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The effects of soil on the taste of wine

GSA TODAY | MAY 2016  Gregory J. Retallack, Dept. of Geological Sciences, University of          in any literal way is scientifically impossible.” Maltman particu-
                      Oregon, Eugene, Oregon 97403, USA, gregr@uoregon.edu; and                  larly decries descriptions of wines as tasting “slaty” or “earthy.”
                      Scott F. Burns, Dept. of Geology, Portland State University,               Between these extremes of literally tasting soils in wine and the
                      Portland, Oregon 97207, USA, burnss@pdx.edu                                implausibility of transferring tastes from soil to wine are
                                                                                                 numerous studies documenting soils’ effects on wine quality
                      ABSTRACT                                                                   (Imre et al., 2012; Costantini et al., 2012; Burns, 2012), including
                                                                                                 this study. Here we address the conventional wisdom of vintners
                        The conventional wisdom of vintners is that alkalinity, and thus         that low-fertility soils produce more profitable wines (Goode,
                      less sour and more rounded taste, are enhanced in wine and                 2014) in a case study of pH as a proxy for soil fertility compared
                      grapes challenged by low-nutrient soils. A common thread here is           with the pH of Pinot Noir wines produced from that soil in the
                      pH, an objectively measurable variable that is both a part of wine         Willamette Valley of Oregon, USA.
                      taste and a proxy for soil fertility. The role of low-pH soils is
                      supported by metadata on Oregon wines from different soils in                Wine within the usual pH range of 3.4–3.8 tastes pleasantly
                      the Willamette Valley of Oregon, USA, which show significant               fresh, brisk or tart, but with too much acid it can be as sour as
                      inverse correlations between minimum pH of the soil and pH of              vinegar, and too little acid leaves it flat and prone to spoilage
                      finished Pinot Noir wine. There is also a direct correlation               (Goode, 2014). Wines are 80%–90% water and 0.1%–20% sugar,
                      between depth of clayey horizons and pH of the finished wine.              with pH determined by a balance between 0.3%–1% acids
                      The minimum pH of these soils is near the base of the clayey (Bw           (tartaric, malic, citric, lactic) and mildly alkaline alcohol
                      or Bt) horizon and is inversely correlated with depth of the clayey        (8%–20% ethanol, glycerol), organic compounds (0.3%–1%
                      horizon. Low soil pH is found in thick middle Pleistocene soils of         flavor compounds, such as anthocyanins, tannins, and flavo-
                      bedrock (Jory, Willakenzie, Laurelwood, and Bellpine soil series)          noids), and mineral cations (0.1%–0.3% potassium, sodium,
                      and high soil pH in thin soils on late Pleistocene and Holocene            calcium, and magnesium; Jackson, 1994). Pinot Noir wine has
                      Missoula Flood deposits and loess (Hazelair, Woodburn, and                 more than 800 distinct organic compounds, which determine
                      Chehulpum soil series). Similar relationships are found between            aroma, color, and flavor (Fang and Qian, 2005).
                      soil pH or depth and the pH of grapes at harvest, which is lower
                      and more varied than pH in finished wine. These relationships are            Soil pH is a convenient proxy for fertility. Moderately acidic
                      especially notable in years of good harvest, but obscured by wine-         soils (pH 4.5–5.8) are low in plant nutrients (Ca2+, Mg2+, Na+, K+)
                      making techniques in years of poor harvest. Good harvest years             because the exchange complex has high amounts of non-nutrient
                      are not necessarily vintages esteemed by wine connoisseurs, which          cations (H+, AlOH2+, Al(OH)2+). Moderately alkaline soils (pH
                      are more strongly correlated with low October precipitation.               8–10) have growth-limiting salts and moisture deficits. The most
                                                                                                 fertile soils for plant growth are between these extremes
                      INTRODUCTION                                                               (Retallack, 2001).

                        The effects of soil on wine are a key component of the French            GEOLOGICAL AND PEDOLOGICAL BACKGROUND
                      concept of gôut de terroir (taste of soil), first codified in 1905 legis-
                      lation of Appellation d’origine contrôlée (Trubek, 2008). The                Oregon’s Willamette Valley is a tectonic forearc basin on the
                      French concept of soil at the time was less scientific than romantic       convergent margin of the northwestern United States, dividing an
                      and political, as revealed by Emile Zola’s (1888) famous novel La          uplifted subduction complex of the Coast Range from active
                      Terre (The Soil). An expanded concept of wine terroir, including           andesitic volcanoes of the Cascade Range. Much of the valley is
                      local climate and winemaking traditions, can be traced back to             underlain by volcaniclastic marine sandstones and siltstones
                      fifteenth-century Burgundian monks, but the naming of local                ranging in age from Eocene to Oligocene, but parts of the region
                      wines after localities and comparisons of their relative quality           were overrun by middle Miocene Grande Ronde and Wanupum
                      were recorded in ancient Rome, Greece, and Egypt, back about               Basalts of the Columbia River Basalt Group (Yeats et al., 1996).
                      4,980 years ago (McGovern, 2003). Nevertheless, it has been diffi-         Large areas of the valley floor are covered by Willamette silts from
                      cult to find a scientific justification for judgements like the            the 15–18 ka Missoula Floods (O’Connor et al., 2001; Allen et al.,
                      following quotation: “The sandy soil will, in general, produce a           2009). These three geological elements determine three main
                      delicate wine, the calcareous soil a spirituous wine and the               kinds of soils planted in vineyards (Moore, 2002; Burns, 2012):
                      decomposed granite a brisk wine” (Busby, 1825, p. 11). In contrast         (1) middle to late Pleistocene soils on Miocene basalt, (2) on Eocene-
                      is the opinion of Maltman (2008, p. 1), “The notion of being able          Oligocene sedimentary rock, and (3) late Pleistocene soils on loess
                      to taste the vineyard geology in the wine—a goût de terroir—is a           or alluvium (Fig. 1).
                      romantic notion that makes good journalistic copy and is mani-
                      festly a powerful marketing tactic, but it is wholly anecdotal and           The thick Jory silt loam was named the state soil of Oregon for
                                                                                                 its importance to the wine industry (Oregon State Legislature,
                                                                                                 2011). It includes two distinct varieties developed on bauxitic
                                                                                                 laterite and basalt. At the type locality of Jory Hill south of Salem,

     GSA Today, v. 26, no. 5, doi: 10.1130/GSATG260A.1.

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