FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Although soils with high pH were traditionally extracted using the Olsen ex tractant, the pH of many soils have currently decreased to pH KCl < 7.0, which means that P from the labile pool increases in availability. If a Bray I or Bray II extraction is now done, excessively high P-concentrations are often obtained. It is therefore suggested that Bray I extractions are 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 using one of the other extractants, the provided norms can be used with reasonable reliability – but the use of Olsen extractions should be avoided. Depending on the clay content of the soil, the P-content should be augmented to the specific norm. For soil preparation the average P-content is determined to 600 mm soil depth. To increase the P-content by 1 mg/kg in the soil for 300 mm depth, 4.5 kg P should be applied, therefore 9 kg P per ha for 600 mm depth. On high pH soils (pH KCl > 7) it may be an option to adjust the recom mended figure downwards and 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 with which the concentration needs to be raised. In the case of high pH soils, where P is easily retained, the annual fertilisation requirement must be calculated and split over three instalments throughout the season. During the harvest 0,7 kg P is removed for each ton of grapes pro duced 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 limit potassi um uptake. Phosphate contents of more than 50 mg/kg in sandy soils, 60 mg/ kg in loamy soils and 70 mg/kg in clayey soils, can be problematic in any pH regime. The stone and gravel volume must therefore always be used in the calculation of the P requirement, to prevent over-fertilisation with P.

3

POTASSIUM (K)

As far as grapevine 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-binding. It is not recommended that K is applied on sandy soils during soil preparation – it could easily leach out on such soils. A broad norm that may be set for K-nutrition on sandy soils, is that annual maintenance fertilisation of 3 kg K per ton of production is applied.

21