FERTILISER GUIDELINES FOR THE WINE INDUSTRY

Potassium (K) It can be considered the most important metal cation in the grapevine. Although it doesn’t form part of any organic compound, it plays a role in various meta bolic processes. It is one of the most mobile elements in the vine and is import ant in the regulation of osmotic pressures and therefore the water relationship of the vine. In particular it also plays a role in the transport and condensation of carbohydrates and therefore is especially important in carbohydrate-rich plants like grapevines. Potassium translocates from the leaves to the grapes during veraison and salinises organic acids, especially tartaric acid, during ripening, which explains why excessive K-nutrition can lead to higher pHs in must. At the same time, however, K induces higher malic and total acid contents in leaves and must. A Kdeficiency accelerates respiration and restricts photosynthesis, impeding protein synthesis and resulting in the accumulation of toxic amines like putrescine and agmatine which cause leaf scorch symptoms, quite similar to excessive Nsymptoms. Acute Kdeficiencies cause incomplete ripening of grapes, with small, straggly bunches and unevenly ripening berries. The importance of K in colour development and as wine quality factor is often stressed, but if Knutrition is sufficient to prevent deficiency symptoms, it is unlikely that additional K will make a positive contribution towards colour and quality. The primary sources of K in the soil are K-feldspars (orthoclase, microcline), micas (biotite, muscovite, vermiculite) and micaceous clays like illite. Usually the K-content of soil exhibits an increase with depth due to erosion in the top layers and leaching-in towards the deeper layers. In materials with low K-con tent it is, however, often the Ahorizon that has the highest Kcontent due to cation circulation and fertilisation practices. Guidelines for Kfertilisation are primarily based on soil analysis, which strives to obtain and maintain a 4% K-saturation of the soil’s exchange complex. It will never be necessary however, to raise the K-content of a soil higher than 120 mg.kg -1 , even if the 4% K-saturation norm indicates a higher value. Where the extent of the exchange complex is not known or, as in the case of soils with high free salt and/or lime contents, it cannot be deducted from the sum of the cations, absolute K-contents, as indicated in Table 2 of Chapter 3, can be aspired to. The uptake of potassium occurs in a prominent peak from bud break to harvest and even though K is mainly required by the grapes, there is another peak in uptake immediately after harvest. However this peak is not as prominent as in the case of N, and constitutes approximately 15% of the annual requirement.

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