FERTILISER GUIDELINES FOR THE WINE INDUSTRY

As already stated, in South Africa the plant-available K is mostly extracted using ammonium acetate (NH 4 Ac), however some laboratories also use Mehlich III. Similar results are obtained between the two extractants for soil with pH KCl < 6.0, where Mehlich III values are approximately 0.9 x NH 4 Ac (Nathan et al ., 2005). For soils with a higher pH (e.g. pH KCl > 6.0), Mehlich III extracts less K than NH 4 Ac (Sawyer, undated). On heavier soils (loamy & clayey soils), the general norms indicated in Table 2 may be used as guidelines for maximum K-values (Conradie 1994). These norms are linked to the differences in clay mineralogical types occurring in the various regions, and are more or less representative of K-contents which constitute 4% of the total interchangeable cations. TABLE 2. Maximum and excessive norms for potassium concentration in soil, as determined using ammonium acetate, to ensure optimal grapevine performance without affecting wine quality (Conradie, 1994).

3

Breede River

Olifants River

Region

Coastal

Karoo Orange River

Maximum norm

70

80

100

100

120

mg/kg

Excessive concentration

105

120

150

150

180

Adjustment of K-concentration during soil preparation is only required in excep tional circumstances. Where deficiencies do occur or are expected in heavy soils, the average K-requirement is determined to a soil depth of 600 mm. For soil with a K-concentration below the norms mentioned above, K-fertilisation should be applied. In the case of production vineyards the K-content is only determined to a soil depth of 300 mm. The requirement per hectare is 4,5 kg of K to increase the K-content in the soil by 1 mg/kg over 300 mm in depth. During soil preparation (to a soil depth of 600 mm), 9 kg K per ha should therefore be applied for each 1 mg/kg increase required in the soil. Since excessive K-contents in the soil may cause problems with colour and pH in wine, over-fertilisation should be avoided.

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