductase and nitroge nase and thus essential for freeliving and symbiotic Nfixation, and also the first step in the Nmetabolism of plants where N is assimilated in nitrate form. Deficiencies are characterised by light green colours similar to Ndeficiency, and in more acute deficiencies it form leafedge scorch, probably due to accu mulation of nitrate in the tissue, which however, in contrast to saline symptoms, remains supple. The necrosis expands until the entire leaf becomes necrotic, with only the sinus part remaining unusually green. Molybdenum deficiencies have not yet been identified in South Africa, although in pot trials with vineyard soil in which Sdeficiencies were identified, there was indeed a reaction to Mo-addition, but only in combination with S. A guideline for molybdenum deficiencies is <0.09mg/kg measured at peak flowering. Molybdenum can be applied as foliar spray with a 0.05%-Na-molybdate solu tion or through a soil application of 150 g ha -1 Na-molybdate in solution, which is evenly sprayed over the entire soil surface. Chlorine (Cl) It was only undisputedly proven by the middle of the previous century that CI in small amounts is essential for plant nutrition. Deficiency symptoms vary considerably between plant types, but usually display wilting symptoms, which may progress to the development of bronze colours, cupformed leaves and specks on and necrosis of leaves, with stunted, short and finely branched roots. Normally plants contain more CI than the minimum requirement and deficien cies have not been reported for grapevines. No fertilisation strategy is in place for vines, but it may be possible for poor CI-nutrition, as in the case of S, to develop over time in high rainfall areas on sandy and highly weathered soils, as KCl is currently the only fertiliser being a Cl-carrier, on account of fertiliser mixtures not containing Cl. In coastal regions problem with Cl-nutrition are not envisaged, because up to more than 100 kg ha -1 j -1 Cl can be carried onto the land by sea winds. The CI-content of chicken and Karoo sheep manure is high, but the potential saline danger thereof is of greater significance than it being a source of CI-nutrition.

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