FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

As already stated, in South Africa plant-available K is generally extracted by means of ammonium acetate (NH 4 Ac), but some laboratories also use Mehlich III. Similar results are obtained for the two extraction agents, with Mehlich III values being approximately 0.9 x NH 4 Ac (Sawyer & Mallarino, 1999; Nathan et al ., 2005). On heavier soils (loamy & clayey soils), the general norms, as 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. In some cases, it can be K contents which constitute 4% of the total exchangeable cations, but preferential use of the 4% norm, as sometimes suggested, is discouraged.

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).

Region

Coastal Breede River Olifants River Karoo Orange River

Maximum norm

70

80

100

100

120

mg kg –1

Excessive concentration

105

120

150

150

180

Adjustment of K levels during soil preparation is only required for heavier soils, where deficiencies are expected. The mean K concentrations is then determined to a soil depth of 600 mm, and for values below the above-mentioned norms, K fertilisation should be applied. In the case of producing vineyards, the K content is determined to a soil depth of 300 mm only. The requirement per hectare is 4.5 kg of K to increase the K content in the soil by 1 mg kg –1 over 300 mm 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 –1 increase required. Since excessive K contents in the soil may cause problems with the uptake of magnesium (Mg) and storage ability, over fertilisation should be avoided. CALC I UM AND MAGNES I UM Both calcium (Ca) and magnesium (Mg) are essential nutrients, required for optimal grapevine performance. Generally, these nutrients are abundant in soils with pH values within the optimal range, making fertilisation of these nutrients unnecessary. Through properly calculated lime requirements and the right choice of lime type, sufficient Ca and Mg to satisfy the nutritional requirements of the grapevine, is applied to the soil (Eksteen, 1969). Nevertheless, some sandy soils with excessive volumes of stone might be deficient in Ca and Mg, even though the soil pH is optimal. Likewise, when the wrong type of lime (e.g. calcitic lime) is used for a Mg deficient soil, applications of Mg might be required.

24 | INTERPRETATION OF SOIL ANALYSIS REPORTS FOR VINEYARDS