FERTILISER GUIDELINES FOR THE WINE INDUSTRY

MICRO- OR TRACE ELEMENTS Elements in this group that are essential for plant growth, include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo) and chlorine (CI). Iron (Fe) The most important function of Fe is as energy supplier in redox reaction en zyme systems. Although it does not form part of the molecule, Fe is essential for the forming of chlorophyll. It is assimilated as the ferrous or ferric ion or as synthetic or natural chelates, but in order to be physiologically active in the plant, Fe must be, at least partially, in reduced form. Most rocks contain an abundance of Fe, with the exception of lime- and certain sandstones, subsequently the Fe-content of most soils is also high. There is no correlation however, between the total Fe-content of soil and the accessibility thereof for plants. The presence of lime in the soil (high pH), in conjunction with cool, wet conditions, can contribute to the accumulation of bicarbonate ions, which, either directly in the soil or indirectly in the vine, negatively affects the solubility of Fe. In Europe, where grapevines are commonly cultivated on soils containing free lime, and are often subject to limeinduced Fedeficiencies (chlorosis), the so-called ‘active lime’ of the soil is determined in order to make an informed decision regarding a suitable rootstock in terms of its tolerance to lime, or lime-chlorosis. This analysis can be further supplemented by simul taneously also determining the accessible Fe and then, combined with the active lime, the Fe-chlorosis potential index for the soil can be calculated.For South African vineyard soils the determination of ‘accessible’ Fe is considered unnecessary as the implication thereof with respect to Fe-supply to the vine is not known. Since Fe is so immobile in the plant and the form in which it occurs is so vital, it is possible that chlorotic leaves can contain even more Fe than healthy leaves. The value of leaf analysis for Fe is therefore questionable. The most characteristic symptom of a Fedeficiency is general interveinal chlo rosis (yellowing) of leaves, first on the younger leaves, with totally yellowed, almost albino-like leaves in severe cases (Fig. 19). In the latter case the leaves brown easily due to sunburn, later fall off, the defoliated shoots die back and secondary shoots develop, giving the vine a bush-like appearance. The crop then dramatically decreases and the vine may even die.

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