Different soil types and relief influence the quality of Merlot grapes in a relatively small area in the Vipava Valley (Slovenia) in relation to the vine water status

Abstract

Besides location and microclimatic conditions, soil plays an important role in the quality of grapes and wine. Soil properties influence the soil water holding capacity, the vegetative growth and the vine water status. Moreover, it is known that the vine water status affects the grape quality, such as concentration and structural characteristics of phenols. In a relatively small area of the Vipava Valley, soil profiles were described on terraced and flat vineyards in the valley bottoms (n=5). Texture, pH, organic matter, available phosphorus, potassium and magnesium were determined for each soil horizon in the soil profile. Viticultural parameters (number of buds, clusters and leaf area) were standardised. Stem water potential (SWP) was measured during the growing season and grapes were randomly sampled during ripening in 2019. The content and structural characteristics of proanthocyanidins were determined in grape skins and seeds. On terraces, SWP was predominantly more negative than on flat vineyards, probably mainly due to the soil ability to retain water. Vineyards on flysch terrace soils consist of up to 90% coarse material and therefore contain less organic matter than the dense and largely non-skeletal loamy alluvial soils of flat vineyards. These two extreme groups of vineyards in the same vine-growing area differ considerably in morphology and soil profile characteristics, as well as microclimatic conditions and relief. The diversity of vineyards is reflected, among other things, in the grape quality. Higher concentration of anthocyanins in the skins and higher galloylation of seed proanthocyanidins was found in grapes from terraced vineyards. Understanding relationships between site, soil and the phenolic ripening of grapes is crucial for determining the best vineyard sites and thus for characterising terroirs suitable for high quality wines.