FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 5

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 molybdenum ions, which are adsorbed by the positive charges at low pH values. 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 nitrogenase and thus essential for free-living and symbiotic N fixation, and also the first step in the N metabolism of plants where N is assimilated in nitrate form. Deficiencies are characterised by light green colours similar to N deficiency, and for more acute deficiencies it forms leaf-edge scorch, probably due to accumulation of nitrate in the tissue, which however, in contrast to saline symptoms, remains flexible. The necrosis expands until the entire leaf becomes necrotic, with only the sinus part remaining uncommonly green. Molybdenum deficiencies have not yet been identified in South Africa, although in pot trials with vineyard soil in which S deficiencies were identified, there was indeed a reaction to Mo addition, but only in combination with S. A guideline for molybdenum deficiencies is < 0.09 mg kg –1 (< 90 µg kg –1 ) in leaves, measured at peak flowering time. Molybdenum can be applied as foliar spray with a 0.05% Na-molybdate solution or through a soil application of 150 g ha –1 Na-molybdate in solution, which is sprayed evenly 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, because it is involved in photosynthesis, osmotic regulation and disease suppression. Deficiency symptoms vary considerably between plant type, but usually display wilting symptoms, which may progress to the development of bronze colours, cup-formed leaves and specks on and necrosis of leaves, with stunted, short and finely branched roots. Plants assimilate Cl as the anion chloride (Cl – ) from the soil solution, where it is usually associated with Na and K as an easily soluble salt. Normally plants contain more CI than the minimum requirement and deficiencies have not been reported for grapevines. No fertilisation strategy is thus in place, but it is possible that insufficient CI nutrition will develop over time, as in the case of S, in high rainfall areas on sandy and highly weathered soils, because KCl is the only Cl containing fertiliser currently being applied – no other fertiliser mixture contain Cl. In coastal regions problems with Cl nutrition are not envisaged, because up to 100 kg Cl per ha –1 yr –1 may be carried onto the land by sea winds. The CI content of chicken and Karoo sheep manure is high, but the potential danger of their salinity is of greater significance than it being a source of CI nutrition. In practice chlorine is more commonly associated with saline conditions, where it, together with Na, prompts the vine to take up water at a higher osmotic potential, but can also have a direct toxic effect. The symptoms of NaCl damage are browning of the

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