FERTILISER GUIDELINES FOR THE WINE INDUSTRY

MICRO-ELEMENTS (B, Mn, Zn, Cu)

Soil analysis reports for vineyards usually indicate zinc (Zn), manganese (Mn), boron (B) and copper (Cu) contents in mg/kg. Zinc, Mn and Cu are extracted with either EDTA, HCl or DTPA, while B is extracted with hot water. As soil pH increases the extraction efficiency of HCl reduces dramatically, making it un suitable for use on soils with a pH KCl > 5.0, and even then it does not extract Mn adequately (Table 4). Extraction levels with DTPA appear to be similar to EDTA.

TABLE 4. Comparison of EDTA and HCl as extractants for micro-nutrients in soils of different pH values (Lambrechts, unpublished).

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Cu (mg/kg)

Cu (mg/kg)

Cu (mg/kg)

Soil-pH KCl

EDTA HCl

EDTA HCl

EDTA HCl

5.0 6.0 7.0

0.36 0.28 0.21

0.45 0.17 0.07

0.64 0.51 0.41

0.80 0.23 0.07

67 34 17

17

5

1.4

Micro-elements are required by the vine in small quantities and the availability thereof is directly dependent on the pH of the soil solution. Where the pH is high, manganese (Mn) and zinc (Zn) are inaccessible to the plant since these elements do not remain in the solution. Sometimes these elements may therefore be present in the soil, but not accessible to the plant. Consequently soil analysis is not a reliable means of determining the availability of micro-el ements. To ascertain whether the metals are excessive or insufficient, leaf analyses should be done. In soils with low pH, boron (B) and Zn deficiencies can also be expected. On the other hand, Mn in low pH soil may be so solu ble that it could become toxic to the vine. Lime applications will solve these problems by making B and Zn more available to the plant and Mn less soluble (Van Schoor, et al ., 2001). In Table 5 the norms for optimal micro-nutrient concentrations in soil are sup plied. In cases where the concentrations of the nutrients in the soil are below the norms, deficiencies may occur. In such cases the vineyard should be monitored visually for symptoms of deficiencies. If there is still any doubt, leaf analyses should be done. In the past, Cudeficiencies very rarely occurred in vineyards, due to the use of fungicides that contain Cu. With the reduction in use of these products, attention should also be given to this micro-nutrient.

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