FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 5

Gypsum application is the most practical and cost-effective solution for S deficiencies, but reparation takes place over more than one season. The annual S requirement of vineyards is comparable to that of P and an application of 1 ton ha –1 gypsum should be sufficient for the lifetime of the vineyard. According to local norms for the sulphur content of soils, a value of 0 – 4 mg kg –1 is considered “low”, against “slightly low” for 4 – 6 mg kg –1 , “satisfactory” for 6 – 8 mg kg –1 , “good” for 8 – 12 mg kg –1 and “high” for >12 mg kg –1 . Because leaf analysis for S entails an extra extraction, it is usually not done. Consequently, accurate norms have not yet been generated, but it is accepted that values of lower than 0.2% in petioles point towards deficiency, irrespective of the time of sampling. MI CRO- OR TRACE-E LEMENTS Elements in this group that are essential for plant growth are iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo) and chlorine (Cl). Iron (Fe) The most important function of Fe is as energy supplier in redox-reaction enzyme systems. Even though it does not form part of the molecule, Fe is essential in the formation 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, at least partially, be in reduced form. Most rocks contain an abundance of Fe, with the exception of lime and certain sand- stones; 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, may contribute to the accumulation of bicarbonate ions, which, either directly in the soil or indirectly in the vine, negatively affects the solubility of Fe, thus leading to Fe deficiency symptoms. This is usually regarded as “lime induced Fe chlorosis”. Since Fe is so immobile within the plant and the form in which it occurs is so vital, it is possible for chlorotic leaves to contain even more Fe than healthy leaves. The value of leaf analysis for Fe is therefore questionable. The most characteristic symptom of Fe deficiency is general interveinal chlorosis (yellowing) of leaves, first on the younger leaves, with totally yellowed, almost albino- like leaves in severe cases (Figure 16). 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 yield then decreases dramatically, and the vine may even die.

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