FERTILISER GUIDELINES FOR THE WINE INDUSTRY

It is estimated that about 3 kg K per production ton is required by the grapevine. This can serve as guideline for maintenance fertilisation and K can, similar to N, be applied in two or three instalments. However, as already stated, the requirement is relatively low in the post-harvest period. Where soil has a clay content of ≥ 10%, Kdeficiencies can be corrected by a single application before soil preparation or even be supplemented in existing vineyards. There are perceptions in California that Kfertilisation is only required when deficiency symptoms become visible on leaves. It has been found there and also locally that in such cases single, massive (shock) applications of up to 500 kg K per hectare are more effective than applications in instalments, probably due to more effective downward movement of K at high salt concentrations. Potassium deficiencies can be readily identified by leaf symptoms. It is a diffuse chlorosis on the edges of basal leaves, which can become necrotic in extreme cases and strongly resembles saline damage. 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.

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FIGURE 6: Diffuse leaf edge chlorosis of basal leaves due to Kdeficiency.

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