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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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