FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 3

Plant-available Ca and Mg can also be extracted by means of NH 4 Ac or Mehlich III. Similar results are obtained for the two extractants for soil with pH KCl < 6.0 (Nathan et al ., 2005). The following general norms may be used as guidelines for minimum Ca and Mg concentrations. The Ca:Mg ratio should preferably not exceed a value of 6.

TABLE 3: Minimum norms for calcium and magnesium concentration in soil, as determined using ammonium acetate, to ensure optimal grapevine performance.

Sandy soil

Clayey soil

Nutrient

mg kg –1

cmol (+) kg –1

mg kg –1

cmol (+) kg –1

Calcium

360

1.80 0.30

500 120

2.50 1.00

Magnesium

40

MI CRO-E LEMENTS (BORON, MANGANESE, ZI NC, COPPER) Soil analysis reports for vineyards usually indicate zinc (Zn), manganese (Mn), boron (B) and copper (Cu) contents as mg kg –1 . Zinc, Mn and Cu are extracted with EDTA, HCl or DTPA, while B is extracted with hot water. As soil pH increases the extraction efficiency of HCl reduces dramatically, making it unsuitable for use on soils with a pH KCl > 5.0. Even at a pH of 5 and lower it does not extract Mn adequately (Table 4). Extraction levels with DTPA appear to be similar to that of EDTA.

TABLE 4: Comparison of EDTA and HCl as extractants for micro-nutrients in soils of different pH values (Lambrechts, unpublished).

Cu (mg kg –1 )

Zn (mg kg –1 )

Mn (mg kg –1 )

Soil pH KCl

EDTA

HCl

EDTA

HCl

EDTA

HCl

5.0 6.0

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

17

34

5

7.0

17

1.4

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