South Africa Wine Technical Yearbook 2025

TABLE 1 Vigour classes 1. Poor

• If poor growth was achieved through the season, apply the full third of the initial estimated nitrogen requirement determined according to crop load, thus 3 - 4 kg nitrogen per tonne of grapes harvested.

- Youngest shoots 30 - 60 cm. - No/few active growing tips after flowering. - Internodes mainly short (<5 cm). - Light/yellow-green leaf colour. 2. Ideal - Shoot length 80 - 120 cm. - No/few active growing tips after veraison. - Internode length 6 - 8 cm. - Pale green to light green leaves. - Shoots thicker than pencil at base (1 - 1.5 cm). 3. Lush (excessively vigorous) - Most shoots longer than 120 cm. - Active growing tips after ripening. - Internode length >10 cm. - Shiny, dark green leaves. - Shoots of thumb thickness at base (>2 cm).

• If the vineyard displayed ideal vigour with good balance, apply 1.5 - 3 kg nitrogen per kilogram grapes harvested.

• With lush or excessively vigorous growth, the post-harvest fertilisation can be omitted.

vine to utilise phosphate sources. The long growing season also allows the vine enough time to absorb adequate amounts of phosphate. Consequently, deficiency symptoms are rarely observed in the field despite the generally low P contents of South African vineyard soils. Phosphate is bound so quickly by clay in the soil, though, that little movement takes place. The phosphate concentration in the soil solution is pH dependent, with the highest concentrations found between pH 3.5 - 6.0. In Figure 3, a peak uptake is nevertheless noticeable after harvest when the most phosphate is absorbed. We can therefore use the following formula as guideline to determine the post-harvest phosphate fertilisation requirement: 0.7 kg x 23% = 0.11 kg P required per tonne of grapes produced. For 30 tonnes per ha, 3.3 kg phosphate can thus be applied as post-harvest fertilisation. In most cases, the requirement is so small that it can be omitted from the post-harvest programme. The fact that this element is not very mobile, also ensures that available phosphate isn’t leached out of the soil and will still be available post-harvest for uptake by the plant.

(reserves) and is therefore particularly important for carbohydrate-rich plants like grapevines. Although potassium is translocated from the leaves to the grapes, there is still a peak uptake immediately after harvest, which constitutes about 15% of the annual requirement. Potassium reacts with organic acids in grapes, especially tartaric acid, to form salts during ripening, which is why excessive K-fertilisation can lead to higher pH’s in must. It is not the only or primary cause of high pH’s in must, however. We can therefore use the following formula as guideline to determine the post-harvest potassium fertilisation: 3 kg x 15% = 0.45kg K required per tonne of grapes produced. For 30 tonnes per ha, 13.5 kg clean potassium is applied as post-harvest maintenance fertilisation. Phosphate Although phosphate is an essential macro-nutrient, it is required in very small amounts by the grapevine. The grapevine is also able to utilise less accessible sources of P due to specific root properties and mycorrhiza associations in the soil and root environment. Soils with sufficient living organisms can thus help the

Table 1 can be used as a guideline to determine the vigour of vineyards for the most recent season, which can then be used to determine the nitrogen requirement for the next season. To calculate the amount of nitrogen required for post-harvest fertilisation, the following formula can be used: 3 kg x 38% = 1.14 kg N per tonne of grapes produced. We can thus apply 1.14 kg x 30 tonnes per ha = 34.2 kg of clean nitrogen. Be sure however, to take the past season’s vigour, as described in Table 1, into consideration to determine the actual amount of N to be applied as maintenance fertilisation. Potassium Potassium plays an important role in several of the plant’s metabolic processes and thus can be considered the most important metal cation. Potassium is taken up from budding to harvest, mainly because it is required by the grapes during this period (Figure 2). It is one of the most mobile elements in the vine and is important for the regulation of osmotic pressures and subsequently water management of the vine. It also plays a role in the transport and condensation of carbohydrates

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TECHNICAL YEARBOOK 2025