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The aim of this work was to evaluate the interactive effects of different rates of nitrogen and potassium fertilisation on the phenolic composition of grape berries during ripening. To achieve this objective, in 2000 a trial was carried out in a Tempranillo vineyard located in the 'Ribera de Duero' Apellation d'Origine area (Spain). The plants were treated with three nitrogen (0, 50 and 200 g N per vine) and three potassium (0, 60 and 120 g K 2 O per vine) doses, applied as ammonium nitrate and potassium sulphate respectively, in a randomised complete block design with four replications. The rate of fertilisation did not significantly affect either the vigour or productive capacity of the vines or the size of the berries. However, high nitrogen doses caused a substantial delay in the accumulation of sugars during ripening compared with other treatments. High potassium supplies caused decreases in the total acidity of the berries but did not affect the chromatic parameters of the must at harvest. Three different stages could be distinguished in the development of total polyphenol contents in grape skins during ripening: a slow initial increase, a sharp accumulation during the fifth and sixth weeks after veraison and a final period of reduction until harvest. The vines with no added nitrogen showed more accumulation of polyphenols in the skin than those with other treatments, especially during the last few weeks of sampling. These differences between nitrogen treatments became less as the potassium fertilisation dose was increased. The nitrogen dose of 50 g N per vine increased the levels of anthocyanins in the skin (600 mg l −1 standard extract) compared with the untreated control vines (532 mg l −1 ), and this significantly increased the colour density of the must. The degree of polymerisation of condensed tannins decreased when the ratio N:K was balanced and the amounts of both nutrients applied were high.

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