FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 7

FIGURE 28: Acute K deficiencies lead to uneven ripening of berries (Photo: P. Raath).

Potassium not only plays an important role in berry growth and accumulation of sugar, but also in the process of shaping and accumulation of colour pigments. On the other hand, there is also convincing evidence that an excess of K fertiliser will make no contribution towards colour development of berries (Morris et al ., 1980; Morris et al ., 1982). As a matter of fact, it is known that an excess of K in the juice of red grapes will be injurious for colour intensity (Morris et al ., 1982; Mpelasoka et al. , 2003). The reason for this is that K induces the tartaric acid to precipitate and thus the tartaric acid: lactic acid ratio in the berry juice is reduced. Tartaric acid is a stronger acid than lactic acid (Rühl, 2000). The result is that the pH of the juice is raised and colour pigments change to a lighter shade, while taste will also change. Tartaric acid has a fresher taste than lactic acid. In an experiment in Paarl it was found that an excess of K fertiliser accelerated sugar accumulation of Prime to some extent, but there were also signs that the occurrence of internal browning was raised (Raath, 2012). Because K is such a mobile element, attempts to increase the K concentration of bunches by means of foliar sprays, are commonly employed. However, if it is ensured, by means of a suitable fertilisation programme, that the grapevines do not experience K deficiency, the possible contribution of these K sprays to the enhancement of ripening and colour, will be negligibly small. For grapevines that are adequately nourished with K, K will translocate readily from leaves to the bunch. CALC I UM Calcium plays an extremely important role during cell division and the initial development phase of the berry, especially as far as the formation of the walls of membranes and

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