FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

Potassium (K) Potassium can be considered the most important metal cation in the grapevine. Although it does not form part of any organic compound, it does play a role in various metabolic processes. It is one of the most mobile elements in the vine and is important in the regulation of osmotic pressures and therefore the water relationship of the vine. A K deficiency accelerates respiration and restricts photosynthesis, impedes protein synthesis and results in the accumulation of toxic amines like putrescine and agmatine which cause leaf scorch symptoms, quite similar to excessive N symptoms (Figure 7). In particular, it plays a role in the transport and condensation of carbohydrates and is therefore important for sugar accumulation and berry size in grapevines. Acute K deficiency causes incomplete ripening of grapes, with small, straggly bunches and uneven berry ripening. The importance of K in colour development is often stressed, but if K nutrition is sufficient to prevent deficiency symptoms, it is unlikely that additional K will make a positive contribution towards colour and quality. On the contrary, K over fertilisation may lead to quality problems like poor colour development (Morris et al ., 1982; Mpelasoka et al ., 2003) and higher occurrence of decay (Raath, 2012). An excessive supply of K may also induce deficiencies of other elements like Ca (Conradie, 2001) and Mg (Raath, 2012). It is thus important to apply the correct amount of K.

FIGURE 7: Diffuse colouring of the edges of older leaves (left) and later on necrosis, due to K deficiency (right) (Photos: D. Saayman).

Potassium deficiencies can be easily identified by leaf symptoms. Symptoms are expressed as a diffuse chlorosis on the edges of basal leaves, which can become necrotic in extreme cases and strongly resembles salinity damage (Figure 8). The leaves also curl up or downwards. In cases of latent deficiencies, browning or purpling of the leaf surface may occur from veraison onwards, which can develop into a superficial necrosis, the so-called black leaf symptom.

52 | MAINTENANCE FERTILISATION