FERTILISER GUIDELINES FOR THE WINE INDUSTRY

concentrations between pH (KCl) 3.5 and 6.0. At lower pHs less soluble Fe- and Al-phosphate compounds form and at higher pHs, few soluble Ca-phosphates. Phosphorus deficiency symptoms are rarely observed in the vine, due to the fact that it is not specific. It is described as diminished growth, dark green leaves, occasional purple colouration of main veins and a smaller angle between the main vein and the petiole. It is actually only identifiable in a direct comparison to an adequately nourished vine and thus cannot serve as a reliable indicator for the necessity of P-nutrition.

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FIGURE 5: No clear deficiency symptoms for Pdeficiency, only diminished growth.

As guideline for determining the P-nutrient status of the grapevine, soil anal yses are predominantly used. Due to the complex relationships between the different forms of P in the soil and the concentrations thereof in the soil solution, as discussed in Chapter 3, different empirical extractants are used in practice to try to imitate the ability of a specific group of plants to acquire P. For grapevines the stronger P-extractants like Bray II or 1%-citric acid are used, with norms as indicated in Table 12.

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