FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 5

The effectiveness and economics of Fe applications on calcareous soil are questionable and, in such cases, only a foliar spray with 0.1 – 0.15% Fe chelate (Fe-EDTA) can be recommended, upon the first appearance of symptoms. This must be repeated at weekly intervals, up to a maximum of six times. In many cases Fe deficiency symptoms, which appear early in the season, will disappear gradually, as soon as the vine’s roots become active, thus making it possible to absorb Fe from the soil. A case of a Fe deficiency has been observed on acidic sandy soil, which developed from grey Table Mountain Sandstone. Here the cause was a total deficiency of Fe in the soil and not the inaccessibility due to high pH, even though excessive liming may have played a role as well. On acidic soil a Fe-EDTA application of about 1 kg ha –1 can be considered. Even though leaf analysis may not always be dependable for Fe, norms for grapevines are indicated in Table 22.

TABLE 22: Norms for the Fe contents (mg kg –1 ) of leaf blades and petioles of grapevines.

Fruit set

Veraison

Leaf blades Petioles Minimum High Minimum High Minimum High Minimum High 60 200 25 200 60 200 25 200 Petioles Leaf blades

Manganese (Mn) Manganese is responsible for activation of a large variety of enzyme systems and is, inter alia also involved in carbohydrate metabolism, phosphorylation, the citric acid cycle etc. It is also essential for chlorophyll formation and is an ingredient of chloroplasts where the Hill reaction tempo (to manufacture oxygen without the presence of carbon dioxide) is proportional to it. It also plays a role in the reduction of nitrates to, inter alia ammonium. An excess of Mn is toxic for the plant. There is a large variety of minerals that contain Mn and it is also found in the organic fraction of soils as humic acid complexes. Manganese deficiencies are predominantly induced by soil conditions where pH is high, where it is masked by Fe deficiencies. It is also often observed on the acidic soils of the Western Cape, especially in the early spring when the soil is still cold. Moderate Mn deficiencies apparently have no effect on growth and yield. The correlation between the total Mn content of soil and the accessibility thereof for plants is poor. The use of EDTA extractors in the determination of “accessible” Mn is an arbitrary process which imitates the ability of the plant to obtain Mn. For soils with a pH KCl of between 5.0 and 6.5, the total Mn content should amount at least 2.0 mg.kg –1 (see Table 5 in Chapter 3).

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