FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 3

A simplified approach is mostly followed by laboratories serving the table grape industry, namely when the soil pH KCl < 7.0, either a citric acid, Bray I, Bray II or Mehlich III extraction is conducted and above-mentioned norms are used (Table 1). For soil with a pH KCl ≥ 7.0, an Olsen extraction is often done, and the norms in Table 1 are still used. The logic is that an Olsen extraction is less aggressive and is done at a higher pH, theoretically reflecting the lower rate of P release in the root zone better at higher soil pH conditions. For the Olsen extraction technique, however, it has been shown that only 5 – 7% of citric acid extractable P is extracted. This means that the total P in the soil can become excessively high (e.g. if extracted with Bray I or Bray II), while the Olsen P remains below the norm. It is therefore suggested that Bray I extractions are to be done on soils with pH KCl > 7.0, and the Bray I norms in Table 1 are used to calculate P requirements. If analysis is done with one of the other extractants, the provided norms can be used with reasonable reliability – but the use of Olsen extractions should preferably be avoided. Depending on the clay content of the soil, the P content should be augmented to the specific norm, as indicated in Table 1. For soil preparation the average P content is determined to a soil depth of 600 mm. To increase the P content by 1 mg kg –1 for 300 mm depth, 4.5 kg P should be applied, therefore 9 kg P per ha for 600 mm depth. On soils with high pH values (pH KCl > 7) it may be an option to adjust the recommended figure downwards and to increase the annual maintenance fertilisation volumes. For production vineyards the P content is calculated to a soil depth of 300 mm only, e.g. 4.5 kg P per ha must be applied for every 1 mg kg –1 with which the concentration must be increased. In the case of soils with high pH values, where P is easily fixed, the calculated, annual fertiliser requirement must be split over three installments and applied throughout the season. During harvest 0.7 kg P is removed for each ton of grapes produced and maintenance fertilisation should be calculated accordingly, except where soil analyses indicate that the P content is optimal or above the norm. It is important to avoid excessive applications of P, since this may restrict potassium uptake (Conradie and Saayman, 1989). Phosphate contents of more than 50 mg kg –1 in sandy soils, 60 mg kg –1 in loamy soils and 70 mg kg –1 in clayey soils, can be problematic at all pH values. The stone and gravel volume should therefore always be used in the calculation of the P requirement, to prevent over fertilisation with P. POTASS I UM As far as the grapevine’s potassium (K) nutrition is concerned, soil texture also plays an important role in the interpretation of soil analyses. Firstly, K is leached very quickly out of sandy soil, and secondly, clay minerals can play an important role in K fixing. Potassium applications are not recommended on sandy soils during soil preparation – leaching can easily occur on such soils. A broad norm that may be used for K nutrition on sandy soils, is an annual maintenance application of 3 kg K per ton of grapes produced.

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