How does soil influence wine quality?
Hugel vineyard – Schoelhammer – in Riquewihr (Alsace, France)
Basically, wine can be made from the grapes grown in virtually every type of soil. The essential requirements are: The soil needs to provide sufficient anchor to the vines, and enough water and nutrients for them to be able to produce grapes that ripen for the harvest. Nevertheless, there are many soil-related factors that will influence wine quality, such as the depth and composition of the soil, the pH, presence of organic matter, macro and micro nutrients, and availability and drainage of water. All these attributions and how viticultural practices can reduce, eliminate, or even optimize these conditions in the attempt of making better wine is what will be discussed in this paper.
Arguably, the 2 most important factors of a soil, for the purpose of growing grapes and making wine, are its structure and texture. These two components will cause a vine to grow and produce grapes differently. In principle, the percentages of clay, sand, silt, loam, and rock present in the soil will determine the grape varieties that would be well suited to produce grapes of the best quality for that specific site. A classic example: “Cabernet Sauvignon grows best in gravel soil. Merlot grows best in clay”. Case in point, Clos d’Estournel in Bordeaux plants Cabernet Sauvignon (a late ripener) on the top and southern slopes vineyards, where gravel is dominant, while Merlot is planted on the eastern slope in a clay limestone soil (which is cooler). The result is that both varieties ripen well and, therefore, wines of high quality are made from them. Just reverse the order and it’s easy to see how soil affects wine quality.
Soil depth (and the correspondent water availability) comes next. 16 inches of water is required by the average vineyard to grow and ripen grapes (some times more and sometimes less), depending on the climatic conditions and each foot in depth of a loamy soil will hold about 3 inches of available water. Thus, such a soil must be 5 feet or more in depth, while sandy soils must be considerably deeper to supply enough water to the vine. How does it affect the quality of a wine? There’s been ample demonstration throughout wine regions of the world that a mild (but not excessive) water stress during the ripening period of the grapes is favorable to wine quality and good viticultural practices can optimize the outcome: A vine bearing a large crop may show distress (and stop the ripening of the grapes which, in turn, will be detrimental to wine quality) from water scarcity, when vines carrying lower yields will show no signs of being stressed. Nevertheless, the ability of the roots to grow deep will make the vine less susceptible to water availability in the top layers. Chile’s Vigno Cooperative in the Maule Valley grows Carignan without irrigation and the roots of those vines have grown deep enough to allow the vines to survive the hot and dry Chilean summers without any major water stress. Not only that, the vines produce grapes of intense color, ripe phenolic compounds, and good concentration of aromatic and flavor pre-cursors, showing that a characteristic of the soil (in this case, depth) can improve the quality of a wine.
Being water availability and drainage of the soil critical factors on the outcome of the crop, it’s worth dedicating a paragraph on how human intervention can optimize (or mitigate) the intrinsic qualities of the soil of a vineyard through the use of irrigation and procedures to provide artificial drainage of excessive water (such as tillage) at critical points of the growing cycle of the vines, and especially when the grapes are ripening. Stephen Skelton MW, a writer from the UK, believes that water availability is a key element in defining the quality of a wine, more so than the chemical content of the soil: “Good soils for viticulture are those that can provide water during the growing season and is fairly dry during ripening”, he says.
Another viticultural practice that can help to improve the soil condition is the use of cover crops. They protect the soil from rain impact (that is especially helpful in hill sides that have a high risk of erosion), improve drainage, and help to build soil organic matter, growth of Arbuscular mycorrhiza fungi (which is essential for the absorption of nutrients by the roots), and to add organic nitrogen to the soil. Federica Stianti from Castello di Volpaia in Tuscany believes that having the rows of their vineyards planted with cover crops has helped to improve phenolic maturity of their grapes and, consequently, the quality of their wines.
For vines, nitrogen is food. They use it to grow and to reproduce and fertilization can help to improve any existent deficiencies of nitrogen (and other nutrients). However, an excess of this compound can be detrimental to fruit-bud formation or to fruit set. Nevertheless, the fact that matters the most regarding the influence of soil in wine quality is that high nitrogen concentration will also aid a strong vegetative growth. This can be detrimental to the production of high quality grapes and, therefore, high quality wines. Two other deficiencies and toxicities can bring similar results: With potassium deficiency, leaf fall is premature and can be so extensive that the fruit will fail to ripen; in chalk and limestone soils (plenty of calcium), vines are susceptible to lime-induced chlorosis; both detrimental to fruit quality. High salinity of the soil can have negative effects as well. When it exceeds certain levels, it affects plant growth and its ability to properly ripen grapes. As calcium is concerned, it’s also important to discuss acidification of vineyard soils. This is making harder to produce quality grapes in some main wine regions in Australia. Grape varieties vary substantially in their optimum pH for growth but, very acidic soils (pH of 5.5 or lower), can affect the ability of the vine to absorb some nutrients, while making others much more available, bringing them to levels of toxicity. This can certainly reduce the quality of the wines, but the application of calcium is a remedy for this condition.
Vineyard of the Sannio region in Campania, Italy.
Some may argue that the presence of soil pests, such as Phylloxera and nematodes, should also be included in the characteristics that influence wine quality. However (and especially for Phylloxera), the counter-argument is that what is at stake is the viability of the vineyard and, only to a much lesser extent, the quality of the wines produced. At any rate, the use of appropriate rootstocks with high resistance to one or the other is the solution to the problem. Along the same line (of debatable factors) lies the concept that the soil can deliver “a taste of minerality” to the wine, but can it really? Some scientists such as Carole Meredith (professor at UC Davis) say that the vine is unable to take up the minerals from the soil. However, some people will passionately argue that a Chablis is better than many “other Chardonnays” because of the flinty minerality impaired to the wines by the Kimmeridgian soils of the region.
In conclusion, the answer is yes. Even if some specialists will argue that the influence of the soil is not as dramatic in wine quality as those presented by the climatic conditions of a region (or, even more so, by the weather conditions of a certain vintage), to a certain extent soil can influence wine quality with its own positive or negative characteristics and the viticultural practices used to optimize the positive ones, and reduce or even eliminate the negative traits. Overall, any description of an “ideal soil” for grape growing must acknowledge the local environment where the vineyard is located, the landscape features, grape varieties planted, and the style and level of quality of the wine to be made.
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